Wide Field Imaging from Space: The Origin and Evolution of Galaxies. R. Michael Rich, UCLA. Stellar Populations Science Case. Assume SNAP fact sheet as the reference mission: 2.0 m telescope, detector area 35X35’, 0.10”/pixel (76xACS)
R. Michael Rich, UCLA
Stellar Populations Science Case Galaxies
Assume SNAP fact sheet as the reference mission:
2.0 m telescope, detector area 35X35’, 0.10”/pixel (76xACS)
0.35-1.0um range; also consider an IR channel of similar FOV.
Star Formation History: Age, metallicity, stellar content of streams, structure, and outer disks of M31, M33 and other Local Group galaxies and their globular clusters. Global
SF history and gradients for dwarf galaxies.
Streams, satellites, metallicity, and age constraints for halos of galaxies to ~10 Mpc
Very long integrations: ages of halos, ages of satellites in
Stellar Populations Goals (cont’d) Galaxies
Are the ages of the oldest stars (~M92) the same in all metal poor systems? Did character of star formation change after
Resolve the stellar populations in low surface brightness galaxies and tidal tails out to ~15 Mpc.
Survey low luminosity stars and mass function in the Galactic halo and bulge. Settle problem of white dwarfs as dark matter.
Cause and nature of the Ultraviolet rising flux.
Precise relative ages, maybe star formation history reconstruction, from white dwarf cooling sequence.
Microlensing survey in M31
Will anyone care in 2012?
Where might we be in 2012? Galaxies
Is science worth doing?
Galaxy evolution and formation will likely be interesting.
JWST and ground-based progress at high redshift will encourage innovative work in the nearby Universe.
The relative roles of gas accretion, interactions, ingestion of companions will best be sorted out for nearby galaxies.
Galaxy evolution and formation in the Local Group may not be representative of either low or high density environments; we will want to conduct detailed studies of stellar populations across the Hubble sequence and across environment. A survey mission can make a critical contribution.
HST: Unlikely to complete fundamental surveys in remaining mission lifetime, even with robotic extension (factor of ~50+ gain for wide-field mission concept).
JWST: Wide IR field and aperture make this a better choice for IR surveys (9x gain and 3x resolution over 2m offsets FOV issues)
Ground-based AO: Variable point spread function, high background, small FOV (even with MCAO at BEST a few arcmin) make this technology uncompetetive.
Deep ground-based imaging with 6-10m telescopes: Best possible optical seeing over wide fields is 0.3” for brief periods. PAN STARRS technology? No examples of ground-based imaging competitive with HST.
AGB 10^7 yr
Branch (RGB) ~ 5x10^8 yr
Horizontal Branch (HB)
~10^8 yr (He burning)
MS turnoff is most reliable age
measure. HB can indicate
Intermediate age vs. old pops.
The AGB tip luminosity still not
a reliable indicator of inter-
mediate age stars, especially
In metal rich populations
Optical vs. IR: IR superior for low luminosity stars and obscured populations (e.g. survey of the inner 100pc of the Galaxy). Because metal lines are in the optical, more sensitivity to temperature (much better age, abundance discrimination).
Absolute mag in V and K as a function of
stellar mass. Infrared colors have a clear
advantage for this problem. At the Galactic
Center, one must reach K=27 to get to the
end of the hydrogen burning stars, whereas
one must reach to V=36 (!) to accomplish
the same in optical colors. This problem
(and others like it) will be done by JWST.
(models from Baraffe et al. 2002)
Applying the White Dwarf Cooling Sequence to determine obscured populations (e.g. survey of the inner 100pc of the Galaxy). Because metal lines are in the optical, more sensitivity to temperature (much better age, abundance discrimination).
Precision relative ages for the Milky Way and LMC/SMC
Globular Clusters and the Galactic Bulge
New cooling models by Hansen (1998) show that the oldest DA
white dwarfs become bluer at the end of their cooling tracks, due
to H_2 molecular opacity, and may be observed at M_V=+18
HST+ACS will likely observe 3-4 clusters (needs 2 epochs for
proper motion cleaning of CMD; 10-50 orbits per epoch)
NGST can do this problem if it can reach the 6000A band, but old
wd suffer the H_2 opacity in the IR.
