We present a two-dimensional (2-D) fitting algorithm (GALFIT) designed to extract structural components from galaxy images, with emphasis on closely modeling light profiles of spatially well-resolved, nearby galaxies observed with the Hubble Space Telescope. Our algorithm improves on previous techniques in two areas, by being able to simultaneously fit a galaxy with an arbitrary number of components, and with optimization in computation speed, suited for working on large galaxy images. We use 2-D models such as the ``Nuker'' law, the Sersic (de Vaucouleurs) profile, an exponential disk, and Gaussian or Moffat functions. The azimuthal shapes are generalized ellipses that can fit disky and boxy components. Many galaxies with complex isophotes, ellipticity changes, and position-angle twists can be modeled accurately in 2-D. When examined in detail, we find that even simple-looking galaxies generally require at least three components to be modeled accurately, rather than the one or two components more often employed. We illustrate this by way of 7 case studies, which include regular and barred spiral galaxies, highly disky lenticular galaxies, and elliptical galaxies displaying various levels of complexities. A useful extension of this algorithm is to accurately extract nuclear point sources in galaxies. We compare 2-D and 1-D extraction techniques on simulated images of galaxies having nuclear slopes with different degrees of cuspiness, and we then illustrate the application of the program to several examples of nearby galaxies with weak nuclei.Comment: 29 pages, 14 figures, abridged version. Full version AJ accepted. For full version with high resolution figures, go to: http://zwicky.as.arizona.edu/~cyp/work/galfit.ps.g

We present a two-dimensional (2-D) fitting algorithm (Galfit, Version 3) with new capabilities to study the structural components of galaxies and other astronomical objects in digital images. Our technique improves on previous 2-D fitting algorithms by allowing for irregular, curved, logarithmic and power-law spirals, ring and truncated shapes in otherwise traditional parametric functions like the Sérsic, Moffat, King, Ferrer, etc., profiles. One can mix and match these new shape features freely, with or without constraints, apply them to an arbitrary number of model components and of numerous profile types, so as to produce realistic-looking galaxy model images. Yet, despite the potential for extreme complexity, the meaning of the key parameters like the Sérsic index, effective radius or luminosity remain intuitive and essentially unchanged. The new features have an interesting potential for use to quantify the degree of asymmetry of galaxies, to quantify low surface brightness tidal features beneath and beyond luminous galaxies, to allow more realistic decompositions of galaxy subcomponents in the presence of strong rings and spiral arms, and to enable ways to gauge the uncertainties when decomposing galaxy subcomponents. We illustrate these new features by way of several case studies that display various levels of complexity.

In the present-day universe, supermassive black hole masses (M BH ) appear to be strongly correlated with their galaxy's bulge luminosity, among other properties. In this study, we explore the analogous relationship between M BH , derived using the virial method, and the stellar R-band bulge luminosity (L R ) or stellar bulge mass (M * ) at epochs of 1 z 4.5 using a sample of 31 gravitationally lensed AGNs and 20 non-lensed AGNs. At redshifts z > 1.7 (10-12 Gyrs ago), we find that the observed M BH -L R relation is nearly the same (to within ∼ 0.3 mag) as it is today. When the observed L R are corrected for luminosity evolution, this means that the black holes grew in mass faster than their hosts, with the M BH /M * mass ratio being a factor of 4 +2 −1 times larger at z > 1.7 than it is today. By the redshift range 1 z 1.7 (8-10 Gyrs ago), the M BH /M * ratio is at most two times higher than today, but it may be consistent with no evolution. Combining the results, we conclude that the ratio M BH /M * rises with look-back time, although it may saturate at ≈ 6 times the local value. Scenarios in which moderately luminous quasar hosts at z 1.7 were fully formed bulges that passively faded to the present epoch are ruled out.

