Search

We combine the exceptional depth of the Hubble Ultra Deep Field (HUDF) images and the deep GRism ACS Program for Extragalactic Science (GRAPES) grism spectroscopy to explore the stellar populations of 34 bulges belonging to late-type galaxies at z=0.8-1.3. The sample is selected based on the presence of a noticeable 4000A break in their GRAPES spectra, and by visual inspection of the HUDF images. The HUDF images are used to measure bulge color and Sersic index. The narrow extraction of the GRAPES data around the galaxy center enables us to study the spectrum of the bulges in these late-type galaxies, minimizing the contamination from the disk of the galaxy. We use the low resolution (R~50) spectral energy distribution (SED) around the 4000A break to estimate redshifts and stellar ages. The SEDs are compared with models of galactic chemical evolution to determine the stellar mass, and to characterize the age distribution. We find that, (1) the average age of late-type bulges in our sample is ~1.3 Gyr with stellar masses in the range log(M)=6.5-10 solar. (2) Late-type bulges are younger than early-type galaxies at similar redshifts and lack a trend of age with respect to redshift, suggesting a more extended period of star formation. (3) Bulges and inner disks in these late-type galaxies show similar stellar populations, and (4) late-type bulges are better fitted by exponential surface brightness profiles. The overall picture emerging from the GRAPES data is that, in late-type galaxies at z=1, bulges form through secular evolution and disks via an inside-out process., 16 pages, emulateapj; Accepted for publication in ApJ

We present a two-pronged approach to the formation of early-type galaxies, using a sample of 18 galaxies at 0.5 < z < 1 from the HST/ACS Ultra Deep Field and GRAPES surveys: 1) We combine slitless low resolution spectroscopy from the GRAPES dataset with simple models of galaxy formation to explore their star formation histories. 2) We also perform an analysis of their surface brightness distribution with the unprecedented details provided by the ACS superb angular resolution and photometric depth. Our spectroscopic analysis reveals that their stellar populations are rather homogeneous in age and metallicity and formed at redshifts z ~ 2-5. Evolving them passively, they become practically indistinguishable from ellipticals at z = 0. Also, their isophotal shapes appear very similar to those observed for nearby ellipticals, in that the percentages of disky and boxy galaxies at z ~ 1 are close to the values measured at z = 0. Moreover,we find that the isophotal structure of z ~ 1 early-type galaxies obeys the correlations already observed among nearby ellipticals, i.e. disky ellipticals have generally higher characteristic ellipticities, and boxy ellipticals have larger half-light radii and are brighter in the restframe B band. In this respect then, no significant structural differences are seen for ellipticals between z = 0 and 1., 21 pages, 13 figures, accepted by ApJ. Higher quality figures are available at http://www.stsci.edu/science/grapes/papers/pasquali05

We present a photometric search for objects with point-source components that are optically variable on timescales of weeks--months in the Hubble Ultra Deep Field (HUDF) to i'(AB)=28.0 mag. The data are split into four sub-stacks of approximately equal exposure times. Objects exhibiting the signature of optical variability are selected by studying the photometric error distribution between the four different epochs, and selecting 622 candidates as 3.0 sigma outliers from the original catalog of 4644 objects. Of these, 45 are visually confirmed as free of contamination from close neighbors or various types of image defects. Four lie within the positional error boxes of Chandra X-ray sources, and two of these are spectroscopically confirmed AGN. The photometric redshift distribution of the selected variable sample is compared to that of field galaxies, and we find that a constant fraction of ~1% of all field objects show variability over the range of 0.1, Comment: 9 pages, 6 figures, accepted for publication in Astrophysical Journal, minor changes to references

We report on a complete sample of 7 luminous early-type galaxies in the Hubble Ultra Deep Field (UDF) with spectroscopic redshifts between 1.39 and 2.47 and to K1.4. Low-resolution spectra of these objects have been extracted from the HST+ACS grism data taken over the UDF by the GRAPES project. Redshift for the 7 galaxies have been identified based on the UV feature at rest frame 2640=1.7 appears to be roughly a factor of 2--3 smaller than that of their local counterparts, further supporting the notion that such massive and old galaxies are already ubiquitous at early cosmic times. Much smaller effective radii are derived for some of the objects compared to local massive ellipticals, which may be due to morphological K corrections, evolution, or the presence of a central point-like source. Nuclear activity is indeed present in a subset of the galaxies, as revealed by them being hard X-ray sources, hinting to AGN activity having played a role in discontinuing star formation., 18 pages, 15 figures, ApJ in press

