1 / 51

Kyle Stewart University of California, Irvine Dissertation Defense, 11-20-2009

Merger Histories of Dark Matter Halos in L CDM and Implications for the Evolution of Milky Way-size Galaxies. Kyle Stewart University of California, Irvine Dissertation Defense, 11-20-2009 Advisor: James Bullock. Outline. Dark Matter Halo Merger Rates Evolution with z, m/M, M DM

becka
Download Presentation

Kyle Stewart University of California, Irvine Dissertation Defense, 11-20-2009

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Merger Histories of Dark Matter Halos in LCDM and Implications for the Evolution of Milky Way-size Galaxies Kyle Stewart University of California, Irvine Dissertation Defense, 11-20-2009 Advisor: James Bullock

  2. Outline • Dark Matter Halo Merger Rates Evolution with z, m/M, MDM Comparison to observations • The Problem of Disk Survivability Defining the problem: too many mergers? Gas-rich mergers: possible solution? • Milky Way Substructure “Stealth” galaxies • Preliminary & Future Work Cool halo gas in a cosmological context

  3. Dark Matter Halo Merger Rates Stewart et al. 2009a

  4. Introduction: • LCDM, DM dominates the matter density of the universe. • Galaxies reside in center of DM halos. • DM halos (and galaxies within them) grow hierarchically over time, by mergers of smaller halos (galaxies). • In order to understand galaxy evolution, we need to first understand dark matter merger statistics (and how mergers affect galaxies). Stewart et al. 2008

  5. DM Merger Histories • DM only, LCDM, N-Body simulation. • 80 h-1Mpc Box, s8=0.9 • 5123 particles, mp~3x108 h-1Msun • Adaptive Refinement Tree (ART) code. 5123 cells, refined to max. of 8 levels. hpeak ~ 1.2 h-1kpc (Kravtsov et al. ‘97) • Focus on host masses ranging from 1011-1013h-1Msun . (~15,000 halos at z=0, ~9,000 halos at z=2.) • Statistics complete to 1010 h-1Msun See, eg. Stewart et al. 2008 (galaxy size halos) Berrier, Stewart et al. 2009 (cluster size halos) Stewart et al. 2008

  6. dN/dt vs z : (Number with merger larger than r = m/M) Predict: Strong evolution with redshift ~ (1+z)^2.2. Worry: does this contradict observational evidence for flat merger fraction with redshift ? (e.g. Lotz et al. ‘08, Jogee et al. ‘08) Stewart et al. 2009a

  7. Merger Fraction in past 500 Myr*. *Sometimes used as an estimated timescale for morphological disruption. Use number density matching to associate halos with ~0.1L* galaxies from observed luminosity function (e.g. Faber et al. 07) Agrees reasonably well with observations, for 1:10 minor + major mergers. Suggests much higher fraction at high redshift. Lotz et al. ‘08 Jogee et al. ‘08 redshift Stewart et al. 2009a

  8. Merger Fraction in past dynamical time*. *Use halo dynamical time as a proxy for morphological dyn. time. Use number density matching to associate halos with ~0.1L* galaxies from observed luminosity function (e.g. Faber et al. 07) Agrees reasonably well with observations, for 1:3 major mergers. Shows relatively flat redshift evolution. Lotz et al. ‘08 Jogee et al. ‘08 redshift Stewart et al. 2009a

  9. Stewart et al. 2008, 2009b Merger Histories vs. Disk Survival

  10. Background: Why is there a concern about disk survival? • Dark Matter Halos form by mergers. • Major mergers (even some minor mergers) turn disk-type galaxies into thick, flared, more bulgy systems. (eg. Mihos & Hernquist ‘94, Kazantzidis et al. ’07, ‘08; Purcell et al. ’08b) • And Yet: Majority of Milky-Way sized DM halos contain thin disk-dominated galaxies (z=0). (eg. Weinmann et al. ’06, ‘09) • Merger Rate increases with redshift. • And Yet: Large disk-like galaxies observed at z~2. (eg. Förster Shreiber ‘06; Genzel et al. ‘06; Shapiro et al. ‘08.) How is all this compatible?

