Wednesday, January 2, 2013

Jerusalem WS lecture notes: 09. the IMF and the SFR

By M. Krumholz, slides here. I especially liked Mark's lectures, because he wades into murky, difficult topics of the ISM physics that many researchers like to leave out and assume something.
  • THE 2 problems
  • IMF:
    • determines stellar feedback (more at top-heavy IMF, etc), abundances, stellar masses
    • Observations:
    • Bastian 2010 -- MW MF plot -- universal IMF in different regions
    • Why a typical star is a few 10ths M_{\odot}? Insensitive to SF environment, metallicity, dwarf/spiral galaxy type
    • Andersen 2009: IMF in MCs (brightest HII region in the Local Group). Sabbi 2008: in SMC (0.2 Z_{\odot})
    • Variation in cDs? van Dokkum & Conroy 2010 (unresolved stars, red & dead galaxies, stars formed at z = 2) -- "The direct detection of the light of low-mass stars implies that they are very abundant in elliptical galaxies, making up over 80% of the total number of stars and contributing more than 60% of the total stellar mass. We infer that the IMF in massive star-forming galaxies in the early Universe produced many more low-mass stars than the IMF in the Milky Way disk, and was probably slightly steeper than the Salpeter form in the mass range 0.1M_{\odot} to 1M_{\odot}"
    • Peak location: non-isothermality is required, comes either from:
    • galactic properties, e.g. Hopkins 2012
    • local non-isothermality approximation:
    • isothermality broken by star formation: accreting star is brighter than non-accreting star --> Krumholz 2012: the characteristic mass is set by deviation from isothermality due to SF, the characteristic mass (peak location) depends on pressure of the core (lower mass at higher p). Pressure is set by balance vs. gravity (surface density)
    • Slope: universal, probably due to turbulence
  • SFR:
    • bathtub model (gas in, gas out) when t_{SF} << t_H
    • Correlation b/ween molecular gas SD and SF on galactic scales
    • sub-galactic scales: cloud mass vs. no of YSOs, IR luminosity vs. amount of gas (Wu 2005)
    • Galaxy metallicity dependence
    • phase dependence
    • SFR is a function of Toomre Q in galaxy. Dobbs 2011 simulation: self-regulation
    • Top-down model is not sufficient
    • Bottom-up model: what matters is the small scales (local SF law)
    • Why is \epsilon_{eff} (gas conversion to stars ratio) so low, ~1%? Federrath & Klessen 2012 --> few % efficiency for turbulent, virialised objects.
    • l_s -- sound length, size scaling
    • metallicity-phase dependence:
    • why most of the gas is atomic? Dissociation by UV, except in areas where UV is shielded by dust or H_2, 'magic number' -- surface density that shields, ~10M_{\odot}/pc^2
    • why SF follows H_2? Photons that are responsible for dissociation of H_2, are the same that heat the gas, so if the gas is shielded, it cools --> H_2
    • extragalactic phase dependence (SMC has a different SF law because of different metallicity). SF is not only self-regulating process, it depends on global galaxy properties

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