This paper presents the Hubble Ultra Deep Field (HUDF), a one million second exposure of an 11 square minute-of-arc region in the southern sky with the Hubble Space Telescope. The exposure time was divided among four filters, F435W (B435), F606W (V606), F775W (i775), and F850LP (z850), to give approximately uniform limiting magnitudes mAB~29 for point sources. The image contains at least 10,000 objects presented here as a catalog. Few if any galaxies at redshifts greater than ~4 resemble present day spiral or elliptical galaxies. Using the Lyman break dropout method, we find 504 B-dropouts, 204 V-dropouts, and 54 i-dropouts. Using these samples that are at different redshifts but derived from the same data, we find no evidence for a change in the characteristic luminosity of galaxies but some evidence for a decrease in their number densities between redshifts of 4 and 7. The ultraviolet luminosity density of these samples is dominated by galaxies fainter than the characteristic luminosity, and the HUDF reveals considerably more luminosity than shallower surveys. The apparent ultraviolet luminosity density of galaxies appears to decrease from redshifts of a few to redshifts greater than 6. The highest redshift samples show that star formation was already vigorous at the earliest epochs that galaxies have been observed, less than one billion years after the Big Bang.Comment: 44 pages, 18 figures, to appear in the Astronomical Journal October 200

This Special Issue of the Astrophysical Journal Letters is dedicated to presenting initial results from the Great Observatories Origins Deep Survey (GOODS) that are primarily, but not exclusively, based on multi-band imaging data obtained with the Hubble Space Telescope (HST) and the Advanced Camera for Surveys (ACS). The survey covers roughly 320 square arcminutes in the ACS F435W, F606W, F814W, and F850LP bands, divided into two well-studied fields. Existing deep observations from the Chandra X-ray Observatory (CXO) and groundbased facilities are supplemented with new, deep imaging in the optical and

We use the bulge Sérsic index n and bulge-to-total ratio (B/T ) to explore the fundamental question of how bulges form. We perform 2D bulge-disk-bar decomposition on H-band images of 143 bright, high mass (M ⋆ ≥ 1.0 × 10 10 M ⊙ ) low-to-moderately inclined (i < 70 • ) spirals. Our results are: (1) Our H-band bar fraction (∼ 58%) is consistent with that from ellipse fits. (5) We compare the results with predictions from a set of ΛCDM models. In the models, a high mass spiral can have a bulge with a present-day low B/T ≤ 0.2 only if it did not undergo a major merger since z ≤ 2. The predicted fraction (∼ 1.6%) of high mass spirals, which have undergone a major merger since z ≤ 4 and host a bulge with a present-day low B/T ≤ 0.2, is a factor of over thirty smaller than the observed fraction (∼ 66%) of high mass spirals with B/T ≤ 0.2. Thus, contrary to common perception, bulges built via major mergers since z ≤ 4 seriously fail to account for the bulges present in ∼ 66% of high mass spirals. Most of these present-day low B/T ≤ 0.2 bulges are likely to have been built by a combination of minor mergers and/or secular processes since z ≤ 4.

We combine HST imaging from the GEMS (Galaxy Evolution from Morphologies and SEDs) survey with photometric redshifts from COMBO-17 to explore the evolution of disk-dominated galaxies since z P1:1. The sample is composed of all GEMS galaxies with Sérsic indices n < 2:5, derived from fits to the galaxy images. We account fully for selection effects through careful analysis of image simulations; we are limited by the depth of the redshift and HST data to the study of galaxies with M V P À20, or equivalently, log M/M ð Þk 10. We find strong evolution in the magnitude-size scaling relation for galaxies with M V P À20, corresponding to a brightening of $1 mag arcsec À2 in rest-frame V band by z $ 1. Yet disks at a given absolute magnitude are bluer and have lower stellar mass-to-light ratios at z $ 1 than at the present day. As a result, our findings indicate weak or no evolution in the relation between stellar mass and effective disk size for galaxies with log M/M ð Þk 10 over the same time interval. This is strongly inconsistent with the most naive theoretical expectation, in which disk size scales in proportion to the halo virial radius, which would predict that disks are a factor of 2 denser at fixed mass at z $ 1. The lack of evolution in the stellar mass-size relation is consistent with an ''inside-out'' growth of galaxy disks on average (galaxies increasing in size as they grow more massive), although we cannot rule out more complex evolutionary scenarios.

We present a study of large-scale bars in the local universe, based on a large sample of 3692 galaxies, with 18:5 M g < À22:0 mag and redshift 0:01 z < 0:03, drawn from the Sloan Digitized Sky Survey. Our sample includes many galaxies that are disk-dominated and of late Hubble types. Both color cuts and Sérsic cuts yield a similar sample of $2000 disk galaxies. We characterize bars and disks by ellipse-fitting r-band images and applying quantitative criteria. After excluding highly inclined (60 ) systems, we find the following results. (1) The optical r-band fraction ( f optÀr ) of barred galaxies, when averaged over the whole sample, is $48%-52%.(2) When galaxies are separated according to half light radius (r e ), or normalized r e /R 24 , which is a measure of the bulge-to-disk (B/D) ratio, a remarkable result is seen: f optÀr rises sharply, from $40% in galaxies that have small r e /R 24 and visually appear to host prominent bulges, to $70% for galaxies that have large r e /R 24 and appear disk-dominated. (3) For galaxies with bluer colors, f optÀr rises significantly (by $30%). A weaker rise (by $15%Y20%) is seen for lower luminosities or lower masses. (4) While hierarchical ÃCDM models of galaxy evolution models fail to produce galaxies without classical bulges, our study finds that $20% of disk galaxies appear to be ''quasi-bulgeless.'' (5) We outline how the effect of a decreasing resolution and a rising obscuration of bars by gas and dust over z ¼ 0:2Y1:0 can cause a significant artificial loss of bars, and an artificial reduction in the optical bar fraction over z ¼ 0:2Y1:0.

