We refine a technique to measure the absorption corrected ultraviolet (UV) luminosity of starburst galaxies using rest frame UV quantities alone, and apply it to Lyman-limit U -dropouts at z ≈ 3 found in the Hubble Deep field (HDF). The method is based on an observed correlation between the ratio of far infrared (FIR) to UV fluxes with spectral slope β (a UV color). A simple fit to this relation allows the UV flux absorbed by dust and reprocessed to the FIR to be calculated, and hence the dust-free UV luminosity to be determined. International Ultraviolet Explorer spectra and InfraRed Astronomical Satellite fluxes of local starbursts are used to calibrate the F FIR /F 1600 versus β relation in terms of A 1600 (the dust absorption at 1600Å), and the transformation from broad band photometric color to β. Both calibrations are almost completely independent of theoretical stellar population models. We show that the recent marginal and non-detections of HDF U -dropouts at radio and sub-mm wavelengths are consistent with their assumed starburst nature, and our calculated A 1600 . This is also true of recent observations of the ratio of optical emission line flux to UV flux density in the brightest U -dropouts. This latter ratio turns out not to be a good indicator of dust extinction. In U -dropouts, absolute magnitude M 1600,0 correlates with β: brighter galaxies are redder, as is observed to be the case for local starburst galaxies. This suggests that a mass-metallicity relationship is already in place at z ≈ 3. The absorption-corrected UV luminosity function of U -dropouts extends up to M 1600,0 ≈ −24 ABmag corresponding to a star formation rate ∼ 200 M ⊙ yr −1 (H 0 = 50 km s −1 Mpc −1 , q 0 = 0.5 assumed throughout). The absorption-corrected UV luminosity density at z ≈ 3 is ρ 1600,0 ≥ 1.4 × 10 27 erg s −1 Hz −1 Mpc −3 . It is still a lower limit since completeness corrections have not been done and because only galaxies with A 1600 ∼ < 3.6 mag are blue enough in the UV to be selected as U -dropouts. The luminosity weighted mean dust absorption factor of our sample is 5.4 ± 0.9 at 1600Å.

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-thanpredicted 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 standard stars have been used to revise the prelaunch estimate of the instrument response curves to best match predicted and observed count rates. Synthetic photometry has been used to determine zero points for all filters in three magnitude systems and to derive interstellar extinction values for the ACS photometric systems. Due to the CCD internal scattering of longwavelength photons, the width of the PSF increases significantly in the near-IR, and the aperture correction for photometry with near-IR filters depends on the spectral energy distribution of the source. We provide a detailed recipe to correct for the latter effect. Transformations between the ACS photometric systems and the UBVRI and WFPC2 systems are presented. In general, two sets of transformations are available: one based on the observation of two star clusters; the other on synthetic photometry. We discuss the accuracy of these transformations and their sensitivity to details of the spectra being transformed. Initial signs of detector degradation due to the HST radiative environment are already visible. We discuss the impact on the data in terms of dark rate increase, charge transfer inefficiency, and "hot" pixel population.

