arXiv subjects

Christopher J. Conselice

Publications and source records attributed to Christopher J. Conselice.

At least 325 records · Page 18Linked to original sources

arXiv2004

The Deep2 Galaxy Redshift Survey: Evolution of Close Galaxy Pairs and Major-Merger Rates Up to z ~ 1.2

We derive the close, kinematic pair fraction and merger rate up to z ~ 1.2 from the initial data of the DEEP2 Redshift Survey. Assuming a mild luminosity evolution, the number of companions per luminous galaxy is found to evolve as (1+z)^{m}, with m = 0.51+-0.28; assuming no evolution, m = 1.60+-0.29. Our results imply that only 9% of present-day $L^{*}$ galaxies have undergone major mergers since z ~ 1.2 and that the average major merger rate is about 4* 10^{-4} h^{3} Mpc^{-3} Gyr^{-1} for z ~ 0.5 - 1.2. Most previous studies have yielded higher values.

astro-ph↗

arXiv2004

The Luminosity, Stellar Mass, and Number Density Evolution of Field Galaxies of Known Morphology from z = 0.5 - 3

The evolution of rest-frame B-band luminosities, stellar masses, and number densities of field galaxies in the Hubble Deep Field North and South are studied as a function of rest-frame B-band morphological type out to redshifts z~3 using a sample of 1231 I < 27 galaxies with spectroscopic and photometric redshifts. We find that the co-moving number, and relative number, densities of ellipticals and spirals declines rapidly at z > 1, although examples exist at z > 2. The number and number fraction of peculiar galaxies consistent with undergoing major mergers rises consistently at redshifts z > 2. Through simulations we argue that this decline is robust at the 4 sigma level against morphological k-corrections and redshift effects. We trace the evolution of rest-frame B-band luminosity density as a function of morphology out to z~3 finding that the luminosity density is steadily dominated by peculiars at z > 1.5 with a peak fraction of 60-90% at z~3. By z~0.5 B-band luminosity fractions are similar to their local values. The stellar mass density follows a similar trend as the luminosity density, with some important exceptions. At high redshifts, z > 2, over 60-80% of stellar mass is attached to peculiars, while at z< 1, 80-95% of stellar mass is attached to ellipticals and spirals. In contrast to the luminosity density, the stellar mass density of ellipticals is greater than spirals at z < 1.5, and the stellar masses in both types grow together at z < 1, while number densities remain constant. (abridged)

astro-ph↗

arXiv2004

High Redshift Supernova Rates

We use a sample of 42 supernovae detected with the Advanced Camera for Surveys on-board the Hubble Space Telescope as part of the Great Observatories Origins Deep Survey to measure the rate of core collapse supernovae to z~0.7 and type Ia supernovae to z~1.6. This significantly increases the redshift range where supernova rates have been estimated from observations. The rate of core collapse supernovae can be used as an independent probe of the cosmic star formation rate. Based on the observations of 17 core collapse supernovae, we measure an increase in the core collapse supernova rate by a factor of 1.6 in the range 0.3 1 compared to low redshift. At higher redshift (z>1), we find a suggested decrease in the type Ia rate with redshift. This evolution of the Ia rate with redshift is consistent with a type Ia progenitor model where there is a substantial delay between the formation of the progenitor star and the explosion of the supernova. Assuming that the type Ia progenitor stars have initial main sequence masses 3-8 M_Sun, we find that 5-7% of the available progenitors explode as type Ia supernovae.

astro-ph↗

arXiv2004

Properties of Spiral and Elliptical Galaxy Progenitors at z > 1

We present the results of a Hubble Space Telescope and ground-based optical and near-infrared study to identify progenitors of spirals and ellipticals at z > 1. We identify these systems through photometric and spectroscopic redshifts, deep K-band imaging, stellar mass measurements, and high resolution imaging. The major modes of galaxy formation, including major mergers, minor mergers, and accretion of intergalactic gas, and their relative contributions towards building up the stellar masses of galaxies, can now be directly measured using these data.

