Norbert Pirzkal

Passively Evolving Early-Type Galaxies at 1.4 ≲ z ≲ 2.5 in the Hubble Ultra Deep Field*

We report on a complete sample of seven luminous early-type galaxies in the Hubble Ultra Deep Field (UDF) with spectroscopic redshifts between 1.39 and 2.47, and to KAB 1:4. Low-resolution spectra of these objects have been extracted from the Hubble Space Telescope (HST) ACS grism data taken over the UDF by the Grism ACS Program for Extragalactic Science (GRAPES) project. Redshifts for the seven galaxies have been identified based on the UV feature at rest frame 2640 < k < 2850 8. This feature is mainly due to a combination of Fe ii ,M gi ,a nd Mgii absorptions, which are characteristic of stellar populations dominated by stars older than � 0.5 Gyr. The redshift identification and the passively evolvingnatureofthesegalaxiesisfurthersupportedbythephotometricredshiftsandbytheoverallspectralenergy distribution (SED), with the ultradeep HST ACS NICMOS imaging revealing compact morphologies typical of

Evidence for TP-AGB Stars in High-Redshift Galaxies, and Their Effect on Deriving Stellar Population Parameters

We explore the effects of using different stellar population models on estimates of star formation histories, ages, and masses of high-redshift galaxies by fitting the SEDs with models by Maraston (hereafter M05) and by Bruzual & Charlot (hereafter BC03). We focus on the thermally pulsing asymptotic giant branch (TP-AGB) phase of stellar evolution, whose treatment is a source of major discrepancy. In this respect, BC03 models are representative of other models whose treatment of the TP-AGB phase is similar. Moreover, M05 and BC03 models adopt stellar tracks with different assumptions on convective overshooting. For our experiment we use a sample of high-z (1.4 z 2.7) galaxies, for which rest-frame UV spectroscopy and spectroscopic redshifts are available, along with Spitzer IRAC and MIPS photometry from GOODS. The mid-UV spectra of these galaxies exhibit features typical of A- or F-type stars, indicative of ages in the range ~0.2-2 Gyr, when the contribution of TP-AGB stars is expected to be maximum. We find that the TP-AGB phase plays a key role in the interpretation of the Spitzer data, where the rest-frame near-IR is sampled. Generally, M05 models give better fits than BC03 models and indicate systematically lower ages and lower masses (by ~60%, on average). Photometric redshifts derived using M05 models are also in better agreement with the spectroscopic ones, especially when the rest-frame near-IR fluxes from Spitzer IRAC are included in the fit. We argue that the different results are primarily a consequence of the different treatment of the TP-AGB phase, although other differences in the input stellar evolution also contribute. This work provides a first direct evidence for a strong contribution by TP-AGB stars to the SED of galaxies in the high-redshift universe (z ~ 2).

Optical-to-Mid-Infrared Observations of Lyα Galaxies at z ≈ 5 in the Hubble Ultra Deep Field: A Young and Low-Mass Population

High redshift galaxies selected on the basis of their strong Lyman-alpha emission tend to be young ages and small physical sizes. We show this by analyzing the spectral energy distribution (SED) of 9 Lyman-alpha emitting (LAE) galaxies at 4.0 < z < 5.7 n the Hubble Ultra Deep Field (HUDF). Rest-frame UV to optical 700A < lambda < 7500A luminosities, or upper limits, are used to constrain old stellar populations. We derive best fit, as well as maximally massive and maximally old, properties of all 9 objects. We show that these faint and distant objects are all very young, being most likely only a few millions years old, and not massive, the mass in stars being ~10^6-10^8 M_sun. Deep Spitzer Infrared Array Camera (IRAC) observations of these objects, even in cases where objects were not detected, were crucial in constraining the masses of these objects. The space density of these objects, ~1.25x10^-4 Mpc^-3 is comparable to previously reported space density of LAEs at moderate to high redshifts. These Lyman-alpha galaxies show modest star formation rates of ~8 M_sun yr^-1, which is nevertheless strong enough to have allowed these galaxies to assemble their stellar mass in less than a few x10^6 years. These sources appear to have small physical sizes, usually smaller than 1 Kpc, and are also rather concentrated. They are likely to be some of the least massive and youngest high redshift galaxies observed to date.High redshift galaxies selected on the basis of strong Ly-α emission tend to be young and have small physical sizes. We show this by analyzing the spectral energy distribution of nine Ly-α-emitting galaxies (LAEs) at 4.0 < z < 5.7 in the Hubble Ultra Deep Field . Rest-frame UV to optical 700Å < λ < 7500Å luminosities, or upper limits, are used to constrain old stellar populations. We derive best fit, as well as maximally massive and maximally old, properties of all nine objects. We show that these faint and distant objects are all very young, most likely only a few million years old, and not massive, the mass in stars being ≈ 10 − 10 M⊙. Deep Spitzer Infrared Array Camera observations of these objects, even in cases where the object was not detected, proved crucial in constraining the masses of these objects. The space density of these objects, ≈ 1.25×10 Mpc, is comparable to previously reported space densities of LAEs at moderate-to-high redshifts. These Ly-α galaxies show modest star formation rates of ≈ 8 M⊙ yr , which is nevertheless strong enough to have allowed them to assemble their stellar mass in less than a few million years. These sources appear to have small physical sizes, usually smaller than 1 kpc, and are also rather concentrated. They are likely to be some of the least massive and youngest high redshift galaxies observed to date. Subject headings: galaxies: evolution, galaxies: high redshift, galaxies: formation, galaxies: structure, surveys, cosmology Space Telescope Science Institute, Baltimore, MD 21218. Affiliated with the Space Science Depatment of the European Space Agency, ESTEC, NL-2200 AG Noordwijk, Netherlands. School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287. Shanghai Institute of Technical Physics, 200083, Shanghai, China.

