Stellar Populations Science

Knut Olsen

The Star Formation Histories of Disk Galaxies

•Context

– Hierarchical structure formation does an excellent job of describing large scale structure

– Galaxy formation is complex and non-linear, depending on processes operating on a huge range of scales

– Star formation histories of simulated disks are sensitive to the input physics, e.g. feedback from stars, as well as to the mass of the parent galaxy – what do observations tell us?

The observed Universe vs. a simulated one (Springel, Frenk, & White 2006)

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Governato et al. (2007)StarsAbadi et al. (2003)

The Star Formation Histories of Disk Galaxies

•Approach

– Target bulges and disks of galaxies with different luminosities and in different environments. Will need to observe several fields in any given galaxy to fully sample radial and stochastic variations in stellar populations

– Determine their star formation and chemical enrichment histories, detailed chemical evolution, and kinematic distributions; through observations of resolved stars; look for differences as a function of galaxy luminosity and environmental density

– Need to get out to d~10 Mpc to sample large range of environmental density, ~100 galaxies

Left: Luminosities of individual galaxies out to 10 Mpc from new Tully catalog

Right: Density of groups as function of distance, from new Tully catalog

The Star Formation Histories of Disk Galaxies

•Measurements

– Photometry down to the crowding limit in bulges and disks of galaxies with different masses and in different environments

– Fit CMD for age and metallicity mix to determine star formation and chemical enrichment history

– R~25000 near-IR spectroscopy of TRGB stars would confirm abundance distribution, probe detailed chemical evolution (Fe, O, Si, Ca, Mg, Ti, C), and measure kinematics with v~1 km s-1 (e.g. Rich et al. 2007)

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The Star Formation Histories of Disk Galaxies

•Limitations

– Crowding

Crowding introduces photometric error through luminosity fluctuations within a single resolution element of the telescope due to the unresolved stellar sources in that element.

– Sensitivity

At K > 19 mag arcsec-2, the time needed to reach the crowding limit (for a diffraction-limited ground-based telescope working in the near-IR) becomes >> 1 hour

~

The Star Formation Histories of Disk Galaxies

•Instrument requirements– Moderate to high Strehl AO imaging; near-IR assumed, but if shorter wavelengths were available, would help; FOV >10 arcsec (~104 diffraction-limited sources at K)– Near-IR spectroscopy with R up to ~25000 for detailed abundances and kinematics

•Role of GSMT– Deeper crowding limit allows us to image stars in wide range of evolutionary stages in both bulges and disks, giving, for the first time, an accurate account of their star formation and chemical enrichment histories– Allows us to measure star formation histories of disks out to ~10 Mpc, increasing the number of available galaxies by an order of magnitude, covering a wide range of morphological types, masses, and environments– Greater sensitivity and resolution allows high resolution spectroscopic analysis of TRGB stars out to ~4 Mpc, making it possible to study the detailed chemistry of galaxies outside the sphere of the Milky Way

•What current generation of 8-10m telescopes can do– Determine star formation and chemical enrichment histories from imaging using bright, evolved stars out to distance of M31– Measure the detailed chemistry of stars in the Milky Way and its nearest dwarf companions, for comparison with more distant galaxies available to GSMT

The Star Formation Histories of Disk Galaxies

•With Gemini North and NIRI+Altair, usefully measure stars as faint as MK = -4 to -5 (includes TRGB) in bulge and inner disk (published in Davidge et al. (2005) and Olsen et al. (2006) )

•Disk 2 field reaches level of horizontal branch

M31’s Bulge and Inner Disk Population Box from Gemini Analysis

•Old ages, nearly solar metallicities dominate

•Metal-poor intermediate-age populations are probably spurious

•Luminosity-weighted age, [Fe/H] = 8 Gyr, 0.0 (-0.5)

•Mass-weighted age, [Fe/H] = 8.3 Gyr, 0.0 (-0.4)

The Disk 2 Field: Preliminary Results

•30% of stellar mass formed within last 250 Myr: prominent signature from the 10 kpc ring!

•35% of the stellar mass appears ancient and metal-poor

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Block et al. (2006): Suggest a collision between M32 and M31 formed the rings ~210 Myr ago

The Star Formation Histories of Disk Galaxies

•Potential gain of increasing aperture to ~ 40m

– Crowding limit deeper by ~0.6 mag compared to 30-m for same observation, reached in same exposure time; incremental progress for most purposes

– 30-m class provides a leap in number and diversity of disk galaxies available for study

– 40-m would reach ~13 Mpc with same quality that 30-m provides at 10 Mpc; 50-m would reach Virgo cluster. Exact gains of 40-m requires detailed modeling, however.

– Another great leap would be provided by a much larger telescope (~50-m to 100-m) or development of robust optical AO for 30-m class telescope

•Key GSMT requirements

– Main gain for imaging is high angular resolution, resulting in deeper crowding limits in bulges and disks of large number of galaxies out to ~10 Mpc

– Main gain for R~25000 near-IR spectroscopy is combination of high angular resolution (deeper crowding limits) and high sensitivity

– Need to preserve diffraction-limited performance of delivered PSFs, stable PSFs are a big plus

The Star Formation Histories of Disk Galaxies

•Site requirements

– High fraction of clear nights

– High fraction of nights with low, stable atmospheric turbulence

•Operations requirements

–Scheduling to take advantage of clear nights and good seeing, necessary for AO operations

– Programs could occupy a few to tens of nights per year

The Star Formation Histories of Disk Galaxies

• Precursor observations– Ground-based optical/near-IR surveys to identify optimal field

pointings– 8-10m AO imaging observations to identify scientifically most

interesting targets and create target lists for spectroscopy

• Followup observations– Repeat observations for studying variables (imaging and

spectroscopy) and rare time-domain events (e.g. microlensing)

• Desirable access to elements of the US system– JWST to probe lower surface brightness regions of same galaxies – 8-10 m observations to select best targets and pointings and to

sharpen scientific questions– LSST for studying variables, placing results within larger context

• Potential archival research– First epoch observations for astrometric followup