Astronomy

Luminosity of the Milky Way compared to Seyfert galaxies

Luminosity of the Milky Way compared to Seyfert galaxies


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This page lists the total luminosity of the Milky Way (MW) Galaxy to be $4 imes10^{10}$ $L_{ m sol}$, and $L_{ m sol}=3.86 cdot 10^{33} { m ergs.sec}^{-1}$; this gives a total luminosity for the MW of $1.54cdot 10^{44}{ m ergs.sec}^{-1}$.

But Seyfert galaxies can have luminosities of $sim 10^{40} - 10^{42}{ m ergs.sec}^{-1}$, and this is due to the high luminosity of the galactic core of a Seyfert.

Since the MW doesn't have an active SMBH, does this mean that the MW is particularly bright due to a high luminosity stellar population? Or am I missing something?


Luminosity of the Milky Way compared to Seyfert galaxies - Astronomy

Context: Recently, a relationship between the water maser detection rate and far infrared (FIR) flux density has been found as a result of a 22 GHz maser survey in a sample comprised of northern galaxies with 100 μm flux density >50 Jy and a declination >-30°.
Aims: The survey has been extended toward galaxies with lower FIR flux densities in order to confirm this correlation and to discover additional maser sources for relevant follow-up interferometric studies.
Methods: A sample of 41 galaxies with 30 Jy < S 100 μ m < 50 Jy and δ > -30° was observed with the 100-m telescope at Effelsberg in a search for the 22 GHz water vapor line. The average 3σ noise level of the survey is 40 mJy for a 1 km s -1 channel, corresponding to a detection threshold for the isotropic maser luminosity of 0.5 L ⊙ at a distance of 25 Mpc.
Results: Two detections are reported: a megamaser with an isotropic luminosity, L , of ≈35 L ⊙ in the Seyfert/Hii galaxy NGC 613 and a kilomaser with L ≈ 1 L ⊙ in the merger system NGC 520. The high luminosity and the presence of a Seyfert nucleus favor an association for NGC 613 with an active galactic nucleus. The kilomaser in NGC 520 was also detected with the Very Large Array, providing a position with subarcsecond accuracy. The H2O emission, originating from a ⪉0.02 pc sized region with a brightness temperature ⪆10 10 K (if the observed variations are intrinsic to the masing cloud(s)), is close to one of the two radio continuum sources located in the inner parsecs of NGC 520. The maser is most likely associated with a young supernova remnant (SNR), although an association with a low-luminosity AGN (LLAGN) cannot be ruled out. The maser detection rate, with 2 new maser sources out of 41 galaxies observed, is consistent with expectations extrapolated from the statistical properties of the S 100 μ m > 50 Jy sample. The H2O kilomasers are “subluminous”, while H2O megamasers tend to be “superluminous” with respect to the FIR luminosity of their parent galaxy, when compared with sites of massive star formation in the Milky Way.

Based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut für Radioastronomie) at Effelsberg.


The H i Distribution of the Milky Way

Neutral atomic hydrogen (H i ) traces the interstellar medium (ISM) over a broad range of physical conditions. Its 21-cm emission line is a key probe of the structure and dynamics of the Milky Way Galaxy. About 50 years after the first detection of the 21-cm line the exploration of the H i distribution of the Milky Way has undergone a true renaissance. This was triggered by several large-scale 21-cm surveys that became available within the past decade. New all-sky surveys unravel the shape and volume density distribution of the gaseous disk up to its borders. High-resolution Galactic plane surveys disclose a wealth of shells, filaments, and spurs that bear witness to the recycling of matter between stars and the ISM. All these observational results indicate that the H i gas traces a dynamical Galactic ISM with structures on all scales, from tens of astronomical units to kiloparsecs. The Galaxy can be considered to be a violent, breathing disk surrounded by highly turbulent extra-planar gas.


Contents

During the first half of the 20th century, photographic observations of nearby galaxies detected some characteristic signatures of AGN emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Some early observations included the first spectroscopic detection of emission lines from the nuclei of NGC 1068 and Messier 81 by Edward Fath (published in 1909), [1] and the discovery of the jet in Messier 87 by Heber Curtis (published in 1918). [2] Further spectroscopic studies by astronomers including Vesto Slipher, Milton Humason, and Nicholas Mayall noted the presence of unusual emission lines in some galaxy nuclei. [3] [4] [5] [6] In 1943, Carl Seyfert published a paper in which he described observations of nearby galaxies having bright nuclei that were sources of unusually broad emission lines. [7] Galaxies observed as part of this study included NGC 1068, NGC 4151, NGC 3516, and NGC 7469. Active galaxies such as these are known as Seyfert galaxies in honor of Seyfert's pioneering work.

The development of radio astronomy was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby active elliptical galaxies such as Messier 87 and Centaurus A. [8] Another radio source, Cygnus A, was identified by Walter Baade and Rudolph Minkowski as a tidally distorted galaxy with an unusual emission-line spectrum, having a recessional velocity of 16,700 kilometers per second. [9] The 3C radio survey led to further progress in discovery of new radio sources as well as identifying the visible-light sources associated with the radio emission. In photographic images, some of these objects were nearly point-like or quasi-stellar in appearance, and were classified as quasi-stellar radio sources (later abbreviated as "quasars").

