25. The Nature of Galaxies25. The Nature of GalaxiesGoalsGoals:1. Characterize the main galaxy

morphological types and recognize some of their main features as well as their relative abundances in the universe.

2. Link the velocity distributions observed in central galaxy components to their physical characteristics.

3. Introduce an alternate classification scheme possibly more directly linked to fundamental characteristics of galaxies.

The Hubble Sequence:The familiar “tuning fork” diagram of Edwin Hubble was an attempt to link the main galaxy types of elliptical (E), spiral (S), barred spiral (SB), and irregular (Ir) classes. The lenticular galaxies (S0, SB0), added later, were a supposed link between spheroidal E galaxies and flattened S and SB galaxies, but unfortunately the diagram was also pictured as an evolutionary sequence. Thus, elliptical galaxies are often referred to as “early-type” galaxies, much like “early-type” OB stars.

A sample of elliptical galaxies, where the ellipticity is the ratio of small and large axes, ε = 1 β/α. E# = E(10ε)

Sample: IC 4296, E0, NGC 777, E1, NGC 1549, E2, NGC 4365, E3, NGC 4564, E6, NGC 4623, E7.

Some are actually lenticulars, S0.

Ellipticals are not all the same type seen in various angles of projection, but often seem to be oblate (top) or prolate (bottom) spheroids viewed at various angles. That is determined by their relative frequencies.

The fact that the “latest” type assigned to an elliptical is E7 indicates that the highest degree of ellipticity must be 0.7 = 1 β/α, i.e. β/α = 0.3, implying that the small diameter is never less than 30% of the large diameter.

But van den Bergh noted that most galaxies classified as E7 have rather “pointy” long axes, suggesting that they are actually lenticular galaxies (S0s) viewed nearly edge-on. If the “latest” type for ellipticals is truly E6, as he suggests, that implies β/α = 0.4, in other words the small diameter is never less than 40% of the large diameter.

It is a difference that is relevant to understanding the dynamical origins of elliptical galaxies.

Early examples of lenticular galaxies.

List: NGC 3245, S0, NGC 4251, S0, NGC 4179, S0:, NGC 5422, S0:, NGC 3203, S0:, NGC 4429, S0/Sa.

Note that lenticular galaxies, named because of their “lens-like” appearance, are actually more like spiral and barred spiral galaxies in having a flattened disk, rather than like elliptical galaxies which mostly possess an ellipsoidal symmetry. The primary feature that distinguishes lenticulars from spirals is that they do not have spiral arms or features. For example, lenticular galaxies seen edge-on do not have a dust lane running along the length of the galaxy.

van den Bergh has suggested a possible origin for lenticulars via collisions between spirals that results in the dust and gas being swept out of the galaxies by ram pressure. Because they have been swept clean of material for making new stars, such galaxies eventually contain only old stars, like ellipticals, and lack the distinguishing features of spirals. The fact that most lenticulars are found in rich clusters of galaxies seems to support the idea.

Hubble Atlas examples of “early-type” spiral galaxies.

NGC 3269, Sa, NGC 7096, Sa, NGC 1350, Sa,NGC 1371, Sa, NGC 488, Sab, NGC 2460, Sab.

Hubble Atlas examples of “late-type” spiral galaxies.

NGC 1566, Sbc, NGC 5247, Sc, NGC 2997, Sc,NGC 3184, Sc, NGC 3938, Sc, NGC 5055, Sbc.

Spiral galaxies are sometimes classified on the basis of the length of the spiral arms and their degree of “tightness.” However, a more universal criterion would appear to be the relative size of the central bulge, which is always largest in the Sa galaxies and smallest at Sc. van den Bergh suggested using disk to bulge ratio, D/B, as a good indicator, with Sa galaxies characterized by D/B ≤ 3, Sb galaxies by 3 ≤ D/B ≤ 10, and Sc galaxies by D/B ≥ 10.

Certainly, the dimensions of the bulge are the important characteristic of spiral galaxies, and bulges account for much of their mass. The length of the spiral arms is another feature that appears to depend upon the dimensions of the disk. Again, van den Bergh suggests that this characteristic can be used as a luminosity indicator for spirals. See van den Bergh 1960, ApJ, 131, 215 & 558.

Early examples of barred and barred ring lenticular galaxies.

