The Star Formation Histories of Disk
Galaxies
Knut Olsen
Collaborators: Bob Blum, Andrew Stephens, Tim
Davidge, Phil Massey, Steve Strom, François
Rigaut, and Joss Bland-Hawthorn
Science with Giant Telescopes: Public
Participation in TMT and GMT
Chicago, June 16, 2008
T.A.Rector and B.A.Wolpa/NOAO/AURA/NSF
Motivation I
– Many lines of evidence showing that
massive galaxies form the bulk of their
stars at high redshift, earlier than less
massive galaxies
– More massive galaxies have heavier
contribution from spheroidal components,
reinforcing the idea that bulges and
elliptical galaxies are old, and disks are
accreted later
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– Massive disk galaxies also exist at high
redshift; may be the galaxies that form
massive spheroids?
Juneau et al. (2005)
Benson et al. (2007)
Stockton et al. (2004)
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Motivation II
– Hierarchical structure formation does an excellent job of
describing large scale structure; history of build-up of dark
matter, however, appears different than that of the
observed stellar mass buildup
Abadi et al. (2003)
320 kpc
– 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 and
merging, as well as to the mass of the parent galaxy
40 kpc
The observed Universe vs. a simulated one
(Springel, Frenk, & White 2006)
Dark matter
Gas
Stars
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Robertson et al. (2004)
•Feedback inhibits rapid collapse of gas
•Feedback regulates star formation
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•Introduces dependence on galaxy mass
•Also expect dependence on environment
Governato et al. (2007)
Approach
The M31 halo with HST (Brown et al. 2007): old
and intermediate-age populations
Milky Way
bulge from
near- and
mid-IR
photometry
(van Loon
et al. 2003):
old stars
dominate
A summary of Local Group dwarf star formation
histories (Grebel & Gallagher 2004): variety is the rule
Why ELTs?
1.
Need a representative sample of
morphological types of galaxies
2.
Need to sample orders of
magnitude in the range of
environmental densities
Both argue for getting out to 10 Mpc
and beyond
With high surface brightnesses and
faint sources, need to consider
both sensitivity and crowding
From Tully group catalog
Modeling 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.
V
I
To calculate the effects of crowding on magnitudes and colors, we
need only consider the Poisson statistics of the luminosity functions
(e.g. Tonry & Schneider 1988)
hi
For magnitudes:
For colors:
8
8
Crowding limits for current and future telescopes
HST (optical)
Gemini North
30-m (I
(near-IR)
and J)
Magnitudes at which 10% photometry is possible in
regions of surface brightness SV=22, SK=19 for
galaxies at the indicated distances.
How big an ELT do we need?
– The main sequence and its turnoff is the
most fundamental indicator of stellar age
and metallicity, but the stars are faint and
extremely crowded
– More advanced phases of stellar
evolution can also be used to determine the
ages and metallicities of populations of
stars, at the expense of more uncertain
theoretical modeling
-2
Using Savg
K=22 mag arcsec
Current Systems: the Bulge and Disk of
M31 with Gemini N and NIRI+Altair
•Nearby:
Can study entire star formation
history from its resolved stars
Complementary to studies of galaxies
with z > 0.5, which are limited to
integrated broadband photometry or IFU
spectroscopy
•Extragalactic:
Can easily trace contributions
from different galactic
components
Milky Way produces important
constraints on the stellar populations of