A 10-30m diffraction limited HST can reach M_V~34, placing the bulge
(m-M)_V=16 and intermediate age LMC/SMC clusters in reach.
The technique has the potential for relative age dating to +/- 1 Gyr
Color-magnitude diagram of M4 HST/WFPC2 obscured populations (e.g. survey of the inner 100pc of the Galaxy). Because metal lines are in the optical, more sensitivity to temperature (much better age, abundance discrimination).
Richer et al. 2002
Full Sample Cluster Field
120 Orbits with WFPC2 -- ~1 orbit 8m HST
A powerful age constraint, insensitive to 0.5 mag distance/reddening error.
Detail of proper-motion cleaned cooling
sequence with selection function and
DB cooling track (red). Note the hint of
a blueward hook (DA track in blue).
Fit of cooling models (including incomplete-
ness, and the wd counts from M4. The best
fit is for 12.5 Gyr. Data in grey area ignored
in fit. Chi-square insensitive to +/-0.5 mag
error in distance/reddening.
(constrained from models of WD luminosity function)
Hansen et al. 2001; LDM=Liebert Dahn Monet
Hansen et al. 2004
(calibrate WD cooling age method)
(e.g. Kalirai et al. 2001) for NGC 2099.
t(wd) = 566 Myr;
t(TO) = 520 Myr
WD luminosity function NGC 2099
Deep IR luminosity function (Rich + 2004).
(Kuijken & Rich 2002
Rich et al. 1996 + many others
Ferguson et al. 2002).
M31 system is huge. 2001;
Ferguson et al. 2002
ACS imaging of 2001;
M31 halo field
(vs. 5 old globular
Brown et al.2003
Ferguson et al.
SNAP could map
With actual MS
Int. Age AGB
CMDs in M31 halo show interesting differences from place to place: (what is nature of blue plume; metal rich populations?)
Survey by Bellazzini et al. 2003 for only 16 WFPC2 fields.
s place: (what is nature of blue plume; metal rich populations?)
Keck spectroscopy of stream fields (Rich, Guhathakurta, place: (what is nature of blue plume; metal rich populations?)
Majewski, Reitzel, Johnston) Wide field spectrographs will give complementary data for wide-field surveys. (Deimos on Keck; IMACS on Magellan).
Deimos survey (Guhathakurta, Rich, Reitzel et al. 2004 place: (what is nature of blue plume; metal rich populations?)
Stream is very cold and at -475km/sec
(M31 at -300 km/sec)
Is G1 associated with remnants of a dwarf spheroidal? place: (what is nature of blue plume; metal rich populations?)
Search for stars with same radial velocity,
Deep HST imaging of the field.
M31 halo field near G1, 32 kpc from nucleus place: (what is nature of blue plume; metal rich populations?)
Rich, Reitzel et al. 2002: Field Populations
are young. (Rich et al. 2004)
Many field stars near G1 have radial velocity of place: (what is nature of blue plume; metal rich populations?)
HI from extended disk of M31 at 30 kpc (Cram et al. 1980)
(Keck spectra, Ca triplet method for abundance/radial vel.)
G1 in M31: Rich et al. 1996;
Meylan et al. 2001
Rich, Shara, and Zurek 2001
(NGC 121 in LMC)
Turnoff Photometry of a large sample of M31 but need at least 50 orbits.
Globular clusters presently impossible with HST
(100 orbits/cluster) but feasible with wide-field
Survey. RR Lyraes and precise distances a bonus.
Rich et al. 2004 (WFPC2 4 orbits)
The Andromeda dwarfs range from [Fe/H]=-2 to -1, but need at least 50 orbits.
and show internal age ranges, but RR Lyrae stars and
BHB demand some old component. They look like
Galactic dwarf spheroidals.
Da Costa et al. 1996, 2000, 2002
What is the use of precision ages for but need at least 50 orbits.
globular clusters in the Local Group?
