Galaxy Half-light Radius Research Articles

Rest-frame Near-infrared Sizes of Galaxies at Cosmic Noon: Objects in JWST's Mirror Are Smaller than They Appeared

Galaxy sizes and their evolution over cosmic time have been studied for decades and serve as key tests of galaxy formation models. However, at z ≳ 1 these studies have been limited by a lack of deep, high-resolution rest-frame infrared imaging that accurately traces stellar mass distributions. Here, we leverage the new capabilities of the James Webb Space Telescope (JWST) to measure the 4.4 μm sizes of ∼1000 galaxies with and 1.0 ≤ z ≤ 2.5 from public CEERS imaging in the Extended Groth Strip deep field. We compare the sizes of galaxies measured from NIRCam imaging at 4.4 μm (λ rest ∼ 1.6 μm) with sizes measured at 1.5 μm (λ rest ∼ 5500 Å). We find that, on average, galaxy half-light radii are ∼9% smaller at 4.4 μm than 1.5 μm in this sample. This size difference is markedly stronger at higher stellar masses and redder rest-frame V − J colors: galaxies with M * ∼ 1011 M ⊙ have 4.4 μm sizes that are ∼30% smaller than their 1.5 μm sizes. Our results indicate that galaxy mass profiles are significantly more compact than their rest-frame optical light profiles at cosmic noon, and demonstrate that spatial variations in age and attenuation are important, particularly for massive galaxies. The trend we find here impacts our understanding of the size growth and evolution of galaxies, and suggests that previous studies based on rest-frame optical light may not have captured the mass-weighted structural evolution of galaxies. This paper represents a first step toward a new understanding of the morphologies of early massive galaxies enabled by JWST’s infrared window into the distant universe.

An excess of globular clusters in Ultra-Diffuse Galaxies formed through tidal heating

ABSTRACT To investigate the origin of elevated globular cluster (GC) abundances observed around Ultra-Diffuse Galaxies (UDGs), we simulate GC populations hosted by UDGs formed through tidal heating. Specifically, GC formation is modelled as occurring in regions of dense star formation. Because star formation-rate densities are higher at high redshift, dwarf galaxies in massive galaxy clusters, which formed most of their stars at high redshift, form a large fraction of their stars in GCs. Given that UDGs formed through environmental processes are more likely to be accreted at high redshift, these systems have more GCs than non-UDGs. In particular, our model predicts that massive UDGs have twice the GC mass of non-UDGs of similar stellar mass, in rough agreement with observations. Although this effect is somewhat diminished by GC disruption, we find that the relationship between GC mass fraction and cluster-centric distance, and the relationship between GC mass fraction and galaxy half-light radius are remarkably similar to observations. Among our model objects, both UDGs and non-UDGs present a correlation between halo mass and GC mass, although UDGs have lower dynamical masses at a given GC mass. Furthermore, because of the effectiveness of GC disruption, we predict that GCs around UDGs should have a more top heavy mass function than GCs around non-UDGs. This analysis suggests that dwarfs with older stellar populations, such as UDGs, should have higher GC mass fractions than objects with young stellar populations, such as isolated dwarfs.

A dark matter profile to model diverse feedback-induced core sizes of ΛCDM haloes

