Jennifer Mack

The Photometric Performance and Calibration of the Hubble Space Telescope Advanced Camera for Surveys

ABSTRACT We present the photometric calibration of the Advanced Camera for Surveys (ACS). The ACS was installed in the Hubble Space Telescope (HST) in 2002 March. It comprises three cameras: the Wide Field Channel (WFC), optimized for deep near‐IR survey imaging programs; the High Resolution Channel (HRC), a high‐resolution imager that fully samples the HST point‐spread function (PSF) in the visible; and the Solar Blind Channel (SBC), a far‐UV imager. A significant amount of data has been collected to characterize the on‐orbit performance of the three channels. We give here an overview of the performance and calibration of the two CCD cameras (WFC and HRC) and a description of the best techniques for reducing ACS CCD data. The overall performance is as expected from prelaunch testing of the camera. Surprises were a better‐than‐predicted sensitivity in the visible and near‐IR for both the WFC and HRC and an unpredicted dip in the HRC UV response at ∼3200 A. On‐orbit observations of spectrophotometric stand...

WFPC2 Observations of the Hubble Deep Field South

The Hubble Deep Field South (HDF-S) observations targeted a high Galactic latitude field near QSO J2233-606. We present Wide Field Planetary Camera 2 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 with a new catalog of the original Hubble Deep Field (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.

Dynamical Friction in dE Globular Cluster Systems

The dynamical friction timescale for globular clusters to sink to the center of a dwarf elliptical galaxy (dE) is significantly less than a Hubble time if the halos have King-model or isothermal profiles and the globular clusters formed with the same radial density profile as the underlying stellar population. We examine the summed radial distribution of the entire globular cluster systems and the bright globular cluster candidates in 51 Virgo and Fornax Cluster dEs for evidence of dynamical friction processes. We find that the summed distribution of the entire globular cluster population closely follows the exponential profile of the underlying stellar population. However, there is a deficit of bright clusters within the central regions of dEs (excluding the nuclei), perhaps due to the orbital decay of these massive clusters into the dE cores. We also predict the nuclear magnitude of each dE assuming that the nuclei form via dynamical friction. The observed trend of decreasing nuclear luminosity with decreasing dE luminosity is much stronger than predicted if the nuclei formed via simple dynamical friction processes. We find that the bright dE nuclei could have been formed from the merger of orbitally decayed massive clusters, but the faint nuclei are several magnitudes fainter than expected. These faint nuclei are found primarily in MV > -14 dEs, which have high globular cluster specific frequencies and extended globular cluster systems. In these galaxies, supernova-driven winds, high central dark matter densities, extended dark matter halos, the formation of new star clusters, or tidal interactions may act to prevent dynamical friction from collapsing the entire globular cluster population into a single bright nucleus.

The Hubble Deep Field South: Formulation of the Observing Campaign

Deep, multiband observations of high Galactic latitude fields are an essential tool for studying topics ranging from Galactic structure to extragalactic background radiation. The Hubble Deep Field (HDF-N) observations obtained in 1995 December established a standard for such narrow, deep surveys. The field has been extensively analyzed by a variety of groups and has been widely studied with imaging and spectroscopy over wavelengths ranging from 10-3 to 2 × 105 μm. We describe here a second deep field campaign (HDF-S), this time in the southern hemisphere, undertaken by the Hubble Space Telescope (HST) in 1998 October in a program very similar to the northern Hubble Deep Field. Imaging and spectroscopy of three adjacent fields in the southern continuous viewing zone were obtained simultaneously for 150 orbits, and a mosaic of flanking fields was imaged for 27 additional orbits. Two important features of the HDF-S distinguish it from the HDF-N: the campaign included parallel observations by the three main HST instruments—WFPC2, STIS, and NICMOS—and the HDF-S location was selected to place a bright z = 2.24 quasar in the STIS field of view. The HDF-S observations consist of WFPC2 images in filters close to U, B, V, and I, a deep STIS image of the field surrounding the quasar, spectroscopy of the quasar with STIS from 1150 to 3560 A, and deep imaging of an adjacent field with NICMOS camera 3 at 1.1, 1.6, and 2.2 μm. All of the HDF-S data were fully reduced and made publicly available within 2 months of the observations, and we describe here the selection of the fields and the observing strategy that was employed. Detailed descriptions of the data and the reduction techniques for each field, together with the corresponding source catalogs, appear in separate papers.