The Advanced Camera for Surveys on-board HST is equipped with a set of one grism and three prisms for low-resolution, slitless spectroscopy in the range 1150 Ang. to 10500 Ang. The G800L grism provides optical spectroscopy between 5500 Ang. and 1 micron with a mean dispersion of 39 Ang./pix and 24 Ang./pix (in the first spectral order) when coupled with the Wide Field and the High Resolution Channels, respectively. Given the lack of any on-board calibration lamps for wavelength and narrow band flat-fielding, the G800L grism can only be calibrated using astronomical targets. In this paper, we describe the strategy used to calibrate the grism in orbit, with special attention to the treatment of the field dependence of the grism flat-field, wavelength solution and sensitivity in both Channels., Comment: 24 pages, 14 figures, accepted by PASP

Following the detection of a cosmic shear signal at the 30'' scale using archival parallel data from the STIS CCD camera onboard HST in Haemmerle et al. (2002), we analyzed a larger data set obtained from an HST GO pure parallel program. Although this data set is considerably larger than the one analyzed previously, we do not obtain a significant detection of the cosmic shear signal. The potential causes of this null result are the multiple systematics that plague the STIS CCD data, and in particular the degradation of the CCD charge transfer efficiency after 4 years in space., 12 pages, 14 figures, submitted to A&A

In direct imaging, broad band flat-fields can easily be applied to correct deviation from uniform sensitivity across the detector field. However for slitless spectroscopy data the flat field is both field and wavelength dependent. The effect of the wavelength dependent flat field for the slitless G800L grism mode of the ACS Wide Field Channel (WFC) has been investigated from observations of a flux calibrator at different positions in the field. The results of various flat-fielding schemes are presented including application of a flat field cube derived from in-orbit broad band filter flat fields. Excellent results are reported with deviations in the extracted spectra, Proceedings of the 2002 HST Calibration Workshop

We present the wavelength solution derived for the G800L grism with the Wide Field Channel from the spectra of two Galactic Wolf-Rayet stars, WR 45 and WR 96. The data were acquired in-orbit during the SMOV tests and the early INTERIM calibration program. We have obtained an average dispersion of 39.2 A/pix in the first order, 20.5 A/pix in the second and -42.5 A/pix in the negative first order. We show that the wavelength solution is strongly field-dependent, with an amplitude of the variation of about 11% from the center of the WFC aperture to the corners. The direction of the field-dependence is the diagonal from the image left top corner (amplifier A) to the bottom right corner (amplifier D). These trends are observed for all grism orders. We also describe the calibration files derived from the SMOV and INTERIM data which are used by the ST-ECF slitless extraction code aXe., Proceeding for The 2002 HST Calibration Workshop, 9 pages

The measurement of cosmic shear requires deep imaging with high image quality on many lines of sight to sample the statistics of large-scale structure. The expected distortion of galaxy images by cosmic shear on the STIS angular scale is a few percent, therefore the PSF anisotropy has to be understood and controlled to an accuracy better than 1%. In this poster we present the analysis of the PSF of STIS and a preliminary cosmic shear measurement using archival data from the STIS pure parallel program to show that the STIS camera on-board HST is well suited for our project. The data reduction and catalog production are described in an accompanying paper (astro-ph/0102330)., Comment: 4 pages, 5 figures, to appear in the proceedings of the STScI 2001 spring symposium "The Dark Universe: Matter, Energy and Gravity" Baltimore April 2-5 2001, acknowledgements added