EVIDENCE FOR A MASSIVE POSTSTARBURST GALAXY AT z 6.5

We describe results from a search for high-redshift J-band dropout galaxies in the portion of the Great Observatories Origins Deep Survey (GOODS) southern field that is covered by extremely deep imaging from the Hubble Ultradeep Field (HUDF). Using observations at optical, near-infrared, and mid-infrared wavelengths from the Hubble and Spitzer Space Telescopes and the European Southern Observatory Very Large Telescope, we find one particularly remarkable candidate, which we designate HUDF-JD2. Its spectral energy distribution has distinctive features that are consistent with those of a galaxy at z ~ 6.5, observed several hundred million years after a powerful burst of star formation that produced a stellar mass of 6 × 1011 M☉ (for a Salpeter IMF). We interpret a prominent photometric break between the near-infrared and Spitzer bandpasses as the 3646 A Balmer discontinuity. The best-fitting models have low reddening and ages of several hundred Myr, placing the formation of the bulk of the stars at z > 9. Alternative models of dusty galaxies at z ≈ 2.5 are possible, but provide significantly poorer fits to the photometric data. The object is detected with Spitzer at 24 μm. We consider interpretations of the 24 μm emission as originating either from an obscured active nucleus or from star formation and find that the 24 μm detection does not help to uniquely discriminate between the z = 6.5 and 2.5 alternatives. We present optical and near-infrared spectroscopy that has, thus far, failed to detect any spectral features. This nondetection helps limit the solution in which the galaxy is a starburst or active galaxy at z ≈ 2.5, but does not rule it out. If the high-redshift interpretation is correct, HUDF-JD2 is an example of a galaxy that formed by a process strongly resembling traditional models of monolithic collapse, in which a very large mass of stars formed within a remarkably short period of time, at very high redshift.

An Overdensity of Galaxies at z = 5.9 ? 0.2 in the Hubble Ultra Deep Field Confirmed Using the ACS Grism

We present grism spectra taken with the Advanced Camera for Surveys (ACS) to identify 29 red sources with (i775 - z850) ? 0.9 in the Hubble Ultra Deep Field (HUDF). Of these, 23 are found to be galaxies at redshifts between z = 5.4 and 6.7, identified by the break at 1216 ? due to intergalactic medium (IGM) absorption; two are late-type dwarf stars with red colors; and four are galaxies with colors and spectral shapes similar to dust-reddened or old galaxies at redshifts z ? 1-2. This constitutes the largest uniform, flux-limited sample of spectroscopically confirmed galaxies at such faint fluxes (z850 ? 27.5). Many are also among the most distant spectroscopically confirmed galaxies (at redshifts up to z = 6.7). We find a significant overdensity of galaxies at redshifts z = 5.9 ? 0.2. Nearly two-thirds of the galaxies in our sample (15/23) belong to this peak. Taking into account the selection function and the redshift sensitivity of the survey, we get a conservative overdensity of at least a factor of 2 along the line of sight. The galaxies found in this redshift peak are also localized in the plane of the sky in a nonrandom manner, occupying about half of the ACS chip. Thus the volume overdensity is a factor of 4. The star formation rate derived from detected sources in this overdense region is sufficient to reionize the local IGM.

Stars in the Hubble Ultra Deep Field

We identified 46 unresolved source candidates in the Hubble Ultra Deep Field (HUDF) down to i775 ¼ 29:5. UnresolvedobjectswereidentifiedusingaparameterS,whichmeasuresthedeviationfromthecurveofgrowthofa point source. Extensive testing of this parameter was carried out, including the effects of decreasing signal-to-noise ratio and of the apparent motions of stars, which demonstrated that stars brighter thani775 ¼ 27:0 could be robustly identified. Low-resolution grism spectra of the 28 objects brighter than i775 ¼ 27:0 identify 18 M and later stellar type dwarfs, two candidate L dwarfs, two QSOs, and four white dwarfs. Using the observed population of dwarfs with spectral type M4 or later, we derive a Galactic disk scale height of 400 � 100 pc for M and L stars. The local white dwarf density is computed to be as high as (1:1 � 0:3) ;10 � 2 stars pc � 3 . On the basis of observations taken 73 days apart, we determined that no object in the field has a proper motion larger than 0B027 yr � 1 (3 � detection limit). No high-velocity white dwarfs were identified inthe HUDF, and all four candidates appear more likely to be part of the Galactic thick disk. The lack of detected halo white dwarfs implies that if the dark matter halo is 12 Gyr old, white dwarfs account for less than 10% of the dark matter halo mass. Subject headingg Galaxy: disk — Galaxy: stellar content — Galaxy: structure — stars: late-type — white dwarfs Online material: color figures

