ASTR 5610 (Majewski) Lecture Notes

OUR PLACE IN THE UNIVERSE: ZOOMING INTO THE LOCAL GROUP

• The earliest redshift surveys revealed the types of stringy structures now obtained in the models.
  

  Map of galaxies from the original CfA redshift survey out to about 230 Mpc. The famous "stick man" (homunculus) is the Coma galaxy cluster.

  

  Cluster of galaxies in Coma Berenices, about 70h^-1 away near the North Galactic Pole. Kitt Peak image.

• Modern surveys, like the Sloan Digital Sky Survey, reveal the filamentary large scale structure -- in this case to 2000 Mpc.
  

  Map of 50,000 galaxies from the Sloan Digital Sky Survey Early Data Release to about 2 Gpc for luminous red galaxies and 1 Gpc for the main galaxy survey. Below is a more complete map from later data releases. The outer circle is at a distance of two billion light years (600 Mpc). The region between the wedges was not mapped by the SDSS because dust in our own Galaxy obscures the view of the distant universe in these directions. This slice contains all galaxies within -1.25 and 1.25 degrees declination. Credit: M. Blanton and the Sloan Digital Sky Survey.

  

• Zooming by a factor of two makes the filaments and "voids" more obvious.
  

  Map of 24,000 galaxies from the Las Campanas Redshift Survey out to about 900 Mpc.

• And still 4X closer, here is a more recent version of the CfA redshift survey, with two hemispheric slices.
  

  Map of galaxies from the CfA-SSRS2 database out to about 230 Mpc.

• The filamentary structure is obvious even in the sky distribution of galaxies:
  

  All sky galaxy distribution in the 2MASS survey. The blue swaths represent the Milky Way disk. From antwrp.gsfc.nasa.gov/ apod/ap030917.html.

• Here is a more complete, albeit schematic view of local superstructures.
  

  Cartoon map of the local universe on similar scale as above, and showing nearest superclusters. From http://homepages.wmich.edu/~korista/phys325.html.

• The nearest 125 Mpc shows that there are vast voids and several large mass concentrations.
  

  From Hudson, 1993, MNRAS, 265, 43.

• The largest nearby large cluster of several thousand galaxies is Virgo, which covers some 100 deg².

  The Virgo Cluster, with bright galaxies named (left; www.star.ucl.ac.uk/ ~apod/apod/ap010126.html) and a zoomed view of the Leo Group in the Virgo cluster (right; from www.randybrewer.net/ virgocluster.htm). Sixteen Messier objects are Virgo galaxies.

• Virgo, at about 17 ± 5 Mpc, is at about the distance limit where some individual "normal" stars (e.g., bright Cepheid variables) can still be resolved with HST, and thus is an important step in the cosmological distance ladder.
  • Obviously, supernovae are stars that can be seen much farther and are keys to the distance ladder, but these are rare.

  • Supernovae in Virgo are important for calibration of the distance scale against Cepheids.

• The enormous mass of Virgo gives rise to a Virgocentric flow, seen as filaments of galaxies falling in towards Virgo:

  The Virgo Cluster has filaments of galaxies apparently connecting to it. On the left is a map of the density of galaxies from Tully (1982, ApJ, 257, 389). On the right is a naming scheme for the filaments from Elmegreen, Galaxies & Galactic Structure. Note that the view is perpendicular to the supergalactic plane.

  The Virgocentric flow results in significant deviations in the local Hubble flow, especially visible in the backside infall.
  

  (a) The Hubble diagram for galaxies to 25 Mpc in the direction of Virgo showing deviation from Hubble flow. (b) After correction for a Virgo infall of about 250 km/s. From John Tonry's page, http://msowww.anu.edu.au/~heron/Tonry/tonry_p3.html.

• The Milky Way and Local Group live on the outskirts of the Virgo supercluster.
  

  The Milky Way and Local Group live on the outskirts of the Virgo supercluster. From http://www.miqel.com/outer-space-info/galactic-matrix-information/position-of-milky-way-in-virgo-supercluster.html.

• That the Local Group is influenced by Virgo is seen by the Cosmic Microwave Background dipole anisotropy:
  

  The CMB dipole anisotropy as seen by COBE. The color scale shows a spread of ±3.5 mK. The anisotropy is actually not pointed at Virgo and is larger than our measured motion with respect to Virgo galaxies. This indicates that more is going on than our infall into the Virgo cluster. From http://zebu.uoregon.edu/1997/ph410/l12.html .

