Role of the Environment in the Evolution of Galaxies Richard Ellis

Transcription

1 Role of the Environment in the Evolution of Galaxies Richard Ellis

2 Hubble s Tuning Fork ( , 1936)..describes a true order among the galaxies, not one imposed by the classifier (Sandage et al 1994)

3 3-D scheme (de Vaucouleurs & de Vaucouleurs 1964) T Incorporates various forms along a single T -type axis Additional dimensions for rings, bars and detailed features of spiral structure

4 The Observers: Deliberate effort was made to find a descriptive classification which is entirely independent of theoretical considerations. E. Hubble (1926) Although taxonomy does not constitute an advanced state of knowledge, it is an essential first step in all fields of science. Without it, we would for ever be studying in detail every individual object without recognizing them as examples of a separate species... Science aims at the general not the particular. The Theorist: How Useful is Morphology? G. de Vaucouleurs (1994) The Hubble system has been extraordinarily useful, but the extensions, although allowing a more accurate description of the optical images, have had only marginal utility. J.P. Ostriker (1976)

5 Merits of Hubble s Scheme Distinguishes dynamically distinct structures: spirals & S0s rotating stellar disks spheroids dense ellipsoidal/triaxial systems with anisotropic velocity dispersions Exist physical variables that govern the sequence: * gas content/integrated color ratio of current to past average star formation rate * inner structures bulge/disk ratio

6 bulge dominated Colors and bulge/disk ratios bluer integrated colors

7 S0 or Lenticular Galaxies Suggestive of spirals which suffered sudden removal of their gas leading to a cessation of star formation..but by what process(es)?

8 Morphological Transformations: Nature vs Nurture Dressler (1980) : proportion of spheroidals is a strong function of projected galaxian density Either: spheroidals formed naturally in high density systems at early times, Or: spheroidals are the results of environmentally-induced activity Dressler Ap J 236, 351 (1980)

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10 The Butcher-Oemler Effect Butcher & Oemler Ap J 226, 559 (1978); Ap J 285, 426 (1984) Rising f B (z) (fraction bluer than red sequence above some luminosity)

11 HST: Evolution in Morphologies Increasing blue fraction f b (z) arises from star-forming spirals Coma z 0 AC118 z=0.31 The `Morphs Project: Dressler, Couch, Ellis, Smail, Oemler

12 Recent Origin of Morphology Density Relation z=0 z=0.5 Spirals transformed to S0s via cluster-based processes? Dressler et al Ap J 490, 577 (1997)

13 Evolution of Morphology Density Relation Smith et al Ap J 620, 78 (2005) f E/S0 f E/S0 Environmental density Σ plays key role in governing morphological mix: - Continued growth in high Σ but delay for lower Σ regions - Slower conversion of spirals to S0s with only Es at z > 1?

14 T- Σ from COSMOS data Capak et al Ap J Supp 172, 284 (2007)

15 Recent Formation of S0s? Fasano et al Ap J 542, 673 (2000) Desai et al Ap J 660, 1151 (2007)

16 Is Dynamical Identification of S0s Feasible? Separating rotational/pressure-supported spheroidals in 0.4<z<0.8 cluster & field samples will verify role of S0s in evolving T- Σ rel n

17 Dynamicallyclassified S0s: Find v/(1-ε) vs σ is best discriminator Using this find: f(s0) ~20±5% at z~0.5 over 0.5< lg Σ < 2.5 At z=0 Dressler found f(s0)>40% in this Σ range: significant growth! z~0.5 Promising method..extending it to larger samples & higher z f SO

18 Why are environmental effects often ignored? It s hard to distinguish between effects of mass (which also depends on Σ) and environmentally-driven processes (accelerated evolution vs environmental evolution) Evolutionary trends (T- Σ) may reflect global trends in the field (e.g. field galaxies show more star formation at high z and so Butcher-Oemler effect may simply reflect infalling field galaxies) Theorists find it hard to model external environmental effects and so have restricted modeling of SF changes to internal feedback effects.

19 SDSS: Importance of Environment: Σ (Mpc -2 ) Mass fn: Φ (M, Σ) f(σ) Fraction in red light ~25% of all stars at z~0 are in ellipticals (75% in spheroids) Most are in massive galaxies whose mass function is correlated with Σ 25% of stellar mass is in dense environments (lg Σ > 0.4) Baldry et al MNRAS 373, 469 (2006)

20 Possible Environmental Processes Galaxy-galaxy interactions: mergers: low speed interactions (Mihos 1995) harassment: numerous high speed encounters (Moore et al 1996) Galaxy-cluster gravitational effects: tidal compression of disk gas (Byrd & Valtonen 1990) tidal truncation of outer disk (Merritt 1983) Galaxy-intracluster medium effects: ram pressure stripping: removal of gas (Gunn & Gott 1972) thermal evaporation: gas heated by hot ICM (Cowie & Songaila 1977) Viscous stripping of interstellar gas (Nulsen 1982) Pressure-triggered star formation (Dressler & Gunn 1983) ICM processes can work at different rates: rapid depletion - quenching slow depletion starvation/strangulation Review: Treu et al Ap J 591, 53 (2003)

21 Removal of gas in cluster environs Halo gas Hot gas Disk gas V=500 km/s T=10 7 K (M cl =10 14 M sun ) M d =6*10 10 M sun,v c =220 km/s,b/d=0.2 More efficient stripping in clusters (F strip depends on M cl, V, T etc). Typically 70% of gas can be removed from galaxy halos. Bekki astro-ph/

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23 Diffuse Starlight in Virgo (Mihos et al)