I Zw 18 Revisited with HST ACS and Cepheids: New Distance and Age*

We present new V- and I-band HST ACS photometry of I Zw 18, the most metal-poor blue compact dwarf (BCD) galaxy in the nearby universe. It has been argued in the past that I Zw 18 is a very young system that started forming stars only 500 Myr ago, but other work has hinted that older (1 Gyr) red giant branch (RGB) stars may also exist. Our new data, once combined with archival HST ACS data, provide a deep and uncontaminated optical color-magnitude diagram (CMD) that now strongly indicates an RGB. The RGB tip (TRGB) magnitude yields a distance modulus (m - M)0 = 31.30 ± 0.17, i.e., D = 18.2 ± 1.5 Mpc. The time-series nature of our observations allows us to also detect and characterize for the first time three classical Cepheids in I Zw 18. The time-averaged Cepheid V and I magnitudes are compared to the VI reddening-free Wesenheit relation predicted from new nonlinear pulsation models specifically calculated at the metallicity of I Zw 18. For the one bona fide classical Cepheid with a period of 8.63 days this implies a distance modulus (m - M)0 = 31.42 ± 0.26. The other two Cepheids have unusually long periods (125.0 and 129.8 days) but are consistent with this distance. The coherent picture that emerges is that I Zw 18 is farther away than previously assumed and older than suggested by some previous works. The presence of an RGB population rules out the possibility that I Zw 18 is a truly primordial galaxy formed recently (z 0.1) in the local universe.

The Frontier Fields: Survey Design

The Frontier Fields are a directors discretionary time campaign with HST and the Spitzer Space Telescope to see deeper into the universe than ever before. The Frontier Fields combine the power of HST and Spitzer with the natural gravitational telescopes of massive high-magnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. Six clusters - Abell 2744, MACSJ0416.1-2403, MACSJ0717.5+3745, MACSJ1149.5+2223, Abell S1063, and Abell 370 - were selected based on their lensing strength, sky darkness, Galactic extinction, parallel field suitability, accessibility to ground-based facilities, HST, Spitzer and JWST observability, and pre-existing ancillary data. These clusters have been targeted by the HST ACS/WFC and WFC3/IR with coordinated parallels of adjacent blank fields for over 840 HST orbits. The Spitzer Space Telescope has dedicated > 1000 hours of directors discretionary time to obtain IRAC 3.6 and 4.5 micron imaging to ~26.5, 26.0 ABmag 5-sigma point-source depths in the six cluster and six parallel Frontier Fields. The Frontier Field parallel fields are the second-deepest observations thus far by HST with ~29th ABmag 5-sigma point source depths in seven optical - near-infrared bandpasses. Galaxies behind the Frontier Field cluster lenses experience typical magnification factors of a few, with small regions near the critical curves magnified by factors 10-100. Therefore, the Frontier Field cluster HST images achieve intrinsic depths of ~30-33 magnitudes over very small volumes. Early studies of the Frontier Fields have probed galaxies fainter than any seen before during the epoch of reionization 6 < z < 10, mapped out the cluster dark matter to unprecedented resolution, and followed lensed transient events.

Distant Cluster Hunting. II; A Comparison of X-Ray and Optical Cluster Detection Techniques and Catalogs from the ROSAT Optical X-Ray Survey