Soviet Armenian astrophysicist Viktor Ambartsumian introduced Active Galactic Nuclei in the early 1950s. [10] At the Solvay Conference on Physics in 1958, Ambartsumian presented a report arguing that "explosions in galactic nuclei cause large amounts of mass to be expelled. For these explosions to occur, galactic nuclei must contain bodies of huge mass and unknown nature. From this point forward Active Galactic Nuclei (AGN) became a key component in theories of galactic evolution." [11] His idea was initially accepted skeptically. [12] [13]

A major breakthrough was the measurement of the redshift of the quasar 3C 273 by Maarten Schmidt, published in 1963. [14] Schmidt noted that if this object was extragalactic (outside the Milky Way, at a cosmological distance) then its large redshift of 0.158 implied that it was the nuclear region of a galaxy about 100 times more powerful than other radio galaxies that had been identified. Shortly afterward, optical spectra were used to measure the redshifts of a growing number of quasars including 3C 48, even more distant at redshift 0.37. [15]

The enormous luminosities of these quasars as well as their unusual spectral properties indicated that their power source could not be ordinary stars. Accretion of gas onto a supermassive black hole was suggested as the source of quasars' power in papers by Edwin Salpeter and Yakov Zeldovich in 1964. [16] In 1969 Donald Lynden-Bell proposed that nearby galaxies contain supermassive black holes at their centers as relics of "dead" quasars, and that black hole accretion was the power source for the non-stellar emission in nearby Seyfert galaxies. [17] In the 1960s and 1970s, early X-ray astronomy observations demonstrated that Seyfert galaxies and quasars are powerful sources of X-ray emission, which originates from the inner regions of black hole accretion disks.

Today, AGN are a major topic of astrophysical research, both observational and theoretical. AGN research encompasses observational surveys to find AGN over broad ranges of luminosity and redshift, examination of the cosmic evolution and growth of black holes, studies of the physics of black hole accretion and the emission of electromagnetic radiation from AGN, examination of the properties of jets and outflows of matter from AGN, and the impact of black hole accretion and quasar activity on galaxy evolution.

For a long time it has been argued [19] that an AGN must be powered by accretion of mass onto massive black holes (10 6 to 10 10 times the Solar mass). AGN are both compact and persistently extremely luminous. Accretion can potentially give very efficient conversion of potential and kinetic energy to radiation, and a massive black hole has a high Eddington luminosity, and as a result, it can provide the observed high persistent luminosity. Supermassive black holes are now believed to exist in the centres of most if not all massive galaxies since the mass of the black hole correlates well with the velocity dispersion of the galactic bulge (the M–sigma relation) or with bulge luminosity. [20] Thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole.

Accretion disc Edit

In the standard model of AGN, cold material close to a black hole forms an accretion disc. Dissipative processes in the accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The expected spectrum of an accretion disc peaks in the optical-ultraviolet waveband in addition, a corona of hot material forms above the accretion disc and can inverse-Compton scatter photons up to X-ray energies. The radiation from the accretion disc excites cold atomic material close to the black hole and this in turn radiates at particular emission lines. A large fraction of the AGN's radiation may be obscured by interstellar gas and dust close to the accretion disc, but (in a steady-state situation) this will be re-radiated at some other waveband, most likely the infrared.

Relativistic jets Edit

Some accretion discs produce jets of twin, highly collimated, and fast outflows that emerge in opposite directions from close to the disc. The direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low. The jets have their most obvious observational effects in the radio waveband, where very-long-baseline interferometry can be used to study the synchrotron radiation they emit at resolutions of sub-parsec scales. However, they radiate in all wavebands from the radio through to the gamma-ray range via the synchrotron and the inverse-Compton scattering process, and so AGN jets are a second potential source of any observed continuum radiation.

Radiatively inefficient AGN Edit

There exists a class of "radiatively inefficient" solutions to the equations that govern accretion. The most widely known of these is the Advection Dominated Accretion Flow (ADAF), [21] but other theories exist. In this type of accretion, which is important for accretion rates well below the Eddington limit, the accreting matter does not form a thin disc and consequently does not efficiently radiate away the energy that it acquired as it moved close to the black hole. Radiatively inefficient accretion has been used to explain the lack of strong AGN-type radiation from massive black holes at the centres of elliptical galaxies in clusters, where otherwise we might expect high accretion rates and correspondingly high luminosities. [22] Radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc.

AGN are a candidate source of high and ultra-high energy cosmic rays (see also Centrifugal mechanism of acceleration).

There is no single observational signature of an AGN. The list below covers some of the features that have allowed systems to be identified as AGN.

  • Nuclear optical continuum emission. This is visible whenever there is a direct view of the accretion disc. Jets can also contribute to this component of the AGN emission. The optical emission has a roughly power-law dependence on wavelength.
  • Nuclear infra-red emission. This is visible whenever the accretion disc and its environment are obscured by gas and dust close to the nucleus and then re-emitted ('reprocessing'). As it is thermal emission, it can be distinguished from any jet or disc-related emission.
  • Broad optical emission lines. These come from cold material close to the central black hole. The lines are broad because the emitting material is revolving around the black hole with high speeds causing a range of Doppler shifts of the emitted photons.
  • Narrow optical emission lines. These come from more distant cold material, and so are narrower than the broad lines.
  • Radio continuum emission. This is always due to a jet. It shows a spectrum characteristic of synchrotron radiation.
  • X-ray continuum emission. This can arise both from a jet and from the hot corona of the accretion disc via a scattering process: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is an excess of soft X-ray emission in addition to the power-law component. The origin of the soft X-rays is not clear at present.
  • X-ray line emission. This is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines, the best-known of which is the iron feature around 6.4 keV. This line may be narrow or broad: relativistically broadened iron lines can be used to study the dynamics of the accretion disc very close to the nucleus and therefore the nature of the central black hole.

It is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud. Radio-loud objects have emission contributions from both the jet(s) and the lobes that the jets inflate. These emission contributions dominate the luminosity of the AGN at radio wavelengths and possibly at some or all other wavelengths. Radio-quiet objects are simpler since jet and any jet-related emission can be neglected at all wavelengths.