GRAPES, Grism Spectroscopy of the Hubble Ultra Deep Field: Description and Data Reduction

We present deep unbiased spectroscopy of the Hubble Ultra Deep Field (UDF) carried out using the slitless grism spectroscopy mode of the Advanced Camera for Surveys on the Hubble Space Telescope (HST). The Grism ACS Program for Extragalactic Science (GRAPES) achieves continuum detection as faint as zAB = 27.2 using 40 orbits (9.2 × 104 s) on HST. The data were taken at four orientation angles to correct for the overlap of spectra. GRAPES data provide a unique, uninterrupted, low-resolution (R = 100) spectral coverage for 5500 A < λ < 10500 A and allow us to detect high-redshift galaxies at 4 < z < 7 whether they have Lyα lines or just show the Lyman break, as well as find low-luminosity active galactic nuclei in an unbiased fashion. This paper describes in detail the observations and the data reduction and examines the quality of the extracted spectra. Subsequent papers will deal with the analysis of the data. The extracted and calibrated GRAPES spectra will be available from MAST at STScI.

Sizing up lyα and lyman break galaxies

We measure the sizes for a sample of 174 Lyα-selected galaxies with broadband imaging with the Hubble Space Telescope. Over the redshift range 2.25 < z < 6, Lyα-selected galaxies have a characteristic, constant, small size in rest-frame ultraviolet (UV) light. Coupled with a characteristic star formation intensity (i.e., UV luminosity per unit area), this can explain their non-evolving ultraviolet continuum luminosity function. This is in contrast to Lyman break galaxies (LBGs) over the same redshift range, which have been previously shown to increase in linear size as H(z)–1. The compact physical size seems to be a critical determining factor in whether a galaxy will show Lyα emission or not. The L * of LBGs and its evolution with redshift can be derived from a simple model where the star formation intensity has an upper limit set by feedback processes, independent of redshift. The increase in L * of LBGs is mainly driven by the increase in linear size over redshifts for z = 2-7. Since Lyα galaxies do not grow in linear size, they do not show an increase in L *.

Cosmic shear analysis of archival HST/ACS data. I. Comparison of early ACS pure parallel data to the HST/GEMS survey

Context. This is the first paper of a series describing our measurement of weak lensing by large-scale structure, also termed “cosmic shear”, using archival observations from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). Aims. In this work we present results from a pilot study testing the capabilities of the ACS for cosmic shear measurements with early parallel observations and presenting a re-analysis of HST/ACS data from the GEMS survey and the GOODS observations of the Chandra Deep Field South (CDFS). Methods. We describe the data reduction and, in particular, a new correction scheme for the time-dependent ACS point-spread-function (PSF) based on observations of stellar fields. This is currently th e only technique which takes the full time variation of the PSF between individual ACS exposures into account. We estimate that our PSF correction scheme reduces the systematic contribution to the shear correlation functions due to PSF distortions to< 2× 10 −6 for galaxy fields containing at least 10 stars, which corresp onds to. 5% of the cosmological signal expected on scales of a single ACS field. Results. We perform a number of diagnostic tests indicating that the remaining level of systematics is consistent with zero for the GEMS and GOODS data confirming the success of our PSF correction sc heme. For the parallel data we detect a low level of remaining systematics which we interpret to be caused by a lack of suffi cient dithering of the data. Combining the shear estimate of the GEMS and GOODS observations using 96 galaxies arcmin −2 with the photometric redshift catalogue of the GOODS-MUSIC sample, we determine a local single field estimate for the mass power spectrum normalisationσ8,CDFS = 0.52 +0.11 −0.15 (stat)± 0.07(sys) (68% confidence assuming Gaussian ! ,

The Galaxy Luminosity Function at z ≃ 1 in the HUDF: Probing the Dwarf Population*

We present a catalog of spectrophotometric redshifts for 1308 galaxies from the Grism ACS Program for Extragalactic Science (GRAPES) observations with the Hubble Space Telescope. These low-resolution spectra between 6000 A and 9500 A are supplemented with U, J, H, and Ks data from various facilities, resulting in redshifts computed with ~40 spectral bins per galaxy. For 75 galaxies in the range 0.5 < z < 1.5 with spectroscopic redshifts, the standard deviation in the fractional error in 1 + z is 0.037. With this catalog, we compute the B-band luminosity function in this redshift range from 72 galaxies. Due to the depth of the GRAPES survey, we are able to accurately constrain the faint-end slope by going to MB -18 mag at z = 1.0 ± 0.2, nearly 2 mag fainter than previous studies. The faint-end slope is α = -1.32 ± 0.07. When compared to numerous published values at various redshifts, we find strong evidence for a steepening of the faint-end slope with redshift, which is expected in the hierarchical formation scenario of galaxies.