arXiv subjects

Stefano Casertano

Publications and source records attributed to Stefano Casertano.

86 records · Page 5Linked to original sources

arXiv2006

Faint U-Band Dropouts in the WFPC2 Parallels of the Hubble Ultra Deep Field

We combine data from the extremely deep Hubble Space Telescope $U$ (F300W) image obtained using WFPC2 as part of the parallel observations of the Hubble Ultra Deep Field campaign, with $BVi$ images from the Great Observatories Origins Deep Survey (GOODS) to identify a sample of Lyman break galaxies in the redshift range $2.0 \lesssim z \lesssim 3.5$. We use recent stellar population synthesis models with a wide variety of ages, metallicities, redshifts, and dust content, and a detailed representation of the \HI cosmic opacity as a function of redshift to model the colors of galaxies in our combination of WFPC2/ACS filters. Using these models, we derive improved color selection criteria that provide a clean selection of relatively unobscured, star forming galaxies in this redshift range. Our WFPC2/F300W image is the deepest image ever obtained at that wavelength. The $10σ$ limiting magnitude measured over 0.2 arcsec$^2$ is 27.5 magnitudes in the WFPC2/F300W image, about 0.5 magnitudes deeper than the F300W image in the Hubble Deep Field (HDF)-N. This extra depth relative to the HDFs allows us to directly probe the luminosity function about 0.5 magnitudes deeper than the depth accessible with the HDF data along an independent line of sight. Our sample of star-forming galaxies with $2.0 \lesssim z \lesssim 3.5$ includes 125 objects, the majority of which show clumpy morphologies. We measure a star formation rate density of 0.18 $M_\odot \rm yr^{-1} Mpc^{-3}$, marginally higher than the value measured for the Hubble Deep Fields.

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arXiv2005

The Fundamental Plane of Quasars

We present results from an archival study of 70 medium-redshift QSOs observed with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. The QSOs have magnitudes M_V < -23 (total nuclear plus host light) and redshifts 0.06 < z < 0.46. A close relationship between QSO host and nucleus is found by examining multiple parameters at once. A principal components analysis shows that 3 nuclear and host properties are related in a kind of fundamental plane: nuclear luminosity and the size and effective surface magnitude of the bulge. Using optical nuclear luminosity, this relationship explains 95.9% of the variance in the overall sample, while 94.9% of the variance is accounted for if we use x-ray nuclear luminosity. The form of this QSO fundamental plane shows similarities to the well-studied fundamental plane of elliptical galaxies, and we examine the possible relationship between them as well as the difficulties involved in establishing this connection. The key to the relationship might lie in the fueling mechanism of the central black hole.

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arXiv2005

The WFPC2 Archival Parallels Project

We describe the methods and procedures developed to obtain a near-automatic combination of WFPC2 images obtained as part of the WFPC2 Archival Pure Parallels program. Several techniques have been developed or refined to ensure proper alignment, registration, and combination of overlapping images that can be obtained at different times and with different orientations. We quantify the success rate and the accuracy of the registration of images of different types, and we develop techniques suitable to equalize the sky background without unduly affecting extended emission. About 600 combined images of the 1,500 eventually planned have already been publicly released through the STScI Archive. The images released to date are especially suited to study star formation in the Magellanic Clouds, the stellar population in the halo of nearby galaxies, and the properties of star-forming galaxies at $ z \sim 3 $.

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arXiv2005

Rest-frame Ultraviolet-to-Optical Properties of Galaxies at z ~ 6 and 5 in the Hubble Ultra Deep Field: from Hubble to Spitzer

We use data from the first epoch of observations with the IRAC/Spitzer for the GOODS to detect and study a collection of LBGs at z ~ 6 to 5 in the HUDF, six of which have spectroscopic confirmation. At these redshifts, IRAC samples rest-frame optical light in the range 0.5 to 0.8 um, where the effects of dust extinction are smaller and the sensitivity to light from evolved stars is greater than at shorter, rest-frame UV observable from the ground or with the HST. As such, it provides useful constraints on the ages and masses of these galaxies' stellar populations. We find that the SEDs for many of these galaxies are best fitted by models of stellar populations with masses of a few x 1e10 M_sun, and with ages of a few hundred Myrs, values quite similar to those derived for typical LBGs at z ~ 3. When the universe was only 1 Gyr old, some galaxies had already formed a mass of stars approaching that of the present-day Milky Way, and that they started forming those stars at z > 7, and in some cases much earlier. We find that the lower limits to the space density for galaxies in this mass range are consistent with predictions from recent hydrodynamic simulations of structure formation in a LCDM universe. All objects in our samples are consistent with having solar metallicity, suggesting that they might have already been significantly polluted by metals. The values for dust reddening derived from the model fitting are low or zero, and we find that some of the galaxies have rest-frame UV colors that are even bluer than those predicted by the stellar population models to which we compare them. These colors might be attributed to the presence of very massive stars (> 100 M_sun), or by weaker intergalactic HI absorption than what is commonly assumed.