AGN terminology is often confusing, since the distinctions between different types of AGN sometimes reflect historical differences in how the objects were discovered or initially classified, rather than real physical differences.

Radio-quiet AGN Edit

    (LINERs). As the name suggests, these systems show only weak nuclear emission-line regions, and no other signatures of AGN emission. It is debatable [23] whether all such systems are true AGN (powered by accretion on to a supermassive black hole). If they are, they constitute the lowest-luminosity class of radio-quiet AGN. Some may be radio-quiet analogues of the low-excitation radio galaxies (see below). . Seyferts were the earliest distinct class of AGN to be identified. They show optical range nuclear continuum emission, narrow and occasionally broad emission lines, occasionally strong nuclear X-ray emission and sometimes a weak small-scale radio jet. Originally they were divided into two types known as Seyfert 1 and 2: Seyfert 1s show strong broad emission lines while Seyfert 2s do not, and Seyfert 1s are more likely to show strong low-energy X-ray emission. Various forms of elaboration on this scheme exist: for example, Seyfert 1s with relatively narrow broad lines are sometimes referred to as narrow-line Seyfert 1s. The host galaxies of Seyferts are usually spiral or irregular galaxies.
  • Radio-quiet quasars/QSOs. These are essentially more luminous versions of Seyfert 1s: the distinction is arbitrary and is usually expressed in terms of a limiting optical magnitude. Quasars were originally 'quasi-stellar' in optical images as they had optical luminosities that were greater than that of their host galaxy. They always show strong optical continuum emission, X-ray continuum emission, and broad and narrow optical emission lines. Some astronomers use the term QSO (Quasi-Stellar Object) for this class of AGN, reserving 'quasar' for radio-loud objects, while others talk about radio-quiet and radio-loud quasars. The host galaxies of quasars can be spirals, irregulars or ellipticals. There is a correlation between the quasar's luminosity and the mass of its host galaxy, in that the most luminous quasars inhabit the most massive galaxies (ellipticals).
  • 'Quasar 2s'. By analogy with Seyfert 2s, these are objects with quasar-like luminosities but without strong optical nuclear continuum emission or broad line emission. They are scarce in surveys, though a number of possible candidate quasar 2s have been identified.

Radio-loud AGN Edit

See main article Radio galaxy for a discussion of the large-scale behaviour of the jets. Here, only the active nuclei are discussed.

  • Radio-loud quasars behave exactly like radio-quiet quasars with the addition of emission from a jet. Thus they show strong optical continuum emission, broad and narrow emission lines, and strong X-ray emission, together with nuclear and often extended radio emission.
  • “Blazars” (BL Lac objects and OVV quasars) classes are distinguished by rapidly variable, polarized optical, radio and X-ray emission. BL Lac objects show no optical emission lines, broad or narrow, so that their redshifts can only be determined from features in the spectra of their host galaxies. The emission-line features may be intrinsically absent or simply swamped by the additional variable component. In the latter case, emission lines may become visible when the variable component is at a low level. [24] OVV quasars behave more like standard radio-loud quasars with the addition of a rapidly variable component. In both classes of source, the variable emission is believed to originate in a relativistic jet oriented close to the line of sight. Relativistic effects amplify both the luminosity of the jet and the amplitude of variability.
  • Radio galaxies. These objects show nuclear and extended radio emission. Their other AGN properties are heterogeneous. They can broadly be divided into low-excitation and high-excitation classes. [25][26] Low-excitation objects show no strong narrow or broad emission lines, and the emission lines they do have may be excited by a different mechanism. [27] Their optical and X-ray nuclear emission is consistent with originating purely in a jet. [28][29] They may be the best current candidates for AGN with radiatively inefficient accretion. By contrast, high-excitation objects (narrow-line radio galaxies) have emission-line spectra similar to those of Seyfert 2s. The small class of broad-line radio galaxies, which show relatively strong nuclear optical continuum emission [30] probably includes some objects that are simply low-luminosity radio-loud quasars. The host galaxies of radio galaxies, whatever their emission-line type, are essentially always ellipticals.

Unified models propose that different observational classes of AGN are a single type of physical object observed under different conditions. The currently favoured unified models are 'orientation-based unified models' meaning that they propose that the apparent differences between different types of objects arise simply because of their different orientations to the observer. [31] [32] However, they are debated (see below).

Radio-quiet unification Edit

At low luminosities, the objects to be unified are Seyfert galaxies. The unification models propose that in Seyfert 1s the observer has a direct view of the active nucleus. In Seyfert 2s the nucleus is observed through an obscuring structure which prevents a direct view of the optical continuum, broad-line region or (soft) X-ray emission. The key insight of orientation-dependent accretion models is that the two types of object can be the same if only certain angles to the line of sight are observed. The standard picture is of a torus of obscuring material surrounding the accretion disc. It must be large enough to obscure the broad-line region but not large enough to obscure the narrow-line region, which is seen in both classes of object. Seyfert 2s are seen through the torus. Outside the torus there is material that can scatter some of the nuclear emission into our line of sight, allowing us to see some optical and X-ray continuum and, in some cases, broad emission lines—which are strongly polarized, showing that they have been scattered and proving that some Seyfert 2s really do contain hidden Seyfert 1s. Infrared observations of the nuclei of Seyfert 2s also support this picture.

At higher luminosities, quasars take the place of Seyfert 1s, but, as already mentioned, the corresponding 'quasar 2s' are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect except through their luminous narrow-line and hard X-ray emission.

