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)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. GlobalSF 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 inVirgo 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 afterreionization?

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

Red Giant Branch (RGB) ~ 5x10^8 yr100-10^3 Lsun

Horizontal Branch (HB)~10^8 yr (He burning)100 Lsun

Main sequence~10^10 yr H-burning

1 Lsun

AGB 10^7 yr

MS turnoff is most reliable agemeasure. HB can indicate Intermediate age vs. old pops.The AGB tip luminosity still nota reliable indicator of inter-mediate age stars, especiallyIn metal rich populations

UVX?

Optical wavelengths are superior for deriving ages and abundances of old 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 ofstellar mass. Infrared colors have a clearadvantage for this problem. At the GalacticCenter, one must reach K=27 to get to theend of the hydrogen burning stars, whereasone must reach to V=36 (!) to accomplishthe 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 determinePrecision 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 DAwhite dwarfs become bluer at the end of their cooling tracks, dueto H_2 molecular opacity, and may be observed at M_V=+18

HST+ACS will likely observe 3-4 clusters (needs 2 epochs forproper motion cleaning of CMD; 10-50 orbits per epoch)

NGST can do this problem if it can reach the 6000A band, but oldwd 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)

Detail of proper-motion cleaned coolingsequence with selection function andDB 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 bestfit is for 12.5 Gyr. Data in grey area ignored in fit. Chi-square insensitive to +/-0.5 magerror in distance/reddening.

A powerful age constraint, insensitive to 0.5 mag distance/reddening error.

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

(Kuijken & Rich 2002

WFPC2 bulgeProper motions

Additional gains: transiting exoplanets (Sahu+ 2004),Deep IR luminosity function (Rich + 2004).

Surprise: the halo of M31 is metal rich! (Durrell et al. 1994;Rich et al. 1996 + many others

Galactic halos are getting more complicated. (Ibata et al. 2001; Ferguson et al. 2002).

Ferguson et al. 2002

M31 system is huge.

ACS imaging ofM31 halo field

(vs. 5 old globular Clusters spanning-2<[Fe/H]<-0.2)

Brown et al.2003

ALL RED

Int. Age AGBBlue/Red

Ferguson et al.2002

SNAP could mapWith actual MSTurnoff ages!

s

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

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. 2004Stream 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 nucleusRich, Reitzel et al. 2002: Field Populationsare young. (Rich et al. 2004)

Many field stars near G1 have radial velocity ofHI 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.

G1 in M31: Rich et al. 1996;Meylan et al. 2001

Rich, Shara, and Zurek 2001(NGC 121 in LMC)

From this--

To this!

Jablonka 1999

Rich et al. 2004 (WFPC2 4 orbits)

Turnoff Photometry of a large sample of M31Globular clusters presently impossible with HST(100 orbits/cluster) but feasible with wide-fieldSurvey. RR Lyraes and precise distances a bonus.

The Andromeda dwarfs range from [Fe/H]=-2 to -1,and show internal age ranges, but RR Lyrae stars andBHB demand some old component. They look likeGalactic dwarf spheroidals.

Da Costa et al. 1996, 2000, 2002

What is the use of precision ages forglobular clusters in the Local Group?

Johnson & B

olte 1999

ApJ

Precise ages relative to parent galaxya test for CDM models (of course, CMBthe best test…

Also seek evidence for an age of commonignition for the oldest stars (e.g. M92-like globular clusters throughout LocalGroup (Harris et al. 1997; Mighell & Rich 1996).

Precision calibration of M(RR) vs [Fe/H],improving distance/age scale, 2nd parameter problem, etc.

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?)

Rich et al. 1997 ApJ

Some young and intermediateage populations can be diag-nosed simply by reaching 1-2mag below the old HB.

This level can be reached for outer halo, dwarf galaxies, tidal streamers, and extratidal regions in the Virgo cluster.

Stephens et al. 2003

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.

Mbol<-5.5

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

Mighell&Rich 1996

Fornax Buonnano et al.

Detailed star formation histories andPopulation gradients in dwarf galaxies:Did star formation change before/after reionization?

Fornax Dwarf Galaxy Coleman et al. 2004

One would like to map age, star formation history ofdwarf galaxies - was there a transition in SF before/afterreionization? SF history vs radius?

Survey of Omega Cen - Ferraro et al. 2004 ApJ L(Poster)

Haris, Harris, Poole 2001

Extend studies of metallicitiesof halo populations

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 ?

MW

Hibbard + Galex Team 2004

Saviane et al. 2004 HST image of “tidal dwarf”

A SNAP could do detailedstudies of unusual stellar populations, such as thosefound in interacting galaxies,tidal tails, etc.

The CMDs at left from WFPC/2Imagery of the tidal dwarfGalaxy candidate in NGC 4038/9

SNAP could map over wholeField of Antennae.

8 associations in the tidal dwarf galaxy candidateIn the Antennae (NGC 4038/9) Saviane. Hibbard, & Rich 2004

CONCLUSIONS

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.