Stellar populations and chemical evolution in nearby galaxies: the Local Group

• Significantly more information about star formation history can be derived from systems where individual stars can be resolved: no problems with luminosity-weighted biases, ability to study abundance ratios in individual stars, etc.

• Photometry for individual stars down to unevolved stars can be obtained only for Local Group galaxies (at M31/M33 distances, even this requires HST). Spectroscopy can be determined from the brightest stars.

• The MW lives in a slightly overdense region known as the Local Group, consisting of some 40 galaxies. Roughly this region is 3 Mpc in diameter. Most of the galaxies are found in two groups, around the MW and M31, though there are several isolated objects. Probably our knowledge of the LG is nearly complete except for the possiblity of objects behind the Galactic plane and perhaps very faint and/or low SB dwarfs; note recent discoveries by Zucker et al. and the current research interest in this question given the possible discrepancy with numerical models of dark halo satellites, e.g. Klypin et al..
  • Local Group Luminosity function

• Deciding membership in the LG is a slightly complicated issue, with both distance and velocity playing a role. Map of LG (Grebel), and velocity membership plot (Grebel).

• The biggest and brightest members of the LG are the MW and M31, large spirals, followed by M33, a Sc spiral.

• a large number of Irr galaxies, with the LMC being the brightest of the group. These contain large amounts of HI.

• No giant ellipticals in the LG, so we are unable to study individual faint stars in any galaxies of this type; note M32 as a possible low luminosity ``giant'' elliptical. Most of the LG galaxies are smaller ellipticals, sometimes called dwarf ellipticals and sometimes dwarf spheroidals. The relationship between these types and between giant ellipticals is best understood through the relationship of their global properties (luminosity and surface brightness, a relation we have seen before) (Bingelli plot). The dSph galaxies have much lower SB than giant ellipticals. The dEs in the Local Group look like brighter dSph's, with low SB; other ellipticals outside the Local Group can exist at similar luminosities but higher surface brightness and thus appear more closely related to like gE's. The dE's and dSph's in the LG have little or no ISM. Nature of these objects is somewhat under debate: primordial or evolutionary?

• Significant relation between the morphology and location of dwarf galaxies - the gas poor dwarfs tend to be found closer to the large galaxies and the gas-rich dwarfs further - but it has exceptions (e.g. LMC/SMC, Tucana).

• dE's and dSph's are found primarily around the MW and M31. Some claims have been made that the MW dSph's appear to be located on two great circles; one of these also includes the MCs. However, the statistical significance of this has been questioned.

  Summary of properties: Gallagher and Wyse table 1, Mateo Tab 1

Stellar populations in Local Group galaxies

• get information on stellar pops in LG galaxies by resolving individual stars, e.g.
  • qualitatively from CMD
  • Quantitatively from fitting Hess diagrams
  • there exists a great diversity of SF histories, both between and within morphological types.
  • Chemistry, i.e. abundance ratios, can also provide important clues

• Milky Way:
  • varying distances and extinction pose challenges
  • Historical: Population I (low velocity relative to Sun, high metallicty, younger) and Population II stars (high velocity, low metallicity, older). However, note Pop II association with low metallicity is now recognized not to be fundamental; inner halo/bulge significantly more metal rich
  • Now 3 components recognized: thin disk, thick disk, halo; inner disk has a bar. Bulge as separate component, or part of bar or halo?
  • Solar neighborhood SFH can be studied for stars with known parallaxes, e.g. Hipparcos sample: roughly constant SFH? Old disk stars exist; note cool white dwarf population
  • Bulge can also be studied, but note foreground population: predominantly old population. bulge CMD, and luminosity function
  • Duration of star formation of different components also probed by abundance ratios: halo and thick disk formed relatively rapidly.
  • Increasing evidence for substructure in halo: current disruption of Sagittarius, plus other new structures recently discovered

• M31:
  • distance and crowding in inner regions pose challenges
  • Halo appears to have very significant intermediate age popultion not seen in MW halo (see Hess diagrams from Brown et al (spheroid, stream, disk)
  • Significant substructure in halo
  • Disk: less information. Outer disk seems to be dominated by intermediate age stars

• M33 disk: work in progress! Note presence of intermediate age globular clusters

• Dwarf Irrs: (Sex B picture)
  • Summary (Grebel SFH Irregulars).
  • LMC/SMC. HST CMDs: continuous star formation over past few Gyr, superposed on older population with somewhat lower SFR (Holtzman SFR).
  • Note that youngest populations (= highest time resolution) can be studied in dIrrs, and these suggest more continuous than episodic star formation (Dohm-Palmer, Skillman et al).
    • IC10 may be experiencing a starburst as it has a very high density of massive stars; some evidence (WR stars) suggests a variation in IMF from other galaxies (Massey et al. AJ 109, 2470 (95)).
  • Abundances of irregulars correlated with luminosity (Skillman, Kennicutt, Hodge Fig2); possible (but not only!) explanation is more mass loss in lower mass galaxies, though note that mass loss obviously isn't total, since there's still gas.
  • ``Transition'' galaxies: Phoenix and LGS3 have gas, but not much, no very young stars, but some moderately young ones.