Radio-loud unification Edit

Historically, work on radio-loud unification has concentrated on high-luminosity radio-loud quasars. These can be unified with narrow-line radio galaxies in a manner directly analogous to the Seyfert 1/2 unification (but without the complication of much in the way of a reflection component: narrow-line radio galaxies show no nuclear optical continuum or reflected X-ray component, although they do occasionally show polarized broad-line emission). The large-scale radio structures of these objects provide compelling evidence that the orientation-based unified models really are true. [33] [34] [35] X-ray evidence, where available, supports the unified picture: radio galaxies show evidence of obscuration from a torus, while quasars do not, although care must be taken since radio-loud objects also have a soft unabsorbed jet-related component, and high resolution is necessary to separate out thermal emission from the sources' large-scale hot-gas environment. [36] At very small angles to the line of sight, relativistic beaming dominates, and we see a blazar of some variety.

However, the population of radio galaxies is completely dominated by low-luminosity, low-excitation objects. These do not show strong nuclear emission lines—broad or narrow—they have optical continua which appear to be entirely jet-related, [28] and their X-ray emission is also consistent with coming purely from a jet, with no heavily absorbed nuclear component in general. [29] These objects cannot be unified with quasars, even though they include some high-luminosity objects when looking at radio emission, since the torus can never hide the narrow-line region to the required extent, and since infrared studies show that they have no hidden nuclear component: [37] in fact there is no evidence for a torus in these objects at all. Most likely, they form a separate class in which only jet-related emission is important. At small angles to the line of sight, they will appear as BL Lac objects. [38]

Criticism of the radio-quiet unification Edit

In the recent literature on AGN, being subject to an intense debate, an increasing set of observations appear to be in conflict with some of the key predictions of the Unified Model, e.g. that each Seyfert 2 has an obscured Seyfert 1 nucleus (a hidden broad-line region).

Therefore, one cannot know whether the gas in all Seyfert 2 galaxies is ionized due to photoionization from a single, non-stellar continuum source in the center or due to shock-ionization from e.g. intense, nuclear starbursts. Spectropolarimetric studies [39] reveal that only 50% of Seyfert 2s show a hidden broad-line region and thus split Seyfert 2 galaxies into two populations. The two classes of populations appear to differ by their luminosity, where the Seyfert 2s without a hidden broad-line region are generally less luminous. [40] This suggests absence of broad-line region is connected to low Eddington ratio, and not to obscuration.

The covering factor of the torus might play an important role. Some torus models [41] [42] predict how Seyfert 1s and Seyfert 2s can obtain different covering factors from a luminosity- and accretion rate- dependence of the torus covering factor, something supported by studies in the x-ray of AGN. [43] The models also suggest an accretion-rate dependence of the broad-line region and provide a natural evolution from more active engines in Seyfert 1s to more “dead” Seyfert 2s [44] and can explain the observed break-down of the unified model at low luminosities [45] and the evolution of the broad-line region. [46]

While studies of single AGN show important deviations from the expectations of the unified model, results from statistical tests have been contradictory. The most important short-coming of statistical tests by direct comparisons of statistical samples of Seyfert 1s and Seyfert 2s is the introduction of selection biases due to anisotropic selection criteria. [47] [48]

Studying neighbour galaxies rather than the AGN themselves [49] [50] [51] first suggested the numbers of neighbours were larger for Seyfert 2s than for Seyfert 1s, in contradiction with the Unified Model. Today, having overcome the previous limitations of small sample sizes and anisotropic selection, studies of neighbours of hundreds to thousands of AGN [52] have shown that the neighbours of Seyfert 2s are intrinsically dustier and more star-forming than Seyfert 1s and a connection between AGN type, host galaxy morphology and collision history. Moreover, angular clustering studies [53] of the two AGN types confirm that they reside in different environments and show that they reside within dark matter halos of different masses. The AGN environment studies are in line with evolution-based unification models [54] where Seyfert 2s transform into Seyfert 1s during merger, supporting earlier models of merger-driven activation of Seyfert 1 nuclei.

While controversy about the soundness of each individual study still prevails, they all agree on that the simplest viewing-angle based models of AGN Unification are incomplete. Seyfert-1 and Seyfert-2 seem to differ in star formation and AGN engine power. [55]

While it still might be valid that an obscured Seyfert 1 can appear as a Seyfert 2, not all Seyfert 2s must host an obscured Seyfert 1. Understanding whether it is the same engine driving all Seyfert 2s, the connection to radio-loud AGN, the mechanisms of the variability of some AGN that vary between the two types at very short time scales, and the connection of the AGN type to small- and large-scale environment remain important issues to incorporate into any unified model of active galactic nuclei.

For a long time, active galaxies held all the records for the highest-redshift objects known either in the optical or the radio spectrum, because of their high luminosity. They still have a role to play in studies of the early universe, but it is now recognised that an AGN gives a highly biased picture of the "typical" high-redshift galaxy.

Most luminous classes of AGN (radio-loud and radio-quiet) seem to have been much more numerous in the early universe. This suggests that massive black holes formed early on and that the conditions for the formation of luminous AGN were more common in the early universe, such as a much higher availability of cold gas near the centre of galaxies than at present. It also implies that many objects that were once luminous quasars are now much less luminous, or entirely quiescent. The evolution of the low-luminosity AGN population is much less well understood due to the difficulty of observing these objects at high redshifts.


Complete Astronomy Unit IX "The Milky Way and Other Galaxies"

Fourth Unit (out of five) in Astronomy B - second semester.

Four inclusive PowerPoint lessons with short video links and slide sorter notes for the instructor four review programs with printable color PDF worksheets student guidelines/notes and activities that go with Unit IX "The Milky Way and Other Galaxies." Two student Internet lab projects or webquests on "Galaxies." Four student guided/textbook activities in Word. Syllabus/activators suggested textbook assignments in Word and an assessment folder - two quizzes and a unit test (multiple choice) with answer keys in both Word and in PDF. The latest chapter readings in PDF are also included. Textbook assignments and activities are based on "Astronomy Today" 8th edition.