Since the Universe is inhomogeneous on scales well below the Hubble radius, light bundles from distant galaxies are deflected and distorted by the tidal gravitational field of the large-scale matter distribution as they propagate through the Universe. Two-point statistical measures of the observed ellipticities, like the dispersion within a finite aperture or the ellipticity cross-correlation, can be related to the power spectrum of the large-scale structure. The measurement of cosmic shear (especially on small angular scales) can thus be used to constrain cosmological parameters and to test cosmological structure formation in the non-linear regime, without any assumptions about the relation between luminous and dark matter. In this paper we will present preliminary cosmic shear measurements on sub-arcminute scales, obtained from archival STIS parallel data. The high angular resolution of HST, together with the sensitivity and PSF-stability of STIS, allows us to measure cosmic shear along many independent lines-of-sight. Ongoing STIS parallel observations, currently being carried out in the frame of a big GO program (8562+9248), will greatly increase the number of available useful fields and will enable us to measure cosmic shear with higher accuracy on sub-arcminute scales., 5 pages, 4 figures, to be published in "Where's the Matter? Tracing Dark and Bright Matter with the New Generation of Large Scale Surveys", June 2001, Treyer & Tresse Eds., Frontier Group, added acknowledgements

In June 1997, parallel observations using the Space Telescope Imaging Spectrograph (STIS) on the HST started to be taken in substantial numbers along many different lines-of-sight. We are using the imaging data to investigate the distortion of background galaxies by the gravitational field of the large scale matter distribution, also known as Cosmic Shear. This poster presents the data and the catalog production that leads to the cosmic shear result presented in the poster "First Cosmic Shear results from STIS parallel program archive data" (Haemmerle et al., this conference). The data is publicly available also at http://www.stecf.org/projects/shear ., Comment: 4 pages, 2 figures, to appear in the proceedings of the STScI 2001 spring symposium "The Dark Universe: Matter, Energy and Gravity" Baltimore April 2-5 2001. Poster in Original format available at http://www.astro.uni-bonn.de/~miralles/Balt_post.htm

We report on the marginal detection of cosmic shear on sub-arcmin scales with archive data from the STIS camera on board HST. For the measurement 121 galaxy fields with a field of view of 51'' x 51'' are used to obtain an rms cosmic shear of ~ 4% with 1.5sigma significance. This value is consistent with groundbased results obtained by other groups on larger scales, and with theoretical predictions for a standard LambdaCDM cosmology. To show the suitability of STIS for weak shear measurements we carefully investigated the stability of the PSF. We demonstrate that small temporal changes do not affect the cosmic shear measurement by more than ~ 10%. We also discuss the influence of various weighting and selection schemes for the galaxy ellipticities., Comment: 14 pages, 15 figures, replaced to match version accepted by A&A, minor changes to the text, last figure replaced

We present deep unbiased spectroscopy of the Hubble Ultra Deep Field (UDF) carried out using the slitless grism spectroscopy mode of the Advance Camera for Surveys on the Hubble Space Telescope (HST). The GRIsm ACS Program for Extragalactic Science (GRAPES) achieves continuum detection as faint as $z_{AB}=27.2$ using 40 orbits ($9.2 \times 10^4$ seconds) 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 $5500��< ��< 10500��$, and allow us to detect high redshift galaxies at $4 < z < 7$ whether they have $\lya$ lines or just show the Lyman Break, as well as find low luminosity AGNs 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., Accepted by ApJS

We explore the effects of stellar population models on estimating star formation histories, ages and masses of high redshift galaxies. The focus is on the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) phase of stellar evolution, whose treatment is a source of major discrepancy among different evolutionary population synthesis. In particular, besides the models usually adopted in the literature, we use models (by Maraston 2005), in which the contribution of the TP-AGB phase is observationally calibrated and is the dominant source of near-IR energy for stellar populations in the age range 0.2 to 2 Gyr. We use a sample of high-z galaxies in the HUDF, with spectroscopic redshifts, and Spitzer IRAC and MIPS photometry from GOODS. We find that the TP-AGB phase plays a key role in the interpretation of Spitzer data for high-z galaxies, when the rest-frame near-IR is sampled. When fitting without dust reddening, the models with the empirically-calibrated TP-AGB phase always reproduce better the observed spectral energy distributions (SEDs). Allowing for dust reddening improves the fits with literature models in some cases. In both cases, the results from Maraston models imply younger ages by factors up to 6 and lower stellar masses (by ~60 % on average). The observed strengths of the MgUV spectral feature compare better to the predicted ones in the case of the Maraston models, implying a better overall consistency of SED fitting. Finally, we find that photometric redshifts improve significantly using these models on the SEDs extending over the IRAC bands. This work provides the first direct evidence of TP-AGB stars in the primeval Universe., 14 pages, 10 figures, 3 tables, submitted to the Astrophysical Journal