• dE's: M32. M32 critical as it's the only compact E in the LG, though it may not be greatly representative (it's too compact, may be stripped, note no globular clusters).
  • Integrated light suggests some component of intermediate age stars (5-10 Gyr), and this was apparently confirmed by detection of AGB stars by Freedman (89). However, HST shows that most of these were blends (Grillmair CMD).
  • Large metallicity spread but dominant number of stars between -0.2 and 0.1 in [Fe/H] ( Grillmair Fig 9 Grillmair Fig 10 Note this is narrower than expected from simple model. This might be explained by variations from the simple model (e.g., variable yield, inhomogeneous enrichment, variable IMF), or alternatively , the lower mass stars may be at larger radius - we await new observations at outer regions.
  • CMD is consistent with integrated light analysis which gives mean age of $\sim$ 8 Gyr; age question still hotly debated.

• diffuse Es: NGC 205, 185, 147.
  • Have lower central velocity dispersion and SB than M32
  • NGC 205 has central OB stars, plus evidence for intermediate age population as well, superposed on an apparently very old halo.
  • NGC 185, NGC 147 do not appear to have very young stars (less than 1 Gyr, maybe a few in NGC 185), but may have an intermediate age populations.
  • Metallicities of all 3 around [Fe/H] $\sim$ -1, with substantial spread.

• dSphs. (Draco photo) (Leo I photo) (UMa cor photo) (HST cmds)
  • (Grebel SFH dsphs)
  • Luminosities in the range of globulars but significantly lower SB.
  • Metallicity also in the range of globulars.
    • However, not single metallicity systems (e.g. Helmi et al), and many have indications of intermediate age population.
    • Abundance ratios differ from those of MW halo stars (e.g. Pritzl et al)
  • Fornax is most massive and even has several globular clusters at a range of metallicity. Field is more metal-rich than GCs, and has evidence for star formation up to 2-3 Gyr ago (CMD).
  • Sagitarius dwarf of comparable luminosity and probably similar stellar pops; likely in the process of disrupting.
  • Leo I has intermediate age stars, possibly no very old stars at all.
  • And I probably has small intermediate age population, possibly larger in And II.
  • Carina has primarily intermediate age stars, and best case in LG for episodic star formation
  • Sculptor mostly old stars, possibly some intermediate age.
  • Ursa Minor and Draco seem to be predominatly all old (HST CMDs); note second parameter problem.
  • Tucana?
  • Problem of missing gas...nature of dSphs.

• Metallicites may be driven by mass-loss from system and not from low yields in low-Z stars. Most massive systems appear to have highest Z. These also appear to have younger stars. However, note Carina: low mass but has younger stars.

• Summary: no LG galaxies are SSPs, even ones that seem predominantly old: note recent developments on elemental abundances, that show even the ``simpler'' systems seem to show significant metallicity spread.

• dark matter problem: from observed velocity dispersions and virial equilibrium, can derive M/L ratios. For dSphs, find a range, with some objects having very high M/L (5-15 in Fornax and Sculptor, up to 100 in Ursa Minor, Sextans, Draco).
  • much higher than one can get from even an entirely old stellar population with a local IMF.
  • Several possible explanations:
    • large dark matter halos,
    • different IMF,
    • not in virial equilibrium.
  • important question in light of galaxy formation models: are dSphs the oldest systems, resulting from the most dense regions in the early universe, or more recently formed, perhaps resulting from interactions? Note that MW dSphs may be found on two great circles. However, also note variations in stellar pop ages, and isolation of Tucana, and general low metallicity of dSphs. (dSph Reference/reading: Gallagher and Wyse, PASP 106, 1225)

Local Group formation

• Historical models of MW formation:
  • Eggen, Lynden-Bell, Sandage scenario
  • Searle Zinn scenario

• CDM and hierarchical clustering.
  • To compare dark matter halos with observed galaxies, need to understand relation of luminous components to dark matter components.
  • missing satellites problem and possible resolutions:
    • missing luminous satellites,
    • missing unrecognized satellites (HVCs),
    • missing dark satellites,
    • star formation suppression in low velocity halos (but why some luminous ones?), Star formation triggers and suppressants:
      • interactions
      • ionizing radiation
      • feedback/SN
      • gas supply
    • evolution of halo properties (current halo circ velocity not representative of past one).

• Dwarf galaxies:
  • ram-pressure stripping
    • Note Magellanic stream
  • tidal stripping/ harassment (Mayer et al 2001; Lin and Murray)?
    • Note Magellanic stream leading arm
  • See review by Gallagher, Grebel, & Harbeck. Do morphologies evolve with time?

• Is the Local Group typical? Other groups, e.g. M81, Sculptor