Galaxies

1. Galaxies • Introduction • Elliptical galaxies • Spiral galaxies • Scaling relations • Central black holes • Luminosity functions • Spectra

2. Introduction Classification:the Hubble sequence spirals ellipticals lenticulars barred spirals

3. Introduction - 2 • The Hubble classification is morphological and influenced by projection effects (2D view, not 3D) • Elliptical galaxies belong to classes En (n = 0,…,7) where b a • Ellipticity is not necessarily an intrinsic property of the galaxy (a cigar or a disk could be classified E0, depending on the viewing angle)

4. Introduction - 3 • Spiral galaxies are classified Sa, Sb, Sc, Sd depending on the importance of the bulge with respect to the disk and the characteristics of the arms • Intermediate classes (Sab, Sbc, Scd) are also introduced • Barred spirals have a similar classification: SBa, SBab, SBb,… • Galaxies not fitting in that scheme are classified irregulars

5. Introduction - 4 Mass / luminosity ratio (ϒorY) • Generally given in solar units → Y = 1 for the Sun • Depends on the spectral band (ex: YV = M/LV) • Extrapolated to bolometric luminosity using spectral models • Applies to stars, star clusters, galaxies, galaxy clusters • Ex: massive stars: Y < 1 gas-rich spiral galaxies: Y ~ 1 – 10 elliptical galaxies: Y ~ 10 – 100 galaxy clusters: Y ~ 300 Why Y > 1 in most galaxies ?

6. Introduction - 5 • Most massive stars (→ most luminous) evolve faster (M ≈ ct but L decreases) → Y increases when galaxy ages (and mostly when star formation slows down) • Hot stars ionizegasaroundthem (Hiiregions, high L for verylowM) → reinforce the variation of Y with age • Stellar remnants have a very high Y • and dark matter an infinite Y…

7. Introduction - 6 Color • The color of an object is measured by a color index (ex: B–R = mB – mR) • After correction for dust reddining (if necessary), it is an intrinsic property of the object B R • An object with a large color index is called red, an object with a low (or negative) color index, blue red object blue objects

8. Introduction - 7 Metallicity • Content in elements from carbon and heavier • Iron is often considered representative • Applies to stars, interstellar matter, galaxies • Depends on the chemical history of matter (previous stellar generations) → generally not homogeneous in a galaxy • Higher metallicity → redder object (since more absorption lines in the blue)

9. Introduction - 8 Magnitudes • For a point-like object: • For an extended object: – either one measures the integrated magnitude – either one measure the magnitude per unit of solid angle where Isurf is the flux received per unit solid angle (μin mag/arcsec2)

10. Introduction - 9 Virial theorem • For an isolated system in dynamical equilibrium: 2 EK + EP = 0 (in absolute value, kinetic energy = ½ potential energy) • Estimate of the mass of a cluster (of galaxies): R = mean distance between 2 galaxies → EP ~ −GM2/2R (*) V = mean velocity of galaxies → EK ~ ½ MV2 (* /2 in order not to count twice the energy associated to a pair of galaxies)

11. Elliptical galaxies (early-type galaxies) Sub-types – gE (giant elliptical) – E (elliptical) – cE (compact elliptical) – dE (dwarf elliptical) (surface brightness of dE lower than cE) ESO 325−G004 (gE) NGC 205 (dE) M 32 (cE)

12. Elliptical galaxies - 2 – cD galaxies: supergiant ellipticals (c) with extended halo → appear diffuse (D) located at the center of some rich clusters Image: NGC 3311 (cD) and NGC 3309 (gE) at the center of Hydra I cluster Note the presence of thousands of globular clusters around these galaxies

13. Elliptical galaxies - 3 – S0 galaxies: lenticulars (intermediate between spirals and ellipticals) ≈ spirals without spiral arms Image: NGC 2784

14. Elliptical galaxies - 4 – dSph galaxies: dwarf spheroidals, very low surface brightness → observable only in the local group (maybe the most frequent galaxies, but very hard to observe) Image: NGC 147

15. Elliptical galaxies - 5 Luminosity profile • The surface brightness decreases from the center to the outskirts according to a simple empirical law (de Vaucouleurs law): or: (r1/4 law) Re = effective radius (contains half the emitted light) Ie = surface brightness at the effective radius

