Galaxy formation and evolution I. (Some) observational facts

Transcription

1 Galaxy formation and evolution I. (Some) observational facts Gabriella De Lucia Astronomical Observatory of Trieste

2 Outline: ü Observational properties of galaxies ü Galaxies and Cosmology ü Gas accretion and cooling ü Star formation ü Feedback (photo-ionization and supernovae) ü AGN feedback ü Environmental processes ü Introduction to galaxy formation models ü Highlights and problems from simulations and semi-analytic models

3 An era of precision cosmology WMAP5 Nolta et al Hinshaw et al t 0 = ± 0.12 Gyr h = ± σ 8 = ± Ω t = 1.02 ± 0.02 Ω m h 2 = ± Ω b = ± Komatsu et al galaxy power spectrum + cosmic shear + Lyα forest + supernovae observations + baryon fraction in clusters +.

4 A complex problem Galaxy formation is a subject of great complexity: it involves physical processes that cover ~23 orders of magnitude in physical size and ~4 orders of magnitudes in timescales. These physical processes are entangled in a complex network of actions, back-reactions and self-regulations. Galaxies are not only interesting in their own right, but also play a crucial role in cosmological studies. Indeed, virtually all cosmological probes use baryons as tracers.

5 The Hubble `tuning-fork diagram What determines galaxy morphologies and the observed correlations with other physical properties and/or with environment?

6 The classification of galaxies The Hubble sequence is based on (visual) morphological classification. Galaxies can be classified according to different properties (e.g. luminosity, gas content, colour). Many of these change systematically along the sequence indicating that it reflects a sequence in the basic physical properties of galaxies. Roberts & Haynes 1994

7 Different environments Grebel et al Springel et al Synthetic galaxy catalogues (galaxy formation models + DM cosmological simulations)

8 The importance of the environment It has been long known that there is a strong correlation between the galaxy morphology and the environment. There is a plethora of physical processes acting in over-dense regions. However, one should consider also that high density regions grow from higher fluctuations in the primordial density field. Dressler (1980) Are the observed trends driven by the local environment or are they imprinted already in the initial conditions? (Nature or Nurture?)

9 Elliptical galaxies - shapes Isophotes of elliptical galaxies commonly fitted by ellipses characterized by minor-to-major axis ratio b/a (ellipticity = 1-b/a) and position angle (this can vary: isophote twisting). Deviations from ellipses are quantified by the Fourier coefficient of the function: a 4 < 0 a 4 > 0 For normal ellipticals: 0.3 b/a 1 corresponding to E0-E7 Boxy and disky ellipticals have different physical properties: boxy ones are bright, rotate slowly, harbor central cores, show stronger than average X- ray emission.

10 Elliptical galaxies - kinematics Kormendy & Bender 1996 When the kinematic structure of elliptical galaxies is examined in detail, a wide range of behaviour is found, evidence of a wide range of formation histories. At the very centre, the velocity dispersion rises more strongly than it can be understood as a result of the gravitational effect presence of a super-massive black hole.

11 Scaling relations: the colour-magnitude Visvanathan & Sandage (1977) It has long been known that the colour of early-type galaxies (in clusters) is strongly correlated with their luminosity. This is usually interpreted as a metallicity relation (more massive galaxies can keep higher fractions of processed gas) but age trends are not excluded (measured) The scatter of the CMR is small interpreted as small range in z_form Coma 5 mag De Propris et al. (1998)

12 Evolution of the CMR The faint-end of the colour magnitude relation appears to be depopulated at high redshifts. Results are qualitatively consistent with a scenario in which the infalling blue galaxies have their star formation histories truncated by the hostile cluster environment. The responsible physical mechanisms and associated time-scale have yet to be identified. Rudnick et al. 2009

13 Scaling relations: the fundamental plane Origin of the fundamental plane is usually interpreted in terms of the virial theorem. GM/<R> = <v 2 > Using R e = k R <R>, σ 0 = k v <v 2 >, and L = 2π<I e >R e 2 one obtains: Re = α σ 0 2 <I e > -1 (M/L) -1 Kormendy & Djorgovski 1989 The observed relation is `tilted with respect to this prediction, which is just another indication that elliptical galaxies do not represent a homologous class.

14 Scaling relations: the BH - mass Gebhardt et al The relatively small scatter of the M-σ relation is generally interpreted to imply some source of mechanical feedback between the growth of supermassive black holes and the growth of galaxy bulges that acts to maintain the connection despite processes like galaxy mergers and gas accretion that are expected to increase the scatter over time.