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arXiv2004

High-Redshift Extremely Red Objects in the HST Ultra Deep Field Revealed by the GOODS IRAC Observations

Using early data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope, taken for the Great Observatories Origins Deep Survey (GOODS), we identify and study objects that are well-detected at 3.6um, but are very faint (and in some cases, invisible) in the Hubble Ultra Deep Field (HUDF) ACS and NICMOS images and in very deep VLT Ks-band imaging. We select a sample of 17 objects with f_nu(3.6um)/f_nu(z_{850})>20. The analysis of their spectral energy-distributions (SEDs) from 0.4 to 8.0um shows that the majority of these objects cannot be satisfactorily explained without a well-evolved stellar population. We find that most of them can be well fitted by a simple two-component model, where the primary component represents a massive, old population that dominates the strong IR emission, while the secondary component represents a low amplitude, on-going star-formation process that accounts for the weak optical fluxes. Their estimated photometric redshifts (z_p) range from 1.6 to 2.9, with the median at z_p=2.4. For the simple star formation histories considered here, their corresponding stellar masses range from 0.1 to 1.6e11 M_sun for a Chabrier Initial Mass Function (IMF). Their median rest-frame Ks-band absolute magnitude is -22.9 mag in AB system, or 1.5xL*(K) for present--day elliptical galaxies. In the scenario of pure luminosity evolution, such objects may be direct progenitors for at least 14 to 51% of the local population of early--type galaxies. Due to the small cosmic volume of the HUDF, however, this simple estimate could be affected by other effects such as cosmic variance and the strong clustering of massive galaxies. A full analysis of the entire GOODS area is now underway to assess such effects.

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arXiv2004

Type Ia Supernova Discoveries at z>1 From the Hubble Space Telescope: Evidence for Past Deceleration and Constraints on Dark Energy Evolution

We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. These objects, discovered during the course of the GOODS ACS Treasury program, include 6 of the 7 highest-redshift SNe Ia known, all at z>1.25, and populate the Hubble diagram in unexplored territory. The luminosity distances to these and 170 previous SNe Ia are provided. A purely kinematic interpretation of the SN Ia sample provides evidence at the > 99% confidence level for a transition from deceleration to acceleration or similarly, strong evidence for a cosmic jerk. Using a simple model of the expansion history, the transition between the two epochs is constrained to be at z=0.46 +/- 0.13. The data are consistent with the cosmic concordance model of Omega_M ~ 0.3, Omega_Lambda~0.7 (chi^2_dof=1.06), and are inconsistent with a simple model of evolution or dust as an alternative to dark energy. For a flat Universe with a cosmological constant. When combined with external flat-Universe constraints we find w=-1.02 + 0.13 - 0.19 (and $<-0.76 at the 95% confidence level) for an assumed static equation of state of dark energy, P = wρc^2. Joint constraints on both the recent equation of state of dark energy, $w_0$, and its time evolution, dw/dz, are a factor of ~8 more precise than its first estimate and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w_0 = -1.0, dw/dz = 0), and are inconsistent with very rapid evolution of dark energy. We address consequences of evolving dark energy for the fate of the Universe.

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arXiv2003

Cosmic Gravitational Shear from the HST Medium Deep Survey

We present a measurement of cosmic shear on scales ranging from $ 10\arcsec $ to $ 2\arcmin $ in 347 WFPC2 images of random fields. Our result is based on shapes measured via image fitting and on a simple statistical technique; careful calibration of each step allows us to quantify our systematic uncertainties and to measure the cosmic shear down to very small angular scales. The WFPC2 images provide a robust measurement of the cosmic shear signal decreasing from $5.2% $ at $ 10\arcsec $ to $2.2% $ at $ 130\arcsec $.