ABSTRACT We analyse the cold dark matter density profiles of 54 galaxy haloes simulated with Feedback In Realistic Environments (FIRE)-2 galaxy formation physics, each resolved within $0.5{{\ \rm per\ cent}}$ of the halo virial radius. These haloes contain galaxies with masses that range from ultrafaint dwarfs ($M_\star \simeq 10^{4.5}\, \mathrm{M}_{\odot }$) to the largest spirals ($M_\star \simeq 10^{11}\, \mathrm{M}_{\odot }$) and have density profiles that are both cored and cuspy. We characterize our results using a new, analytic density profile that extends the standard two-parameter Einasto form to allow for a pronounced constant density core in the resolved innermost radius. With one additional core-radius parameter, rc, this three-parameter core-Einasto profile is able to characterize our feedback-impacted dark matter haloes more accurately than other three-parameter profiles proposed in the literature. To enable comparisons with observations, we provide fitting functions for rc and other profile parameters as a function of both M⋆ and M⋆/Mhalo. In agreement with past studies, we find that dark matter core formation is most efficient at the characteristic stellar-to-halo mass ratio M⋆/Mhalo ≃ 5 × 10−3, or $M_{\star } \sim 10^9 \, \mathrm{M}_{\odot }$, with cores that are roughly the size of the galaxy half-light radius, rc ≃ 1−5 kpc. Furthermore, we find no evidence for core formation at radii $\gtrsim 100\ \rm pc$ in galaxies with M⋆/Mhalo < 5 × 10−4 or $M_\star \lesssim 10^6 \, \mathrm{M}_{\odot }$. For Milky Way-size galaxies, baryonic contraction often makes haloes significantly more concentrated and dense at the stellar half-light radius than DMO runs. However, even at the Milky Way scale, FIRE-2 galaxy formation still produces small dark matter cores of ≃ 0.5−2 kpc in size. Recent evidence for a ∼2 kpc core in the Milky Way’s dark matter halo is consistent with this expectation.

What determines the sizes of bars in spiral galaxies?

ABSTRACT I use volume- and mass-limited subsamples and recently published data from the Spitzer Survey of Stellar Structure in Galaxies (S4G) to investigate how the size of bars depends on galaxy properties. The known correlation between bar semimajor axis a and galaxy stellar mass (or luminosity) is actually bimodal: for $\log \, (M_{\star }/\mathrm{M}_{\odot })\lesssim 10.1$, bar size is almost independent of stellar mass ($a \propto M_{\star }^{0.1}$), while it is a strong function for higher masses ($a \propto M_{\star }^{0.6}$). Bar size is a slightly stronger function of galaxy half-light radius Re and (especially) exponential disc scale length h (a ∝ h0.8). Correlations between stellar mass and galaxy size can explain the bar-size–M⋆ correlation – but only for galaxies with $\log \, (M_{\star }/\mathrm{M}_{\odot })\lesssim 10.1$; at higher masses, there is an extra dependence of bar size on M⋆ itself. Despite theoretical arguments that the presence of gas can affect bar growth, there is no evidence for any residual dependence of bar size on (present-day) gas mass fraction. The traditional dependence of bar size on Hubble type (longer bars in early-type discs) can be explained as a side effect of stellar mass–Hubble-type correlations. Finally, I show that galaxy size (Re or h) can be modelled as a function of stellar mass and both bar presence and bar size: barred galaxies tend to be more extended than unbarred galaxies of the same mass, with larger bars correlated with larger sizes.

NIHAO XX: the impact of the star formation threshold on the cusp–core transformation of cold dark matter haloes

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the impact of the threshold for star formation on the response of the dark matter (DM) halo to baryonic processes. The fiducial NIHAO threshold, $n=10\, {\rm cm}^{-3}$, results in strong expansion of the DM halo in galaxies with stellar masses in the range $10^{7.5} < M_{star} < 10^{9.5} M_{\odot}$. We find that lower thresholds such as $n=0.1$ (as employed by the EAGLE/APOSTLE and Illustris/AURIGA projects) do not result in significant halo expansion at any mass scale. Halo expansion driven by supernova feedback requires significant fluctuations in the local gas fraction on sub-dynamical times (i.e., < 50 Myr at galaxy half-light radii), which are themselves caused by variability in the star formation rate. At one per cent of the virial radius, simulations with $n=10$ have gas fractions of $\simeq 0.2$ and variations of $\simeq 0.1$, while $n=0.1$ simulations have order of magnitude lower gas fractions and hence do not expand the halo. The observed DM circular velocities of nearby dwarf galaxies are inconsistent with CDM simulations with $n=0.1$ and $n=1$, but in reasonable agreement with $n=10$. Star formation rates are more variable for higher $n$, lower galaxy masses, and when star formation is measured on shorter time scales. For example, simulations with $n=10$ have up to 0.4 dex higher scatter in specific star formation rates than simulations with $n=0.1$. Thus observationally constraining the sub-grid model for star formation, and hence the nature of DM, should be possible in the near future.