galactic components, but from large and
heterogeneous datasets
Local Group Survey (Massey et al. 2002) image
Observations
Gemini N+Altair/NIRI SV
observations, 18-19 Nov 2003
(one night photometric)
Disk 2 field observed 14 Sep
2006: 0.´´2 - 0.´´3 seeing,
photometric
NIRI/Altair provided near
diffraction-limited imaging in HK
over 22.´´5  22.´´5 field
We also include published
HST/NICMOS data from
Stephens et al. (2003)
1:1:
2:540s
960s
320sH,
J,J,960s
320sH,H,
1040sKK
Disk 2:
Bulge
520s
3420s
480s
K3480s
880s
0.”059 H (~30%
0.”066
(~40%KK
Strehl)
0.”11
J, 0.”09
0.”085
H,H,K0.”10
0.”09
0.”15Strehl),
0.”09
Analysis
•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
Photometry
•PSF-fitting photometry with
DAOPHOT/ALLSTAR
Fits the core of the PSF (0.”44
diameter), neglecting the halo
•Corrections applied to account for:
-difference between PSF and aperture
magnitudes out to a diameter of 0.”66
(30 pixels): ~0.3 mags
-difference between 0.”66 diameter
aperture magnitudes and 4.”4 diameter
aperture magnitudes: ~0.4 - 0.6 mags
-spatial variability of the aperture
correction
-transformation of magnitudes to
standard system
Photometric error analysis
Bulge
Disk 22field
field
•Completeness and photometric
errors calculated from extensive
Monte Carlo simulations
•Both simulations and analytical
crowding calculation (Olsen,
Blum, & Rigaut 2003) indicate
that crowding dominates errors for
bulge and inner disk; do not go as
deep as expected in Disk 2 field
•Restrict analysis to magnitudes
with >50% completeness
Deriving the population mix
•Build models from isochrones (Girardi et al. 2002):
Age = 1, 3, 5, 10 Gyr; Z=0.0001, 0.0004, 0.001, 0.008, 0.019, 0.03; Salpeter IMF for bulge and
inner disk; finer age grid for Disk 2 field
•Apply photometric errors and incompleteness to models
•Fit model mix to LFs using maximum likelihood analysis (Dolphin 1997, Olsen 1999, Dolphin
2002); assume E(H-K) from IRAS/ISO; solve age and Z; (m-M)0 = 24.45
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Results
Example: two
fields with
Bulge/Disk ~ 1
Fits are dominated
by the oldest
populations
Fit to LF: P ~ 6 - 17%
M31’s Bulge and Inner Disk
Population Box
•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)
Radial Trends
•Both bulge and disk are
dominated by older stars
•The bulge has nearly
solar metallicity, in
agreement with other
studies
•The lower disk
metallicities are in
general agreement with
other studies
The Disk 2 Field
Block et al. (2006): Suggest that a
collision between M32 and M31
formed the rings ~210 Myr ago
•30% of stellar mass formed within last ~100300 Myr: prominent signature from the 10
kpc ring!
•35% of the stellar mass appears ancient and
metal-poor
An M31 Survey: 20-m
Name
F1
Bulge1
F177
F174
F3
Bulge2
F4
F5
F170
Disk2
F2
F280
Disk1
r()
1.97
2.05
2.79
2.59
3.80
3.83
3.98
5.84
6.08
9.09
11.9
20.5
56.9
SK
15.0
15.1
15.4
15.4
15.8
16.0
16.1
16.4
16.5
17.1
17.8
18.4
19.6
B/D
7.4
6.7
5.5
5.3
3.8
3.1
2.7
2.0
1.9
1.1
0.3
0.2
0.0
Klim Time(s)
24.1 21.8
24.1 24.9
24.4 38.3
24.4 38.3
24.7 66.9
24.9 87.8
25.0 100
25.2 147
25.3 168
25.7 348
26.1 776
26.6 1767
27.8 17476
1 hour exposure, S/N=5:
J: 28.9
H: 28.0
KHB~23.5, KMSTO~27
K: 27.0
Local Group Survey (Massey et al. 2002) image
An M31 Survey: 30-m
Name
F1
Bulge1
F177
F174
F3
Bulge2
F4
F5
F170
Disk2
F2
F280
Disk1
r()
1.97
2.05
2.79
2.59
3.80
3.83
3.98
5.84
6.08
9.09
11.9
20.5
56.9
SK
15.0
15.1
15.4
15.4
15.8
16.0
16.1
16.4
16.5
17.1
17.8
18.4
19.6
B/D
7.4
6.7
5.5
5.3
3.8
3.1
2.7
2.0
1.9
1.1
0.3
0.2
0.0
Klim
24.8
24.9
25.1
25.1
25.4
25.5
25.6
25.8
25.9
26.2
26.8