astro-ph↗

arXiv2004

Unveiling the Formation of Massive Galaxies

Massive galaxies, such as nearby ellipticals, have relatively low number densities, yet they host the majority of the stellar mass in the universe. Understanding their origin is a central problem of galaxy formation. Age dating of stellar populations found in modern ellipticals, and observations of star formation in high redshift galaxies, allow us to determine roughly when these systems formed. These age diagnostics however do not tell us what triggered star formation, or how galaxies form as opposed to simply when. Recent analyses of the structures of z > 2 ultraviolet selected galaxies reveal that major galaxy mergers are a likely method for forming some massive galaxies. There are however galaxy populations at high redshift (z > 2), namely infrared and sub-millimeter bright systems, whose evolutionary relationship to modern ellipticals is still uncertain. An improved characterization of these and other high redshift galaxy populations is achievable with large infrared imaging and spectroscopic surveys.

astro-ph↗

arXiv2004

The DEEP2 Galaxy Redshift Survey: Clustering of Galaxies in Early Data

We measure the two-point correlation function xi(r) using a sample of 2219 galaxies in an area of 0.32 degrees^2 at z=0.7-1.35 from the first season of the DEEP2 Galaxy Redshift Survey. We find that xi(r) can be approximated by a power-law, xi(r)=(r/r_0)^-gamma, on scales 0.1-20 Mpc/h. In a sample with an effective redshift of z_eff=0.82, for a Lcdm cosmology we find r_0=3.53 +/-0.81 Mpc/h (comoving) and gamma=1.66 +/-0.12, while in a higher-redshift sample with z_eff=1.14 we find r_0=3.14 +/-0.72 Mpc/h and gamma=1.61 +/-0.11. We find that red, absorption-dominated, passively-evolving galaxies have a larger clustering scale length, r_0, and more prominent ``fingers of God'' than blue, emission-line, actively star-forming galaxies. Intrinsically brighter galaxies also cluster more strongly than fainter galaxies at z~1, with a significant luminosity-bias seen for galaxies fainter than M*. Our results are suggestive of evolution in the galaxy clustering within our survey volume and imply that the DEEP2 galaxies, with a median brightness one magnitude fainter than M* have an effective bias b=0.97 +/-0.13 if sigma_{8 DM}=1 today or b=1.20 +/-0.16 if sigma_{8 DM}=0.8 today. Given the strong luminosity-dependence in the bias that we measure at z~1, the galaxy bias at M* may be significantly greater. We note that our star-forming sample at z~1 has very similar selection criteria as the Lyman-break galaxies at z~3 and that our red, absorption-line sample displays a clustering strength comparable to the expected clustering of the Lyman-break galaxy descendants at z~1. Our results demonstrate that the clustering properties in the galaxy distribution seen in the local Universe were largely in place by z~1.

astro-ph↗

arXiv2003

The Ionized Gas in Local Starburst Galaxies: Global and Small--Scale Feedback from Star Formation

Abridged: The small-- and intermediate--scale structure and the fraction of the ISM ionized by non--radiative processes is investigated in a small sample of four local starburst galaxies, imaged with the HST WFPC2. The sample comprises three dwarf galaxies, NGC3077, NGC4214, and NGC5253, and one giant spiral, NGC5236 (M83). The galaxies span a range in metallicity, luminosity, and environment, enabling the investigation of non--radiative ionization processes in a variety of galactic conditions. For this purpose, the four galaxies were imaged in the lines of H-beta(4861 A), [OIII](5007 A), H-alpha(6563 A), and [SII](6717,6731 A). The non--photoionized gas in these galaxies has been traced on scales ranging from a few tens of pc to a few hundred pc, providing a full budget for this ionized gas component. Using the `maximum starburst line' of Kewley et al. (2001) to discriminate between photoionized and non--photoionized gas, we find that in all four galaxies non--photoionization processes are responsible for a small fraction of the total H-alpha emission, at the level of 3%--4%. The starbursts in the three dwarf galaxies deposit a significant fraction, 70%-100%, of their mechanical energy into the surrounding interstellar medium and require time--extended (a few x 10^7 yr to ~10^8 yr) star formation, in order to account for the observed luminosity of the non--photoionized gas. In the massive spiral, the non--photoionized gas is concentrated in localized areas surrounded by active star--formation, with no evidence for extended structures (as those in the dwarfs). This confirms the picture that starbursts remain confined events in massive galaxies, likely due to the deep potential well.

astro-ph↗

arXiv2003

Are Cluster Dwarfs Recycled Galaxies?