ZENS. IV. SIMILAR MORPHOLOGICAL CHANGES ASSOCIATED WITH MASS QUENCHING AND ENVIRONMENT QUENCHING AND THE RELATIVE IMPORTANCE OF BULGE GROWTH VERSUS THE FADING OF DISKS*

ABSTRACT We use the low-redshift Zurich Environmental Study (ZENS) catalog to study the dependence of the quenched satellite fraction at 10 10.0 M &CircleDot; &rarr; 10 11.5 M &CircleDot; ?> , and of the morphological mix of these quenched satellites, on three different environmental parameters: group halo mass, halo-centric distance, and large-scale structure (LSS) overdensity. Within the two mass bins into which we divide our galaxy sample, the fraction of quenched satellites is more or less independent of halo mass and the surrounding LSS overdensity, but it increases toward the centers of the halos, as found in previous studies. The morphological mix of these quenched satellites is, however, constant with radial position in the halo, indicating that the well-known morphology–density relation results from the increasing fraction of quenched galaxies toward the centers of halos. If the radial variation in the quenched fraction reflects the action of two quenching processes, one related to mass and the other to environment, then the constancy with radius of the morphological outcome suggests that both have the same effect on the morphologies of the galaxies. Alternatively, mass and environment quenching may be two reflections of a single physical mechanism. The quenched satellites have larger bulge-to-total ratios (B/T) and smaller half-light radii than the star-forming satellites. The bulges in quenched satellites have very similar luminosities and surface brightness profiles, and any mass growth of the bulges associated with quenching cannot greatly change these quantities. The differences in the light-defined B/T and in the galaxy half-light radii are mostly due to differences in the disks, which have lower luminosities in the quenched galaxies. The difference in galaxy half-light radii between quenched and star-forming satellites is however larger than can be explained by uniformly fading the disks following quenching, and the quenched disks have smaller scale lengths than in star-forming satellites. This can be explained either by a differential fading of the disks with galaxy radius or the disks being generally smaller in the past, both of which would be expected in an inside-out disk growth scenario. The overall conclusion is that, at least at low redshifts, the structure of massive quenched satellites at these masses is produced by processes that operate before the quenching takes place. A comparison of our results with semianalytic models argues for a reduction in the efficiency of group halos in quenching their disk satellites and for mechanisms to increase the B/T of low-mass quenched satellites.

Non-parametric analysis of the rest-frame UV sizes and morphological disturbance amongstL*galaxies at 4 &lt;z&lt; 8

We present the results of a study investigating the sizes and morphologies of redshift 4 < z < 8 galaxies in the CANDELS GOODS-S, HUDF and HUDF parallel fields. Based on non-parametric measurements and incorporating a careful treatment of measurement biases, we quantify the typical size of galaxies at each redshift as the peak of the log-normal size distribution, rather than the arithmetic mean size. Parameterizing the evolution of galaxy half-light radius as $r_{50} \propto (1+z)^n$, we find $n = -0.20 \pm 0.26$ at bright UV-luminosities ($0.3L_{*(z=3)} < L < L_*$) and $n = -0.47 \pm 0.62$ at faint luminosities ($0.12L_* < L < 0.3L_*$). Furthermore, simulations based on artificially redshifting our z~4 galaxy sample show that we cannot reject the null hypothesis of no size evolution. We show that this result is caused by a combination of the size-dependent completeness of high-redshift galaxy samples and the underestimation of the sizes of the largest galaxies at a given epoch. To explore the evolution of galaxy morphology we first compare asymmetry measurements to those from a large sample of simulated single S\'ersic profiles, in order to robustly categorise galaxies as either `smooth' or `disturbed'. Comparing the disturbed fraction amongst bright ($M_{UV} \leq -20$) galaxies at each redshift to that obtained by artificially redshifting our z~4 galaxy sample, while carefully matching the size and UV-luminosity distributions, we find no clear evidence for evolution in galaxy morphology over the redshift interval 4 < z < 8. Therefore, based on our results, a bright ($M_{UV} \leq -20$) galaxy at z~6 is no more likely to be measured as `disturbed' than a comparable galaxy at z~4, given the current observational constraints.

