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Motivation
· 40 orbits of UDF observations with the ACS grism · Spectra for every source in the field. · Good S/N continuum detections to I(AB) ~ 27; about 30% of UDF sources. · Spectral identification of every 4 ­ high equivalent width lines; ­ Real z > 6 LBGs among the very red, faint stuff.


GRism ACS Program for Extragalactic Science (GRAPES)
Deepest Unbiased Spectroscopy yet. I(AB) < 27 (UDF)

Team: S. Malhotra, A. Cimatti, S. Cristiani, E. Daddi, H. Ferguson, J. Gardner, C. Gronwall, Z. Haiman, A. Koekemoer, A. Pasquali, N. Panagia, L. Petro, N. Pirzkal, J. Rhoads, M. Stiavelli, S. di Serego Aligheri, Z. Tsvetanov J. Vernet, J. Walsh, R. Windhorst, H.J. Yan


ACS Grism Characteristics
(G800L + WFC)
· Dispersion: 40å/pixel, Resolution: ~ 80å (point source; scales with image size). · Wavelength calibration is accurate ~10å or Dz~0.001 · Wavelength coverage: ~ 550 nm to 1050 nm at "zero" response; 600 to 930 nm at half max.


Advantages of HST/ACS combination:
· Low sky background from space · Red sensitivity of the ACS · High redshift galaxies are compact, HST PSF helps · Contiguous wavelength/redshift coverage, unlike ground based instruments.


Science Goals
· Probe reionization era by determining luminosity functions of lyman- emitters, lyman-break galaxies at z=4-7 and low-luminosity AGNs. · Study star-formation and galaxy assembly at 1

The Epoch of Reionization
· The detection of Gunn-Peterson trough(s) in z ~ 6 quasars show the late stages of H reionization (Becker et al. 2001, Fan et al. 2002.) · WMAP results indicate substantial reionization at z~15 · Was the universe reionized twice (Cen 2002)?


Epoch of Galaxy formation?
· Early stages of galaxy formation are presumably ongoing at z~6. · Our current samples at this redshift are small: A half dozen z>5 galaxies, 6 at z>6, quasars at the very brightest end of luminosity function. · We would like to determine the epoch and pace of reionization as well as the luminosity function of sources (galaxies/AGNs) responsible for the photons.


Testing Reionization with Lyman- emitters
Low luminosity Lyman- sources should not before reionization: · Lyman alpha photons resonantly scatter in universe. · This means they should not be apparent as sources, I.e., we expect a sharp drop in the source counts at reionization. be visible a neutral compact Lyman alpha

(Miralda-Escude 1998; Miralda-Escude & Rees 1998; Haiman & Spaans 1999; Loeb & Rybicki 1999)

· Higher luminosity sources (e.g. quasars) create a local ionized bubble allowing the Lyman- photons to escape.


Resonant Scattering Before Reionization
Neutral IGM

Continuum Photons
To Observer

H II region Young starburst

Lyman photons


Constraining Reionization
· We still see expected number/luminosity of Lyman alpha emitters in our z=5.7 sample.Thus, the reionization redshift is z > 5.7.
(Rhoads & Malhotra 2001, ApJ Lett 563, L5)

· ... extended to z(reionization) > 6.6?
(Hu et al. 2002, Kodaira et al. 2003, Lilly et al. 2003, Cuby et al. 2003, Rhoads et al. 2003)


HII Regions in z > 5 Lyman Samples
· An HII region must be > 1.2 Mpc (non-comoving) to reduce the line center optical depth to < 1. · This requires a minimum value of (Li t fesc). · We can constrain all of these quantities in LALA using the observed line luminosities and equivalent width distribution. · We find that Li t fesc is < 30% of threshold for the z=5.7 sources, i.e. the sources are too faint to create a large enough HII region. · The limit for the Hu et al source is similar, thanks to its lower physical luminosity.


The Lyman Test, First Order Concerns: ~2 < ~
· Our threshold HII region size was based on 0 = 1 at emitted line center. Lines have finite width, and < 0 in the red wing. · The Hu et al source could be embedded in a fully neutral IGM and still get 10 to 20% of its Lyman flux out (Haiman 2002).


