H.-W. Rix

Nearly 5000 Distant Early-Type Galaxies in COMBO-17: A Red Sequence and Its Evolution since z ~ 1

We present the rest-frame colors and luminosities of ~25,000 mR 24 galaxies in the redshift range 0.2 < z ≤ 1.1 drawn from 0.78 deg2 of the COMBO-17 survey (Classifying Objects by Medium-Band Observations in 17 Filters). We find that the rest-frame color distribution of these galaxies is bimodal at all redshifts out to z ~ 1. This bimodality permits a model-independent definition of red early-type galaxies and blue late-type galaxies at any given redshift. The colors of the blue peak become redder toward the present day, and the number density of blue luminous galaxies has dropped strongly since z ~ 1. Focusing on the red galaxies, we find that they populate a color-magnitude relation. Such red sequences have been identified in galaxy cluster environments, but our data show that such a sequence exists over this redshift range even when averaging over all environments. The mean color of the red galaxy sequence evolves with redshift in a way that is consistent with the aging of an ancient stellar population. The rest-frame B-band luminosity density in red galaxies evolves only mildly with redshift in a Λ-dominated cold dark matter universe. When we account for the change in stellar mass-to-light ratio implied by the redshift evolution in red galaxy colors, the COMBO-17 data indicate an increase in stellar mass on the red sequence by a factor of 2 since z ~ 1. The largest source of uncertainty is large-scale structure, implying that considerably larger surveys are necessary to further refine this result. We explore mechanisms that may drive this evolution in the red galaxy population, finding that both galaxy merging and truncation of star formation in some fraction of the blue star-forming population are required to fully explain the properties of these galaxies.

Detailed Decomposition of Galaxy Images. II. Beyond Axisymmetric Models

We present a two-dimensional (2D) fitting algorithm (GALFIT, ver. 3) with new capabilities to study the structural components of galaxies and other astronomical objects in digital images. Our technique improves on previous 2D fitting algorithms by allowing for irregular, curved, logarithmic and power-law spirals, ring, and truncated shapes in otherwise traditional parametric functions like the Sersic, Moffat, King, Ferrer, etc., profiles. One can mix and match these new shape features freely, with or without constraints, and apply them to an arbitrary number of model components 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 Sersic index, effective radius, or luminosity remains 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.

Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions*

We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed-up with either the Subaru or the Isaac Newton Telescopes. They include four probable new dwarf galaxies--one each in the constellations of Coma Berenices, Canes Venatici, Leo and Hercules--together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii and color-magnitude diagrams for all five satellites. The morphological features of the color-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the last two years, a total of ten new Milky Way satellites with effective surface brightness {mu}{sub v} {approx}> 28 mag arcsec{sup -2} have been discovered in SDSS data. They are less luminous, more irregular and appear to be more metal-poor than the previously-known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between {approx} 40 pc and {approx} 100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.

The James Webb Space Telescope

The James Webb Space Telescope (JWST) is a large (6.6 m), cold (<50 K), infrared (IR)-optimized space observatory that will be launched early in the next decade into orbit around the second Earth–Sun Lagrange point. The observatory will have four instruments: a near-IR camera, a near-IR multiobject spectrograph, and a tunable filter imager will cover the wavelength range, 0.6 < ; < 5.0 μ m, while the mid-IR instrument will do both imaging and spectroscopy from 5.0 < ; < 29 μ m.The JWST science goals are divided into four themes. The key objective of The End of the Dark Ages: First Light and Reionization theme is to identify the first luminous sources to form and to determine the ionization history of the early universe. The key objective of The Assembly of Galaxies theme is to determine how galaxies and the dark matter, gas, stars, metals, morphological structures, and active nuclei within them evolved from the epoch of reionization to the present day. The key objective of The Birth of Stars and Protoplanetary Systems theme is to unravel the birth and early evolution of stars, from infall on to dust-enshrouded protostars to the genesis of planetary systems. The key objective of the Planetary Systems and the Origins of Life theme is to determine the physical and chemical properties of planetary systems including our own, and investigate the potential for the origins of life in those systems. Within these themes and objectives, we have derived representative astronomical observations.To enable these observations, JWST consists of a telescope, an instrument package, a spacecraft, and a sunshield. The telescope consists of 18 beryllium segments, some of which are deployed. The segments will be brought into optical alignment on-orbit through a process of periodic wavefront sensing and control. The instrument package contains the four science instruments and a fine guidance sensor. The spacecraft provides pointing, orbit maintenance, and communications. The sunshield provides passive thermal control. The JWST operations plan is based on that used for previous space observatories, and the majority of JWST observing time will be allocated to the international astronomical community through annual peer-reviewed proposal opportunities.