• However, the direction of the dipole is not exactly lined up in the direction of Virgo, which suggests that Virgo is not the only major mass influencing the Local Group.
  • It has been postulated that there is yet another, more distant, larger mass concentration that is influencing even Virgo.

  • Unfortunately, the supposed "Great Attractor" is in a heavily obscured part of the Milky Way and not easily visible.
    

    Copyright C. Kraan-Korteweg and Ofer Lahav, Scientific American (October 1998): pp. 50-57.

  • Supposedly the core of the Great Attractor lies along the "Centaurus Wall", which is a "Great Wall" seen edge-on from our perspective.
    The GA would be near "A3627" in this figure, which shows the Zone of Avoidance of the Milky Way.
    

    Copyright Anthony P. Fairall, An Atlas of Nearby Large-scale Structures" .

  • Evidence that the Great Attractor exists comes from more non-Hubble flow. However, the question of GA-associated backside infall is still debated.
    

    From www.ap.stmarys.ca/ ~gaw/ast413/.

  

  A breakdown of possible motions leading to the net motion of the Local Group, including possible motion of the entire Local Supercluster toward the Hydra-Centaurus Supercluster. From http://zebu.uoregon.edu/1997/ph410/l12.html .

CENSUS OF THE LOCAL GROUP

• The Milky Way system is the closer of the two clumps in the center foreground and at the center of the SG coordinate system.

• The Virgo cluster lies near SGX=0 and SGY=15000 kpc.

• Though the view does not give a representation of the relative masses of galaxies it gives an impression of the relative number density of galaxies and how the Milky Way and Andromeda subgroups represent a local maximum in density and how the local neighborhood is of relatively low density.

From the Encyclopaedia Galactica, http://www.astro.utu.fi/EGal/elg/ELG3Dl.html. The units shown are in kpc, and the green/blue vertical lines show height above/below the supergalactic plane. Other lines show projections on different supergalactic cardinal planes. Red dot sizes correspond to relative distance from the viewer. See this website for the individual galaxy identifications and a larger version of this image.

From Sparke & Gallagher, Galaxies in the Universe.

• Distances: As already mentioned, the definition of the boundary of the LG varies.
  But this is only part of the problem, since accurate distances to some potential LG members remain to be obtained, and may only be known to a factor of two.

• Resolvability: Present technological capability is such that we can resolve the brightest normal (e.g., upper red giant branch) stars only on Mpc distance scales (but this is now being pushed to 10 Mpc scales both with HST and even from the ground).
  Thus, for systems with poor distance determinations, the ability to resolve these stars can be used as a crude indicator of locality.
  

  Color-magnitude diagram of M31's bulge made with HST (through a treasury proposal). This CMD reaches to the MSTO, after which photometric errors significantly increase. This was the deepest CMD ever obtained of another galaxy at the time (see deeper CMDs of M31 by this group on the previous webpage), and shows the limitations of exploring resolved stellar populations even within the Local Group. From Brown et al. (2003, ApJL, 529, L17) .

  (Recall that Baade used a similar criterion to establish distances, and therefore luminosities, of some galaxies in the M31 system.)

• Dynamics: On could look for objects that are part of the LG on the basis of dynamical considerations.
  • Obviously, such systems would have lower velocity with respect to the LG barycenter than systems participating in the Hubble flow.

  • To estimate the zero-velocity surface, which separates the LG from the Hubble flow, requires an estimate of the total mass of the LG -- which can be approximated by assuming the MW and M31 dominate the LG mass -- and the LG velocity dispersion (~60 km/s). A surface at 1.8 Mpc is obtained (Sandage 1986, ApJ, 307, 1; Mateo 1998, ARAA, 36. 446).

  • Note that the inferred size and dispersion implies a crossing time longer than a Hubble time, so not all galaxies are infalling yet.

  • One could look for members as those apparently nearby galaxies with low (v ²R / G), since these imply low mass within their orbits. (I.e., those systems implying higher enclosed mass must be on extreme orbits).
    

    One method for dynamically finding members of the Local Group by the Doppler velocity relative to the Sun (which, of course, is moving within the Milky Way, and thus gives rise to the sloping definition in this plot). From Mateo (1998, ARAA, 36, 441).

  • Note the recent discovery of two galaxies -- And XII (Martin et al. 2007, Chapman et al. 2007) and Andromeda XIV (Majewski, Beaton et al. 2007) that look to be "within" the Local Group spatially, but not presently bound to any large galaxy (i.e. M31) and possibly falling into LG for first time.
    