24 Some Numbers for a Typical Cluster Virial radius: r V ~2 Mpc Stripping radius: r S ~ 0.8 Mpc (L* galaxy loses its gas) Cluster density profile r < r V Infalling galaxy arrives with velocity Equations of motion then determine v i (r), t i (r) Ram pressure condition (Gunn & Gott)

25 Some Numbers for a Typical Cluster.. contd Tidal radius: r tt ~ 200 kpc (removing halo mass) treat halo and cluster as isothermal spheres Harassment rate:

26 Rough Timescales (Center to virial radius: ~ 1 Gyr on radial orbit) Ram-pressure stripping: 50 Myr Mergers: 100 Myr Tidal truncation: ~ few Gyr Gas starvation: ~ few Gyr Harassment: ~ few Gyr Observational diagnostics: Emission lines (instantaneous guide to star formation) GALEX NUV <100 Myrs Balmer absorption lines (A stars) < 1 Gyr Morphology changes ~ Gyr

27 Where the Environmental Effects Occur

28 Detailed Studies in Two Contrasting Clusters Cl (z=0.40) MS (z=0.54) HST, GALEX, CFHT/Palomar BVRJK, MIPS to turn-around r~5 Mpc Morphologies for 4000 galaxies to F814W(Vega)=22.5 DEIMOS spectra of 3000 gals, 800 members (σ, v rot for ~240 members) Moran Ap J 671, 1503 (2007)

29 Cl0024 rich in substructure Actively assembling At least two distinct infalling large groups M 200 ~8.7x10 14 M (Kneib et al. 2003) R VIR ~1.7 Mpc (Treu et al. 2003) MS 0451 well-virialized Smooth galaxy distribution in redshift space But elongated spatial distribution M 200 ~1.4x10 15 M (Donahue et al. 2003) R VIR ~2.6 Mpc Two Contrasting Clusters

30 Two Contrasting Clusters ICM mass density Clusters chosen to be very different in their X-ray properties MS has 8 luminosity and 2 temperature of Cl Influence of ICM on infalling galaxies will occur at different radii

31 Spectra of Infalling Spheroidals Scale radius from GALFIT on HST images ~100 E/S0s per cluster to I 814 =21-22; hr exposures 600 l/mm S/N > 8 Å -1 gives velocity dispersions σ to ±10%

32 Radial Trends in Infalling Spheroidals Sharp onset of [OII] emitters at R VIRIAL ~1.5Mpc Subsequently enhanced Balmer absorption (Hδ) Recently-arrived field galaxies interact with ICM in cluster environs Model illustration: 200 Myr burst at R VIRIAL involving 1% burst by mass Environmental rejuvenation as important as mass-dependent trends E+A Hδ + Hγ [OII] R (Mpc) Field spheroidals undergo tidal compression during infall

33 Radial Dependence of FP in Cl (z=0.40) Coma Offset c.f. Coma: Δlog M/L V = ± 0.02; scatter 2 Coma Radial trend in offset even for fixed mass (not `downsizing ) Spheroidals in core are passively evolving since z F > 2 Spheroids in periphery have younger stellar populations Moran et al Ap J 634, 977 (2005)

34 Other Tests of Environmental Dependence of FP Treu et al Ap J 622, L5 (2005) v Dokkum & vd Marel Ap J 655, 30 (2007) clusters field SDSS log Σ Only marginal trends seen with density (including SDSS): - mass-dependent growth is a galactic-scale phenomenon? - age differential (field vs cluster) ~ 0.4 z~0 (4%)

35 Spectra of Spirals: Tully-Fisher Relation Rotation curves for ~50 spirals/cluster to F814W=23.0

36 Disturbed Dynamics in Cluster Spirals: - I Field Cluster Field Cluster Scatter in cluster TF relation is 2 that in field! Effect same in both V and K band so not due to M/L (or dust): kinematic origin M K M V Moran et al Ap J 659, 1138 (2007)

37 Disturbed Dynamics in Cluster Spirals: - II High Scatter seen in both V and K: Enhanced cluster scatter is not due to variations in SFR or dust: Kinematic disturbance of spiral disks Internal spiral densities (~V 2 /R) show a sharp break at R VIR Harassment? Only the most dense spirals survive

38 GALEX and the Origin of S0s NUV (2500Å) observed for 15ks (Cl ) & 45ks (MS ) Probes MS star formation on finer timescales ( yrs) Cl0024 GALEX NUV GALEX NUV - CFHT I

39 `Passive Spirals and Gas Starvation No optical emission lines lack of star formation Visible spiral structure no major disruption to the structure of the galaxy (e.g. major merger) But strong UV emission! Passive spirals suggest decline in SFR via starvation Moran et al. Ap J 641, L97 (2006)

40 Passive Spirals: Cl0024 vs MS0451 Passive spirals less concentrated in MS0451 as expected for gas starvation

41 The S0 Bulge Problem: A Possible Resolution Passive spirals are sufficiently abundant over 0<z<1 to be precursors of local cluster S0s But local S0s have more prominent bulges than their spirals cohorts. So how can one be a faded version of the other? Spitzer IRS data reveals a population of circumnuclear starburst galaxies which may precede the passive spiral stage, building up the bulge mass Geach et al Ap J 691, 783 (2009) 12 cluster LIRGS in Cl0024 S24 < 0.6mJy

42 Discussion Topics Ram pressure stripping in clusters - The Classic Paper: Gunn & Gott Ap J 176, 1 (1972) - Review: van Gorkom astro-ph/ Recent simulations in different environs: Bekki astro-ph/ Controversies regarding the recent origin of S0s - Brief revew: Aragon-Salamanca astro-ph/ K-band luminosity functions: Burstein et al Ap J 621, 246 (2005) - Passive spirals the missing link: Moran et al Ap J 641, L97 - Dust-obscured star formation: Geach et al Ap J 691, 783 (2009)