NIHAO project – I. Reproducing the inefficiency of galaxy formation across cosmic time with a large sample of cosmological hydrodynamical simulations

We introduce project NIHAO (Numerical Investigation of a Hundred Astrophysical Objects), a set of 100 cosmological zoom-in hydrodynamical simulations performed using the gasoline code, with an improved implementation of the SPH algorithm. The haloes in our study range from dwarf to Milky Way masses, and represent an unbiased sampling of merger histories, concentrations and spin parameters. The particle masses and force softenings are chosen to resolve the mass profile to below 1% of the virial radius at all masses, ensuring that galaxy half-light radii are well resolved. Using the same treatment of star formation and stellar feedback for every object, the simulated galaxies reproduce the observed inefficiency of galaxy formation across cosmic time as expressed through the stellar mass vs halo mass relation, and the star formation rate vs stellar mass relation. We thus conclude that stellar feedback is the chief piece of physics required to limit the efficiency of star formation in galaxies less massive than the Milky Way.

GalPaK3D: A BAYESIAN PARAMETRIC TOOL FOR EXTRACTING MORPHOKINEMATICS OF GALAXIES FROM 3D DATA

ABSTRACT We present a method to constrain galaxy parameters directly from three-dimensional data cubes. The algorithm compares directly the data with a parametric model mapped in x , y , &lambda; ?> coordinates. It uses the spectral line-spread function and the spatial point-spread function (PSF) to generate a three-dimensional kernel whose characteristics are instrument specific or user generated. The algorithm returns the intrinsic modeled properties along with both an “intrinsic” model data cube and the modeled galaxy convolved with the 3D kernel. The algorithm uses a Markov Chain Monte Carlo approach with a nontraditional proposal distribution in order to efficiently probe the parameter space. We demonstrate the robustness of the algorithm using 1728 mock galaxies and galaxies generated from hydrodynamical simulations in various seeing conditions from 0.″6 to 1.″2. We find that the algorithm can recover the morphological parameters (inclination, position angle) to within 10% and the kinematic parameters (maximum rotation velocity) to within 20%, irrespectively of the PSF in seeing (up to 1.″2) provided that the maximum signal-to-noise ratio (S/N) is greater than ∼3 pixel−1 and that the ratio of galaxy half-light radius to seeing radius is greater than about 1.5. One can use such an algorithm to constrain simultaneously the kinematics and morphological parameters of (nonmerging) galaxies observed in nonoptimal seeing conditions. The algorithm can also be used on adaptive optics data or on high-quality, high-S/N data to look for nonaxisymmetric structures in the residuals.

Bent radio jets reveal a stripped interstellar medium in NGC 1272

We report the discovery of bent double jets in the elliptical galaxy NGC 1272, a member of the Perseus cluster. The jets have a radius of curvature of ~2 kpc, much smaller than the galaxy half-light radius of ~11 kpc. This bending is likely a result of ram pressure, and indicates that the intracluster gas enters deep within the galaxy and that the interstellar medium of NGC 1272 has been significantly removed. X-ray observations of the surrounding intracluster medium allow us to constrain the pressure within the jet. We find that the standard assumptions of equipartition often used in interpreting other jets underestimate the pressure in the jets by a factor ~30.

How tidal erosion has shaped the relation between globular cluster specific frequency and galaxy luminosity

We quantify to what extent tidal erosion of globular clusters (GCs) has contributed to the observed u-shaped relation between GC specific frequencies S_N and host galaxy luminosity M_V. We used our MUESLI code to calculate GC survival rates for typical early-type galaxy potentials covering a wide range of observed galaxy properties. We do this for isotropic and radially anisotropic GC velocity distributions. We find that the calculated GC survival fraction, f_s, depends linearly on the logarithm of the 3D mass density, rho_3D, within the galaxy's half light radius, with f_s proportional to (rho_3D)^(-0.17). For a given galaxy, survival rates are lower for radially anisotropic configurations than for the isotropic GC cases. We apply these relations to a literature sample of 219 early-type galaxies from Harris et al. (2013) in the range M_V=[-24.5:-15.5] mag. The expected GC survival fraction ranges from ~50% for the most massive galaxies with the largest radii to ~10% for the most compact galaxies. We find that intermediate luminosity galaxies M_V=[-20.5:-17.5] mag have the strongest expected GC erosion. Within the considered literature sample, the predicted GC survival fraction therefore defines a u-shaped relation with M_V, similar to the relation between specific frequency S_N and M_V. As a consequence, the u-shape of S_N vs. M_V gets erased almost entirely when correcting the S_N values for the effect of GC erosion. We conclude that tidal erosion is an important contributor to the u-shaped relation between GC specific frequency and host galaxy luminosity. It must be taken into account when inferring primordial star cluster formation efficiencies from observations of GC systems in the nearby universe.