    The escape velocity as a function of distance from the center of M31, as estimated by deriving the mass from its rotation curve and extrapolations based on an NFW profile. The Andromeda satellites are shown plotted at their radial velocity (i.e. ignoring the unknown tangential components), with these velocities adjusted to put them all projected onto the same vector along the M31-Milky Way axis. Andromeda XIV is the outlier point, and even its plotted position is a very lower limit, because it is plotted at its projected separation from M31 (for the reason that the distance to And XIV is not yet clearly established). Note that Andromeda XII (not plotted) is similarly an outlier in the plot. From Majewski, Beaton et al. (2007, ApJL, 670, L9).

Local Group galaxies shown to the same linear and surface brightness scales. By B. Bingelli, from http://www.usm.uni-muenchen.de/people/saglia/dm/galaxien/alldt/node11.html.

Distribution of galaxy types by their central surface brightness and total luminosity. By B. Bingelli, taken from Sparke & Gallagher, Galaxies in the Universe.

Spheroidal/elliptical characteristics by Kormendy, taken from Sparke & Gallagher, Galaxies in the Universe

• For example, elliptical galaxies, bulges and globular clusters have similar central surface brightnesses, but globulars are much less bright (much less massive) and much more concentrated (thus, GCs do not simply have scaled down versions of the light distributions of E's and bulges).
  This property of globular clusters is what makes them attractive targets when one looks at stellar components of extragalactic systems --- one can more easily identify and get spectra of extragalactic globulars (with all of the light integrated together as one source) than for individual stars.

• Globular clusters and dSph's can have similar total luminosities, but the latter have significantly lower (of order 7 mags/arcsec²) central surface brightnesses -- an indication that their light is MUCH more spread out than globular clusters (over several orders of magnitude larger core radius).

• Three or four spirals, depending on how you categorize the Large Magellanic Cloud.
  • Three of these spirals are barred: M31, MW, LMC.

  • The Milky Way and Andromeda are large spirals that dominate the LG. With M33, these three spirals emit 90% of the Local Group light. Both show some disk warping, as does M33.

  • Disks rotationally supported: e.g., V / σ ~ (220 km/s) / (30 km/s) ~ 7 for MW.

  • M31 is thought to more luminous, and is more massive (rotation speed of 260 km/s and double the scalelength of the Milky Way) and of an earlier galaxy type than the Milky Way (SBb compared to SBbc for Milky Way).
    It also has a much larger central spheroid/bulge (often somewhat errantly called its "inner halo", but more correctly associated with the bulge) than does the Milky Way.

  • M33 is even later, Sc or Sd.

  • The LMC is often considered the prototype of the "latest" type of spiral, the "Magellanic Spiral" class, or SBm, but is also sometimes called a simple Irr.
    May be a transitional type between spirals and Irr -- has a disk and bar, but V / σ ~ 4 and less organized.

  • M33 and LMC are an order of magnitude smaller than the large LG spirals.

    The Local Group small Sc spiral M33 (from www.seds.org/messier/ xtra/mootw/m033_021208.html) and SBm/Irr spiral/irregular Large Magellanic Cloud (from http://www.ifa.hawaii.edu/~wang/gallery/picutres/LMC.html).

  • By the way, there have been several recent (and extremely difficult) measurements of the proper motions of M31 and M33. This has made it possible to generate a dynamical model of the orbits of three primary masses in the LG, and this suggests that the Milky Way and M31 will collide in about 4 billion years.

    Cartoon images of the M31/Milky Way interaction from the HSTPROMO group measuring proper motions of LG galaxies using HST images. (from http://www.stsci.edu/~marel/hstpromo.html).

  • Note also, that by use of the so-called "Local Group Timing Argument", based on the relative velocities and distances of the MW from M31, one can derive a lower limit to the mass of the LG (~3.7 x 10¹² solar masses).

• One elliptical galaxy: M32 (though this might also be called a dwarf elliptical (dE).
  • In many ways an odd system -- tends often to show up as an exception in many plots and trends studied for galaxies.

  • Unlike other LG systems, does not have a flat inner light profile, but rises to a cusp.

  • Brightest central surface brightness known for any galaxy.

  • Dynamical analyses as well as x-ray and radio imaging suggest there may have a central, 2.5 million solar mass black hole (compared to the Milky Way's 4 million solar mass central BH) -- albeit a rather quiescent one.

  • Partly rotationally supported: V / σ ~ 1.