Although cluster dwarf galaxies are often neglected due to their faintness, recent observations demonstrate that they are critical for understanding the physical processes behind galaxy and cluster formation. Dwarfs are the most common galaxy type and are particularly abundant in clusters. Recent observational results suggest that dwarfs in dense environments do not all form early in the universe, as expected from hierarchical structure formation models. Many of these systems appear to be younger and more metal rich than dwarfs in lower density areas, suggesting they are possibly created by a tidal process. Several general galaxy cluster observations, including steep luminosity functions and the origin of intracluster light, are natural outcomes of these processes.

astro-ph↗

arXiv2003

The Relationship between Stellar and Halo Masses of Disk Galaxies at z = 0.2 - 1.2

We present the results of a study to determine the co-evolution of the virial and stellar masses for a sample of 83 disk galaxies between redshifts z = 0.2 - 1.2. The virial masses of these disks are computed using measured maximum rotational velocities from Keck spectroscopy and scale lengths from Hubble Space Telescope imaging. We compute stellar masses based on stellar population synthesis model fits to spectral energy distributions including K-band magnitudes. We find no apparent evolution with redshift from z = 0.2 - 1.2 in the relationship between stellar masses and maximum rotational velocities through the stellar mass Tully-Fisher relationship. We also find no evolution when comparing disk stellar and virial masses. Massive disk galaxies therefore appear to be already in place, in terms of their virial and stellar masses, out to the highest redshifts where they can be morphologically identified.

astro-ph↗

arXiv2003

The Formation of the Hubble Sequence

The history of galaxy formation via star formation and stellar mass assembly rates is now known with some certainty, yet the connection between high redshift and low redshift galaxy populations is not yet clear. By identifying and studying individual massive galaxies at high-redshifts, z > 1.5, we can possibly uncover the physical effects driving galaxy formation. Using the structures of high-z galaxies, as imaged with the Hubble Space Telescope, we argue that it is now possible to directly study the progenitors of ellipticals and disks. We also briefly describe early results that suggest many massive galaxies are forming at z > 2 through major mergers.

astro-ph↗

arXiv2003

A Slow Merger History of Field Galaxies Since z~1

Using deep infrared observations conducted with the CISCO imager on the Subaru Telescope, we investigate the field-corrected pair fraction and the implied merger rate of galaxies in redshift survey fields with Hubble Space Telescope imaging. In the redshift interval, 0.5 < z < 1.5, the fraction of infrared-selected pairs increases only modestly with redshift to 7% +- 6% at z~1. This is nearly a factor of three less than the fraction, 22% +- 8%, determined using the same technique on HST optical images and as measured in a previous similar study. Tests support the hypothesis that optical pair fractions at z~1 are inflated by bright star-forming regions that are unlikely to be representative of the underlying mass distribution. By determining stellar masses for the companions, we estimate the mass accretion rate associated with merging galaxies. At z~1, we estimate this to be 2x10^{9 +- 0.2} solar masses per galaxy per Gyr. Although uncertainties remain, our results suggest that the growth of galaxies via the accretion of pre-existing fragments remains as significant a phenomenon in the redshift range studied as that estimated from ongoing star formation in independent surveys.

astro-ph↗

arXiv2003

Image Simulation with Shapelets

We present a method to simulate deep sky images, including realistic galaxy morphologies and telescope characteristics. To achieve a wide diversity of simulated galaxy morphologies, we first use the shapelets formalism to parametrize the shapes of all objects in the Hubble Deep Fields. We measure this distribution of real galaxy morphologies in shapelet parameter space, then resample it to generate a new population of objects. These simulated galaxies can contain spiral arms, bars, discs, arbitrary radial profiles and even dust lanes or knots. To create a final image, we also model observational effects, including noise, pixellisation, astrometric distortions and a Point-Spread Function. We demonstrate that they are realistic by showing that simulated and real data have consistent distributions of morphology diagnostics: including galaxy size, ellipticity, concentration and asymmetry statistics. Sample images are made available on the world wide web. These simulations are useful to develop and calibrate precision image analysis techniques for photometry, astrometry, and shape measurement. They can also be used to assess the sensitivity of future telescopes and surveys for applications such as supernova searches, microlensing, proper motions, and weak gravitational lensing.