16. Elliptical galaxies - 6 • For cD galaxies, there is an excess brightness at large radii compared to the r1/4 profile → cD ≈ gE + extended luminous halo • The extended halos of the cD galaxies could be the remains of many small galaxies `swallowed´ by the giant elliptical • The de Vaucouleurs law can be generalized to elliptical isophotes where ae and be are the major and minor semi axes

17. Elliptical galaxies - 7 Composition • old stars, little gas → no more star formation • sometimes dust bands (remains from absorbed spirals?) Centaurus A NGC 7049

18. Elliptical galaxies - 8 Ellipticity • Why have ellptical galaxies kept their shape and did not all become spherical? • Rotation as in spirals? • Rotation flattening significant if vrot ~ σv where However, vrot<<σv + triaxial galaxies → rotation can not explain the observed ellipticity → shape is a testimony of history The center of the Virgo cluster

19. Elliptical galaxies - 9 Stability of the ellipsoidal shape • Collisions between stars tend to increase the symmetry of the system • The time needed for this `relaxation´ can be estimated by: trelax = characteristic time for direction change due to collisions tcross = crossing time of the system N = number of stars in the system • With tcross ~ 108 years and N ~ 1012 → trelax ~ 1018 years >> age of the Universe → ellipsoid is stable

20. Elliptical galaxies - 10 Departures from ellipsoidal shape • Generally: isophotes ≈ concentric ellipses • But: − ellipticity ε not always constant with radius − major axis orientation may vary: isophote twisting • Twisting can be a projection effect if ε varies (apparent direction of major axis seems to vary more if ε→ 0)

21. Elliptical galaxies - 11 Shells and waves • Complex structures sometimes visible at low surface brightness • Signs of complex evolution, probably linked to merging of galaxies Image: NGC 474

22. Spiral galaxies (late-type galaxies) Sub-types – spirals: Sa, Sb, Sc, Sd (+ intermediates Sab, Sbc, Scd) – barred spirals: SBa, SBb… (+ intermediates SBab, SBbc…) M74 NGC1365

23. Spiral galaxies - 2 • Sub-classes a, b, c correspond to differences in: bulge θ arm • Barred spirals (± as numerous as spirals) have a similar classification

24. Spiral galaxies - 3 Sa M 104 « Sombrero »

25. Spiral galaxies - 4 Sab Sb M 81 M 63

26. Spiral galaxies - 5 Sbc Sc / Sd NGC 3184 NGC 300

27. Spiral galaxies - 6 SBb SBb M91 M 95

28. Spiral galaxies - 7 SBbc SBc NGC 1300 M 109

29. Spiral galaxies - 8 `intermediate´ (embryo of a bar) M 83

30. Spiral galaxies - 9 Luminosity profile • The (mean) surface brightness of the disk decreases with distance from the center according to an exponential law: or: • μ0not directly measurable (center inside the bulge) → extrapolation • μ0 nearly constant in `normal´ galaxies: (Freeman law) • The surface brightness of the bulge follows the same law as elliptical galaxies

31. Spiral galaxies - 10 Rotation curves • If the galaxy is not seen face-on: where i = inclination (angle between the galactic plane and the plane of the sky) • vrad measured by spectroscopy (Doppler effect) • i determined by assuming that the disk is circular (apart from spiral arms…)

32. Spiral galaxies - 11 • The rotation velocity in the outer parts is too high for the estimated mass (stars + interstellar matter) → one postulates the existence of a dark matter halo

33. Spiral galaxies - 12 • Modelling: one assumes circular orbits in the disk (+ spherical halo) where M(R) = mass included inside the radius R One estimates the amount of `normal´ (luminous) matter Mlum from L(R) and an estimated M/L ratio → gives a predicted rotation curve → the amount of dark matter Mdark is what we need to add to explain the rotation curve:

34. Spiral galaxies - 13 • The different components are modelled separately (disk, bulge, halo, central black hole) R z r The gravitational potential F(R) is defined by Parameters Mdisk, a, ρ0, R0… are adjusted to fit the observations

35. Spiral galaxies - 14 Composition 1. Stars: Later type → more young stars → more massive stars → bluer color (In agreement with the reduced importance of the bulge, redder and containing older stars) NGC 300 M 81

