Gas accretion in Galaxies

Transcription

1 Massive Galaxies Over Cosmic Time 3, Tucson 11/2010 Gas accretion in Galaxies Dušan Kereš TAC, UC Berkeley Hubble Fellow Collaborators: Romeel Davé, Mark Fardal, C.-A. Faucher-Giguere, Lars Hernquist, Phil Hopkins, Neal Katz, Chung-Pei Ma, Ben Oppenheimer, Volker Springel, Mark Vogelsberger, David Weinberg

2 *ρ * z=0 Gas supply is needed Galaxies are actively forming stars at all epochs. What is the source of fuel for star formation? Stars form from molecular gas. Molecular gas consumption timescales are << t_h > Additional gas reservoir is needed to support long term star formation. Dense atomic phase (slow consumption)? Amount of HI in DLAs at high-z is much less than the mass locked in stars at z=0. Dense atomic phase also needs to be constantly re-supplied. Galaxies contain <10% of baryons, huge reservoir available in the IGM Gas from the IGM supplies galaxies at all epochs (see also Bauermeister et al 2010). Prochaska&Wolfe 08

3 Indirect evidence is widespread Star formation rate in the Solar neighborhood has been relatively constant for several Gyrs (Binney et al. 2000). Implies not much change in gas density despite large gas depletion Deuterium in local ISM appears to be re-supplied (e.g. Linsky et al. 2006) Infall needed Gas depletion timescales are much shorter than the Hubble time, but star formation sequence is evident at all times (Daddi et al. 2007, Noeske et al. 2007, Salim et al. 2007) Star forming galaxies are part of the star forming sequence and have long duty cycles of ongoing star formation. -> star formation in the bulk of galaxies is not bursty. Some continuous process keeps galaxies supplied with gas over Hubble time.

4 Global Accretion Kereš et al We use cosmological SPH simulations with Gadget (2,3) (Springel 2005). Gas cooling and star formation, no outflows. We count particles that joined galaxies between two outputs Galaxies grow through mergers and smooth gas accretion Smooth gas accretion dominates global gas supply at all times dominates growth of central galaxies in < M halos Star formation follows smooth gas accretion -> trivial consequence of short star formation timescales. Mergers re-distribute material: globally important after z=1

5 Gas accretion in the standard model E.g. (Rees & Ostriker 1977, White & Rees 1978) Gas falling into a dark matter halo, shock heats to the virial temperature T vir at the R vir, and continuously forms quasi-hydrostatic equilibrium halo. T vir =10 6 (V circ / 167 km/s) 2 K. Hot, virialized gas cools, starting from the central parts, it loses its pressure support and settles into centrifugally supported disk > the (spiral) galaxy. The base for cooling prescriptions used in Semi- Analytic models SAMs (e. g. White & Frenk 1991). Tvir

6 Temperature history of accretion We utilize the Lagrangian nature of our code We follow each accreted gas particle in time and determine its maximum temperature - T max before the accretion event. In the standard model one expects T max ~ T vir Shocked T max =? IGM Galactic gas

7 Empirical division at 2.e5K Evolution of gas properties of accreted particles Gas that was not heated to high T -> COLD MODE ACCRETION K Galaxies accrete fresh gas directly from cold dense intergalactic filaments. Gas heated to high T -> HOT MODE accretion; Cooling of the HOT virialized atmospheres. Kereš et al Katz, Keres et al. 2002

8 Examples from SPH simulations - Gadget-3 (Springel 2005), m_p ~10 6 M, resolution ~300pc (at z=3) - Low mass halos contain mostly cold non-virialized gas, infaling in filaments. This cold filamentary accretion dominates at high redshift and in halos below 3e11Msun (Katz, Keres et al. 2003, Keres et al. 2005) - 1D Shock stability analysis shows that virial shock does not reach Rvir In halos below ~1e11Msun (Birnboim&Dekel 2003) Kereš et al. 2009

9 -Massive halos contain mostly hot shock-heated gas, but cold mode accretion might still operate. Kereš et al. 2009