A video worksheet folder is now included which provides the video link to youtube in the title if the DVD or video tape is not available. A kahoot.it and a quizziz.com 20-question interactive review are included at the end of each PowerPoint lesson (links are in the last slide). The 20 or so question quiz/review is lesson-specific and a fun way to check for student understanding of the material.

Individual lessons/programs (not assessments) can also be found separately in the AstronomyDad store.

Terms in this unit include:

Milky Way, Galaxy, Spiral galaxy, Galactic structure, galactic size, galactic disk, galactic bulge, galactic halo, Arcturus, NGC 6744, variable stars, RR Lyrae stars, Cepheids, pulsating variable stars, Period-Luminosity relationship, standard candles, galactic distances, galactic year, Sun, Milky Way formation, Hubble Space Telescope, Universe, halo stars, spiral arms, galactic halo, galaxy formation, radio mapping, Orion arm, spiral density waves, spiral structure, Kepler’s Third Law, dark halo, dark matter, baryonic matter, gravitational lensing, missing mass, galaxy clusters, black holes, supermassive black holes, solar mass, Large Magellanic Cloud, Small Magellanic Cloud, LMC, SMC, dwarf galaxies, NGC 1512, NGC 1365, M87, Charles Messier, Edwin Hubble, Ferdinand Magellan, satellite galaxies, Hubble’s Law, Hubble’s Constant, redshifting, barred spirals, Virgo Cluster, superclusters, Local Group, Cerro Tololo Observatory, Atacama Desert, standard candles, Cepheid Variables, RR Lyrae stars, Type Ia Supernovae, V = H*D, universe age, universe size, active galaxies, normal galaxies, Seyfert galaxies, radio galaxies, nonstellar radiation, synchrotron radiation, core-halo structure, quasars, "Tadpole" galaxy UGC 10124, galaxy mergers, NGC 7318, high-energy jets, accretion disk, Abell 52, galaxy evolution, Andromeda, Milkomeda, Milkdromeda

Complete Astronomy Unit IX "The Milky Way and Other Galaxies" by astronomydad is licensed under a Creative Commons Attribution 3.0 Unported License.


16 Galaxies

(a) A collection of many galaxies, each consisting of hundreds of billions of stars. Called the Coma Cluster, this group of galaxies lies more than 100 million pc from Earth.

(b) A recent Hubble Space Telescope image of part of the cluster.

Coma Cluster. The blue spiked object at top right is a nearby star. The rest of objects are all galaxies.

Hubble in 1924 used 2.4 m telescope at Mount Wilson to classify galaxies. Using the photographics plates, he divided all the galaxies into four main groups which is known as Hubble Galaxy Classification : Spirals (S), Barred Spirals (SB), Ellipticals (E), Irregulars (Irr)

They are classified according to the size of their central bulge.

As we progress from type Sa to Sb to Sc, the bulges become smaller and the spiral arms tend to become less tightly wound.

The components of spiral galaxies are the same as in our own galaxy: disk, core, halo, bulge, and spiral arms.

Sombrero Galaxy The Sombrero Galaxy, a spiral system seen edge-on. Officially cataloged as M104, this galaxy has a dark band composed of interstellar gas and dust. The large size of this galaxy’s central bulge marks it as type Sa , even though its spiral arms cannot be seen from our perspective. The inset shows this galaxy in IR , highlighting its dust content in false-colored pink.

The variation from SBa to SBc is similar to that for the spirals except that now the spiral arms begin at either end of a bar through the galactic center.

In frame (c), the bright star is a foreground object in our own Galaxy the object at top center is another galaxy that is probably interacting with NGC 6872.

(a) S0 (or lenticular) galaxies contain a disk and a bulge, but no interstellar gas and no spiral arms. They are in many respects intermediate between E7 ellipticals and Sa spirals in their properties.

(b) SB0 galaxies are similar to S0 galaxies, except for a bar of stellar material extending beyond the central bulge.

Ellipticals are classified according to their shape from E0 (almost spherical ) to E7 (the most elongated )

They lack spiral structure, and neither shows evidence of cool interstellar dust or gas, although each has an extensive X-ray halo of hot gas that extends far beyond the visible portion of the galaxy.

Note in (c): M110 is a dwarf elliptical companion to the much larger Andromeda Galaxy.

The Magellanic Clouds are dwarf irregular (Irr I) galaxies, gravitationally bound to our own Milky Way Galaxy. They orbit our Galaxy and accompany it on its trek through the cosmos.

Both have distorted, irregular shapes, although some observers claim they can discern a single spiral arm in the Large Cloud.

Bottom panel: Some irregular (Irr II) galaxies.

(a) The strangely shaped galaxy NGC 1427A is probably plunging headlong into a group of several other galaxies (not shown), causing huge rearrangements of its stars, gas, and dust.

(b) The galaxy M82 seems to show an explosive appearance.

Cepheids extend the distance scale to 25 Mpc. However,

Some galaxies have no Cepheids.

Most of them are farther away then 25 Mpc

Therefore a new distance measures are needed: Tully–Fisher relation correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its luminosity .

A galaxy’s rotation causes some of the radiation it emits to be blueshifted and some to be redshifted (relative to what the emission would be from an unmoving source).

From a distance, when the radiation from the galaxy is combined into a single beam and analyzed spectroscopically, the redshifted and blueshifted components combine to produce a broadening of the galaxy’s spectral lines.

The amount of broadening is a direct measure of the rotation speed of the galaxy.

Type I Supernovae have about the same luminosity, as the process by which they happen doesn’t allow for much variation.