THE MILKY WAY'S CIRCULAR VELOCITY CURVE TO 60 kpc AND AN ESTIMATE OF THE DARK MATTER HALO MASS FROM THE KINEMATICS OF ∼2400 SDSS BLUE HORIZONTAL-BRANCH STARS

We derive new constraints on the mass of the Milky Ways dark matter halo, based on 2401 rigorously selected blue horizontal-branch halo stars from SDSS DR6. This sample enables construction of the full line-of-sight velocity distribution at different galactocentric radii. To interpret these distributions, we compare them to matched mock observations drawn from two different cosmological galaxy formation simulations designed to resemble the Milky Way. This procedure results in an estimate of the Milky Ways circular velocity curve to ~60 kpc, which is found to be slightly falling from the adopted value of 220 km s?1 at the Suns location, and implies -->M( Vcir(r) , derived in statistically independent bins, is found to be consistent with the expectations from an NFW dark matter halo with the established stellar mass components at its center. If we assume that an NFW halo profile of characteristic concentration holds, we can use the observations to estimate the virial mass of the Milky Ways dark matter halo, -->Mvir = 1.0+ 0.3?0.2 ? 1012 M?, which is lower than many previous estimates. We have checked that the particulars of the cosmological simulations are unlikely to introduce systematics larger than the statistical uncertainties. This estimate implies that nearly 40% of the baryons within the virial radius of the Milky Ways dark matter halo reside in the stellar components of our Galaxy. A value for -->Mvir of only ~ -->1 ? 1012 M? also (re)opens the question of whether all of the Milky Ways satellite galaxies are on bound orbits.

THE STAR FORMATION HISTORY OF MASS-SELECTED GALAXIES IN THE COSMOS FIELD

We explore the redshift evolution of the specific star formation rate (SSFR) for galaxies of different stellar mass by drawing on a deep 3.6 µm-selected sample of > 10 5 galaxies in the 2 deg 2 COSMOS field. The average star formation rate (SFR) for sub-sets of these galaxies is estimated with stacked 1.4 GHz radio continuum emission. We separately consider the total sample and a subset of galaxies that shows evidence for substantive recent star formation in the rest-frame optical spectral energy distributions. At redshifts 0.2 2, at least for high-mass (M� & 4 � 10 10 M� ) systems where our conclusions are most robust. Our data show that there is a tight correlation with power-law dependence, SSFR / M� � , between

Probing the coevolution of supermassive black holes and galaxies using gravitationally lensed quasar hosts

In the present-day universe, supermassive black hole masses (MBH) appear to be strongly correlated with their galaxy’s bulge luminosity, among other properties. In this study, we explore the analogous relationship between MBH, derived using the virial method, and the stellar R-band bulge luminosity (LR) 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 MBH–LR relation is nearly the same (to within ∼ 0.3 mag) as it is today. When the observed LR are corrected for luminosity evolution, this means that the black holes grew in mass faster than their hosts, with the MBH/M∗ mass ratio being a factor of & 4 +21 times larger at z > 1.7 than it is today. By the redshift range 1 . z . 1.7 (8–10 Gyrs ago), the MBH/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 MBH/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. Subject headings: galaxies: evolution — galaxies: quasars — galaxies: fundamental parameters — galaxies: structure — galaxies: bulges