10 What have we learned from theory? Based on the cosmological simulations and analytic arguments (Kereš, Katz, Birnboim, Dekel, Brooks, Ocvirk, Teyssier, Agertz, Ceverino and others), we know that: Filamentary cold mode accretion of non-virialized gas is the dominant way of gas supply into high redshift galaxies. Accretion from the IGM gas provides continuous fuel source and is driving high star formation rates of these galaxies (e.g. Genzel et al.) Filamentary gas is dense; virial shocks cannot propagate trough the filaments even in halos that start developing hot atmospheres, at least at high redshift. Cold mode provides gas supply to galaxies in < few 10^11Msun halos at lower redshifts. Properties and geometry of filaments at fixed mass are changing with redshift. Drop in cosmic density -> decreases of gas supply with time -> lower star formation rates at late times.

11 Robust global picture but many open questions Do different simulation techniques agree in the study of gas infall? What happens at much higher resolution? What happens in massive halos, do filaments survive? How to *directly* detect cold mode accretion? With all this theoretical evidence for infalling gas, why we only see outflows? Do outflows affect infall? How does the infall stop? Do we need feedback at the massive end and what causes galaxies to be red-and-dead?

12 Different simulation techniques

13 PRELIMINARY (work in progress) So far good agreement between AMR and SPH codes in cold mode studies. SPH Gadget-3 (Springel 05) and moving mesh code Arepo (Springel 10) Same gravity, same ICs, better instabilities, naturally adaptive, Galilean invariant code 10/h Mpc box 2x128^3 particles. Global properties of cold halo gas are largely insensitive to simulation technique However, Arepo simulations show higher hot mode cooling rates at late times and more extended gas rich disks whose stripping contributes to cold gas. Gadget-3 Arepo Kereš, Vogelsberger, Springel, Hernquist; in preparation

14 Higher Resolution

15 Halo mass at z=0: ~7x10 11 M_sun Gas particle mass: ~4x10 4 M_sun Force resolution ~70/h pc at z=3, ~30pc peak hydro resolution ~8 million particles within Rvir at z~0.

16 Halo clouds at z=0 - At high resolution infalling gas forms clouds at z <1-2 (depending on mass) - Clouds infall from a flattened distribution in the disk plane. - Large fraction of the infall is co-rotating with the galaxy n > 4x10-4 cm /h kpc box Kereš & Hernquist 09

17 Origin of halo clouds Most clouds form from 1-1.5e5K infalling gas Leftovers of cold mode accretion Stripping of the satellite gas A fraction forms from hot halo (likely sensitive to halo gas metallicity) Penetrating filaments are cooling unstable and create density inversions in a gravitational field, susceptible to R-T instabilities. Infalling gas is also compressed by the surrounding hot medium and shocks from structure formation. Cloud masses are <1e6-1e7Msun: high resolution is needed to resolve them. Properties do depend on resolution: not yet clear what determined the mass spectrum? Clouds form earlier in massive halos. In early type remnants cloud formation or their infall has to prevented in order to produce passive systems!

18 Observational Signatures

19 Nearby galaxies Only direct detection of the accreting gas comes from local HI observations. infalling high-velocity HI clouds in the Milky Way Similar clouds around nearby galaxies. Simulated clouds broadly consistent with observations, in terms of mass and column density Estimates of infall rates based purely on HVCs are biased low: Large fraction of accretion is co-rotating with the disk, hard to separate in velocity space. HI observations probe only dense, innermost clouds, the rest is ionized. Work in progress M31, Thilker et al. 2004

20 Higher redshift Weiner et al Steidel et al Absorption studies, both directly from galaxies and using background sources are used as probes of the halo gas (e.g. Weiner et al. 2009, Steidel et al. 2010) Spectra show signature of outflows. Metal lines, but very sensitive (would detect dense cold gas with even small traces of metals) Models that try to predict this absorption can also explain absorption at higher impact parameters, without need for gas infall (Steidel et al. 2010) Without ionizing radiation transfer, low resolution simulations suggested large covering fractions of infall (e.g. Dekel et al. 2009). However, detailed predictions are missing-> time to improve this (check the astro-ph tonight!)