EVIDENCE THAT GAMMA-RAY BURST 130702A EXPLODED IN A DWARF SATELLITE OF A MASSIVE GALAXY

GRB 130702A is a nearby long-duration gamma-ray burst (LGRB) discovered by the Fermi satellite whose associated afterglow was detected by the Palomar Transient Factory. Subsequent photometric and spectroscopic monitoring has identified a coincident broad-lined Type Ic supernova (SN), and nebular emission detected near the explosion site is consistent with a redshift of z=0.145. The SN-GRB exploded at an offset of ~7.6" from the center of an inclined r=18.1 mag red disk-dominated galaxy, and ~0.6" from the center of a much fainter r=23 mag object. We obtained Keck-II DEIMOS spectra of the two objects and find a 2{\sigma} upper limit on their line-of-sight velocity offset of ~<60 km/s. If we project the SN-GRB coordinates onto the plane of the inclined massive disk galaxy, the explosion would have a ~61+-10 kpc offset, or ~6 times the galaxy's half-light radius. This large estimated nuclear offset suggests that the faint source is not a star-forming region of the massive red galaxy but is instead a dwarf galaxy. The star-formation rate of the dwarf galaxy is ~0.05 solar masses per year, and we place an upper limit on its oxygen abundance of 12 + log(O/H) < 8.16 dex. The identification of an LGRB in a dwarf satellite of a massive, metal-rich primary galaxy suggests that recent detections of LGRBs spatially coincident with metal-rich galaxies may be, in some cases, superpositions.

STELLAR ARCHEOLOGY IN THE GALACTIC HALO WITH ULTRA-FAINT DWARFS. VII. HERCULES

We present the first time-series study of the ultra-faint dwarf galaxy Hercules. Using a variety of telescope/instrument facilities we secured about 50 V and 80 B epochs. These data allowed us to detect and characterize 10 pulsating variable stars in Hercules. Our final sample includes 6 fundamental-mode (ab-type) and 3 first overtone (c-type) RR Lyrae stars, and one Anomalous Cepheid. The average period of the ab-type RR Lyrae stars, < Pab >= 0.68 d (sigma = 0.03 d), places Hercules in the Oosterhoff II group, as found for almost the totality of the ultra-faint dwarf galaxies investigated so far for variability. The RR Lyrae stars were used to obtain independent estimates of the metallicity, reddening and distance to Hercules, for which we find: [Fe/H] = -2.30+-0.15 dex, E(B -V) = 0.09+-0.02 mag, and (m-M)o = 20.6+-0.1 mag, in good agreement with the literature values. We have obtained a V, B - V color-magnitude diagram (CMD) of Hercules that reaches V ~ 25 mag and extends beyond the galaxy's half-light radius over a total area of 40' {\times} 36'. The CMD and the RR Lyrae stars indicate the presence of a population as old and metal-poor as (at least) the Galactic globular clusters M68.

