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Wide Field Imaging from Space: The Origin and Evolution of Galaxies

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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)

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Stellar Populations Science Case

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

Virgo cluster.

Stellar Populations Goals (cont’d)

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?

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.

Will other missions/technologies solve these problems first

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.

the hr diagram and the age ladder
The HR diagram and the Age Ladder

AGB 10^7 yr

Red Giant

Branch (RGB) ~ 5x10^8 yr

100-10^3 Lsun

Horizontal Branch (HB)

~10^8 yr (He burning)

100 Lsun


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

Main sequence

~10^10 yr


1 Lsun

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

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

Richer et al. 2002

Full Sample Cluster Field

120 Orbits with WFPC2 -- ~1 orbit 8m HST

Constraining the Age of the Globular Cluster M4 (Hansen et al. 2001)

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.

Best fit age and formation redshift for M4 and the disk

(constrained from models of WD luminosity function)

Hansen et al. 2001; LDM=Liebert Dahn Monet

Hansen et al. 2004

Proper motion clean white dwarfs in open clusters in 1 yr

(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

Additional gains: transiting exoplanets (Sahu+ 2004),

Deep IR luminosity function (Rich + 2004).

WFPC2 bulge

Proper motions

(Kuijken & Rich 2002

M31 system is huge.

Ferguson et al. 2002

ACS imaging of

M31 halo field

(vs. 5 old globular

Clusters spanning


Brown et al.2003



Ferguson et al.


SNAP could map

With actual MS

Turnoff ages!

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.


Keck spectroscopy of stream fields (Rich, Guhathakurta,

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

Stream is very cold and at -475km/sec

(M31 at -300 km/sec)

Is G1 associated with remnants of a dwarf spheroidal?

Search for stars with same radial velocity,

Deep HST imaging of the field.

M31 halo field near G1, 32 kpc from nucleus

Rich, Reitzel et al. 2002: Field Populations

are young. (Rich et al. 2004)

Many field stars near G1 have radial velocity of

HI from extended disk of M31 at 30 kpc (Cram et al. 1980)

(Keck spectra, Ca triplet method for abundance/radial vel.)

Reitzel, Rich,

Guhathakurta 2004

m31 reach the main sequence turnoff anywhere in the halo but need at least 50 orbits
M31: reach the main sequence turnoff anywhere in the halo - but need at least 50 orbits.

From this--

To this!

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

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)

Jablonka 1999

The Andromeda dwarfs range from [Fe/H]=-2 to -1,

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

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.

Kent 1989

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 gradients in dwarf galaxies:

Did star formation change before/after


Mighell&Rich 1996

Fornax Buonnano et al.

One would like to map age, star formation history of

dwarf galaxies - was there a transition in SF before/after

reionization? SF history vs radius?

Fornax Dwarf Galaxy Coleman et al. 2004

Extend studies of metallicities

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.

Could make maps of interaction streamers and dwarf galaxies over wide range Hubble type and luminosity

Spiral Galaxy halos

Ferguson, Rich, Brown, Mouhcine, Smith (2004)

Implication: How can halos be accretion of low mass low metallicity satellites ?


A SNAP could do detailed

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


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.