Testing Reionization
· Statistical test remains: The observed number of Lyman emitters above a fixed threshold will show a dramatic drop at reionization. · Equivalent width test: Also the equivalent width of the Lyman- line will also drop at reionization. · At present 5/6 sources at z>6 have rest equivalent width of the Lyman- line <50 å, whereas at z=4.5 median equivalent width is 200 å (Malhotra & Rhoads 2002). So the line may well be attenuated by the damping wings of the neutral gas. The sixth source has EW>85 å (Rhoads et al. 2003)


The Lyman Test, First Order Concerns: Evolution
· The Lyman test is based on of redshift. · Strong evolution could cause decrease in n(z) could mimic · However, the intrinsic n(z) is decrease at reionization: number counts as a function trouble. In particular, a a neutral IGM. more likely to increase than

­ Star formation in small halos is suppressed at reionization; ­ Lyman galaxies appear to be primitive objects (Malhotra & Rhoads 2002) and should be a larger fraction of galaxies at high z.

· Lyman break galaxies offer a control sample, if we can go deep enough to find them.


Ultradeep Field Grism Expectations: Lyman Break Galaxies
· Extrapolating from shallower grism data, we estimate that Lyman break galaxies can be reliably identified to I(AB) = 26.9 for breaks near the throughput peak. · Effective redshift limit around 6.7 (to have some useful wavelength coverage redward of break). · Compare limit at redshift < 6.0 for i', z' two-filter detections. · Predicted Counts... shortly.


Ultradeep Field Grism Expectations: Lyman Galaxies
· Isolated emission lines near peak sensitivity would be detected to
­ 8x10-18 erg/cm2/s for compact galaxies ­ 6x10-18 erg/cm2/s for point sources

· Lyman- galaxy density: ~1 per sq. arcminute per unit z above flux 2x10-17 erg/cm2/s (z=4.5, Rhoads et al 2000). · A very steep luminosity function observed expect many objects.


Ultradeep Field Grism Expectations: Lyman- Galaxies
Number-flux relations for Lyman galaxies from the LALA survey. Scaled to redshift z=5.7. Black: z=4.5 data; Green: z=5.7 data. Red line: N ~ f-3


Expected Numbers of z>6 Lyman- emitters
· Adapted from Stiavelli et al. 2003. Upper limits from Hu et al. and detection from the Large Area Lyman Alpha (LALA) survey. · Grapes should see 3-30 z>6 Lyman- emitters.

LALA


Ultradeep Field Grism Expectations: High Redshift Galaxy Counts
4.5

Ultradeep Field Grism Expectations: Foreground (z ~ 1) Galaxies
· Most galaxies with a well detected emission line or 4000å break will yield a redshift.
­ Two lines Þ redshift; ­ One line synergy with photo-z (grism redshift is more precise but may be less accurate) and with ground based followup (more wavelength coverage)

· Star formation history from emission lines at 0

Ultradeep Field Grism Expectations: Faint Quasars?
· We have detected a couple of promising high redshift quasar candidates in our deepest cycle 11 parallel grism data. · Many AGN lines broad redshift coverage · Morphology + spectroscopy (+variability) reliable identifications, completeness. · Caveat: modest sample size


APPLES (ACS Pure Parallel Lyman- Emission Survey)


APPLES First-Look Lyman Break Galaxies


APPLES First-Look Emission Line Objects


Synergy with Ground Based Spectra
Grapes will help ground based spectroscopy in at least two ways: · "Instant" redshifts for ~ 300 to 500 objects; potential savings of time. · Identification of which faint objects are worth a slitlet. Conversely, ground based data yield more flexible wavelength coverage and higher resolution, offering physical information unavailable with the grism.


Spectrum Overlap and Roll Angles
· Overlap is potentially a problem in slitless spectroscopy. · Extrapolation from shallower grism fields and simulated GRAPES data both imply 20% contamination in any roll angle. · Multiple roll angles will resolve source confusion due to overlap for almost all sources.


Science and Data Products
Primary Data Product: Reduced, extracted spectra to go in the public domain. Science products: · Spectral identification of galaxies between 4 Clean studies of galaxy evolution. · Galaxies with old stellar populations, HII region lines or both identified at z~1. · M-dwarfs, Supernovae ...