A significant population of red, near-infrared-selected high-redshift galaxies

We use very deep near-infrared photometry of the Hubble Deep Field-South taken with ISAAC on the Very Large Telescope to identify a population of high-redshift galaxies with rest-frame optical colors similar to those of nearby galaxies. The galaxies are chosen by their infrared colors Js-Ks > 2.3, aimed at selecting galaxies with redshifts above 2. When applied to our data set, we find 14 galaxies with Ks < 22.5, corresponding to a surface density of 3 ± 0.8 arcmin-2. The photometric redshifts all lie above 1.9, with a median of 2.6 and an rms of 0.7. The spectral energy distributions of these galaxies show a wide range. One is very blue in the rest-frame UV and satisfies the normal Lyman break criteria for high-redshift, star-forming galaxies. Others are quite red throughout the observed spectral range and are extremely faint in the optical, with a median V = 26.6. Hence, these galaxies would not be included in photometric samples based on optical ground-based data, and spectroscopic follow-up is difficult. The spectral energy distributions often show a prominent break, identified as the Balmer break or the 4000 A break. The median age is 1 Gyr when fitted with a constant star formation model with dust or 0.7 Gyr when fitted with a single burst model. Although significantly younger ages cannot be excluded when a larger range of models is allowed, the results indicate that these galaxies are among the oldest at these redshifts. The volume density to Ks = 22.5 is half that of Lyman break galaxies at z ≈ 3. Since the mass-to-light ratios of the red galaxies are likely to be higher, the stellar mass density is inferred to be comparable to that of Lyman break galaxies. These red galaxies may be the descendants of galaxies that started to form stars at very high redshifts, and they may evolve into the most massive galaxies at low redshift.

The Luminosity Function of the Milky Way Satellites

We quantify the detectability of stellar Milky Way satellites in the Sloan Digital Sky Survey (SDSS) Data Release 5. We show that the effective search volumes for the recently discovered SDSS–satellites depend strongly on their luminosity, with their maximum distance, Dmax, substantially smaller than the Milky Way halo’s virial radius. Calculating the maximum accessible volume, Vmax, for all faint detected satellites, allows the calculation of the luminosity function for Milky Way satellite galaxies, accounting quantitatively for their detectability. We find that the number density of satellite galaxies continues to rise towards low luminosities, but may flatten at MV ∼ −5; within the uncertainties, the luminosity function can be described by a single power law dN/dMV = 10 × 10 0.1(M V +5) , spanning luminosities from MV = −2 all the way to the luminosity of the Large Magellanic Cloud. Comparing these results to several semi-analytic galaxy formation models, we find that their predictions differ significantly from the data: either the shape of the luminosity function, or the surface brightness distributions of the models, do not match. Subject headings: Galaxy: halo – Galaxy: structure – Galaxy: formation – Local Group

Nonaxisymmetric structures in the stellar disks of galaxies

We study the azimuthal structure of the stellar disks of 18 face-on spiral galaxies, using K-band photometry to trace the stellar surface mass density. Assuming the disks are co-planar, we characterize their deviation from axisymmetry by the fractional amplitudes, A_i and phases of the azimuthal Fourier components at radii R about the photometric galaxy center. We find that most disks exhibit a wealth of non-axisymmetric structures, specifically: (1) that about one third of them are substantially lopsided (A_1/A_0>= 0.20) at 2.5 disk exponential scale length, (2) that almost one half of them have strong two-armed spirals with an arm/interarm surface-brightness contrasts of order unity, and (3) that typical disks have some intrinsic ellipticity. We estimate that in the disk plane the characteristic ellipticity of the underlying potential is 0.045+-0.03. However, the spiral pattern couples significantly to the estimate of the intrinsic ellipticity, and our measurement may represent an upper limit on the ``true potential triaxiality. We estimate the radial streaming motions of the disk stars,