15 Disk galaxies and dark haloes For massive galaxies, rotation curves typically rise rapidly at small radii and then are almost constant over most of the disk. In dwarf and lower surface brightness systems a slower central rise is common. There is considerable variation from system to system, and features in rotation curves are often associated with disk structures such as bars or spiral arms Begeman et al. 1989

16 Scaling relation: the Tully-Fisher At infrared wavelengths, L ~ V 4 (similar to the Faber- Jackson relation for ellipticals, common origin)? Giovanelli et al. (1997) At 0-th order, it can be explained by the virial theorem, assuming M/ L~const. In reality, however, the deviates from virial theorem prediction and depends on the waveband. N.B. The scatter is quite small -> distance indicator Tully & Fisher (1977)

17 The galaxy luminosity function Driver (2004)

18 Dependency on environment Binggelli et al Elliptical and lenticular galaxies represent a larger fraction of the cluster galaxies population. They dominate the bright end of the luminosity function. Spiral and irregular galaxies are more numerous at intermediate and faint luminosity. In the cluster environment, a large contribution at the faint end of the luminosity function is given by dwarf elliptical galaxies, which represent a negligible contribution in the field Are galaxies of different classes trasforming into one another due to the cluster environment?

19 Evolution of the luminosity function From the VVDS Deep Survey deeper but smaller volume From the zcosmos Survey larger volume but shallower Zucca et al. (2009) It is now possible to study the evolution of the luminosity function as a function of galaxy type and environment.

20 Bivariate distributions: colour bimodality ~183,000 galaxies at z~0 0.2 from the Sloan Digital Sky Survey Blanton et al. (2003)

21 The mass-metallicity relation Tremonti et al Not clear whether it is a sequence of enrichment or of depletion: if more massive galaxies form fractionally more stars in a Hubble time than their low-mass counterparts, then the observed relation represents a sequence in astration. If galaxies form similar fractions of stars, then the relation could imply that metals are selectively lost from galaxies with small potential wells via galactic winds.

22 The mass-metallicity relation Tremonti et al No metals lost Prediction from a closed box model: Z = yield * ln(1/gas mass fraction) Results indicate that galaxies do not evolve as closed boxes and the effective yield decreases by a factor ~10 from the most massive galaxies to the dwarfs. This is interpreted as consequence of metal loss by galactic winds

23 Downsizing At low redshift, few galaxies have strong emission lines, i.e. they are forming stars at low rates. At high redshift, more galaxies are forming stars at high rates particularly among the most luminous ones. Cowie et al It appears that at the present time there is almost no galaxy formation, but that as recently as z~0.2, low-mass galaxies were forming; progressively higher galaxy masses are seen in formation at higher redshift The word downsizing is today used to refer to different observational trends that might not necessarily represent different manifestations of the same process.

24 Archeological downsizing Thomas et al More massive galaxies are older and more metal-rich. In addition, their alpha/iron ratios are higher than those of their lower mass counterparts. Alpha elements are produced mainly by SN II with short delay times while Fe is mainly produced by SNIa with delay times that vary between few Myr to several Gyr. Trend is usually interpreted as evidence for shorter formation time-scales for higher mass galaxies.

25 Evolution of the stellar mass function No significant evolution of the massive end of the mass function, while significant increase of the number density of low-mass galaxies BUT: observational estimates of stellar mass have large uncertainties and there are very few galaxies at the most massive end. Results appear to be in qualitative agreement with model prediction at the massive end. Marchesini et al. 2009

26 Evolution of the mass-metallicity relation Maiolino et al Evolution of the mass-metallicity relation appears to be faster for low-mass galaxies. Important caveats: different selections for galaxy samples at different redshifts might introduce a bias; different tracers of metallicity are usually employed which can introduce differences in the slope and normalization of the mass-metallicity relation; intrinsically different galaxy population (i.e. at high redshift, not necessarily the progenitors of low-z galaxies)

27 Galaxies as tracers of large scale structure Wang et al The excess number of galaxy pairs of a given separation, r, relative to that expected from a random distribution: ξ(r) =DD(r)/RR(r) 1.

28 Baryon acoustic oscillations Eisenstein et al ~ 47,000 galaxies from the Sloan Digital Sky Survey (SDSS) The acoustic peaks occur because cosmological perturbations excite sound waves in the reltivistic plasma of the early Universe (Peebles & Yu 1970, Sunyaev & Zel dovich 1970). Recombination (z~1000) decreases the sound speed effectively ending the wave propagation.

29 BAOs (not only) and the dark energy Medium-size mission (M2) selected by Goal: understand the nature of dark energy and dark matter by accurate measurement of the accelerated expansion of the Universe through two different independent methods (Weak gravitational Lensing + BAOs) Telescope is a 1.2 m Korsch, with two instruments: VIS (visible imager), NISP (NIR imager and slit-less spectrograph) Launch foreseen for ~2020, mission duration 6 years