astro-ph↗

arXiv2003

The DEEP2 Galaxy Redshift Survey: Spectral classification of galaxies at z~1

We present a Principal Component Analysis (PCA)-based spectral classification, eta, for the first 5600 galaxies observed in the DEEP2 Redshift Survey. This parameter provides a very pronounced separation between absorption and emission dominated galaxy spectra - corresponding to passively evolving and actively star-forming galaxies in the survey respectively. In addition it is shown that despite the high resolution of the observed spectra, this parameter alone can be used to quite accurately reconstruct any given galaxy spectrum, suggesting there are not many `degrees of freedom' in the observed spectra of this galaxy population. It is argued that this form of classification, eta, will be particularly valuable in making future comparisons between high and low-redshift galaxy surveys for which very large spectroscopic samples are now readily available, particularly when used in conjunction with high-resolution spectral synthesis models which will be made public in the near future. We also discuss the relative advantages of this approach to distant galaxy classification compared to other methods such as colors and morphologies. Finally, we compare the classification derived here with that adopted for the 2dF Galaxy Redshift Survey and in so doing show that the two systems are very similar. This will be particularly useful in subsequent analyses when making comparisons between results from each of these surveys to study evolution in the galaxy populations and large-scale structure.

astro-ph↗

arXiv2003

Observing the Formation of the Hubble Sequence in the Great Observatories Origins Deep Survey

Understanding the physical formation of the Hubble sequence remains one of the most important unsolved astrophysical problems. Searches for proto-disks and proto-ellipticals can now be effectively done using deep wide-field Hubble Space Telescope images taken with the new Advanced Camera for Surveys. Through an analysis of concentrations (C), asymmetries (A) and clumpiness values (S) (CAS) of galaxies found in the GOODS Field South, we are able to identify objects possibly forming onto the Hubble sequence. Using this approach, we detect a sizeable population of star forming luminous diffuse objects and star forming luminous asymmetric objects between redshifts 0.5 < z < 2. These galaxies have extremely low light concentrations, or high asymmetries, with absolute magnitudes M_B < -19. The luminous diffuse objects are found in abundance between z = 1 - 2, with fewer objects at z > 2 and z < 1. The luminous asymmetric objects are found at a similar abundance, with a peak at z~1. We argue that these galaxies are a subset of modern disks and ellipticals in formation. The co-moving volume density of the luminous diffuse objects between z = 1 - 2 is similar to the local density of bright disk galaxies, with values ~5*10^{5} Gpc^{-3}. The SEDs of these objects are mostly consistent with starbursts, or star-forming normal galaxies, with average uncorrected for extinction star formation rates of ~4 M_0 yr^{-1}. These galaxies also host 35-40% of the star formation activity at 1 < z < 2. We briefly discuss the implications of these objects for understanding the origin of the Hubble sequence.

astro-ph↗

arXiv2003

The Size Evolution of High Redshift Galaxies

Hubble Space Telescope images of high-redshift galaxies selected via color and photometric redshifts are used to examine the size and axial-ratio distribution of galaxies as a function of redshift at lookback times t > 8 Gyr. These parameters are measured at rest-frame UV wavelengths (1200-2000 Angstroms) on images with a rest-frame resolution of less than 0.8 kpc. Galaxy radii are found to scale with redshift approximately as the inverse of the Hubble parameter H(z). This is in accord with the theoretical expectation that the typical sizes of the luminous parts of galaxies should track the expected evolution in the virial radius of dark-matter halos. The mean ratio of semi-major to semi-minor axis for a bright well-resolved sample of galaxies at z ~ 4 is b/a = 0.65, suggesting that these Lyman break galaxies are not drawn from a spheroidal population. However the median concentration index of this sample is C = 3.5, which is closer to the typical concentration indices, C ~ 4, of nearby elliptical galaxies than to the values, C<2 for local disk galaxies of type Sb and later.