  • No young stars or gas, but enriched to about solar metallicity -- as seen in larger Es.

  • No known globular clusters.

  

  M32 seen against M31's disk. From Jim Burnell's webpage: www.jburnell.com/M32.html.

  

  "The inset is a combined radio/optical/X-ray image of M32. The diffuse purple is the visible starlight of M32. The central white object in the inset is the center of M32, where there is both radio and X-ray emission. Red shows radio-emitting objects, including a pair of presumed planetary nebulae (left center), and the green is an object strongly emitting X-rays. Credit: Bill Saxton, NRAO/AUI/NSF; Yang et al.; NASA, ESA, Digitized Sky Survey 2 (Acknowledgement: Davide DeMartin)." Caption and image from https://public.nrao.edu/news/bashful-black-hole/.

• Three dE: NGC 147, NGC 185 and NGC 205.
  • Note that all four E/dE are associated with M31.

  • Diffuse systems. Some (like NGC 205) subject to tidal stripping.

  • No ordered, internal rotation, pressure supported: V / σ ~ 0.1.

  • Trace amounts of gas and dust.

  • Have intermediate-old populations, but small numbers of younger stars near nuclei in NGC 205 and NGC 185.

  • All three have globular clusters, NGC185 also has youngish star clusters.

  NGC185 (from http://www.ipac.caltech.edu/2mass/gallery/largegal/ngc185/ngc185_mosaic_c.jpg) on the left. NGC 205 and M31's disk in middle imagw (from J.-C. Cuillandre (CFHT Staff), CFHT on antwrp.gsfc.nasa.gov/ apod/ap001023.html). The right image is NGC 147 from http://martingermano.com/N147_14.htm.

• Of order ten Irr or dIrr systems.
  • Division between Irr and dIrr at ~10⁸ solar luminosity.

  • Characterized by large gas fractions and current star formation and bright, young, blue stars and HII regions.

  • Regions between star formation are of low surface brightness (resembling dSph in this way).

  • Diffuse (like the dSphs below).

  • If you stripped recent star formation away, would probably be as faint as dSph.

  • Mostly far from Milky Way and M31 (see below).

  • Irregular shapes partly a result of bright, blotchy star formation regions across them (but interactions -- e.g., LMC/SMC/Milky Way -- may also have played a role).

  • Generally rotationally supported: V / σ ~ 1-6.

  • Generally 1/10 solar metallicity or less.

  Local Group Irr and dIrr galaxies: the Irr Small Magellanic Cloud (from www.seds.org), dIrr system Sextans A (from Van Dyk et al., as shown at antwrp.gsfc.nasa.gov/ apod/ap981103.html), the Leo A dIrr (from www.naoj.org/Gallery/ press release.html), and the WLM Irr/dIrr (DDO221; from http://www.lowell.edu/users/dah/wlmubv.jpg).

• Several "transitional" types, dIrr/dSph.
  • Seem to have some properties intermediate between dSph and dIrr, suggesting the possibility of an evolutionary link: dIrr --> dIrr/dSph --> dSph (a rate of SF sequence? dictated by environment?).

  • V / σ ~ 1.7 in Pegasus (only one measured so far).

  • A little gas and a few young stars but most stars are intermediate-old.

  The Local Group dIrr/dSph systems Antlia (from http://www.eso.org/outreach/press-rel/pr-1999/phot-10a-99-preview.jpg), Pegasus (DDO216) and Pisces (LGS3) (the latter two from www.lowell.edu/ users/dah/ubv.html).

• Dozens of dSph galaxies.
  • Sculptor and Fornax first ones discovered (by Shapley) in 1938.

  • Characterized by extremely low surface brightness (the "invisible" galaxies in the Bingelli figure) and diffuse extended light profiles.

  • No ordered, internal rotation, pressure supported: V / σ ~ 0.1.

  • No signs of current or recent star formation.

  • Little or no detectable hydrogen.

  • Generally all show an old (~13 Gyr) stellar population; most show younger (e.g., 1-10 Gyr) populations. Bursts of star formation suggested.

  • Intermediate to low metallicities.

  • As seen in the Kormendy plots, dSphs have similar sizes as dE galaxies, but are much less luminous, and so have lower stellar density (though not necessarily lower MASS density).

  • Characterized by substantial, sometimes enormous dark matter fractions.

  • Some seem to be undergoing tidal disruption by their parent spiral.

  • Mostly clustered around either Milky Way or M31.