ABSTRACT. The Spitzer Survey of Stellar Structure in Galaxies (S 4 G) is an Exploration Science Legacy Program approved for the Spitzer post-cryogenic mission. It is a volume-, magnitude-, and size-limited (d < 40 Mpc, jbj > 30°, m Bcorr < 15:5, and D 25 > 1 0 ) survey of 2331 galaxies using the Infrared Array Camera (IRAC) at 3.6 and 4.5 μm. Each galaxy is observed for 240 s and mapped to ≥1:5 × D 25 . The final mosaicked images have a typical 1σ rms noise level of 0.0072 and 0:0093 MJy sr À1 at 3.6 and 4.5 μm, respectively. Our azimuthally averaged surface brightness profile typically traces isophotes at μ 3:6μm ðABÞð1σÞ ∼ 27 mag arcsec À2 , equivalent to a stellar mass surface density of ∼1 M ⊙ pc À2 . S 4 G thus provides an unprecedented data set for the study of the distribution of mass and stellar structures in the local universe. This large, unbiased, and extremely deep sample of all Hubble types from dwarfs to spirals to ellipticals will allow for detailed structural studies, not only as a function of stellar mass, but also as a function of the local environment. The data from this survey will serve as a vital testbed for cosmological simulations predicting the stellar mass properties of present-day galaxies. This article introduces the survey and describes the sample selection, the significance of the 3.6 and 4.5 μm bands for this study, and the data collection and survey strategies. We describe the S 4 G data analysis pipeline and present measurements for a first set of galaxies, observed in both the cryogenic and warm mission phases of Spitzer. For every galaxy we tabulate the galaxy diameter, position angle, axial ratio, inclination at μ 3:6μm ðABÞ ¼ 25:5, and 26:5 mag arcsec À2 (equivalent to ≈μ B ðABÞ ¼ 27:2 and 28:2 mag arcsec À2 , respectively). These measurements will form the initial S 4 G catalog of galaxy properties. We also measure the total magnitude and the azimuthally averaged radial profiles of ellipticity, position angle, surface brightness, and color. Finally, using the galaxy-fitting code GALFIT, we deconstruct each galaxy into its main constituent stellar components: the bulge/spheroid, disk, bar, and nuclear point source, where necessary. Together, these data products will provide a comprehensive and definitive catalog of stellar structures, mass, and properties of galaxies in the nearby universe and will enable a variety of scientific investigations, some of which are highlighted in this introductory S 4 G survey paper.

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 report on a program to obtain Hubble Space T elescope (HST ) observations of galaxy-mass gravitational lens systems at optical and infrared wavelengths. Here we discuss the properties of 10 two- We Ðt a set of four standard lens models to each lens that had sufficient constraints to compare isothermal dark matter and constant mass-to-light lens models, and to explore the e †ects of local tidal shears.

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.

The Next Generation Virgo Cluster Survey (NGVS) is a program that uses the 1 deg 2 MegaCam instrument on the Canada-France-Hawaii Telescope to carry out a comprehensive optical imaging survey of the Virgo cluster, from its core to its virial radius-covering a total area of 104 deg 2-in the u * griz bandpasses. Thanks to a dedicated data acquisition strategy and processing pipeline, the NGVS reaches a point-source depth of g ≈ 25.9 mag (10σ) and a surface brightness limit of μ g ∼ 29 mag arcsec −2 (2σ above the mean sky level), thus superseding all previous optical studies of this benchmark galaxy cluster. In this paper, we give an overview of the technical aspects of the survey, such as areal coverage, field placement, choice of filters, limiting magnitudes, observing strategies, data processing and calibration pipelines, survey timeline, and data products. We also describe the primary scientific topics of the NGVS, which include: the galaxy luminosity and mass functions; the color-magnitude relation; galaxy scaling relations; compact stellar systems; galactic nuclei; the extragalactic distance scale; the large-scale environment of the cluster and its relationship to the Local Supercluster; diffuse light and the intracluster medium; galaxy interactions and evolutionary processes; and extragalactic star clusters. In addition, we describe a number of ancillary programs dealing with "foreground" and "background" science topics, including the study of highinclination trans-Neptunian objects; the structure of the Galactic halo in the direction of the Virgo Overdensity and Sagittarius Stream; the measurement of cosmic shear, galaxy-galaxy, and cluster lensing; and the identification of distant galaxy clusters, and strong-lensing events.