They can be used as “standard candles” and which can therefore be used to determine distance using their apparent magnitude.

It is made up of nearly 50 galaxies within approximately 1 Mpc of our Milky Way Galaxy.

Only a few are spirals most of the rest are dwarf-elliptical or irregular galaxies, only some of which are shown here.

The inset map (top right) shows the Milky Way in relation to some of its satellite galaxies.

The photographic insets (top left) show two well-known neighbors of the Andromeda Galaxy (M31): the spiral galaxy M33 and the dwarf elliptical galaxy M32 (also visible in Figure 23.2a, a larger-scale view of the Andromeda system).

Such a group of galaxies, held together by its own gravity, is called a galaxy cluster .

It contains about 3500 galaxies . It is about 17 Mpc away from Earth.

Many large spiral and elliptical galaxies can be seen.

The inset shows several galaxies surrounding the giant elliptical known as M86.

An even bigger elliptical galaxy, M87, noted at the bottom.

It contains huge numbers of galaxies and resides roughly 1 billion parsecs from Earth.

Virtually every patch of light in this photograph is a separate galaxy .

We also see many galaxies colliding, some tearing matter from one another, others merging into single systems.

All galaxies (with a couple of nearby exceptions) seem to be moving away from us, with the redshift of their motion correlated with their distance

Optical spectra of several galaxies named on the right.

Both the extent of the redshift (denoted by the horizontal red arrows) and the distance from the Milky Way Galaxy to each galaxy (numbers in center column) increase from top to bottom.

The vertical yellow arrow in each spectrum highlights a particular spectral feature (a pair of dark absorption lines).

The horizontal red arrows indicate how this feature shifts to longer wavelengths in spectra of more distant galaxies.

The white lines at the top and bottom of each spectrum are laboratory references.

Plots of recessional velocity against distance:

(a) for nearby galaxies shown above

(b) for numerous other galaxies within about 1 billion pc of Earth.

The relationship (slope of the line) is characterized by Hubble’s constant H 0:

recessional velocity = H 0 × distance

The currently accepted value for Hubble’s constant

H 0 = 70 km/s/Mpc

Measuring distances using Hubble’s law actually works better on farther away objects random motions are overwhelmed by the recessional velocity.

Hubble’s law tops the hierarchy of distance measurement techniques.

It is used to find the distances of astronomical objects all the way out to the limits of the observable universe.

About 20-25% of all galaxies don't fit to Hubble Classification.

They are far too luminous .

They are different than "Normal Galaxies": Luminosity & Type of Radiation.

Excess luminosity is due to Non-stellar Radiation .

When a galaxy interact with its neighbor it might trigger an outburst of star formation - called starbust galaxies .

Excess luminosity might be also due to activity in and around the galactic center .

Star Formation Rings around the center

The active galaxy NGC 7742 . It resembles a fried egg, with a ring of blue star-forming regions surrounding a very bright yellow core that spans about 1 kpc. It combines star formation with intense emission from its central nucleus.

They resemble normal spiral galaxies

But their cores are thousands of times more luminous.

(a) The Circinus galaxy with a bright compact core.

It is one of the closest active galaxies.

(b) The rapid variations in the luminosity indicate that the core must be extremely compact.

The variation is Radio wavelength. However, optical and X-ray luminosities vary as well.

(a) Centaurus A Radio Lobes

(b) It has a giant radio-emitting lobes extending a million parsecs or more beyond the central galaxy.

This entire object is thought to be the result of a collision between two galaxies that took place about 500 million years ago.

The lobes are shown here in pastel false colors, with decreasing intensity from red to yellow to green to blue.

The inset (at right) shows an X-ray image of one of the lobes up close, proving that at least the jets in the inner parts of the lobes emit X rays .

(Left) M86. The radio emission comes from a bright central nucleus, which is surrounded by an extended, less-intense radio halo.

(Right) A central energy source produces high-speed jets of matter that interact with intergalactic gas to form radio lobes.

The system may appear to us as either radio lobes or a core-dominated radio galaxy , depending on our location with respect to the jets and lobes.

The giant elliptical galaxy M87 (also called Virgo A) is displayed here in a series of zooms.

(a) A long optical exposure of the galaxy’s halo and embedded central region.

(b) A short optical exposure of its core and an intriguing jet of matter , on a somewhat smaller scale.

(c) An IR image of M87’s jet, examined more closely compared with (b).

The bright point at left marks the bright nucleus of the galaxy the bright blob near the center of the image corresponds to the bright “knot” visible in the jet in (b).

They are named as Quasi Stellar Objects they are starlike in apparance but have very unusual spectral lines.

(a) The bright quasar 3C 273 displays a luminous jet of matter, but the main body of the quasar is starlike in appearance.

(b) The jet extends for about 30 kpc and can be seen better in this high-resolution image.

(Right - Top) Optical spectrum of the distant quasar 3C 273.

Notice both the redshift and the widths of the three hydrogen spectral lines marked as Hβ, Hγ, and Hδ.

The redshift indicates the quasar’s enormous distance . The width of the lines implies rapid internal motion within the quasar.

(Right - Bottom) Typical Quasar

1) Quasars are the most luminous objects in the universe.

2) Quasars's much greater distance makes it appear fainter than the stars - but intrinsically it is much, much birghter.

high luminosity

nonstellar energy emission

variable energy output, indicating small nucleus

jets and other signs of explosive activity

broad emission lines , indicating rapid rotation

The energy source in AGN holds that these objects are powered by material accreting onto a supermassive black hole .

As matter spirals toward the hole, it heats up, producing large amounts of energy.

At the same time, high-speed jets of gas may be ejected perpendicular to the accretion disk, forming the jets and lobes seen in many active objects.