GALAXY PAIRS IN THE SLOAN DIGITAL SKY SURVEY. I. STAR FORMATION, ACTIVE GALACTIC NUCLEUS FRACTION, AND THE LUMINOSITY/MASS-METALLICITY RELATION

We present a sample of 1716 galaxies with companions within Δv <500 km s–1, rp < 80 h–1 70 kpc and stellar mass ratio 0.1 < M 1/M 2 < 10 from the Sloan Digital Sky Survey Data Release 4. The galaxy pairs are selected from the Main Galaxy Sample using stringent and well-understood criteria for redshift, spectral quality, available stellar masses, and metallicities. In agreement with previous studies, we find an enhancement in the star-formation rate (SFR) of galaxy pairs at projected separations <30-40 h–1 70 kpc. In addition, we find that this enhancement is highest (and extends to the greatest separations) for galaxies of approximately equal mass, the so-called major pairs. However, SFR enhancement can still be detected for a sample of galaxy pairs whose masses are within a factor of 10 of each other. Based on these results, we define a sample of close pairs (Δv <500 km s–1, rp < 30 h–1 70 kpc, and 0.1 < M 1/M 2 < 10) which we use to investigate interaction-induced effects in the luminosity-metallicity (LZ) relation. In agreement with the one previous study of the LZ relation in paired galaxies, we find an offset to lower metallicities (by ~0.1 dex) for a given luminosity for galaxies in pairs compared to the control sample. We also present the first mass-metallicity (MZ) relation comparison between paired galaxies and the field and again find an offset to lower metallicities (by ~0.05 dex) for a given mass. The smaller offset in the MZ relation indicates that both higher luminosities and lower metallicities may contribute to the shift of pairs relative to the control in the LZ relation. We show that the offset in the LZ relation depends on galaxy half-light radius, rh . Galaxies with rh 3 h–1 70 kpc and with a close companion show a 0.05-0.1 dex downward offset in metallicity compared to control galaxies of the same size. Larger galaxies do not show this offset and have LZ and MZ relations consistent with the control sample. We investigate the physical impetus behind this empirical dependence on rh and consider the galaxy's dynamical time and bulge fractions as possible causes. We conclude that the former is unlikely to be a fundamental driver of the offset in the LZ relation for paired galaxies, but that bulge fraction may play a role. Finally, we study the active galactic nucleus (AGN) fraction in both the pair and control sample and find that whilst selecting galaxies in different cuts of color and asymmetry yields different AGN fractions, the fraction for pairs and the control sample are consistent for a given set of selection criteria. This indicates that if AGNs are ignited as a result of interactions, this activity begins later than the close pairs stage (i.e. once the merger is complete).

Imprints of Environment on Cluster and Field Late-Type Galaxies atz ~ 1

We present a comparison of late-type galaxies (Sa and later) in intermediate-redshift clusters and in the field using images from the Advanced Camera for Surveys aboard the Hubble Space Telescope. Cluster and field galaxies are selected by matching photometric and spectroscopic catalogs of four cluster fields: Cl 0152� 1357, Cl 1056� 0337 (MS 1054), Cl 1604+4304, and Cl 1604+4321. Concentration, asymmetry, and clumpiness parameters are calculated for each galaxy in blue (F606Wor F625W) and red (F775Wor F814W) filters. Galaxy half-light radii, disk scale lengths, colorgradients, and overall color are compared. We find marginally significant differences in the asymmetry distributions of spiral and irregular galaxies in the X-ray-luminous and X-ray-faint clusters. The massive clusters contain fewer galaxies with large asymmetries. The physical sizes of the cluster and field populations are similar; no significant differences are found in half-light radii or disk scale lengths. The most significant dif

Toward a Unified Model for the “Diffuse Ionized Medium” in Normal and Starburst Galaxies

We analyze H$\alpha$ images and long-slit spectra of samples of normal and starburst galaxies to better understand the nature of the diffuse, low-surface-brightness gas in these galaxies. We find that in both samples there is a strong inverse correlation between the H$\alpha$ surface-brightness ($\Sigma_{H\alpha}$) and the [SII]/H$\alpha$ line ratio at a given location in the galaxy. However, the correlation for the starbursts is offset brightward by an order-of-magnitude in H$\alpha$ surface-brightness at a given line ratio. In contrast, we find that all the galaxies (starburst and normal alike) define a universal relation between line ratio and the relative H$\alpha$ surface brightness ($\Sigma_{H\alpha}/\Sigma_e$, where $\Sigma_e$ is the mean H$\alpha$ surface brightness within the galaxy half-light radius). We show that such a universal correlation is a natural outcome of a model in which the DIM is photoionized gas that has a characteristic thermal pressure ($P$) that is proportional to the mean rate of star-formation per unit area in the galaxy ($\Sigma_{SFR}$). Good quantitative agreement with the data follows if we require the constant of proportionality to be consistent with the values of $P$ and $\Sigma_{SFR}$ in the local disk of the Milky Way. Such a scaling between $P$ and $\Sigma_{SFR}$ may arise either because feedback from massive stars heats the ISM or because $\Sigma_{SFR}$ is determined (or limited) by the mean gas pressure.

