JSI Dwarf Galaxy Conference (Day 2)

Annapolis, Maryland – Today’s talks focus on dwarf galaxy formation

Volker Bromm (UT-Austin), “The First Stars and Galaxies”

  • Discussed Clark+ (2010), Greif+ (2011) simulations, looking at disk fragmentation in the case without radiation feedback.  Points to a flat IMF.
  • Stacy+ (2011) included proto-stellar radiative feedback and found that the final mass was 30-40 Msun in a binary.  Hosogawa+ (2011) found a similar result in a 2.5D rad-hydro simulation.
  • Jeon+ (2011): HMXB gives negative feedback locally and positive feedback globally.
Nick Gnedin, “How Well Do We Know Stellar Populations?”
  • Discussing their HI-H2 relation and its implementation in galaxy formation simulations.
  • K-S law depends on the dust-to-gas ratios.  Talking about the transition from atomic to molecular hydrogen, which occurs around 1-10 cm^-3 in their model and in the MW.
  • Low-Z dwarf galaxies (NGC 2915, UGC 5288) have a very large HI shroud.
  • Bimodality of galaxy metallicities, separated at ~10^8 Msun in stars in their model, but observations show a smooth mass-metallicity trend.
  • Proposing a new stellar population “Pop A” that forms in metal-enriched gas without dust in stellar systems with M* < 10^8 Msun.
  • Little dust observed at z>5 (Bouwens+ 2010, Zafar+ 2011).
Alyson Brooks, “Forming DM Cores in CDM”
  • Discussing Brook+ (2011) about removal of low AM gas.
  • Galaxies in the THINGS survey have an average DM slope of -0.3 at 500 pc.  There is a scatter because of SF strength.
  • Lower mass galaxies don’t undergo repeated bursts of SF, and they retain their cusps.
  • Including baryon physics and stellar feedback creates shallower potential well, and V_circ is much lower than DM-only simulations.
Michael Kuhlen, “How Faint Must We Go?”
  • The original Bouwens+ (2011) model for reionization needs M_lim > -12 to match WMAP7 tau.  But it is possible to have models that have lower limits of -16 and even -18.  Makes the conclusion that the optical depth from WMAP is not enough to constraint the properties of the reionizers.
  • Can get constraints from low-z LAF data because the ionizing background must keep the IGM ionized.  See Faucher-Giguère+ (2008a-d).
  • There is a discrepancy between matching pre-ionization and post-ionizing photon models.  Pre-ionization f_esc must be high to match tau, but post-ionization f_esc must be low (1%) to match LAF.  This points to an evolving f_esc.
  • Only models with M_lim > -12 can match high-z and low-z constraints.
Mia Bovill, “Probing CDM and the First Galaxies in the Local Group”
  • Undetected dwarfs – dynamics: Use the Walker+ (2009) and Wolf+ (2010) mass estimators.
  • Looking at the largest satellites, they overproduce their number density in a MW-like galaxy.  Something is missing, but still need to keep the uFDs stellar-poor.
Michael Boylan-Kolchin, “Massive Failures in the Milky Way”
  • Explanations of “missing satellites problem”: (1) observed stellar dynamics don’t reflect the grav. potential because they’re deeply embedded in the halo; (2) reionization suppresses SF in low-mass halos; (3) feedback strongly suppresses SF in low-mass halos.
  • All of the massive subhalos in N-body simulations are much more massive than the bright satellites in the MW, which have V_circ (at the 1/2 light radius) of 10-20 km/s.
  • From the Aquarius simulations, one would expect 7-15 subhalos with >40 km/s.
  • Why don’t we observe any massive satellite halos between 30-50 km/s circular velocity?  What’s missing?  Baryon / feedback effects?
  • Invoking reionization probably don’t solve this problem because they’re already massive – above 2-3 x 10^9 solar masses at z > 6.
  • Halo-galaxy relation at low masses: Oh+ (2011) THINGS dwarfs.  All MW dwarfs, THINGS isolated dwarfs, and Magellanic clouds don’t match this relation.
  • Solutions?  (1) simulations aren’t representative – not enough scatter; (2) wrong MW mass, causing a mismatch between obs/sim – problem also exists in VL2; (3) MW is anomalous – M31 has a similar satellite population; (4) stochastic galaxy formation; (5) baryons strongly modify CDM small-scale structure – can enough DM be ejected from the inner subhalos?; (6) DM is something different than CDM.

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