We provide a systematic measurement of the rest-frame UV continuum slope β over a wide range in redshift (z ∼ 2-6) and rest-frame UV luminosity (0.1 L * z=3 to 2 L * z=3 ) to improve estimates of the star formation rate (SFR) density at high redshift. We utilize the deep optical and infrared data (Advanced Camera for Surveys/NICMOS) over the Chandra Deep Field-South and Hubble Deep Field-North Great Observatories Origins Deep Survey fields, as well as the UDF for our primary UBV i "dropout" Lyman Break Galaxy sample. We also use strong lensing clusters to identify a population of very low luminosity, high-redshift dropout galaxies. We correct the observed distributions for both selection biases and photometric scatter. We find that the UV-continuum slope of the most luminous galaxies is substantially redder at z ∼ 2-4 than it is at z ∼ 5-6 (from ∼−2.4 at z ∼ 6 to ∼−1.5 at z ∼ 2). Lower luminosity galaxies are also found to be bluer than higher luminosity galaxies at z ∼ 2.5 and z ∼ 4. We do not find a large number of galaxies with β's as red as −1 in our dropout selections at z ∼ 4, and particularly at z 5, even though such sources could be readily selected from our data (and also from Balmer Break Galaxy searches at z ∼ 4). This suggests that star-forming galaxies at z 5 almost universally have very blue UV-continuum slopes, and that there are not likely to be a substantial number of dust-obscured galaxies at z 5 that are missed in "dropout" searches. Using the same relation between UV-continuum slope and dust extinction as has been found to be appropriate at both z ∼ 0 and z ∼ 2, we estimate the average dust extinction of galaxies as a function of redshift and UV luminosity in a consistent way. As expected, we find that the estimated dust extinction increases substantially with cosmic time for the most UV luminous galaxies, but remains small ( 2 times) at all times for lower luminosity galaxies. Because these same lower luminosity galaxies dominate the luminosity density in the UV continuum, the overall dust extinction correction remains modest at all redshifts and the evolution of this correction with redshift is only modest. We include the contribution from ultra-luminous IR galaxies in our SFR density estimates at z ∼ 2-6, but find that they contribute only ∼20% of the total at z ∼ 2.5 and 10% at z 4.

Hubble Space Telescope ultraviolet (UV) images of nine starburst galaxies reveal them to be highly irregular, even after excluding compact sources (clusters and resolved stars). Most (7/9) are found to have a similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area. All starbursts in our sample contain UV bright star clusters indicating that cluster formation is an important mode of star formation in starbursts. On average about 20% of the UV luminosity comes from these clusters. The brightest clusters, or super star clusters (SSC), are preferentially found at the very heart of starbursts. The size of the nearest SSCs are consistent with those of Galactic globular clusters. The luminosity function of SSCs is well represented by a power law with a slope alpha ~ -2. There is a strong correlation between the far infrared excess and the UV spectral slope. The correlation is well modeled by a geometry where much of their dust is in a foreground screen near to the starburst, but not by a geometry of well mixed stars and dust.Comment: 47 pages, text only, LaTeX with aaspp.sty (version 3.0), compressed postscript figures available at ftp://eta.pha.jhu.edu/RecentPublications/meurer

We measure the morphology-density relation ( MDR) and morphology-radius relation (MRR) for galaxies in seven z $ 1 clusters that have been observed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. Simulations and independent comparisons of our visually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to z 850 ¼ 24, corresponding to L /L Ã ¼ 0:21 and 0.30 at z ¼ 0:83 and 1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0:8 < z < 1:2 and that observed at z $ 0, consistent with recent work; specifically, the growth in the bulge-dominated galaxy fraction, f EþS0 , with increasing density proceeds less rapidly at z $ 1 than it does at z $ 0. At z $ 1 and AE ! 500 galaxies Mpc À2 , we find h f EþS0 i ¼ 0:72 AE 0:10. At z $ 0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities AE k 40 galaxies Mpc À2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Sp+Irr galaxies relative to the local galaxy population. The f E -density relation exhibits no significant evolution between z ¼ 1 and 0. We find mild evidence to suggest that the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations.

We analyze deep multicolor Advanced Camera images of the largest known gravitational lens, A1689. Radial and tangential arcs delineate the critical curves in unprecedented detail, and many small counterimages are found near the center of mass. We construct a flexible light deflection field to predict the appearance and positions of counterimages. The model is refined as new counterimages are identified and incorporated to improve the model, yielding a total of 106 images of 30 multiply lensed background galaxies, spanning a wide redshift range, 1:0 < z < 5:5. The resulting mass map is more circular in projection than the clumpy distribution of cluster galaxies, and the light is more concentrated than the mass within r < 50 kpc h À1 . The projected mass profile flattens steadily toward the center with a shallow mean slope of dlog AE=dlog r ' À0:55 AE 0:1, over the observed range r < 250 kpc h À1 , matching well an NFW profile, but with a relatively high concentration, C vir ¼ 8:2 þ2:1 À1:8 . A softened isothermal profile (r core ¼ 20 AE 2 00 ) is not conclusively excluded, illustrating that lensing constrains only projected quantities. Regarding cosmology, we clearly detect the purely geometric increase of bend angles with redshift. The dependence on the cosmological parameters is weak owing to the proximity of A1689, z ¼ 0:18, constraining the locus, M þ Ã 1:2. This consistency with standard cosmology provides independent support for our model, because the redshift information is not required to derive an accurate mass map. Similarly, the relative fluxes of the multiple images are reproduced well by our best-fitting lens model.

