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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
– However, 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; also expect dependence on
density of the environment – what do observations tell us?
The observed Universe vs. a
simulated one (Springel, Frenk, &
White 2006)
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Abadi et al. (2003)
Stars
Governato et al. (2007)
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
•Limitations to Making Use of Resolved Stars
– 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. There is thus a hard limit to
photometric depth, which gets worse with
increasing surface brightness and lower
telescope resolution
~
– Sensitivity
At K > 19 mag arcsec-2, the time needed to
reach the crowding limit (for a diffractionlimited ground-based telescope working in
the near-IR) becomes >> 1 hour. The pure
disk components of galaxies are thus often
sensitivity-limited, while the bulges and
inner disks are crowding-limited.
Approximate crowding limits vs distance
for different apertures, assuming K = 19
mag arcsec-2 and 0.1 mag photometric
error
The Star Formation Histories of Disk
Galaxies
•Measurements
– Photometry down to near 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)
The disk of M31 with
Gemini North and
NIRI+Altair (NGS mode)
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 diffractionlimited 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 of bulges and
disks 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