Critical insights on galaxy evolution stem from the study of bars. With the advent of high redshift HST surveys that trace bars in the rest-frame optical band out to z ∼ 1, it becomes increasingly important to provide a reference baseline for bars at z ∼ 0 in the optical band. We present results on bars at z ∼ 0 in the optical and near-infrared bands, based on 180 spirals in the OSUBSGS survey. (1) The deprojected bar fraction at z ∼ 0 is f NIR1 ∼ 60% ± 6% in the near-infrared H band, and f optical1 ∼ 44% ± 6% in the optical B-band images. The latter likely miss bars obscured by dust and star formation. (2) The results before and after deprojection are similar, which is encouraging for high redshift studies that forego deprojection.(3) Studies of bars at z ∼ 0.2-1.0 (lookback times of 3-8 Gyr) have reported an optical bar fraction of f optical2 ∼ 30% ± 6%, after applying cutoffs in absolute magnitude (M V <-19.3), bar size (a bar ≥ 1.5 kpc), and bar ellipticity (e bar ≥ 0.4) in order to ensure a complete sample, adequate spatial resolution, and reliable bar identification out to z ∼ 1. Applying these exact cutoffs in magnitude, bar size, and bar ellipticity to the OSUBSGS data yields a comparable optical B-band bar fraction at z ∼ 0 of f optical3 ∼ 34% ± 6%. This rules out scenarios where the optical bar fraction in bright disks declines strongly with redshift. (4) We investigate bar strengths at z ∼ 0 using the maximum bar ellipticity (e bar ) as a guide. Most (∼ 70%) bars have moderate to high ellipticity (0.50 ≤ e bar ≤ 0.75), and only a small fraction (7%-10%) have 0.25 ≤ e bar ≤ 0.40. There is no bimodality in the distribution of e bar . The H-band bar fraction and e bar show no substantial variation across RC3 Hubble types Sa to Scd. (5) RC3 bar types should be used with caution. Many galaxies with RC3 types 'AB' turn out to be unbarred and RC3 bar classes 'B' and 'AB' have a significant overlap in e bar . (6) Most (68% in B and 76% in H) bars have sizes below 5 kpc. Bar and disk sizes correlate, and the ratio (a bar /R 25 ) lies primarily in the range 0.1 to 0.5. This suggests that the growth of bars and disks is intimately tied. 2 We assume in this paper a flat cosmology with Ω M = 1 − Ω Λ = 0.3 and H 0 =70 km s −1 Mpc −1 .

We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z ∼ 0.24-0.80 (lookback time T b ∼ 3-7 Gyr) using ∼3600 (M 1 × 10 9 M ) galaxies with GEMS Hubble Space Telescope, COMBO-17, and Spitzer data. Our results are as follows. (1) Among ∼790 high-mass (M 2.5 × 10 10 M ) galaxies, the visually based merger fraction over z ∼ 0.24-0.80, ranges from 9% ± 5% to 8% ± 2%. Lower limits on the major merger and minor merger fraction over this interval range from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first, albeit approximate, empirical estimate of the frequency of minor mergers over the last 7 Gyr. Assuming a visibility timescale of ∼0.5 Gyr, it follows that over T b ∼ 3-7 Gyr, ∼68% of high-mass systems have undergone a merger of mass ratio >1/10, with ∼16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous "major or minor" mergers. The average merger rate is ∼ a few ×10 −4 galaxies Gyr −1 Mpc −3 . Among ∼2840 blue-cloud galaxies of mass M 1.0 × 10 9 M , similar results hold. (2) We compare the empirical merger fraction and merger rate for high-mass galaxies to three Λ cold dark matter-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of 5 dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among ∼3698 M 1.0 × 10 9 M galaxies, we find that the mean star formation rate (SFR) of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z ∼ 0.24-0.80. Visibly merging systems only account for a small fraction (<30%) of the cosmic SFR density over T b ∼ 3-7 Gyr. This complements the results of Wolf et al. over a shorter time interval of T b ∼ 6.2-6.8 Gyr, and suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.

Stellar bars drive gas into the circumnuclear (CN) region of galaxies. To investigate the fate of the CN gas and star formation (SF), we study a sample of barred nonstarbursts and starbursts with high-resolution CO, optical, H, radio continuum, Br, and HST data, and find the following. (1) The inner kiloparsec of bars differs markedly from the outer disk. It hosts molecular gas surface densities AE gas-m of 500-3500 M pc À2 , gas mass fractions of 10%-30%, and epicyclic frequencies of several 100-1000 km s À1 kpc À1. Consequently, in the CN region gravitational instabilities can only grow at high gas densities and on short timescales, explaining in part why powerful starbursts reside there.(2) Across the sample, we find bar pattern speeds with upper limits of 43-115 km s À1 pc À1 and outer inner Lindblad resonance radii of >500 pc. (3) Barred starbursts and nonstarbursts have CN SF rates of 3-11 and 0.1-2 M yr À1 , despite similar CN gas masses. The AE gas-m value in the starbursts is larger (1000-3500 M pc À2) and close to the Toomre critical density over a large region. (4) Molecular gas makes up 10%-30% of the CN dynamical mass and fuels large CN SF rates in the starbursts, building young, massive, high-V/ components. Implications for secular evolution along the Hubble sequence are discussed.