21 Why we see only outflows? 3e11Msun We use zoom-in simulations with peak hydro resolution of ~30pc, 4e4Msun gas particles Covering fraction of DLAs, neutral hydrogen with N_HI > 2e20/cm^2, is very small at z~2, only few few percent of the Rvir. Relevant for metals, several low-ion species originate from this self shielded gas. Given very large covering factor of outflows, signatures of gas accretion, especially in stacked spectra will be washed out. Faucher-Giguere & Kereš e11Msun

22 Halo gas in emission? Direct Ly_alpha emission from the infalling gas in z~3 halos: Cooling radiation form the infalling gas is a strong source of Ly_alpha emission (e.g. Katz &Gunn 1991, Fardal et al. 2001, Dijkstra&Loeb 2009, Goerdt et al. 2010) Other possibilities: star formation, outflows, AGN. Complex: Need to precisely identify self-shielded regions and the gas temperature To get line profiles and surface brightness distributions need to do Ly-alpha line radiative transfer Regardless of the origin of such emission blobs contain information about massive galaxy formation at high-z!

23 Luminosity of Lyα blobs Faucher-Giguere, Kereš et al Predictions are extremely sensitive on the correct treatment of star forming and self-shielded regions (strong temperature dependence at 1-2e4K): Orders of magnitude difference if not careful! Need to be careful with treatment of self-shielding and star forming gas! We apply proper ionizing radiation transfer, both in post-processing and on the fly. With radiation coming out of star forming regions excluded, cooling radiation is insufficient to power brightest blobs, need additional sources to get to ~few 1e44ergs/s. Lower luminosity extended sources can be powered by the cooling radiation.

24 Do we need feedback?

25 Mass function without feedback -No feedback simulations, produce too many stars in galaxies -Problems at the high mass and low mass ends -Removal of hot mode does not make much of a difference, in nofeedback simulations -All of the galaxy formation happens early. Kereš et al. 2009

26 Which mode is responsible? Majority of galaxies were built mostly by cold mode M > 5x10 10 M (0.5-1x10 12 M halo) the contribution of cold mode increases growing merger contribution Mergers of smaller galaxies built the more massive ones, not the recent accretion These small galaxies were built through cold mode Without metal cooling and feedback hot mode not important for mass accumulation: enables mergers (minor and major) to dominate mass growth Early feedback in low mass galaxies will affect massive halos at late times Kereš et al. 2009

27 Low mass end solution? Momentum driven galactic winds and gas re-accretion help with the low mass end Re-accretion of ejected material increases gas accretion in massive halos and ruins the high mass end Solution to the high mass end depends on the low mass end feedback. Oppenheimer, Dave, DK et al. 2009

28 Possible high-mass end solution? Salim et al simulations Kereš et al X Bell et al Hopkins et al Problems with simulated massive galaxies: Current star formation (colors) Mass of galaxies (and morphology) Can gas rich major mergers stop star formation? Trigger quasar mode of AGN Shock heat the halo gas Produce burst of star formation Grow massive black holes, condition necessary to have long term maintenance in radio mode In hot halos, all of these are efficient. Right scaling with mass to produce proper mix of morphological types. However, merger driven feedback needs to be carefully implemented in cosmological environment to see if it can solve the high mass problem.

29 Summary Cold mode accretion is now a theoretically robust scenario. Star formation in the Universe is driven by continuous accretion of gas Predictions for the direct observability of gas infall are coming out, We need more careful work, cannot just observe simulations! Future observations with the next generation of meter telescopes will help avoid stacking and increase sample of background sources probing galactic vicinity. This will help constrain different models of gas accretion and feedback in a very direct way. Feedback is needed to regulate galactic growth! Infall and outflows co-exists and likely interact -> interesting complex physics to model and observe. Merger driven bursts are not important for cosmic star formation but major merger events are important in regulating galaxy evolution in massive systems.

30 Lyα emission from simulated halos We incorporated, ionizing radiation R-T, self-shielding & Ly_alpha line R-T. Example: 2.5e11Msun halo at z=3, from zoom-in simulation. Cooling radiation from the halo gas emission is detectable and contributes to Ly_alpha emission from extended blobs. Faucher-Giguere, Keres et al. 2010