The Local High-Redshift Dwarf Galaxies
Evan Kirby UC Irvine Southern California Center for Galaxy Evolution
credit: John Wise
Dwarf galaxies come in two main types: dwarf spheroidals and dwarf irregulars
dSphs
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log D (kpc) Grcevich & Putman 2009, ApJ, 696, 385 2/15 A catalog of multi-element abundances in dwarf galaxies
MW dSph N Fornax 675 Leo I 827 Keck + DEIMOSdIrr N Sculptor 376 NGC 6822 282 Leo II 258 IC 1613 125 Sextans 141 VV 124 52 Draco 298 Pegasus 94 Canes Venatici I 174 Leo A 39 Ursa Minor 212 Leo T 18 Hercules 21 Total 610 Ursa Major I 28 Leo IV 12 Canes Venatici II 15 Plus, over 1000 stars Ursa Major II 9 in satellites of M31. Coma Berenices 18 Segue 2 25 Total 3089 EK et al. 2010-2013 Simon & Geha 2007, ApJ, 670, 313 3/15 Detailed abundances may be measured from R ≈ 7000 spectra.
EK et al. 2009, ApJ, 705, 328
4/15 Galaxies obey a one-parameter mass-metallicity relationship.
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spectroscopic metallicities: Fe lines
log (Ltot/L⊙) EK et al. 2008,2013b, ApJL, ApJ, 779,685, 102L43 EK et al. 2011a, ApJ, 727, 78 5/15 … but the gas-phase mass-metallicity relation does evolve!
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Peeples & Somerville 2013, MNRAS, 428, 1766 6/15 The [α/Fe] ratio indicates the star formation timescale.
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[Fe/H] Draco was terrible at its job.
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[Fe/H] 8/15 Sculptor was slightly better at its job.
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α SFR = A* (Mgas) -t/τ Mgas(t) = Mgas(0) + Ain t e – Aout (RIa + RII) – SFR
Maoz et al. 2010, ApJ, 722, 1879
Nucleosynthetic yields: Type II SNe: Nomoto et al. 2006, NuPhA, 777, 424 Type Ia SNe: Iwamoto et al., 1996, ApJS, 125, 439 AGB stars: Karakas 2010, MNRAS, 403, 1413 10/15 Low-L galaxies lost a lot of gas.
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[Fe/H] t (Gyr) EK et al. 2011b, ApJ, 727, 79
11/15 Mid-L galaxies didn't lose quite so much gas.
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[Fe/H] t (Gyr) EK et al. 2011b, ApJ, 727, 79
12/15 The mass-metallicity relation is really a sequence of gas loss.
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E log M* (M⊙) 13/15 We haven't found dSphs at z ~ 2 … yet.
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c lookback time (Gyr) Weisz et al. 2011, ApJ, 739, 5 log M* (M⊙) Belli et al. 2013, ApJ, 722, 141 14/15 Conclusions
● Nearly all galaxies in the local universe obey a universal stellar mass–stellar metallicity relation.
● Smaller dwarf galaxies are less efficient at turning gas into stars than larger galaxies.
● Gas and metal loss plague all dwarf 8 galaxies with M* < 10 M⊙.
15/15