arXiv subjects

Peter R. M. Eisenhardt

Publications and source records attributed to Peter R. M. Eisenhardt.

66 records · Page 4Linked to original sources

arXiv2012

IDCS J1426.5+3508: Cosmological implications of a massive, strong lensing cluster at Z = 1.75

The galaxy cluster IDCS J1426.5+3508 at z = 1.75 is the most massive galaxy cluster yet discovered at z > 1.4 and the first cluster at this epoch for which the Sunyaev-Zel'Dovich effect has been observed. In this paper we report on the discovery with HST imaging of a giant arc associated with this cluster. The curvature of the arc suggests that the lensing mass is nearly coincident with the brightest cluster galaxy, and the color is consistent with the arc being a star-forming galaxy. We compare the constraint on M200 based upon strong lensing with Sunyaev-Zel'Dovich results, finding that the two are consistent if the redshift of the arc is z > 3. Finally, we explore the cosmological implications of this system, considering the likelihood of the existence of a strongly lensing galaxy cluster at this epoch in an LCDM universe. While the existence of the cluster itself can potentially be accomodated if one considers the entire volume covered at this redshift by all current high-redshift cluster surveys, the existence of this strongly lensed galaxy greatly exacerbates the long-standing giant arc problem. For standard LCDM structure formation and observed background field galaxy counts this lens system should not exist. Specifically, there should be no giant arcs in the entire sky as bright in F814W as the observed arc for clusters at z \geq 1.75, and only \sim 0.3 as bright in F160W as the observed arc. If we relax the redshift constraint to consider all clusters at z \geq 1.5, the expected number of giant arcs rises to \sim15 in F160W, but the number of giant arcs of this brightness in F814W remains zero. These arc statistic results are independent of the mass of IDCS J1426.5+3508. We consider possible explanations for this discrepancy.

astro-ph.CO↗

arXiv2011

Baryon Content of Massive Galaxy Clusters at z=0-0.6

We study the relationship between two major baryonic components in galaxy clusters, namely the stars in galaxies, and the ionized gas in the intracluster medium (ICM), using 94 clusters that span the redshift range 0-0.6. Accurately measured total and ICM masses from Chandra observations, and stellar masses derived from the Wide-field Infrared Survey Explorer and the Two-Micron All-Sky Survey allow us to trace the evolution of cluster baryon content in a self-consistent fashion. We find that, within r_{500}, the evolution of the ICM mass--total mass relation is consistent with the expectation of self-similar model, while there is no evidence for redshift evolution in the stellar mass--total mass relation. This suggests that the stellar mass and ICM mass in the inner parts of clusters evolve differently.

astro-ph.CO↗

arXiv2011

WISE Discovery of Low Metallicity Blue Compact Dwarf Galaxies

We report two new low metallicity blue compact dwarf galaxies (BCDs), WISEP J080103.93+264053.9 (hereafter W0801+26) and WISEP J170233.53+180306.4 (hereafter W1702+18), discovered using the Wide-field Infrared Survey Explorer (WISE). We identified these two BCDs from their extremely red colors at mid-infrared wavelengths, and obtained follow-up optical spectroscopy using the Low Resolution Imaging Spectrometer on Keck I. The mid-infrared properties of these two sources are similar to the well studied, extremely low metallicity galaxy SBS 0335-052E. We determine metallicities of 12 + log(O/H) = 7.75 and 7.63 for W0801+26 and W1702+18, respectively, placing them amongst a very small group of very metal deficient galaxies (Z < 1/10 Zsun). Their > 300 Angstrom Hbeta equivalent widths, similar to SBS 0335-052E, imply the existence of young (< 5 Myr) star forming regions. We measure star formation rates of 2.6 and 10.9 Msun/yr for W0801+26 and W1702+18, respectively. These BCDs, showing recent star formation activity in extremely low metallicity environments, provide new laboratories for studying star formation in extreme conditions and are low-redshift analogs of the first generation of galaxies to form in the universe. Using the all-sky WISE survey, we discuss a new method to identify similar star forming, low metallicity BCDs.