The integrated bolometric effective surface brightness S e distributions of starbursts are investigated for samples observed in 1. the rest frame ultraviolet (UV), 2. the farinfrared and Hα, and 3. 21cm radio continuum emission. For the UV sample we exploit a tight empirical relationship between UV reddening and extinction to recover the bolometric flux. Parameterizing the S e upper limit by the 90th percentile of the distribution, we find a mean S e,90 = 2.0 × 10 11 L ⊙ kpc −2 for the three samples, with a factor of three difference between the samples. This is consistent with what is expected from the calibration uncertainties alone. We find little variation in S e,90 with effective radii for R e ∼ 0.1 − 10 kpc, and little evolution out to redshifts z ≈ 3. The lack of a strong dependence of S e,90 on wavelength, and its consistency with the pressure measured in strong galactic winds, argue that it corresponds to a global star formation intensity limit (Σ e,90 ∼ 45 M ⊙ kpc −2 yr −1 ) rather than being an opacity effect. There are several important implications of these results: 1. There is a robust physical mechanism limiting starburst intensity. We note that starbursts have S e consistent with the expectations of gravitational instability models applied to the solid body rotation portion of galaxies. 2. Elliptical galaxies and spiral bulges can plausibly be built with maximum intensity bursts, while normal spiral disks can not. 3. The UV extinction of high-z galaxies is significant, implying that star formation in the early universe is moderately obscured. After correcting for extinction, the observed metal production rate at z ∼ 3 agrees well with independent estimates made for the epoch of elliptical galaxy formation.

Many of the results in modern astrophysics rest on the notion that the Initial Mass Function (IMF) is universal. Our observations of a sample of H I selected galaxies in the light of Hα and the far-ultraviolet (FUV) challenge this result. The extinction corrected flux ratio F Hα / f FUV from these two tracers of star formation shows strong correlations with the surface-brightness in Hα and the R band: Low Surface Brightness (LSB) galaxies have lower F Hα / f FUV ratios compared to High Surface Brightness (HSB) galaxies as well as compared to expectations from equilibrium models of constant star formation rate (SFR) using commonly favored IMF parameters. Weaker but significant correlations of F Hα / f FUV with luminosity, rotational velocity and dynamical mass are found as well as a systematic trend with morphology. The correlated variations of F Hα / f FUV with other global parameters are thus part of the larger family of galaxy scaling relations. The F Hα / f FUV correlations can not be due to residual extinction correction errors, while systematic variations in the star formation history can not explain the trends with both Hα and R surface brightness nor with other global properties. The possibility that LSB galaxies have a higher escape fraction of ionizing photons seems inconsistent with their high gas fraction, and observations of color-magnitude diagrams of a few systems which indicate a real deficit of O stars. The most plausible explanation for the correlations is the systematic variations of the upper mass limit M u and/or the slope γ which define the upper end of the IMF. We outline a scenario of pressure driving the correlations by setting the efficiency of the formation of the dense star clusters where the highest mass stars preferentially form. Our results imply that the star formation rate measured in a galaxy is highly sensitive to the tracer used in the measurement. A non-universal IMF would also call into question interpretation of metal abundance patterns in dwarf galaxies as well star formation histories derived from color magnitude diagrams.