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arXiv2003

Optimal Galaxy Shape Measurements for Weak Lensing Applications Using the Hubble Space Telescope Advanced Camera for Surveys

We present three-epoch multiband ($V_{606}$, $i_{775}$, $z_{850}$) measurements of galaxy shapes using the ``polar shapelet'' or Laguerre-expansions method with the Hubble Space Telescope ($HST$) Advanced Camera for Surveys (ACS) data, obtained as part of the {\it Great Observatories Origin Deep Survey} (GOODS). We take advantage of the unique features of the GOODS/ACS Fields to test the reliability of this relatively new method of galaxy shape measurement for weak lensing analysis and to quantify the impact of the ACS Point Spread Function (PSF) on $HST$/ACS data. We estimate the bias introduced by the sharp PSF of the ACS on shape measurement. We show that the bias in the tangential shear due to galaxy-galaxy lensing can be safely neglected provided only well-resolved galaxies are used, while it would be comparable to the signal level (1--3%) for cosmic shear measurements. These results should of be general utility in planning and analyzing weak lensing measurements with $HST$/ACS data.

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arXiv2003

Morphologies and Spectral Energy Distributions of Extremely Red Galaxies in the GOODS-South Field

Using U'- through Ks-band imaging data in the GOODS-South field, we construct a large, complete sample of 275 ``extremely red objects'' (EROs; K_s<22.0, R-K_s>3.35; AB), all with deep HST/ACS imaging in B_435, V_606, i_775, and z_850, and well-calibrated photometric redshifts. Quantitative concentration and asymmetry measurements fail to separate EROs into distinct morphological classes. We therefore visually classify the morphologies of all EROs into four broad types: ``Early'' (elliptical-like), ``Late'' (disk galaxies), ``Irregular'' and ``Other'' (chain galaxies and low surface brightness galaxies), and calculate their relative fractions and comoving space densities. For a broad range of limiting magnitudes and color thresholds, the relative number of early-type EROs is approximately constant at 33-44%, and the comoving space densities of Early- and Late-type EROs are comparable. Mean rest-frame spectral energy distributions (SEDs) at wavelengths between 0.1 and 1.2 um are constructed for all EROs. The SEDs are extremely similar in their range of shapes, independent of morphological type. The implication is that any differences between the broad-band SEDs of Early-type EROs and the other types are relatively subtle, and there is no robust way of photometrically distinguishing between different morphological types with usual optical/near-infrared photometry.

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arXiv2003

Identification of Type Ia Supernovae at Redshift 1.3 and Beyond with the Advanced Camera for Surveys on HST

We present discoveries of SNe Ia at z > 1 and the photometric diagnostic used to discriminate them from other types of SNe detected during the GOODS Hubble Space Telescope Treasury program with the Advanced Camera for Surveys (ACS). Photometric redshift measurements of the hosts combined with deep f606w, f775w, and f850lp imaging discriminates hydrogen-rich SNe II from SNe I at z > 1 by exploiting the ultraviolet (UV) deficit in the energy distributions of SNe Ia. This sorting allows rapid follow-up of space-based discoveries. Subsequent spectroscopy of 11 GOODS SNe Ia obtained from the ground and with the grism on ACS confirmed the reliability of our photometric screening. We present the highest-redshift spectrum of any supernova published to date, SN Ia 2002fw at z = 1.3 observed near maximum brightness with the ACS grism. The lack of UV flux for true SNe Ia provides an effective tool for our ongoing efforts to build a sample of SNe Ia at 1.2 < z < 1.8 which will extend the useful range of the magnitude-redshift relation of SNe Ia.

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arXiv2002

The Hubble Deep Field South Flanking Fields

As part of the Hubble Deep Field South program, a set of shorter 2-orbit observations were obtained of the area adjacent to the deep fields. The WFPC2 flanking fields cover a contiguous solid angle of 48 square arcminutes. Parallel observations with the STIS and NICMOS instruments produce a patchwork of additional fields with optical and near-infrared (1.6 micron) response. Deeper parallel exposures with WFPC2 and NICMOS were obtained when STIS observed the NICMOS deep field. These deeper fields are offset from the rest, and an extended low surface brightness object is visible in the deeper WFPC2 flanking field. In this data paper, which serves as an archival record of the project, we discuss the observations and data reduction, and present SExtractor source catalogs and number counts derived from the data. Number counts are broadly consistent with previous surveys from both ground and space. Among other things, these flanking field observations are useful for defining slit masks for spectroscopic follow-up over a wider area around the deep fields, for studying large-scale structure that extends beyond the deep fields, for future supernova searches, and for number counts and morphological studies, but their ultimate utility will be defined by the astronomical community.