astro-ph↗

arXiv2003

The Internal Ultraviolet-to-Optical Color Dispersion: Quantifying the Morphological K-Correction

We present a quantitative measure of the internal color dispersion within galaxies, which quantifies differences in morphology as a function of wavelength. We apply this statistic to a local galaxy sample with archival images at 1500 and 2500 Angstroms from the Ultraviolet Imaging Telescope, and ground-based B-band observations to investigate how the color dispersion relates to global galaxy properties. The intenal color dispersion generally correlates with transformations in galaxy morphology as a function of wavelength, i.e., it quantifies the morphological K-correction. Mid-type spiral galaxies exhibit the highest dispersion in their internal colors, which stems from differences in the bulge, disk, and spiral-arm components. Irregulars and late-type spirals show moderate internal color dispersion, which implies that young stars generally dominate the colors. Ellipticals, lenticulars, and early-type spirals generally have low or negligible internal color dispersion, which indicates that the stars contributing to the UV-to-optical emission have a very homogeneous distribution. We discuss the application of the internal color dispersion to high-redshift galaxies in deep, Hubble Space Telescope images. By simulating local galaxies at cosmological distances, many of the galaxies have luminosities that are sufficiently bright at rest--frame optical wavelengths to be detected within the limits of the currently deepest near-infrared surveys even with no evolution. Under assumptions that the luminosity and color evolution of the local galaxies conform with the measured values of high-redshift objects, we show that galaxies' intrinsic internal color dispersion remains measurable out to z ~ 3.

astro-ph↗

arXiv2003

Evidence for a Major Merger Origin of High Redshift Sub-millimeter Galaxies

Sub-mm detected galaxies located at redshifts $z > 1$ host a major fraction of the bolometric luminosity at high redshifts due to thermal emission from heated dust grains, yet the nature of these objects remains a mystery. The major problem in understanding their origin is whether the dust heating mechanism is predominantly caused by star formation or active galactic nuclei, and what triggered this activity. We address this issue by examining the structures of a sample of 11 sub-mm galaxies imaged with STIS on the Hubble Space Telescope. We argue that ~61pm21% of these sub-mm sources are undergoing an active major merger using the CAS quantitative morphological system. We rule out at ~5sigma confidence that these sub-mm galaxies are normal Hubble types at high redshift. This merger fraction appears to be higher than for Lyman-break galaxies undergoing mergers at similar redshifts. Using reasonable constraints on the stellar masses of Lyman-Break galaxies and these sub-mm sources, we further argue that at redshifts z~2-3 systems with high stellar masses are more likely than lower mass galaxies to be involved in major mergers.

astro-ph↗

arXiv2003

A Direct Measurement of Major Galaxy Mergers at z < 3

This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z ~ 3. We identify major mergers using the model-independent CAS (concentration, asymmetry, clumpiness) physical morphological system on galaxies detected, and photometrically selected, in the WFPC2 and NICMOS Hubble Deep Field North. We specifically use the asymmetric distributions of rest-frame optical light measured through the asymmetry parameter (A) to determine the fraction of galaxies undergoing major mergers as a function of redshift (z), stellar mass (M_{*}), and absolute magnitude (M_{B}). We find that the fraction of galaxies consistent with undergoing a major merger increases with redshift for all galaxies, but most significantly, at 5 - 10 sigma confidence, for the most luminous and massive systems. The highest merger fractions we find are 40% - 50% for galaxies with M_{B} < -21, or M_{*} > 10^10 M_0 at z > 2.5, i.e., objects identified as Lyman-break galaxies. We model the merger fraction evolution in the form: f x (1+z)^m, finding m values ~4-6 for the most luminous and massive galaxies. We use these merger fractions combined with merger time scales calculated from N-body simulations to derive galaxy merger rates to z ~ 3. We also use stellar masses of HDF-N galaxies to determine the mass accretion rate of field galaxies involved in major mergers, and how this changes with redshift. We further find that the fraction of stellar mass density in galaxies involved in major mergers increases with redshift. By comparing merger fractions predicted in Cold Dark Matter semi-analytic models with our results there is a reasonably good agreement for the largest and brightest systems, although there are more low-mass galaxy mergers at lower redshifts than what these models predict. (abridged)

astro-ph↗