astro-ph.CO↗

arXiv2010

The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance

The all sky surveys done by the Palomar Observatory Schmidt, the European Southern Observatory Schmidt, and the United Kingdom Schmidt, the InfraRed Astronomical Satellite and the 2 Micron All Sky Survey have proven to be extremely useful tools for astronomy with value that lasts for decades. The Wide-field Infrared Survey Explorer is mapping the whole sky following its launch on 14 December 2009. WISE began surveying the sky on 14 Jan 2010 and completed its first full coverage of the sky on July 17. The survey will continue to cover the sky a second time until the cryogen is exhausted (anticipated in November 2010). WISE is achieving 5 sigma point source sensitivities better than 0.08, 0.11, 1 and 6 mJy in unconfused regions on the ecliptic in bands centered at wavelengths of 3.4, 4.6, 12 and 22 microns. Sensitivity improves toward the ecliptic poles due to denser coverage and lower zodiacal background. The angular resolution is 6.1, 6.4, 6.5 and 12.0 arc-seconds at 3.4, 4.6, 12 and 22 microns, and the astrometric precision for high SNR sources is better than 0.15 arc-seconds.

astro-ph.IM↗

arXiv2010

The Formation of Massive Cluster Galaxies

We present composite 3.6 and 4.5 micron luminosity functions for cluster galaxies measured from the Spitzer Deep, Wide-Field Survey (SDWFS) for 0.3 1.3) we also witness a statistically significant (>5sigma) disagreement between the measured luminosity function and the continuation of the passive evolution model from lower redshifts. After considering potential systematic biases that might influence our highest redshift data points, we interpret the observed deviation as potential evidence for ongoing mass assembly at this epoch.

astro-ph.CO↗

arXiv2010

Ultracool Field Brown Dwarf Candidates Selected at 4.5 microns

We have identified a sample of cool field brown dwarf candidates using IRAC data from the Spitzer Deep, Wide-Field Survey (SDWFS). The candidates were selected from 400,000 SDWFS sources with [4.5] <= 18.5 mag and required to have [3.6]-[4.5] >= 1.5 and [4.5] - [8.0] <= 2.0 on the Vega system. The first color requirement selects objects redder than all but a handful of presently known brown dwarfs with spectral classes later than T7, while the second eliminates 14 probable reddened AGN. Optical detection of 4 of the remaining 18 sources implies they are likely also AGN, leaving 14 brown dwarf candidates. For two of the brightest candidates (SDWFS J143524.44+335334.6 and SDWFS J143222.82+323746.5), the spectral energy distributions including near-infrared detections suggest a spectral class of ~ T8. The proper motion is < 0.25 "/yr, consistent with expectations for a luminosity inferred distance of >70 pc. The reddest brown dwarf candidate (SDWFS J143356.62+351849.2) has [3.6] - [4.5]=2.24 and H - [4.5] > 5.7, redder than any published brown dwarf in these colors, and may be the first example of the elusive Y-dwarf spectral class. Models from Burrows et al. (2003) predict larger numbers of cool brown dwarfs should be found for a Chabrier (2003) mass function. Suppressing the model [4.5] flux by a factor of two, as indicated by previous work, brings the Burrows models and observations into reasonable agreement. The recently launched Wide-field Infrared Survey Explorer (WISE) will probe a volume ~40x larger and should find hundreds of brown dwarfs cooler than T7.

astro-ph.SR↗

arXiv2009

The Cosmic Evolution of AGN in galaxy clusters

We present the surface density of luminous active galactic nuclei (AGN) associated with a uniformly selected galaxy cluster sample identified in the 8.5 square degree Bootes field of the NOAO Deep Wide-Field Survey. The clusters are distributed over a large range of redshift (0 < z < 1.5) and we identify AGN using three different selection criteria: mid-IR color, radio luminosity, and X-ray luminosity. Relative to the field, we note a clear overdensity of the number of AGN within 0.5 Mpc of the cluster centers at z > 0.5. The amplitude of this AGN overdensity increases with redshift. Although there are significant differences between the AGN populations probed by each selection technique, the rise in cluster AGN surface density generally increases more steeply than that of field quasars. In particular, X-ray selected AGN are at least three times more prevalent in clusters at 1 < z < 1.5 compared to clusters at 0.5 < z < 1. This effect is stronger than can be explained by the evolving median richness of our cluster sample. We thus confirm the existence of a Butcher-Oemler type effect for AGN in galaxy clusters, with the number of AGN in clusters increasing with redshift.