Magnetic fields generated in the disk are carried by the jets out to the radio lobes, where they play a crucial role in producing the observed radiation.

The accretion disk is whole clouds of interstellar gas and dust they may radiate away as much as 10–20% of their mass before disappearing.

(a) A combined optical/radio image of the giant elliptical galaxy NGC 4261, in the Virgo Cluster, shows a white visible galaxy at center, from which blue-orange (false-color) radio lobes extend for about 60 kpc.

(b) A close-up photograph of the galaxy’s nucleus reveals a 100-pc-diameter disk surrounding a bright hub thought to harbor a black hole .

Recent images and spectra of M87 support the idea of a rapidly whirling accretion disk at the galaxy’s heart.

(a) An image of the central region of M87 shows the galaxy’s bright nucleus and jet.

(b) A magnified view of the nucleus suggests a spiral swarm of stars, gas, and dust.

(c) Spectral-line features observed on opposite sides of the nucleus show opposite Doppler shifts.

The implication is that an accretion disk spins perpendicular to the jet and that at its center is a black hole having some 3 billion times the mass of the Sun.

The accretion disk surrounding a massive black hole consists of hot gas at many different temperatures (hottest nearest the center).

When viewed from above or below , the disk radiates a broad spectrum of electromagnetic energy extending into the X-ray band.

However, the dusty infalling gas that ultimately powers the system is thought to form a rather fat, donut-shaped region outside the accretion disk (shown here in dull red), which effectively absorbs much of the high-energy radiation reaching it, re-emitting it mainly in the form of cooler, infrared radiation .

Thus, when the accretion disk is viewed from the side , strong infrared emission is observed.

For systems with jets (as shown here), the appearance of the jet, radiating mostly radio waves and X rays, also depends on the viewing angle.


Galaxy Geometry

There are many geometries of galaxies including the spiral galaxy characteristic of our own Milky Way. The above image is a segment of the remarkable deep field photograph made by the Hubble Space Telescope, every visible object except for the one obvious foreground star seems to be another galaxy. An even deeper view was obtained in the Hubble Ultra Deep Field in 2003-04. The Extreme Deep Field dated 2012 added greater resolution.


Elliptical galaxies usually have very little gas or dust and hence little evidence of new star formation. The spiral galaxies may have an abundance of gas and dust and show evidence of star formation in the form of lots of hot blue stars.


Characteristics

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a higher than normal luminosity over portions of the electromagnetic spectrum. A galaxy having an active nucleus is called an active galaxy. Active galactic nuclei are the most luminous sources of electromagnetic radiation in the Universe, and their evolution puts constraints on cosmological models. Depending on the type, their luminosity varies over a timescale from a few hours to a few years. The two largest subclasses of active galaxies are quasars and Seyfert galaxies, the main difference between the two being the amount of radiation they emit. In a typical Seyfert galaxy, the nuclear source emits at visible wavelengths an amount of radiation comparable to that of the whole galaxy's constituent stars, while in a quasar, the nuclear source is brighter than the constituent stars by at least a factor of 100. [1][23]  Seyfert galaxies have extremely bright nuclei, with luminosities ranging between 10 8  and 10 11  solar luminosities. Only about 5% of them are radio bright their emissions are moderate in gamma rays and bright in X-rays. [24]  Their visible and infrared spectra shows very bright emission lines of hydrogen, helium, nitrogen, and oxygen. These emission lines exhibit strong Doppler broadening, which implies velocities from 500 to 4,000 km/s (310 to 2,490 mi/s), and are believed to originate near an accretion disc surrounding the central black hole. [25]

Eddington luminosity

A lower limit to the mass of the central black hole can be calculated using the Eddington luminosity. [27]  This limit arises because light exhibits radiation pressure. Assume that a black hole is surrounded by a disc of luminous gas. [28]  Both the attractive gravitational force acting on electron-ion pairs in the disc and the repulsive force exerted by radiation pressure follow an inverse-square law. If the gravitational force exerted by the black hole is less than the repulsive force due to radiation pressure, the disc will be blown away by radiation pressure. [29][note 1]

The image shows a model of an active galactic nucleus. The central black hole is surrounded by an accretion disc, which is surrounded by a torus. The broad line region and narrow line emission region are shown, as well as jets coming out of the nucleus.

Emissions

The emission lines seen on the spectrum of a Seyfert galaxy may come from the surface of the accretion disc itself, or may come from clouds of gas illuminated by the central engine in an ionization cone. The exact geometry of the emitting region is difficult to determine due to poor resolution of the galactic center. However, each part of the accretion disc has a different velocity relative to our line of sight, and the faster the gas is rotating around the black hole, the broader the emission line will be. Similarly, an illuminated disc wind also has a position-dependent velocity. [30]

The narrow lines are believed to originate from the outer part of the active galactic nucleus, where velocities are lower, while the broad lines originate closer to the black hole. This is confirmed by the fact that the narrow lines do not vary detectably, which implies that the emitting region is large, contrary to the broad lines which can vary on relatively short timescales. Reverberation mapping is a technique which uses this variability to try to determine the location and morphology of the emitting region. This technique measures the structure and kinematics of the broad line emitting region by observing the changes in the emitted lines as a response to changes in the continuum. The use of reverberation mapping requires the assumption that the continuum originates in a single central source. [31]  For 35 AGN, reverberation mapping has been used to calculate the mass of the central black holes and the size of the broad line regions. [32]

In the few radio-loud Seyfert galaxies that have been observed, the radio emission is believed to represent synchrotron emission from the jet. The infrared emission is due to radiation in other bands being reprocessed by dust near the nucleus. The highest energy photons are believed to be created by inverse Compton scattering by a high temperature corona near the black hole. [33]


GRB 110328A: First ever observation of a newly formed quasar!