    Local Group dSph galaxies Pegasus dSph (from Grebel & Guhathakurta, antwrp.gsfc.nasa.gov/ apod/ap990122.html), Andromeda V (by Armandroff et al.; from http://www.seds.org/~spider/spider/LG/Add/lg_pix.html), and Sculptor (from David Malin; http://www.ast.cam.ac.uk/AAO/images/deep_html/sculptor_d.html).

  • These are the main type of Local Group galaxy still being discovered today.
    Many new Local Group dSph members found in the past few years, especially around the M31 system (with several dozen or so known dSph companions to date).
    

    Some of the newly found dSphs around the M31 system.

• Ultrafaint Dwarf Galaxies.
  • The Sloan Digital Sky Survey spawned an explosion in the discovery of Milky Way dwarf galaxies of much fainter luminosity than "classical" dSph galaxies.

  • More than doubled the known census of Milky Way satellites.

    Top: Map of the SDSS imaging footprint and identifying the Milky Way satellites (both previously known and new discoveries), modified from Belokurov et al. (2006). Right: List of the MW satellites as of 2008, and the year in which they were discovered. Bottom: Same as top, but with images of a few of the newfound satellites.

  • SDSS footprint is 1/5 of sky, so can expect 5 times as many to really exist!

  • The Dark Energy Survey (DES) survey in the Southern Hemisphere is also uncovering numerous new ones.
    

    Locations of eight dwarf galaxy candidates discovered in 2015 with the Dark Energy Survey, reported here (red triangles) along with nine previously reported dwarf galaxy candidates in the DES footprint (red circles), five recently discovered dwarf galaxy candidates located outside the DES footprint (green diamonds), and twenty-seven Milky Way satellite galaxies known prior to 2015 (blue squares). Systems that have been confirmed as satellite galaxies are individually labelled. The figure is shown in Galactic coordinates (Mollweide projection) with the coordinate grid marking the equatorial coordinate system (solid lines for the equator and zero meridian). Illustration credit: Dark Energy Survey Collaboration. From http://astronomynow.com/2015/08/18/dark-energy-survey-finds-eight-more-galactic-neighbours/.

  • The figure below shows the very sparsely populated, SDSS-generated color-magnitude diagram of the Ursa Major I dwarf compared to the SDSS CMD for a "classical" dSph over a similar area of sky.
    

    The CMD of the field surrounding the Ursa Major I dwarf galaxy on the left, after statistical subtraction of a background in the second panel, and compared to the CMD for the classical Sextans dSph, with all data from the Sloan Digital Sky Survey in the third panel. On the far right is the sky distribution of the stars in the Ursa Major red giant branch, showing the very loose concentration and low luminosity of this system. From Willman et al. 2005, ApJL, 626, L85.

    

    A comparison of the sky appearance of the classical dSph Leo I to the ultrafaint dwarf Bootes I.

  • One can see that the ultrafaints are much fainter than classical dwarfs, with luminosities as low as ~100 solar luminosities or less.

  • Despite their low luminosities, the velocity dispersions of these systems are still relatively large, so that extremely large dark matter contents are inferred.
    

    The velocity dispersions of some ultrafaint dwarf galaxies, which are more or less constant with luminosity. From Simon & Geha (2007, ApJ, 670, 313).

  • But the distorted shapes of these ultrafaints gives pause that they may be undergoing tidal disruption.
    

    The CMD and spatial distribution of Ursa Major II. The latter shows a distorted morphology of the type typical of a disrupted satellite.

  • Of course, the distorted shapes often seen for ultrafaint dwarfs may also be just because the low density of stars lends poor S/N measurements of the true morphology.
    

    Morphologies of various ultrafaint galaxies on the top. At the bottom the lefthand panels show the morphology of Bootes I, compared to models simulating the shape of a system with the same number of stars as Bootes, showing variations caused by differences in statistical sampling that can arise simply from small numbers of stars.

• Within the Local Group, environmental differences in the distribution of galaxy types is evident:
  

  A view of the Local Group in 3-D by Eva Grebel (circa 2004). The dashed ellipses are centered on the LG barycenter (462 kpc away, towards Andromeda). Distances of galaxies from the the arbitrarily chosen plane through the Milky Way are indicated by solid lines (above the plane) and dotted lines (below). The small ellipses are large spirals. The dEs are in yellow, dSphs in orange, the dIrrs are in blue, dSph/dIrr transition types are in green. Note how most dIrrs are in fairly isolated regions away from large mass concentrations, and dSphs cluster around the spirals. From http://nedwww.ipac.caltech.edu/level5/Grebel/frames.html.