astro-ph.CO↗

arXiv2008

Clusters of Galaxies in the First Half of the Universe from the IRAC Shallow Survey

We have identified 335 galaxy cluster and group candidates, 106 of which are at z > 1, using a 4.5 um selected sample of objects from a 7.25 deg^2 region in the Spitzer Infrared Array Camera (IRAC) Shallow Survey. Clusters were identified as 3-dimensional overdensities using a wavelet algorithm, based on photometric redshift probability distributions derived from IRAC and NOAO Deep Wide-Field Survey data. We estimate only ~10% of the detections are spurious. To date 12 of the z > 1 candidates have been confirmed spectroscopically, at redshifts from 1.06 to 1.41. Velocity dispersions of ~750 km/s for two of these argue for total cluster masses well above 10^14 M_sun, as does the mass estimated from the rest frame near infrared stellar luminosity. Although not selected to contain a red sequence, some evidence for red sequences is present in the spectroscopically confirmed clusters, and brighter galaxies are systematically redder than the mean galaxy color in clusters at all redshifts. The mean I - [3.6] color for cluster galaxies up to z ~ 1 is well matched by a passively evolving model in which stars are formed in a 0.1 Gyr burst starting at redshift z_f = 3. At z > 1, a wider range of formation histories is needed, but higher formation redshifts (i.e. z_f > 3) are favored for most clusters.

astro-ph↗

arXiv2006

The Mass Assembly History of Field Galaxies: Detection of an Evolving Mass Limit for Star Forming Galaxies

We characterize the mass-dependent evolution in a large sample of more than 8,000 galaxies using spectroscopic redshifts drawn from the DEEP2 Galaxy Redshift Survey in the range 0.4 < z < 1.4 and stellar masses calculated from K-band photometry obtained at Palomar Observatory. Using restframe (U-B) color and [OII] equivalent widths, we distinguish star-forming from passive populations in order to explore the nature of "downsizing''--a pattern in which the sites of active star formation shift from high mass galaxies at early times to lower mass systems at later epochs. Over the redshift range probed, we identify a mass limit, M_Q, above which star formation appears to be quenched. The physical mechanisms responsible for downsizing can thus be empirically quantified by charting the evolution in this threshold mass. We find that M_Q decreases with time by a factor of ~3 across the redshift range sampled according with a redshift dependence of (1+z)^3.5. To further constrain possible quenching mechanisms, we investigate how this downsizing signal depends on local galaxy environment. For the majority of galaxies in regions near the median density, there is no significant correlation between downsizing and environment. However, a trend is observed in the comparison between more extreme environments that are more than 3 times overdense or underdense relative to the median. Here, we find that downsizing is accelerated in overdense regions which host higher numbers of massive, early-type galaxies and fewer late-types as compared to the underdense regions. Our results significantly constrain recent suggestions for the origin of downsizing and indicate that the process for quenching star formation must, primarily, be internally driven. (Abridged)

astro-ph↗

arXiv2006

The Stellar Masses and Star Formation Histories of Galaxies at z ~ 6: Constraints from Spitzer Observations in the Great Observatories Origins Deep Survey

Using the deep Spitzer IRAC observations of the Great Observatories Origins Deep Survey (GOODS), we study the stellar masses and star formation histories of galaxies at z ~ 6. Our study is based on the i-band dropout sample selected from the GOODS southern and northern fields (~ 330 arcmin^2 in total), several of which already have spectroscopic confirmations. In total, we derive stellar masses for 53 i-band dropouts that have robust IRAC detections. These galaxies have typical stellar masses of ~ 1e10 M_sun and typical ages of a couple of hundred million years, consistent with earlier results based on a smaller sample of z ~ 6 galaxies in the HUDF. We also study 79 i-band dropouts that are invisible in the IRAC data and find that they are typically less massive by a factor of ten. Based on our mass estimates, we derive a lower limit to the global stellar mass density at z ~ 6. Considering the range of systematic uncertainties in the derived stellar masses, this lower limit is 1.1--6.7e6 M_sun/Mpc^3 (co-moving), which is 0.2--1.1% of the present-day value. The prospect of detecting the progenitors of the most massive galaxies at yet higher redshifts is explored: a deep, wide-field near-IR survey using our current technology could possibly result in positive detections at z > 7. We also investigate the implication of our results for reionzation, and find that the progenitors of the galaxies comparable to those in our sample, even in the most optimized (probably unrealistic) scenario, cannot sustain the reionization for a period longer than ~ 2 million years. Thus most of the photons required for reionization must have been provided by other sources, such as the progenitors of the dwarf galaxies that are far below our current detection capability. (Abridged)