  11. How often do mergers occur in 1012h-1M halos? Stewart et al. 2008 By strict mass cut, in last ~10 Gyrs : ~ 70% of halos: m > 1.0x1011 ~ 50% of halos: m > 1.5x1011 ~ 30% of halos: m > 2.5x1011

  12. Could a disk have formed afterwards?(Mass accretion after last major merger) Given a halo with mass M0 at z=0… which we know has experienced at least 1 merger of mass > m … What fraction of M0 was accreted AFTER the most recent merger > m? At most, only ~30% Stewart et al. 2008 These systems probably cannot subsequently re-grow a sizeable disk (from newly accreted material from subsequent galaxy mergers)

  13. Summary: the problem ~70% of Milky Way-sized halos have had a > 1011 h-1Msun merger in the past 10 Gyr. This is twice the mass of the disk! But observations show that most Milky Way size halos are disk-dominated (Weinmann et al. ‘06, ‘09) theorist Thus, typical Milky Way-size disks must be able to survive a 1011 h-1Msun DM halo merger somehow… or we have a serious problem! observations

  14. Gas Rich Mergers: the Solution? • Gas rich minor mergers help form rotationally supported gaseous disk galaxies. • Given a sufficiently high gas fraction (fgas> 50%), even major mergers (3:1) quickly reform into a disk. (Robertson et al. ’06, Hopkins et al. ‘08) Example: Observed disk galaxy at z~2 resembles simulated gas-rich merger remnant: Observation (Genzel et al. ’06) Simulation (Robertson & Bullock ’08)

  15. Estimating Baryonic Content • Dark Matter Merger Trees • Empirical Stellar Mass—Halo Mass relation (Conroy & Wechsler 2008) • Empirical Gas Mass—Stellar Mass relation (McGaugh 2005; Erb et al. 2006) Stewart et al. 2009b

  16. Log (Mgas/Mstar) Log Stellar Mass (Msun) Step 2: Stellar Masses. z = 0 • Use number density matching to statistically assign an average stellar mass, given DM mass (and redshift). (data from Conroy & Wechsler 2008.) Log Stellar Mass (Msun) z = 2 Step 3: Gas Masses. Log Halo Mass (Msun) • Use observations of galaxies at z~0 (McGaugh ‘05) and z~2 (Erb et al. ‘06) to estimate Mgasvs. Mstar (for z=0-2). z = 2 z = 0

  17. Notice, since halos at high z are gas rich, total galaxy mass (cold baryons) per halo doesn’t evolve as much with z as stellar mass does…

  18. Merger Fraction revisited: (> 1/3 mergers that hit the disk) Squares, Xs: early-type fraction observations (Weinmann et al. 06, 09) But what if we only look at gas rich* vs. gas poor* mergers? Small halos  gas rich mergers Large halos  gas poor mergers May explain disk survival? (Robertson et al. ‘06) • * Definitions: • “Gas Poor” : both galaxies with gas fraction < 50% • “Gas Rich” : both galaxies with gas fraction > 50% Stewart et al. 2009b

  19. Section Sum-up : Merger rate high, but nearly ALL of them are very gas rich. • Consider the DM merger rate for a 1012 M halo: May explain assembly of massive, gas-rich disk galaxies at z~2. (Robertson & Bullock 2008) Merger rate low. Mergers gas poor (destroys disks) Merger rate increasing. So is the gas rich merger fraction.

  20. An Aside: Defining “Merger Ratio” Stewart 2009 Note: since relation btwn. MDM and Mstar is nontrivial, so is m/M. e.g. 1:3 DM ratio at MDM ~1011 Msun 1:20 stellar mass ratio or: 1:3 DM ratio at MDM ~1013 Msun 2:3 stellar mass ratio

  21. Abundance Matching to Explore Milky Way Substructure Bullock, Stewart et al. 2009

  22. MW Dwarfs: Observed Properties • Suggestive of unobserved population(s) of satellites: • Distant and faint galaxies (somewhat expected) • Spatially extended (higher Re, or lower total DM mass) and faint. Obs. data from Strigari et al. ’08; Wolf et al. ’09

  23. MW Dwarfs: Observed Properties (cont.) • Again, suggestive of unobserved population(s) of satellites: • High velocity dispersion, faint galaxies • Low DM mass, faint galaxies. Obs. data from Strigari et al. ’08; Wolf et al. ’09

  24. Use VL2 simulation + Mstar(MDM) relation: Also predicts many more “stealth” galaxies below luminosity & SB detection limits. “Sanity check” 1: does the model match the observed luminosity function?... Yes!