DETECTION, PHOTOMETRY, AND COMPLETENESS OF FOCAS CATALOGS FOR THE HST MEDIUM-DEEP SURVEY

ABSTRACT We discuss the methods which we have developed and applied to optimize the detection efficiency of faint galaxies with the HST Wide Field and Planetary Cameras both prior and subsequent to the HST refurbishment mission in December 1993. Our methods are based on the FOCAS (Tyson and Jarvis 1979) automated detection software package, which we have applied to compile galaxy catalogs. Using this package, we examine photometry and detection efficiency as a function of image convolution filter width (rho). We measure completeness and reliability for the catalogs from image simulations, and we also compare a WF/PC data frame with an exposure of the same field obtained with the 3.6-m Canada-France-Hawaii Telescope. Further, we investigate the quality of galaxy scale lengths and axial ratios derived by FOCAS on WF/PC data. We summarize several results applicable to WF/PC and WFPC2 data. The object detection efficiency increases monotonically with increasing rho, independent of galaxy half-light radius (rhl), although only marginal improvement in efficiency occurs for rho > rhi. Secondly, for both WF/PC and WFPC2, the FOCAS 95% completeness limit shifts toward brighter apparent magnitude as a function of increasing rhl, best fit by a second order polynomial but roughly equivalent to a 0.8 mag per 1" increment in half-light radius. As a result of the elimination of the HST spherical aberration and the use of filters with higher throughput, the 95% completeness limits for WFPC2 F606W and F814W data are ≥ 1.1 mag fainter than those for WF/PC F555W and F785LP data, respectively, for a given integration time and sky background level. Fixed-aperture magnitudes grow fainter with increasing rhl for a given intrinsic flux. Finally, 1"-diamter aperture photometry in WF/PC and WFPC2 data serves to parameterize the detection efficiency fairly independently of galaxy scale-lengths for 0.2" ≤ rhl ≤ 2", i.e., the spread in detection efficiency over the range 0.2" ≤ rhl ≤ 2", for galaxies of a given fixed aperture magnitude, is mimimized when the diameter of the fixed aperture is 1".

The Hubble Space Telescope Medium Deep Survey with the Wide Field and Planetary Camera. 1: Methodology and results on the field near 3C 273

We present results from the Medium Deep Survey (MDS), a Key Project using the Hubble Space Telescope (HST). Wide Field Camera (WFC) images of random fields have been taken in 'parallel mode' with an effective resolution of 0.2 sec full width at half maximum (FWHM) in the V(F555W) and I(F785LP) filters. The exposures presented here were targeted on a field away from 3C 273, and resulted in approximately 5 hr integration time in each filter. Detailed morphological structure is seen in galaxy images with total integrated magnitudes down to V approximately = 22.5 and I approximately = 21.5. Parameters are estimated that best fit the observed galaxy images, and 143 objects are identified (including 23 stars) in the field to a fainter limiting magnitude of I approximately = 23.5. We outline the extragalactic goals of the HST Medium Deep Survey, summarize our basic data reduction procedures, and present number (magnitude) counts, a color-magnitude diagram for the field, surface brightness profiles for the brighter galaxies, and best-fit half-light radii for the fainter galaxies as a function of apparent magnitude. A median galaxy half-light radius of 0.4 sec is measured, and the distribution of galaxy sizes versus magnitude is presented. We observe an apparent deficit of galaxies with half-light radii between approximately 0.6 sec and 1.5 sec, with respect to standard no-evolution or mild evolution cosmological models. An apparent excess of compact objects (half-light radii approximately 0.1 sec) is also observed with respect to those models. Finally, we find a small excess in the number of faint galaxy pairs and groups with respect to a random low-redshift field sample.