astro-ph↗

arXiv2005

Rest-frame Ultraviolet-to-Optical Properties of Galaxies at z ~ 6 and 5 in the Hubble Ultra Deep Field: from Hubble to Spitzer

We use data from the first epoch of observations with the IRAC/Spitzer for the GOODS to detect and study a collection of LBGs at z ~ 6 to 5 in the HUDF, six of which have spectroscopic confirmation. At these redshifts, IRAC samples rest-frame optical light in the range 0.5 to 0.8 um, where the effects of dust extinction are smaller and the sensitivity to light from evolved stars is greater than at shorter, rest-frame UV observable from the ground or with the HST. As such, it provides useful constraints on the ages and masses of these galaxies' stellar populations. We find that the SEDs for many of these galaxies are best fitted by models of stellar populations with masses of a few x 1e10 M_sun, and with ages of a few hundred Myrs, values quite similar to those derived for typical LBGs at z ~ 3. When the universe was only 1 Gyr old, some galaxies had already formed a mass of stars approaching that of the present-day Milky Way, and that they started forming those stars at z > 7, and in some cases much earlier. We find that the lower limits to the space density for galaxies in this mass range are consistent with predictions from recent hydrodynamic simulations of structure formation in a LCDM universe. All objects in our samples are consistent with having solar metallicity, suggesting that they might have already been significantly polluted by metals. The values for dust reddening derived from the model fitting are low or zero, and we find that some of the galaxies have rest-frame UV colors that are even bluer than those predicted by the stellar population models to which we compare them. These colors might be attributed to the presence of very massive stars (> 100 M_sun), or by weaker intergalactic HI absorption than what is commonly assumed.

astro-ph↗

arXiv2004

High-Redshift Extremely Red Objects in the HST Ultra Deep Field Revealed by the GOODS IRAC Observations

Using early data from the Infrared Array Camera (IRAC) on the Spitzer Space Telescope, taken for the Great Observatories Origins Deep Survey (GOODS), we identify and study objects that are well-detected at 3.6um, but are very faint (and in some cases, invisible) in the Hubble Ultra Deep Field (HUDF) ACS and NICMOS images and in very deep VLT Ks-band imaging. We select a sample of 17 objects with f_nu(3.6um)/f_nu(z_{850})>20. The analysis of their spectral energy-distributions (SEDs) from 0.4 to 8.0um shows that the majority of these objects cannot be satisfactorily explained without a well-evolved stellar population. We find that most of them can be well fitted by a simple two-component model, where the primary component represents a massive, old population that dominates the strong IR emission, while the secondary component represents a low amplitude, on-going star-formation process that accounts for the weak optical fluxes. Their estimated photometric redshifts (z_p) range from 1.6 to 2.9, with the median at z_p=2.4. For the simple star formation histories considered here, their corresponding stellar masses range from 0.1 to 1.6e11 M_sun for a Chabrier Initial Mass Function (IMF). Their median rest-frame Ks-band absolute magnitude is -22.9 mag in AB system, or 1.5xL*(K) for present--day elliptical galaxies. In the scenario of pure luminosity evolution, such objects may be direct progenitors for at least 14 to 51% of the local population of early--type galaxies. Due to the small cosmic volume of the HUDF, however, this simple estimate could be affected by other effects such as cosmic variance and the strong clustering of massive galaxies. A full analysis of the entire GOODS area is now underway to assess such effects.

astro-ph↗