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The Extragalactic Perspective

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The Extragalactic Perspective Ast 777: Star and Planet Formation Jonathan Williams, University of Hawaii The (extra)galactic perspective https://www.nasa.gov/feature/goddard/2017/messier-51-the-whirlpool-galaxy The geometry is much more favorable for studying the (spiral) structure of galaxies and the relation between the ISM and star-forming regions. But due to the great distance, we only see these properties integrated over spatial scales of several hundred pc > molecular cloud scales Williams Measuring the molecular mass schematic from Williams see Bolatto Ann Rev for many more details Measuring the star formation rate Kennicutt & Evans 2012 HI —> H2 —> stars —> HII over ~20-30Myr 1-5% of the ISM converts into stars on each Galactic rotation Williams Star-formation relations Kennicutt-Schmidt law but the scaling (A) and index (N) depend on which gas you’re considering Kennicutt-Schmidt law percentage of gas used up in 100 Myr Independent of Linear dependence “Canonical” atomic gas? on molecular gas N ~ 1.5 Bigiel et al. 2008 Gas depletion timescale Similar from centers to edges of an individual galaxy and from one galaxy to another => (1) on kpc scales, star formation proceeds in a fairly regular way at a rate that depends only on the available molecular reservoir => (2) about 1% of the molecular gas is used up in 20 Myr ~ free-fall timescale of a GMC: this is the McKee/Krumholz preferred definition of star formation efficiency => (3) the molecular gas is depleted in ~ 2 Gyr Bigiel et al. 2008 What is the role of HI? • HI disks extend well beyond the stellar and H2 disks in spiral galaxies -2 • Threshold at ΣHI ~ 3 M⊙ pc where there is insufficient shielding of dissociating radiation, little molecular gas, and therefore no star formation -2 • Saturation at ΣHI ~ 9 M⊙ pc where conversion to molecular gas is efficient and leads to high SFR densities • Thus, HI is fairly uniform, varying only by about a factor of 3 in column density, and serves as a background reservoir to sustain galactic star formation beyond the 2 Gyr molecular depletion time Bigiel et al. 2008 Galaxy collisions https://www.spacetelescope.org/images/heic0206b/ If the Sun were the size of an orange, its nearest neighbor would be at the distance of California If a spiral galaxy were the size of a CD, it would typically have neighbors within a few arms length Galaxy collisions are common and a defining characteristic of their evolution Stellar number density Geometric cross section mean-free path of a single star path length of stellar disk => chance of collision during encounter ~ i.e., maybe you get one stellar collision when the Milky Way and Andromeda collide in ~4 Gyr https://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html The situation is a little different from an ISM / star formation perspective • Two slabs of gas, ~ 50 kpc wide, ~3 kpc thick crash into each other at supersonic speeds (>> 102 km/s) • Shocks! => heating & ionization • But recombination and molecule formation occur much faster than the ~100 Myr crossing time • Conversion of most of ISM to dense, cold, molecular gas • Prolific star formation • Random stellar orbits and lack of gas —> elliptical galaxy (and possible AGN fueling) https://www.nasa.gov/mission_pages/hubble/science/milky-way-collide.html The first stars Bromm/Larson 2004 (SciAm and AnnRev) The first stars • The first stars formed in metal-free environments: no C, O, etc to cool down clouds and lower their pressure • But the expanding Universe cools adiabatically —> recombination at z ~ 1100 (380,000 yr after Big Bang) • At these temperatures, T = 2.7(1+z) K ~ 3000 K, H2 can survive if it can form • no dust grains, but free electrons left over from recombination can catalyze a slower reaction • This is (quadratically) enhanced where primordial density fluctuations are high. H2 quadrupole transitions, ΔJ=2, cool the gas • Once H2/H > 10-3, cooling time < dynamical time at 104 cm-3 and stable molecular cores at T ~ 200 K form. Thermal pressure vs gravity implies large Jeans mass, MJ ~ 350 M⊙ • Massive Population III stars form ~108 yr after Big Bang Bromm/Larson 2004 (SciAm and AnnRev) … but some very low metallicity dwarfs have been found in the halo => low mass stars formed at early times out of almost pure hydrogen* * notably carbon enhanced, however => fallback-supernova? Nordlander et al. 2019 Further reading / viewing Kennicutt & Evans 2012 Annual Reviews Star Formation in the Milky Way and Nearby Galaxies https://ui.adsabs.harvard.edu/abs/2012ARA%26A..50..531K/abstract Ast 777: Star and Planet Formation In the last third of the course, we will focus on what happens with the abundant metals in the present-day Universe… Asynchronous lecture Williams & Cieza 2011 Annual Reviews Protoplanetary Disks and their Evolution https://ui.adsabs.harvard.edu/abs/2011ARA%26A..49...67W/abstract See class website for pdf; I’ll email you discussion questions.
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