NGC 3384, SB0:, NGC 5473, SB0:, NGC 1574, SB0,NGC 4340, RSB0, NGC 2859, RSB0, NGC 3945, RSB0.

van den Bergh (1960) developed his luminosity classification scheme using galaxies in clusters of galaxies, eliminating distance as a complicating factor. He noticed that the degree to which spiral structure is developed in “late-type” galaxies is a function of absolute magnitude. The most luminous galaxies have the most strongly developed spiral structure, with the correlation being strong enough to define a luminosity classification system. The luminosity classes are like those used for spectral types, i.e. I, II, III, IV, V, corresponding to supergiant galaxies, bright giant galaxies, giant galaxies, subgiant galaxies, and dwarf galaxies.

van den Bergh applied the scheme to late-type spirals and barred spirals, as well as irregular galaxies. Note the types applying to well-known objects: M31 (Sb I-II), LMC (SBm III), SMC (Im IV-V), M33 (Sc II-III), as listed in the Observer’s Handbook. The Milky Way is a supergiant galaxy by this criterion.

Some of van den Bergh’s classifications for Sc and SBb galaxies.

The Large Magellanic Cloud (SBm III) from UK Schmidt plates by David Malin. An irregular galaxy that is an incipient barred spiral in the making?

The Small Magellanic Cloud (Im IV-V) from UK Schmidt plates by David Malin. An “inverse C” shape.

The Andromeda Galaxy NGC 224 (M31, Sb I-II) by Robert Gendler, 2002. The companions are NGC 205 (M110, S0/E5pec) and NGC 221 (M32, E2).

M32

M110

The Triangulum Galaxy NGC 598 (M33, Sc II-III) from Kitt Peak National Observatory. Note the small bulge and restricted length of the spiral arms.

The Sculptor dwarf spheroidal galaxy (dE) from an image obtained by the Anglo Australian Observatory.

The Leo II dwarf spheroidal galaxy (dE or dSph).

van den Bergh’s classification scheme for galaxies, from 1976, ApJ, 206, 883.

van den Bergh’s classification scheme for galaxies, from 1976, ApJ, 206, 883.

An alternate schematic for van den Bergh’s scheme linking the a, b, c types to D/B ratio.

NGC 4921, an example of a spiral galaxy in the Coma cluster intermediate in type between a normal barred spiral and an anemic barred spiral.

Luminosity Differences Among E Galaxies.The I, II, II, IV, V luminosity scheme was never applied to E galaxies because there are no obvious distinguishing features in ellipticals suitable for that purpose. All Es are composed of old, low-mass stars. Differences in luminosity are evident, however, from E galaxies in clusters and of known redshift. The differences have given rise to separate morphological classes for ellipticals:cD galaxies. Huge elliptical galaxies dominating some clusters of galaxies, so-named because the cD designation stands for “cluster dominating” galaxy. Some appear to be incredibly large, massive, and luminous.Normal ellipticals. Standard E galaxies comparable in luminosity to supergiant spiral galaxies.Dwarf ellipticals, dE. Lower luminosity elliptical galaxies comparable in absolute magnitude to giant and subgiant spiral galaxies.

Dwarf spheroidal galaxies, dSph. Very low luminosity elliptical galaxies only found, so far, in nearby regions of the Local Group. Such galaxies often appear much like rich globular clusters.Blue compact dwarf galaxies, BCD. Such galaxies appear like ellipticals because of their compact nature, yet they contain abundant quantities of gas and massive blue stars with active star formation occurring.E Properties.Type cD E dE dSph BCD

MB 22/25 15/23 13/19 8/15 14/17

M 1013-1014 108-1013 107-109 107-108 ~109

D(kpc) ~1000 1-200 1-10 0.1-0.5 <3

M/L ~750? 7100 ~10 5100 0.110

<SN> ~15 ~5 4.8 ±1.0 n/a ?

Gas and Dust Properties of Galaxies.

Gas and dust content increases towards “later-type” galaxies.Likewise, the content of young stars also increases monotonically towards “later” types of galaxies. The exceptions are interesting objects in their own right.

K Corrections.The absolute magnitudes of galaxies are derived in similar fashion to those of stars, i.e.:m M = 5 log d 5 + Am ,where Am is the amount of absorption by intergalactic and interstellar extinction. Most galaxies are at high Galactic latitudes, so local extinction is small. But distant galaxies suffer from large redshifts, which stretches the spectrum and shifts the distribution to longer λs. Hence the need for K corrections.

The technique is still evolving as galaxy flux distributions are combined with filter passband functions to compute K corrections suitable for the individual galaxies under investigation.

Schematic Schematic since since

stretching stretching also also

occurs!occurs!

