Stellar Populations in the Local Group Recall what we ve learned from the Milky Way: Age and metallicity tend to be correlated: older -> lower heavy element content younger -> greater heavy element content Evidence for some episodic star formation in the disk Disk metallicity distribution shows few very low metallicity stars => disk has accreted gas Evidence for both accretion events and a monolithic collapse early in the Galaxy s history Bulge is old, broad range of metallicities Halo is old, mainly low metallicity 1
Resolved Stellar Populations in Galaxies Component Mv Distance Limits (Mpc) Spectra GB Imaging HST Imaging (V<22) (V<24) (V<27) A Supergiants -10 25 60 250 OB, M Supergiants -8 10 25 100 Red Giant Tip -3 1 3 10 Cepheids -4 2 5 20 RR Lyrae, HB 0 0.25 0.6 2 MS Turnoff +5 0.025 0.06 0.25 Local Group 0.01 0.1 1 10 100Mpc LMC M31 M81 Virgo Coma SMC M33 Fornax 2
IMF from Local Galaxies Only locations outside the MW with a measurement of the IMF are a few galaxies of the Local Group. Only the high mass end has been measured. Measurement tricky because the brightest stars at V are net the highest mass stars. No evidence for a slope of other than 2.35 (or 1.35 in log m ) From Massey 2003 ARAA 41 15 3
Challenges to Stellar Evolutionary Models Rare and/or short-lived stages of a massive star s life can be probed in the Local Group. Relative numbers of red supergiants as compared to blue supergiants or as compared to Wolf- Rayet stars not correctly predicted by models. Some key physics missing for red supergiants (mass loss, convection, mixing?) From Massey 2003 ARAA 41 15 4
The Local Group The list has grown remarkably since 1980. There is still some debate about how many members there are in total. Definition of what constitutes the gravitational boundary of the Local Group is tricky. van den Bergh 2000, PASP, 112, 529 5
3D map by E. Grebel 6
LMC D=49kpc M~10 10 M 7
LMC as seen by Spitzer 3.6µm = blue 8µm = green 24µm=red 8
Zaritsky et al. 1997, AJ, 114, 1002 Alcock et al. 2000, AJ, 119, 2194 9
LMC disk bar difference Dark = higher density in bar Smecker-Hane et al. 2002, ApJ, 566, 239 10
Holtzman et al. 1999, AJ, 118, 2262 Constrained age-metallicity=small dispersion 11
LMC Star Formation History Smecker-Hane et al. 2002, ApJ, 566, 239 M /yr Bar SF history Shaded = 10x Disk 12
Field stars LMC: Results LMC contains outer halo of old field stars and globular clusters evidence for large star formation events 3-10 Gyr ago peak in star formation occurred in last 3 Gyr Clusters and associations age distribution is bimodal, with gap between 3-10 Gyr (field stars show different behavior) LMC contains population of massive young blue globular clusters, and supergiant HII regions 13
47 Tuc SMC D=57kpc 14
Interactions Must bear in mind that the LMC and SMC are beginning to suffer from being so close to the MW. Tidal forces may influence the star formation history. Depiction of HI streamers. 15
SF & Metallicity Histories for the Magellanic Clouds Olszewski et al. 1996 ARAA Holtzman et al. 1999 AJ LMC SF appears to have been more constant for the SMC. The LMC also shows a much stronger agemetallicity relation than the SMC. 16
M31 17
Halo has relatively high metallicity ([Fe/H]>-0.5) and a large age spread (6-13 Gyr). Is this the result of a merger 6Gyr ago? Brown et al. 2003, ApJ, 592, L13 18
More on M31 Star Formation History Olsen et al. 2006 AJ 132 271 19
Olsen et al. 2006 AJ 132 271 20
Bulge dominated by old (>6 Gyr) solar-metallicity stars Bulge and disk indistinguishable in age but disk has lower metallicity Olsen et al. 2006 AJ 132 271 Solid lines are a theoretical disk model. 21
M33: A Dwarf Spiral 22
High Mass Stars: M31 Compared to M33 Massey et al. 2006 AJ 131 2478 23
Dwarf Spheroidal Galaxies 24
Dwarf Spheroidal Galaxies SF history from HST CMDs young population is absent (by definition) old population ubiquitous at least one purely old galaxy (Ursa Minor) --------> intermediate-age population varies from 0% --> >90% Ursa Minor compared to M92 => Stars v.old and metal-poor. Mighell, Burke 1999, AJ, 118, 366 25
Carina dwarf elliptical 10-13 Gyr old population: 10-20% ~7 Gyr old population: 80-90% Smecker-Hane et al. 1994, AJ, 108, 507 26
Carina Had Bursts Hurley-Keller et al 1998, AJ, 115, 1840 27
Leo I Relatively steady SF from 10-13 Gyr to ~1 Gyr ago Gallart et al. 1999, ApJ, 514, 665 Gallart et al. 1999, AJ, 118, 2245 28
Dwarf Irregular Galaxies less well studied than DSph use main sequence or blue He sequence to age date stars can reconstruct SFH as function of position, trace past star formation in space and time NGC 6822 Dohm-Palmer et al. 2002, AJ, 123, 813 29
Case Study: Sextans A Dohm-Palmer et al. 2002, AJ, 123, 813 30
31
NGC 6822 Studies of gas densities and kinematics reveal that the Toomre Q criterion matches the star formation best but a simple constant density threshold also works for this galaxy. (de Blok and Walter 2006 AJ 131 363) Dwarfs provide a unique look at the threshold question because of the lack of spiral arms and bars. From Hutchings et al. IAU Symp. No. 92 1999 32
Local Group Dwarfs: Varied SFHs Mateo 1998 ARAA 36 435 33
Another View: Reionization Fossil = dsph w/ M dm <10 9 M that formed before reionization. From Ricotti and Gnedin 2005 ApJ 629 259 34