36. Spiral galaxies - 15 2. Gas: Later type → larger proportion of gas (necessary for star formation) 3. Dust: Mass of dust ~1% mass of gas If dust heated by hot stars → emission in far IR (FIR) → mainly in late-type spirals M104 in false colors: blue = visible (HST) red = FIR (Spitzer)

37. Spiral galaxies - 16 Structure 1. Spiral arms: Higher contrast in blue but arms also seen in red → imply all components of disk but excess of young stars Density waves (amplitude ~10 – 20%) that propagate at a speed different from that of matter Perturbation amplified by dynamic evolution Various theories to explain their appearance: chaotic phenomenon, tidal effect from a companion, triggering of star formation by differential rotation…

38. Spiral galaxies - 17 2. Bar: Stable over several rotation periods Triggered by instability in the disk NGC 6050 and IC 1179

39. Scaling relations Scaling relation = • relation between several characteristic properties of a class of objects • determined empirically in the nearby Universe • that can be applied to remote objects for which the determination of one of these properties would necessitate the knowledge of distance Ex: → allows to estimate the distance of these remote objects δv L depends on d independent of d

40. Scaling relations - 2 Tully – Fisher relation (spiral galaxies) vmax = maximum rotation velocity (in the `plateau´ – measured e.g. by the 21cm H line) L = integrated luminosity α = exponent varying with wavelength (α increases with λ) • nearbygalaxies: spatially resolved spectrum • remote galaxies: integrated spectum W

41. Scaling relations - 3 Interpretation:

42. Scaling relations - 4 Interpretation (2): Since L is roughly proportional to M*, Tully-Fisher links M* and v4 However, in some galaxies (less massive ones, which have the lowest star formation rate), Mgas should be taken into account One gets indeed a better correlation between log vmax and log(Mdisk= M*+Mgas)than with M* alone → suggests that the M/L ratio (and thus the fraction of dark matter) is ± constant in a large range of galactic masses (disk-halo conspiracy) Mdisk M*

43. Scaling relations - 5 Faber – Jackson relation (elliptical galaxies) σ0 = velocity dispersion in the center of the galaxy L = integrated luminosity Dispersion around the relation larger than for Tully-Fisher → suggests that (at least) another parameter plays a role

44. Scaling relations - 6 Fundamental plane (elliptical galaxies) • One seeks a relation between 3 parameters in order to reduce the dispersion • It is empirically found that where Re= effective radius (contains half of the luminous flux) and = mean flux inside Re → suggests to seek a relation between , Re et σ0

45. Scaling relations - 7 I

46. Central black holes Black hole = solution of the general relativity equations for a `point mass´ • escape speed vesc: • Schwarzschild radius: RS = R for vesc = c • black hole = object for which R < RS • all sufficiently massive galaxies seem to contain a central supermassive black hole (SMBH, M ~ 105 – 109 MO)

47. Central black holes - 2 Detection in inactive galaxies • Dynamical effect can be measured in a region where black hole (BH) potential dominates R0 = radius of the sphere in which the black hole potential dominates σ0 = velocity dispersion at the center of the (elliptical) galaxy or of the bulge (in a spiral) • Angular resolution needed: → possible in nearby galaxies with the best instruments

48. Central black holes - 3 • Increase of velocity dispersion σor of the rotation velocity vrot in the central region (R < R0) • No direct proof that it is due to a black hole but no alternative solution (huge mass in a limited volume) spectrum x image of the galactic center spectrograph slit λ

49. Central black holes - 4 Correlations • Estimates of the SMBH mass in a sample of galaxies → study of correlations with galactic properties → one observes a correlation between the mass of the black hole and the mass of the bulge: MSMBH / Mbulge ~ 0.002 → joint evolution? or result of galactic mergers?

50. Central black holes - 5 Sagittarius A* • At the center of our Galaxy: compact star cluster centered on the radio source SgrA* • The proper motions and radial velocities of ~1000 stars in that cluster could be measured (inside 10 arcseconds around SgrA*) → imply the presence of a mass M = (3.6±0.4) 106 MO in R < 0.01 pc (2000 AU) centered on SgrA*