Nascent Quasar GRB 110328A

When a formerly quiescent galactic nucleus is observed by astronomers to suddenly begin radiating high energy emission, it is probably natural for them at first to avoid interpreting the sighting as the birth of a quasar and instead propose something on a far smaller less dramatic scale. Knowing very well the psychology of his astronomer peers, Sir Fred Hoyle forsaw a similar sighting downplay in his science fiction story The Inferno (1973). His story was about astronomers first sighting the explosion of our own galactic nucleus, its sudden activation into the quasar state. A passage from his book describes how some members on the astronomical discovery team at first wrongly concluded that what they had discovered was a supernova explosion:

“Except this supernova does seem unusually bright,” interjected Tom Cook.
“Has brightened up still more,” announced Bill Gaynor, who had just come in. “Didn’t go to bed. I stayed up till it rose—in the east, about an hour ago.”
“What is it now?”
“I’d say about minus eight.” [25 times brighter than Venus]
There was a whistle around the common room.
“More like a bloody quasar than a supernova,” muttered someone.
A long silence followed this remark. It was broken by Almond. “Which would explain something that’s been worrying the hell out of me.”
“What’s that, Dr. Almond?” Gaynor asked, his eyes red with lack of sleep.
“Why the position of the thing is so precisely the same as the Galactic center. It’s obvious really, isn’t it? The center of the Galaxy has blown up.” Almond’s deep voice was grave as he made this pronouncement.

The Inferno, Sir Fred Hoyle and Geoffrey Hoyle
passage quoted in Earth Under Fire by P. LaViolette

We may be seeing the same sequence of events playing out in real life with the discovery of the source GRB 110328A which may actually prove to be a quasar, the first ever to be seen turning on. The initial appearance of this X-ray and gamma ray source was first detected by the Swift telescope on March 28, 2011. It was found to be located at the center of a galaxy in the constellation of Draco situated about 3.8 billion light years away (z = 0.35).

Seeing that the source continued its highly energetic activity even days afterward, astronomers began to realize that what they had been observing was something other than a mere gamma ray burst (GRB). Most gamma ray bursts, on the other hand, last from a minute or so to several hours at most. But in seeking an alternate interpretation, astronomers have leaned towards the less dramatic and proposed that we are observing a “supermassive black hole” that is in the process of tidally disrupting and consuming a passing star.

For example, on April 14th, after the source had been active for over two weeks, astronomers Almeida and De Angelis proposed just this in a paper they had submitted for publication to Astronomy and Astrophysics journal. They propose that we are seeing a black hole having a mass of

10 7 solar masses ripping apart and consuming a red giant star of mass 0.5 to 5 solar masses which had happened to orbit too close to it. They state that if their theory is correct, we should expect that the intense X-ray emission from GRB 110328A to not last more than a few weeks to a few months, i.e., the time taken for the red giant star’s mass to become completely consumed. In fact in their April 14th paper to Astronomy and Astrophysics, they state that the emission should be seen to begin to fade within a few days to a few months.

Now more than a week has passed since the date they posted their paper, so the predicted lower limit of a “few days” has been well exceeded. If the source continues its current variable activity after a few months from now, then like Dr. Almond in Hoyle’s novel, astronomers will be forced to consider the inevitable, that what we are seeing is more like a “bloody quasar” than the transitory burp of a black hole!

I predict that GRB 110328A is a quasar and that we just happen to be viewing it at a point in its cycle when it has happened to turn on. I would prefer not to call it a supermassive black hole as has become customary in astronomy for the reason that I don’t believe in the existence of black holes. I prefer to use the more neutral term galactic core or alternatively supermassive mother star. Here are some facts to consider that favor the interpretation that GRB 110328A is a quasar:

1) the X-ray emission is coming from the exact center of the host galaxy, hence from its core. Similarly, quasars are known to be galactic nuclei in their active state, hence a galactic core observed during its active phase.

2) the average long-term emission coming from GRB 110328A is seen to have an intensity in the range of what is observed to come from a quasar. That is, quasars typically have X-ray luminosities that range from 10 43 to 10 48 ergs/s whereas Almeida and De Angelis report that this object has an average X-ray luminosity of about 2.5 X 10 47 erg/s. So GRB 110328A is near the upper end of the quasar luminosity range.

3) Whereas the X-ray luminosity from quasars is observed to erratically vary by many fold over a period of anywhere from hours to weeks, similarly the emission from GRB 110328A has been observed to vary erratically on a timescale of a few hours to a day, very similar to a more rapidly varying quasar.

4) Like a quasar, GRB 110328A emits synchrotron radio emission. Radio emission from this source was reported on April 11th by Brunthaler et al.

X-ray intensity light curve for source GRB 110328A

X-ray intensity lightcurve for quasar PDS 456.

Considering that we may be observing for the first time the onset of a quasar, there are several interesting things that we can learn from GRB 110328A.
First we can get an idea about the rapidity of the onset of the quasar state. The observed event occurred without prior warning and reached maximum intensity within 15 minutes . I have previously stated that we could expect a similar sneak attack from the core of our own Galaxy. (GRB 110328A instead lies several billion light years away. So we need not worry about it.)
Second, when it initially turned on, its luminosity was about 20 times greater than the value it attained days later. At its initial onset it achieved a luminosity of around 5 X 10 48 ergs/s in two peak events separated a day apart. Hence in its first days it would have been one of the most luminous quasars in the sky. This is very significant. For it implies that a first strike from our own galactic core may deliver its most deadly effects in the first day or two, with intensities an order of magnitude greater than what we would later be exposed to.



Comments:

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  2. Godofredo

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  3. Geteye

    What audacity!



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