  25. Use VL2 simulation + Mstar(MDM) relation: OBSERVABLE ONLY! ALL MODEL DATA “Sanity check” 2: does the model fit the Strigari (2008) plot, (M300 vs L)?… Yes. Again, predicts many more “stealth” galaxies at low M300 and low Luminosity.  Flatness of Strigari plot a selection effect!

  26. Preliminary & Future Work • How do galaxies acquire their cool gas? • Cold flows? Cloud Fragmentation? (e.g. Keres et al. ‘09, Dekel & Birnboim ‘06, Maller & Bullock ’04) • Gas rich mergers? (Stewart et al. 2009b) • How do we test these ideas? • Absorption systems: probes of cold halo gas

  27. Observing Gas Around Galaxies: QSO (Mg II) D ~ 100 kpc (or less) Image from Tripp & Bowen (2005) 2) Cloud vs. Galaxy Kinematics • Covering Fraction

  28. Observing Gas Around Galaxies: • Covering Fraction Mg II Cf ~20-80% 2) Cloud vs. Galaxy Kinematics But what ARE they? Spherical halo gas? Cold Filaments? Pressure-confined gas clouds? Outflowing winds? Tidal Streams? e.g. Tripp & Bowen ’05; Tinker & Chen ‘08; Kackprzak et al. '08

  29. Observing Gas Around Galaxies: Kacprzak et al. ‘09 (submitted) • Covering Fraction 2) Cloud vs. Galaxy Kinematics Majority (7/10) Mg II absorbers show velocities that co-rotate with galaxy

  30. Our Simulation: Log rHI [Msun/pc3 ]= [-8, -1] ; width 300 kpc + Diemand et al. ‘08 Wadsley et al. ‘04 Via Lactea II (ICs) GASOLINE (sph code) Some stats: WMAP3 cosmo: W0=0.24, L=0.76, h=0.73, σ8=0.77, Wb=0.042 mDM, mgas, mstar ~3e5, 4e5, 1e5 Msun, Np~4 million. Sph smooth len: 332 pc. Final halo mass Mvir~2.e12 Msun ‘Blast-wave’ feedback of Stinson et al. ‘06; Haardt & Madau ‘96 UV field; NOTE: no momentum driven blow-out winds Log rstars [Msun/pc3 ] = [-7, 1]

  31. Results: Covering Fraction Router ~ 50 kpc (comoving) Ngrid ~ 1000 Rinner ~ 5 kpc (comoving) LOS “covered” if N(HI) >1016,18,20 atoms/cm2

  32. 1) Cold flows: high Cf. 2) & 3) Major Mergers: “bump”drop 4) Gas-rich minor merger: gashalo. ↑Cf 5) Cloud infall / fragmentation: Keeps Cf high. 6 & 7) Choose quiescent hist: ↓Cf. rapid SF. Results: Covering Fraction (averaged over 3 projections) Covering Fraction Depends Strongly on Recent Gas Accretion!

  33. What about Gas and Galaxy Kinematics? z~0.8, Image width ~60 kpc Log rHI = [-7, 1] LOS velocity: +/- 250 km/s

  34. Gas and Galaxy Kinematics: z~0.8, Image width ~60 kpc Log rHI = [-7, 1] LOS velocity: +/- 250 km/s Co-rotation (as observed) is a natural result of cosmological gas accretion & fragmentation

  35. Summary: • Merger Rates: • Merger rate agrees fairly well with observed “morphologically disturbed” fractions, for first order estimates of merger timescale. Merger rates depend on z, m/M, M: well fit by a “universal” function. • “Major Mergers” (1:3 ratio) defined by DM vs. stars not trivially related. • Disk Survival: • Milky Way-size disks must be able to survive some major* mergers (*either merger ratio > 1:3, or m > 1011 h-1M ). • Gas rich/poor merger histories seem promising for disk survival (explains mass-morphology relation?) provided gas-rich mergers result in disks. • MW Substructure: • We predict a population of “stealth” dwarf galaxies in the halo of the Milky Way, with SB’s too low for current techniques. • Preliminary Work—Cosmological hydrodynamic simulations: • High CF (~50%) and cloud co-rotation both consistent with observations, from infalling gas (cold flows/mergers/clouds). No momentum winds! • Covering fraction depends strongly on recent gas accretion.