From Sparke & Gallagher, Galaxies in the Universe.

• The power law part of the Schechter function, (L / L* )^α, dominates the shape of the function for fainter galaxies, by the power law index α.

• The exponential part of the Schechter function, exp(-L / L* ), describes the shape of the bright end of the luminosity function.

• Φ* sets the normalization.

• L* (or M *) is a characteristic luminosity brighter than which the number of galaxies falls quickly.

• For reference, the Milky Way is roughly an L* galaxy.

• Even though there are less of them, L ~ L* galaxies contribute the most optical light (see the dashed curve in the plot above).
  Thus, the Milky Way is "typical" in luminosity-weighted terms...

• Dwarf galaxies are often considered to be those with L < 0.1 L*.

• First, the dwarf to giant ratio of galaxies greatly varies by environment.
  • For giants defined as (M_B < -16) and dwarfs as (-16 < M_B < -11) the difference in dwarf/giant is very different:

    System	Dwarf/Giant ratio
    Local Group	~3*
    Virgo Cluster	~4-5
    Coma Cluster	~12

    * THIS NUMBER DOES NOT INCLUDE RECENT (UP TO 2007) DISCOVERIES, WHICH ARE FAINTER THAN -11 TYPICALLY.

  • Though the statistics for the Local Group are in the realm of "small number" when binned into a luminosity function, can still see differences in luminosity function compared to higher density environments.:
    

    The luminosity functions of galaxy groups and clusters from Trentham & Hodgkin 2002, MNRASm, 333, 423. Note that this is missing the recent spate of discoveries of very low luminosity dwarfs with M_B > -10.

• But it is not just the relative numbers of bright and faint galaxies that varies by environment, but also the morphologies:
  • As density increases the number of E's and SO galaxies rise, the number of S and Irr's goes down.
    

    The distribution of galaxy types by their environmental density. From Dressler, 1980, ApJ, 236, 351.

  • The Local Group is typical of "galaxy groups" in being rich in later type galaxies and poor in early type galaxies.

  • Galaxy groups like the Local Group can be considered to represent a "typical" galaxy environment (at least by volume) in the Universe: It is a bound system of galaxies but not as dense as the galaxy clusters and superclusters.

• Normal and Giant Ellipticals (only very small E, M32, nearby).
  • Smooth light profiles.

  • No current star formation.

  • Most devoid of cool gas, trace amounts of dust in cores.

  • Stellar velocity dispersions very high, σ > 300 km/s.

  • Typical elliptical galaxy contains large amounts of hot ionized gas, visible in x-ray. Gas probably shed by old stars, heated by shocks from gas/stars at high velocity dispersion.

  • Large range of V / σ.

  • Central regions fairly metal rich, ~solar.

  • Some show shells, arcs and other tidal features around them, suggesting mergers.

  • Tend to have a high globular cluster specific frequency (number of globular clusters per unit galaxy luminosity), many showing redder cluster population perhaps from more metal rich, apparently younger clusters that may have come from a merger process.
    Thus, show "bimodal" color distribution for globular clusters (see below).

  • Note in Kormendy plot that brighter ellipticals tend to be larger and actually have lower central surface brightnesses than dE.
    This suggests that dE/dSph form differently than large Es -- perhaps by collisions of galaxies.

  Evidence for a merger history of large elliptical galaxies. (Left) A shell galaxy, NGC 3923 (from http://abyss.uoregon.edu/~js/lectures/cannibalism/cannibalism_5.html). (Center) The color distribution of the globular clusters in NGC 3923, from Zepf et al. (1995, ApJ, 443, 570). (Right) The elliptical galaxy NGC5128 (Centaurs A) has a ring of dust and cool gas is thought to be captured from a merging late type galaxy From http://www.atnf.csiro.au/news/press/images/trw_alliance/Centaurus_A.jpg.

• Early spirals/SO.
  • SO and Sa tend to be more luminous, more massive, and redder and to have higher central surface brightness, less star formation, less gas than later types.

  • Spiral arms tight (Sa) or absent (SO).

  

  Two versions of the Hubble Tuning Fork classification of galaxies. From http://skyserver.sdss.org/dr1/en/proj/advanced/galaxies/tuningfork.asp and www.dovada.com/ orbital_data.htm.

• Classical AGN not represented in LG.
  • But M31, MW and M32 seem to have central supermassive black holes.