Galaxy Isophotes.The brightness distribution in galaxies is notoriously difficult to establish. For one thing, the brightness of the night sky amounts to ~23 magnitudes/arcsecond2 in blue (B) light, whereas new detectors are capable of imaging galaxies to B ~ 29. It has been convenient to model the brightness distributions with a de Vaucouleurs profile. The central bulges of spiral galaxies appear to follow a light distribution characterized by:

A Sérsic profile uses 1/n rather than ¼. Finding a parameter characterizing the apparent radius of a galaxy is not obvious. The Holmberg radius corresponds to a point where the brightness levels reach μ = 26.5 B mag./arcsec2, but a more logical parameter is the effective radius, re, the isophote containing ½ the galaxy’s light.

13268.8)(

41

ee r

rr

Tully-Fisher Relation.The rotation curves of spiralgalaxies were first studied byVera Rubin, who discoveredthe flat nature of theirdistributions. That wasoriginally startling, and onlylater was it also found toapply to our own Galaxy.Prior expectations of aKeplerian drop-off tendedto dominate previousstudies of the rotation curvefor the Galaxy. Note howthe peak value for vR varieswith the specific type and luminosity class for the spiral galaxy: Sa, Sb, Sc, I–III.

The last characteristic was formulized by Brent Tully and Richard Fisher when they used cluster galaxies and galaxies of known redshift to show a dependence on MB.

Radius-Luminosity Relation.This is essentially van den Bergh’s luminosity classification relation for late-type galaxies formulized into a mathematical equation. It is given by:

where R25 is the radius corresponding to a brightness level of 25 B mag./arcsec2, and is measured in kpc.

00.4249.0log 2510 BMR

Colours and the Abundance of Gas and Dust.Mean BV colours for galaxies have long been used to deduce characteristics of the galaxies themselves, in particular the relative proportions of early-type (young) stars that they contain. Bluer galaxies are assumed to contain a higher frequency of bluer stars, thereby implying the existence of ongoing star formation, which can only take place if there is interstellar gas in the galaxy.

Of the various galaxy types, Es and S0s are the reddest, S and SBs are bluer, and Irrs are generally the bluest (objects like the Large and Small Magellanic Clouds).

Metallicity and Colour Gradients.Individual spiral galaxies exhibit colour gradients in their disks, with the bulge regions being redder, presumably because they are dominated by red giants, i.e. old stars. They may also exhibitmetallicity gradientslike that in the MilkyWay. There is also anobserved correlationbetween metallicityand luminosity, in thesense that the mostluminous galaxies arethe most metal-rich.

Globular Cluster Specific Frequencies.Spiral and elliptical galaxies both contain globular cluster systems. In general it appears that the globular clusters, consisting primarily of an older population of stars, exhibit the same luminosity distribution for all galaxies. Low luminosity globulars consist of fewer stars, and are the first to evaporate, so their numbers are small. High luminosity globulars consist of large numbers of stars, and are relatively rare. They have not had time to lose many stars, so appear almost as rich as when they were formed. The distribution peaks for globulars of intermediate luminosity.

Work in this area has been dominated by Canadian researchers, such as Dave Hanes (Queen’s) and Bill Harris (McMaster).

Examples of the luminosity distributions for globular clusters in our own Galaxy and other galaxies. From Harris 1991, ARA&A, 29, 543.

Globular clusters are more plentiful the more massive and more luminous the galaxy. Spot the globulars.

Specific frequency is a toolfor normalizing the globularcluster distributions forindividual galaxies so thatthey are characterized bythe same total number ofglobular clusters, Nt. IfLV = the galaxy’s luminosityand L15 is the luminositycorresponding to an absolutevisual magnitude of 15,then:

Note how SN varies withGalaxy type. Should be ~1.5for the Milky Way.

VtN L

LNS 15

Potential problems arise in some cases. For example, the globular cluster population of the LMC includes young clusters as well as old, which also appears to be true of the globular cluster population of M31 (below left). The “globulars” in M31 also appear to display a rather large velocity dispersion (below right).

Central Black Hole Characteristics.Many nearby, luminous galaxies exhibit velocity dispersions near their centres suggestive of the presence of extremely massive compact objects, i.e. “black holes.” Masses for the black holes are estimated from kinematic studies, as well as from estimates made using the Virial Theorem, namely:

See relation at right.There appears to be adirect correlationbetween the mass of agalaxy’s central blackhole and its luminosity.

G

RM 15

virial

5

The bulge mass also appears to correlate with the mass estimated for a galaxy’s central black hole.