Precise ages relative to parent galaxy
a test for CDM models (of course, CMB
the best test…
Also seek evidence for an age of common
ignition for the oldest stars (e.g. M92-
like globular clusters throughout Local
Group (Harris et al. 1997; Mighell & Rich
Precision calibration of M(RR) vs [Fe/H],
improving distance/age scale, 2nd
parameter problem, etc.
Johnson & Bolte 1999 ApJ
For Local Group, possible to work in the outer M31, M33 disks; measure star formation history to the main sequence turnoff.
Contrast SFH of disks, halos, dwarf galaxies.
Is there a substantial age dispersion in elliptical galaxies? (Trager
et al.; Worthey, Faber et al.) Or is something else going on?
(A blue horizontal branch, or blue stragglers?)
Some young and intermediate
age populations can be diag-
nosed simply by reaching 1-2
mag below the old HB.
This level can be reached for outer halo, dwarf galaxies, tidal streamers, and extratidal regions in the Virgo cluster.
Rich et al. 1997 ApJ
The AGB stars in the M31 bulge would be resolvable at 1.6um with a 2m telescope; possibility of tracing age gradient and superposed intermediate age populations.
This is exorbitantly hard with current AO.
Stephens et al. 2003
Composite of all NICMOS images finds no evidence for a population of extremely luminous AGB stars.
But old metal rich populations have AGB stars reaching Mbol=-5.5
A younger (8 Gyr) population such as seen in the halo might not be distinguishable.
Galactic bulge Zoccali et al. 2003
Detailed star formation histories and population of extremely luminous AGB stars.
Population gradients in dwarf galaxies:
Did star formation change before/after
Fornax Buonnano et al.
One would like to map age, star formation history of population of extremely luminous AGB stars.
dwarf galaxies - was there a transition in SF before/after
reionization? SF history vs radius?
Fornax Dwarf Galaxy Coleman et al. 2004
Survey of Omega Cen - Ferraro et al. 2004 ApJ L(Poster) population of extremely luminous AGB stars.
Extend studies of metallicities population of extremely luminous AGB stars.
of halo populations
Haris, Harris, Poole 2001
Wide field surveys of Local Group halos could reach to below the horizontal branch and allow structural and relative star formation history studies.
Galaxy halos can be resolved to 10 Mpc. the horizontal branch and allow structural and relative star formation history studies.
Could make maps of interaction streamers and dwarf galaxies over wide range Hubble type and luminosity
Spiral Galaxy halos the horizontal branch and allow structural and relative star formation history studies.
Ferguson, Rich, Brown, Mouhcine, Smith (2004)
Implication: How can halos be accretion of low mass low metallicity satellites ?
Hibbard + Galex Team 2004 the horizontal branch and allow structural and relative star formation history studies.
Saviane et al. 2004 HST image of “tidal dwarf” the horizontal branch and allow structural and relative star formation history studies.
A SNAP could do detailed the horizontal branch and allow structural and relative star formation history studies.
studies of unusual stellar
populations, such as those
found in interacting galaxies,
tidal tails, etc.
The CMDs at left from WFPC/2
Imagery of the tidal dwarf
Galaxy candidate in NGC 4038/9
SNAP could map over whole
Field of Antennae.
8 associations in the tidal dwarf galaxy candidate
In the Antennae (NGC 4038/9) Saviane. Hibbard, & Rich 2004
CONCLUSIONS the horizontal branch and allow structural and relative star formation history studies.
A Wide Field Imager could make fundamental, breakthrough-level contributions in the subject of stellar populations.
Need: wide field, small pixels, optical
Wide area proper motion surveys would give maps of Local Group galaxies, with turnoff ages, over huge regions to constrain assembly, star formation histories. Major discoveries guaranteed.
Out to 10 Mpc, deep integrations give detailed stellar populations and ages of associations and young stars. Tidal streams, satellites in the halos of massive galaxies to 10+ Mpc.
This comes at a price. Need long, deep integrations 50-100 orbits;
This is only a 2m telescope.