  36. Future Work: • Run a suite of SPH simulations, for a systematic sampling of “common” merger histories, from “quiescent” to “active.” • Which properties depend strongly on merger history? • Can “cold flows” be observed? Is high CF ubiquitous? • What trends in CF vs. time are universal? Which aren’t? • What do properties of the gaseous halo tell us about galaxy formation? • Compare systematic sample of simulated galaxies to DM merger statistics. Can we infer broader statements about galaxy populations? • Side project: simulating an “invisible” major merger : • DM mass ratio 1:3, stellar ratio 1:20. (MDM~1011Msun) • Is the faint galaxy distinguishable, observationally?

  37. Thank You!

  38. Extra Slides

  39. dN/dz vs. z • To first order, dN/dz is consistent with completely flat redshift evolution. • To second order, dN/dz proportional to d(dc)/dz. • Similar to findings in Fakhouri & Ma ‘08

  40. Galaxy Merger Rates Given observational/theoretical errors, agrees with close-pair counts

  41. Where does a halo’s mass come from? • Comparison to theoretical EPS predictions reasonably close to N-Body, • considering mass definitions • M ~ 0.1*M0 • Largest contribution to final halo mass comes from mergers with m/M0 ~ 10% Stewart et al. ‘08 • 1013 h-1M halos built up from ~ 1012 h-1M mergers • 1012 h-1M halos built up from ~ 1011 h-1M mergers • 1011 h-1M halos built up from ~ 1010 h-1M mergers

  42. Is there a trend with mass? (from 1011-1013) Stewart et al. 2008 1 word answer: “Nope.” 2 word answer: “Only slightly.”

  43. Using different merger ratios:

  44. Baryonic Mass AssemblyHow do galaxies get their mass (in mergers)? ~30% of cold baryons in MW-mass galaxies accreted directly in major (>1:3) mergers since z~2 (~20% gas, 10% stars) Cumulative effect of major mergers is a significant source of final baryons! Stewart et al. 2009b

  45. Mass Assembly (threshold case):How do galaxies get their mass (in mergers)? ~30%  20% accreted directly in major mergers since z~2 (~10% gas, 10% stars) Cumulative effect of major mergers still significant, but less so for low mass. Stewart et al. 2009b

  46. Gas Rich/Poor Merger Fractions vs. z 0.5 z = 1.0 z = 0.5 Note transition mass above/below which gas rich/poor mergers dominate. (<1011.2, z=0 ; ~1011.6, z=0.5 ; ~1012.7, z=1) Gas rich mergers at high redshift  “cold flows” ? z = 0.0 Merger Fraction (1 dyn. time) gas rich all gas poor Log(Halo Mass) Stewart et al. 2009b

  47. Notice, since halos at high z are gas rich, total galaxy mass (cold baryons) per halo doesn’t evolve as much with z as stellar mass does…

  48. Number density matching for VL2: Note: this extrapolation goes well below the regime where this mapping is robust (MDM>1010 Msun). “Threshold” Scenario: Mstar=0 for MDM<5e8 Msun (black dotted line).

  49. “Threshold Scenario” • Impose SF threshold in Mstar(MDM) model. • Mstar (MDM < 5x108 Msun) = 0 • Are there still “stealth” galaxies? • Also fits observations, but no significant population of “stealth” galaxies. • Finding these low SB galaxies (or not) tells us about galaxy formation!

  50. Galaxies Probing Galaxies Rubin et al. ‘09 z~0.5 z~0.7 Keck/LRIS absorption spectrum Spatially-extended complex of cool clouds at d>17kpc from galaxy (with high velocity width) Cool gas ejected from host galaxy during past merger?

More Related