X-Ray Luminosity.Most nearby galaxies are detected in X-rays as well as at other wavelengths. There is apparently a surprisingly tight correlation between the X-ray luminosity of spiral galaxies, LX, and their blue luminosities, LB, in particular:

which is interpreted to indicate that the X-rays originate from a particular class of object in spiral galaxies that constitutes a constant proportion of the stellar content.X-ray binary systems, which consist of compact objects (black holes?) in close orbit about a binary companion, have been postulated as a potential origin, since they should increase in number in proportion to the stellar mass of a galaxy.

710B

X

L

L

Spiral Structure.The textbook has a lengthy section on the density wave model for the propagation of spiral arm patterns in disk galaxies, which you can read over yourself. Also included are interesting comments about the question of trailing or leading spiral arms (most appear to be trailing) and about early dynamical models to simulate spiral galaxies. Among the latter are the creation of bar instabilities in the central regions of galaxies and the question of whether or not all spirals contain central bars of some type. Two-armed spirals appear to be common, although there are examples of more exotic objects, including one 3-armed spiral? Stochastic self-propagating star formation is the term used to describe galaxies in which spiral features are the natural consequence of sequential star formation and normal galactic rotation.

Faber-Jackson Relation.Dwarf ellipticals, dwarf spheroidals, normal ellipticals, and the bulges of spiral galaxies all appear to share one feature in common, namely a relationship between the central radial velocity dispersion and MB, namely:

or

or, for thefundamentalplane:

BM1.0log 010

40L

65.065.20 erL

Rotation and Galaxy Shapes.It appears from the study of elliptical galaxies that their shapes are not related to rotation. Instead, most luminous E galaxies represent triaxial ellipsoids with their shapes established by the velocity dispersions of their stars along the three primary axes. Low luminosity E galaxies, and the bulges of spiral galaxies, on the other hand, do appear to have shapes governed by the rotation of the galaxy or bulge. Much effort is expended in matching galaxy shapes to ellipsoids to establish whether or not they display “boxiness,” which is taken to indicate bar-like structure.

Relative Numbers of Galaxies vs Hubble Type.From Binggeli et al. 1988,ARA&A, 26, 509.

The numbers of lowluminosity galaxies haveprobably been vastlyunderestimated.

Note that normal spiralsoutnumber lenticulars,and ellipticals are themost common type athigh luminosity.

Sample QuestionsSample Questions1. Neglect extinction effects and K-corrections and show that the surface brightness of a galaxy is independent of its distance from the observer.

Answer: By definition:Flux of a galaxy = f = E/s at all wavelengths crossing a unit area of sky, in units of W/m2. Surface brightness of a galaxy = SB = flux received per unit solid angle, in units of W/m2/steradian.Luminosity = L = total power at all wavelengths radiated (or in some defined spectral range).

So: for the light

distributed over a galaxy, which means:

So the flux from a distant galaxy varies as the inverse square of its distance. What about surface brightness?

Surface brightness is defined by:

where Ω is the solid angle subtended by the source.

But Ω = πθ2 for small θ.

Solid angle is defined by Ω = A/r2, where A is the area of the source (the gray area). For small θ, A πl2. Thus, we have Ω = πl2/r2, where l is the fixed size of the source. So the surface brightness is given by:

which is independent of distance.

2. How would you classify the galaxies in Hickson Compact Group 87?

Answer. Try ityourself.

3. Use the rotation curve for stars within 1" of the centre of M32 shown in Fig. 25.14 to estimate the mass lying within that region. Compare your answer with the value obtained using the velocity dispersion data in Example 25.2.1 of the textbook and with the estimated range cited in that example.

Answer. Wedid an exampleof using theorbital velocityto estimate galaxy mass inChapt. 24 forthe Galaxy.Here, vr =50 km/s andr = 3.7 pc.

Use Kepler’s 3rd Law in Newtonian fashion,i.e. (MG + Mstar) = a3/P2, for a in A.U. and P in years.For an orbital speed of 50 km/s and orbital radius of 3.7 pc the orbital period is:

The semi-major axis is:

So the mass of the Galaxy is:

as compared to an estimate of ~107 M obtained from the velocity dispersion and values ranging from 1.5−5106 M from the rotation curve, similar to the present value.

yr105463.4

s/yr103.1558km/s05

km/AU10496.1AU/pc206265pc7.32

km/s50

2

5

7

8

R

P

AU106318.7AU/pc206265pc7.3 5a

Sun

6Sun25

35

G 101506.2105463.4

106318.7MMM