We present and analyze the optical and X-ray catalogs of moderate-redshift cluster candidates from the ROSAT Optical X-Ray Survey, or ROXS. The survey covers the sky area contained in the fields of view of 23 deep archival ROSAT PSPC pointings, 4.8 square degrees. The cross-correlated cluster catalogs were constructed by comparing two independent catalogs extracted from the optical and X-ray bandpasses, using a matched-filter technique for the optical data and a wavelet technique for the X-ray data. We cross-identified cluster candidates in each catalog. As reported in Paper I, the matched-filter technique found optical counterparts for at least 60% (26 out of 43) of the X-ray cluster candidates; the estimated redshifts from the matched filter algorithm agree with at least 7 of 11 spectroscopic confirmations (Δz 0.10). The matched filter technique, with an imaging sensitivity of mI ~ 23, identified approximately 3 times the number of candidates (155 candidates, 142 with a detection confidence >3 σ) found in the X-ray survey of nearly the same area. There are 57 X-ray candidates, 43 of which are unobscured by scattered light or bright stars in the optical images. Twenty-six of these have fairly secure optical counterparts. We find that the matched filter algorithm, when applied to images with galaxy flux sensitivities of mI ~ 23, is fairly well-matched to discovering z ≤ 1 clusters detected by wavelets in ROSAT PSPC exposures of 8000-60,000 s. The difference in the spurious fractions between the optical and X-ray (30% and 10%, respectively) cannot account for the difference in source number. In Paper I, we compared the optical and X-ray cluster luminosity functions and we found that the luminosity functions are consistent if the relationship between X-ray and optical luminosities is steep (LX ∝ L). Here, in Paper II, we present the cluster catalogs and a numerical simulation of the ROXS. We also present color-magnitude plots for several of the cluster candidates, and examine the prominence of the red sequence in each. We find that the X-ray clusters in our survey do not all have a prominent red sequence. We conclude that while the red sequence may be a distinct feature in the color-magnitude plots for virialized massive clusters, it may be less distinct in lower mass clusters of galaxies at even moderate redshifts. Multiple, complementary methods of selecting and defining clusters may be essential, particularly at high redshift where all methods start to run into completeness limits, incomplete understanding of physical evolution, and projection effects.

Distant Cluster Hunting: A Comparison Between the Optical and X-Ray Luminosity Functions from an Optical/X-Ray Joint Survey

We present a comparison of X-ray and optical luminosities and luminosity functions of cluster candidates from a joint optical/X-ray survey, the ROSAT Optical X-Ray Survey. Completely independent X-ray and optical catalogs of 23 ROSAT fields (4.8 deg2) were created by a matched-filter optical algorithm and by a wavelet technique in the X-ray. We directly compare the results of the optical and X-ray selection techniques. The matched-filter technique detected 74% (26 out of 35) of the most reliable cluster candidates in the X-ray-selected sample; the remainder could be either constellations of X-ray point sources or z > 1 clusters. The matched-filter technique identified approximately 3 times the number of candidates (152 candidates) found in the X-ray survey of nearly the same sky (57 candidates). While the estimated optical and X-ray luminosities of clusters of galaxies are correlated, the intrinsic scatter in this relationship is very large. We can reproduce the number and distribution of optical clusters with a model defined by the X-ray luminosity function and by an LX-Λcl relation if H0 = 75 km s-1 Mpc-1 and if the LX-Λcl relation is steeper than the expected LX ∝ Λ. On statistical grounds, a bimodal distribution of X-ray luminous and X-ray faint clusters is unnecessary to explain our observations. Follow-up work is required to confirm whether the clusters without bright X-ray counterparts are simply X-ray faint for their optical luminosity because of their low mass or youth or are a distinct population of clusters that do not, for some reason, have dense intracluster media. We suspect that these optical clusters are low-mass systems, with correspondingly low X-ray temperatures and luminosities, or that they are not yet completely virialized systems.

Starbursts in the Local Universe: New Hubble Space Telescope Advanced Camera for Surveys Observations of the Irregular Galaxy NGC 4449

We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V), and F814W (I) broadband filters, and in the F658N (Hα) narrowband filter. Our photometry includes ≈300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V 28, and ≈400,000 objects in the (V, I) CMD, down to I 27. A subsample of ≈200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (0.2 dex). Also, we detect an interesting resolved star cluster on the west side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Hα emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D = 3.82 ± 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I) broad-band filters, and in the F658N (Halpha) narrow-band filter. Our photometry includes ~300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V < 28, and ~400,000 objects in the (V, I) CMD, down to I < 27 . A subsample of ~200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (< 0.2 dex). Also, we detect an interesting resolved star cluster on the West side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Halpha emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D=3.82 \pm 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.

Starbursts in the Local Universe: new HST/ACS observations of the irregular galaxy NGC 4449

We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V), and F814W (I) broadband filters, and in the F658N (Hα) narrowband filter. Our photometry includes ≈300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V 28, and ≈400,000 objects in the (V, I) CMD, down to I 27. A subsample of ≈200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (0.2 dex). Also, we detect an interesting resolved star cluster on the west side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Hα emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D = 3.82 ± 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I) broad-band filters, and in the F658N (Halpha) narrow-band filter. Our photometry includes ~300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V < 28, and ~400,000 objects in the (V, I) CMD, down to I < 27 . A subsample of ~200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (< 0.2 dex). Also, we detect an interesting resolved star cluster on the West side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Halpha emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D=3.82 \pm 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.