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arXiv2002

The Luminosity Function of QSO Host Galaxies

We present some results from our HST archival image study of 71 QSO host galaxies. The objects are selected to have z < 0.46 and total absolute magnitude M_V < -23 (H_0=50 km/s/Mpc). This initial study investigates host morphology and radio loudness, and it derives the QSO host galaxy luminosity function. We have analyzed available WFPC2 broad band images, using a uniform set of procedures. The host galaxies span a narrow range of luminosities and are exceptionally bright, much more so than normal galaxies, usually L > L*_V. The QSOs are almost equally divided among radio-loud QSOs with elliptical hosts, radio-quiet QSOs with elliptical hosts, and radio-quiet QSOs with spiral hosts. Using a weighting procedure, we derive the combined luminosity function of QSO host galaxies. We find that the luminosity function of QSO hosts differs in shape from that of normal galaxies but that they coincide at the highest luminosities. The ratio of the number of quasar hosts to the number of normal galaxies at a luminosity L_V is R = [L_V/(11.48 L*_V)]^2.46, where L*_V corresponds to M*_V = -22.35, and a QSO is defined to be an object with total nuclear plus host light M_V < -23. This ratio can be interpreted as the probability that a galaxy with luminosity L_V will host a QSO at redshift z ~ 0.26. (Abridged)

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arXiv2001

The Farthest Known Supernova: Support for an Accelerating Universe and a Glimpse of the Epoch of Deceleration

We present photometric observations of an apparent Type Ia supernova (SN Ia) at a redshift of ~1.7, the farthest SN observed to date. SN 1997ff, was discovered in a repeat observation by the HST of the HDF-), and serendipitously monitored with NICMOS on HST throughout the GTO campaign. The SN type can be determined from the host galaxy type:an evolved, red elliptical lacking enough recent star formation to provide a significant population of core-collapse SNe. The class- ification is further supported by diagnostics available from the observed colors and temporal behavior of the SN, both of which match a typical SN Ia. The photo- metric record of the SN includes a dozen flux measurements in the I, J, and H bands spanning 35 days in the observed frame. The redshift derived from the SN photometry, z=1.7+/-0.1, is in excellent agreement with the redshift estimate of z=1.65+/-0.15 derived from the U_300,B_450,V_606,I_814,J_110,J_125,H_160, H_165,K_s photometry of the galaxy. Optical and near-infrared spectra of the host provide a very tentative spectroscopic redshift of 1.755. Fits to observations of the SN provide constraints for the redshift-distance relation of SNe~Ia and a powerful test of the current accelerating Universe hypothesis. The apparent SN brightness is consistent with that expected in the decelerating phase of the preferred cosmological model, Omega_M~1/3, Omega_Lambda~2/3. It is inconsistent with grey dust or simple luminosity evolution, candidate astro- physical effects which could mimic past evidence for an accelerating Universe from SNe Ia at z~0.5.We consider several sources of possible systematic error including lensing, SN misclassification, selection bias, and calibration errors. Currently, none of these effects appears likely to challenge our conclusions.

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arXiv2000

WFPC2 Observations of the Hubble Deep Field-South

The Hubble Deep Field-South observations targeted a high-galactic-latitude field near QSO J2233-606. We present WFPC2 observations of the field in four wide bandpasses centered at roughly 300, 450, 606, and 814 nm. Observations, data reduction procedures, and noise properties of the final images are discussed in detail. A catalog of sources is presented, and the number counts and color distributions of the galaxies are compared to a new catalog of the HDF-N that has been constructed in an identical manner. The two fields are qualitatively similar, with the galaxy number counts for the two fields agreeing to within 20%. The HDF-S has more candidate Lyman-break galaxies at z > 2 than the HDF-N. The star-formation rate per unit volume computed from the HDF-S, based on the UV luminosity of high-redshift candidates, is a factor of 1.9 higher than from the HDF-N at z ~ 2.7, and a factor of 1.3 higher at z ~ 4.

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