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Evolution of Normal Galaxies with ACS
Richard Ellis, Caltech


Main Points
· Science goals: beyond basic exploration towards a physical understanding of how galaxies form & evolve · Lessons learnt from HDF: deep versus wide, role of colors · Future ground-based instrumentation: how to prepare · Pointers for splinter group discussion meeting

· Concerned with optical survey imaging of moderate & high redshift objects · Disregarding studies of specific objects: clusters, nearby galactic systems · Assume others will emphasize role of non-optical/IR facilities


Science Goals for ACS: Deep, Wide or Something New?
· What is the case for deeper-than-HDF optical exploration?
No. count slope flattens beyond B=24, I=24, K=19 Most of deep HDF data is still unexploited

· Going beyond simple exploration & "census studies" towards physical understanding of galaxy formation
How to better "connect" samples at various redshifts to get a meaningful "big picture" Require more detailed & representative datasets at HDF depth targeting physically-relevant parameters

Let's THINK not just POINT & SHOOT deep


How do we connect samples observed at various z?
We have the capability of isolating well-defined samples at various epochs: · Lyman/Balmer break samples: star-forming galaxies z1-3 · Red, possibly passively-evolving, field ellipticals z>1 on what physical basis can we intercompare these samples?

z1 z 2.5

z 0


Merging Dark Matter Halos & Structure Formation

Dark matter "halos" act as seeds for galaxies to form

Physical clustering at different epochs provides one basis for intercomparisons: HST can enrich such datasets but only if offered over much wider fields


Galaxy Masses: More Useful than Star Formation Rates
Dynamics: rotation & line widths Grav. lensing IR-based stellar masses

K

Stellar c.f Dynamical Masses (0.5
M(dyn)/M(stars) M(stars)

HST data must match complementary ground-based spectra & IR imaging


Stellar Mass Distribution by Morphology

Stellar mass density from Kband data

Decline in mass density in late-types occurs at the expense of a modest growth in regular spirals & ellipticals (Brinchmann & Ellis 2000) (N 500g from 30 WFPC fields with redshifts )


Limited overlap of HST & ground-based redshifts
Contiguous WFPC-2 fields (probably incomplete)

Hawaii SSA fields (Cowie): 8 WFPC-2 fields in 4 areas CFRS (Lilly) LDSS (Broadhurst) Groth strip HDF-N/S 12 WFPC-2 fields in 4 areas 6 WFPC-2 fields in 2 areas 28 WFPC-2 fields in 1 area 16 WFPC-2 fields in 2 areas

Total: 70 WFPC-2 fields (350 arcmin2); <1500 redshifts Modest considering the unique opportunities!


HDF has shown us new possibilities!
What lessons have we learnt? Advantages: · Depth (but not dramatically so) · Four bands with unrivalled photometric precision (photo-z's) · Resolved color images for galaxies at all z Limitations: · Restricted field may be unrepresentative, e.g for z<2 samples · Field mismatch c.f. essential/complementary facilities · Large fraction of data yet to be effectively exploited · Was the U band data worth the extraordinary effort?


Two HDFs (Hubble vs Herschel)

Can expect reasonably deep multi-band imaging from 8-10m telescopes in 0.2-0.5 arcsec resolution photo-z's etc


Advantages of Resolved Color Data: I

Identification and photometric study of key diagnostic sub-components: (e.g. bars, bulges etc)

Discovering evidence for continued growth in field ellipticals


Advantages of Resolved Color Data: II

Exploiting 2-D information:
· Sophisticated photo-z techniques: improved precision, role of dust etc · SF characteristics for objects of known z

Targets for AO instruments:
· IFU spectrographs capable of dynamical and excitation studies of SF galaxies


HDF-N Statistics (after Ferguson et al ARAA 38, 667)
Galaxies with UBVI Galaxies with K Spectroscopic redshifts R<24
z >0.5 ellipticals 25 face-on barred spirals 10 chain galaxies 1
3000 300 150

Radio galaxies Lenses

16
Volume to z=2 equivalent to one containing 30-80 L* galaxies


Spectra: <25 HDF-N high z ellipticals within reach of Keck

LRIS z=0.98

Ellis, van Dokkum & Abraham: Keck LRIS 600/7500 gr 7.5 hours I<23


Decline in Fraction of Barred Spirals z > 0.5?

Abraham, Merrifield, Ellis et al (2000): HDF-N & HDF-S


Exploiting Investment in Ground-based Spectroscopy
Deep-1HS: 480 x (4 x 16 arcmin) Deep-3HS: 60 x (3.5 x 16 arcmin) Multiplex gain: < 130; R=5000 50,000g I<23.5; 5000g I<24

VIMOS

VIMOS: <145 x (56 x 4 arcmin) Multiplex gain: 750; R<2500 130,000g I<22.5; 50,000g I<24 1000g I<26


Pointers for Discussion
· What is the physical motivation for deeper-than-HDF imaging in 1-2 small fields given high fraction of HDF data remains unexploited? · Significant limitation lies in connecting data at different epochs: suggested ways forward require wide field data, e.g. clustering in representative fields and extensive associated g-based data. · Community has been slow to exploit resolved color data (perhaps as it is restricted largely to HDF); however significant benefits likely even if expensive in HST time. · How should panoramic ground-based surveys influence what HST does? Multi-color HST exploitation of such surveys represents a significant requirement