27.5
29.1
Time(s)
17.6
19.7
27.9
27.9
44.2
55.3
62.0
86.3
96.8
188
543
1732
32500
1 hour exposure, S/N=5:
J: 29.8
H: 28.9
KHB~23.5, KMSTO~27
K: 27.9
Local Group Survey (Massey et al. 2002) image
An M31 Survey: 50-m
Name
F1
Bulge1
F177
F174
F3
Bulge2
F4
F5
F170
Disk2
F2
F280
Disk1
r()
1.97
2.05
2.79
2.59
3.80
3.83
3.98
5.84
6.08
9.09
11.9
20.5
56.9
SK
15.0
15.1
15.4
15.4
15.8
16.0
16.1
16.4
16.5
17.1
17.8
18.4
19.6
B/D
7.4
6.7
5.5
5.3
3.8
3.1
2.7
2.0
1.9
1.1
0.3
0.2
0.0
Klim Time(s)
25.6 12.8
25.7 14.1
25.9 18.9
25.9 18.9
26.1 27.9
26.2 34.4
26.3 38.8
26.6 58.6
26.6 68.7
27.2 194
28.1 849
28.9 4170
30.4 69500
1 hour exposure, S/N=5:
J: 30.7
H: 29.8
KHB~23.5, KMSTO~27
K: 28.9
Local Group Survey (Massey et al. 2002) image
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A GMT and TMT disk galaxy program
•Imaging
GMT HRCAM and TMT IRIS broad-band imaging of ~10–
100 galaxies out to 10 Mpc
~10 pointings per galaxy
Having one telescope in each hemisphere would be ideal!
•Spectroscopy
R~3500 @ 0.85 (Ca triplet) with <~0.”05 resolution is ideal
for abundance gradients and velocities of RGB stars out to
~10 Mpc; TMT IRIS and IRMOS, GMT NIRMOS
High resolution R~25000 – 50000 spectroscopy will be
provide important detailed abundances in the nearest
galaxies; GMTNIRS, TMT NIRES and HROS (Smith talk)
Program needs 10–100 clear nights
Other Facilities
Ibata et al. (2007)
The low surface brightness regions far
from galaxy centers are ideal places to
study the late accretion histories of disk
galaxies; needs wide fields (LSST,
PanSTARRS), excellent site (PILOT),
and deep imaging (JWST)
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Bland-Hawthorn et al. (2005)
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Closing Thoughts
The age and metallicity distributions of stars in bulges and disks are sensitive
indicators of galaxy formation physics.
Resolved stellar populations can be used to measure the entire star formation
and chemical enrichment histories of galaxies.
Spatial resolution is the most critical capability needed to measure the star
formation histories of massive galaxies; we are just beginning to probe these
galaxies.
Photometry with ground-based adaptive optics on current and future large
telescopes are excellent tools to allow us to measure the star formation
histories in the bright components of massive galaxies.
A program of measuring star formation histories from resolved stars in disk
galaxies out to ~10 Mpc will provide an exciting and unique perspective on
galaxy formation!
Can we trust star formation histories
derived from only evolved stars?
J-K
•Compare star formation
histories derived from
2MASS J-K, K CMD of
the LMC Bar (Olsen, in
prep.) to that derived
from HST/WFPC2
(Dolphin 2002)
An NGC 3379 Survey: 20-m
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Name
Re
3Re
Rtot
r() SK
30 17.0
90 19.3
190 22.5
Klim Time(s)
24.6 197
27.2 22620
31.1 
KHB~29, KMSTO~32.5
Jarrett et al. (2002)
An NGC 3379 Survey: 30-m
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Name
Re
3Re
Rtot
r () SK Klim Time(s)
30 17.0 25.7 282
90 19.3 28.5 47200
190 22.5 31.6 
KHB~29, KMSTO~32.5
Jarrett et al. (2002)
An NGC 3379 Survey: 50-m
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Name
Re
3Re
Rtot
r() SK
30 17.0
90 19.3
190 22.5
Klim Time(s)
27.0 496
30.2 175800
32.6 
KHB~29, KMSTO~32.5
Jarrett et al. (2002)
An NGC 3379 Survey: 100-m
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Name
Re
3Re
Rtot
r() SK
30 17.0
90 19.3
190 22.5
Klim Time(s)
28.8 1724
31.3 170000
34.5 
KHB~29, KMSTO~32.5
Jarrett et al. (2002)