Lei Bai

The 24 Micron Source Counts in Deep Spitzer Space Telescope Surveys

Galaxy source counts in the infrared provide strong constraints on the evolution of the bolometric energy output from distant galaxy populations. We present the results from deep 24 μm imaging from Spitzer surveys, which include ≈5 × 10^4 sources to an 80% completeness of ≃ 60 μJy. The 24 μm counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with a super-Euclidean rate between 0.4 and 4 mJy, and converge below ~0.3 mJy. The 24 μm counts exceed expectations from nonevolving models by a factor of ≳10 at S_ν ~ 0.1 mJy. The peak in the differential number counts corresponds to a population of faint sources that is not expected from predictions based on 15 μm counts from the Infrared Space Observatory. We argue that this implies the existence of a previously undetected population of infrared-luminous galaxies at z ~ 1-3. Integrating the counts to 60 μJy, we derive a lower limit on the 24 μm background intensity of 1.9 ± 0.6 nW m^(-2) sr^(-1) of which the majority (~60%) stems from sources fainter than 0.4 mJy. Extrapolating to fainter flux densities, sources below 60 μJy contribute 0.8^(+0.9)_(-0.4) nW m^(-2) sr^(-1) to the background, which provides an estimate of the total 24 μm background of 2.7^(+1.1)_(-0.7) nW m^(-2) sr^(-1).

IR Observations of MS 1054–03: Star Formation and Its Evolution in Rich Galaxy Clusters

We study the IR properties of galaxies in the cluster MS 1054-03 at z = 0.83 by combining MIPS 24 μm data with spectra of more than 400 galaxies and a very deep K-band-selected catalog. Nineteen IR cluster members are selected spectroscopically, and an additional 15 are selected by their photometric redshifts. We derive the IR luminosity function of the cluster and find strong evolution compared to the similar-mass Coma Cluster. The best-fitting Schechter function gives L = 11.49 L☉ with a fixed faint-end slope, about 1 order of magnitude larger than that in Coma. The rate of evolution of the IR luminosity from Coma to MS 1054-03 is consistent with that found in field galaxies, and it suggests that some internal mechanism, e.g., the consumption of the gas fuel, is responsible for the general decline of the cosmic SFR in different environments. The mass-normalized integrated SFR within 0.5R200 in MS 1054-03 also shows evolution compared with other rich clusters at lower redshifts, but the trend is less conclusive if the mass selection effect is considered. A nonnegligible fraction (13% ± 3%) of cluster members are forming stars actively, and the overdensity of IR galaxies is about 20 compared to the field. It is unlikely that clusters only passively accrete star-forming galaxies from the surrounding fields and have their star formation quenched quickly afterward; instead, many cluster galaxies still have large amounts of gas, and their star formation may be enhanced by the interaction with the cluster.

Far-infrared Source Counts at 70 and 160 Microns in Spitzer Deep Surveys

We derive galaxy source counts at 70 and 160 μm using the Multiband Imaging Photometer for Spitzer (MIPS) to map the Chandra Deep Field-South (CDF-S) and other fields. At 70 μm, our observations extend upward about 2 orders of magnitude in flux density from a threshold of 15 mJy, and at 160 μm they extend about an order of magnitude upward from 50 mJy. The counts are consistent with previous observations on the bright end. Significant evolution is detected at the faint end of the counts in both bands, by factors of 2-3 over no-evolution models. This evolution agrees well with models that indicate that most of the faint galaxies lie at redshifts between 0.7 and 0.9. The new Spitzer data already resolve about 23% of the cosmic far-infrared background at 70 μm and about 7% at 160 μm.

Confusion of Extragalactic Sources in the Mid- and Far-Infrared: Spitzer and Beyond

We use the source counts measured with the Multiband Imaging Photometer for Spitzer at 24, 70, and 160 μm to determine the 5 σ confusion limits due to extragalactic sources: 56 μJy, 3.2 mJy, and 40 mJy at 24, 70, and 160 μm, respectively. We also make predictions for confusion limits for a number of proposed far-infrared missions of larger aperture (3.5-10 m diameter).

The Infrared Luminosity Functions of Rich Clusters

We present MIPS observations of the cluster A3266. About 100 spectroscopic cluster members have been detected at 24 µm. The IR luminosity function (LF) in A3266 is very similar to that in the Coma cluster down to the detection limit LIR ∼ 10 43 ergs s −1 , suggesting a universal form of the bright end IR LF for local rich clusters with M ∼ 10 15 M⊙. The shape of the bright end of the A3266-Coma composite IR LF is not significantly different from that of nearby field galaxies, but the fraction of IR-bright galaxies (SFR > 0.2M⊙ yr −1 ) in both clusters increases with cluster-centric radius. The decrease of the blue galaxy fraction toward the high density cores only accounts for part of the trend; the fraction of red galaxies with moderate SFRs (0.2 M⊙ yr −1 < SFR < 1 M⊙ yr −1 ) also decreases with increasing galaxy density. These results suggest that for the IR bright galaxies, nearby rich clusters are distinguished from the field by a lower star-forming galaxy fraction, but not by a change in L ∗ . The composite IR LF of Coma and A3266 shows strong evolution when compared with the composite IR LF of two z ∼ 0.8 clusters, MS 1054 and RX J0152, with L ∗ ∝ (1+z) 3.2 +0.7 0.7,� ∗ IR ∝ (1+z) 1.7 +1.0 1.0 . This L ∗ evolution is indistinguishable from that in the field, and the � ∗ evolution is stronger, but still consistent with that in the field. The similarity of the evolution of bright-end IR LF in very different cluster and field environments suggests either this evolution is driven by the mechanism that works in both environments, or clusters continually replenish their star-forming galaxies from the field, yielding an evolution in the IR LF that is similar to the field. The mass-normalized integrated star formation rates (SFRs) of clusters within 0.5R200 also evolve strongly with redshift, as (1 + z) 5.3 . Subject headings: galaxies: clusters: individual (Abell 3266) — galaxies: luminosity function — infrared: galaxies

A SPECTROSCOPICALLY CONFIRMED EXCESS OF 24 μm SOURCES IN A SUPER GALAXY GROUP AT z = 0.37: ENHANCED DUSTY STAR FORMATION RELATIVE TO THE CLUSTER AND FIELD ENVIRONMENT

To trace how dust-obscured star formation varies with environment, we compare the fraction of 24 μm sources in a super galaxy group to the field and a rich galaxy cluster at z ~ 0.35. We draw on multi-wavelength observations9Based on observations made with (1) The ESO telescopes at Paranal Observatories under program IDs 072.A-0367, 076.B-0362, 078.B-0409; (2) the NASA/ESA Hubble Space Telescope (GO-10499); STScI is operated by the association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555; (3) the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA; support for this work was provided by NASA through an award issued by JPL/Caltech (GO-20683); (4) the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060; and (5) the Magellan 6.5 m telescope operated by OCIW. that combine Hubble, Chandra, and Spitzer imaging with extensive optical spectroscopy (>1800 redshifts) to isolate galaxies in each environment and thus ensure a uniform analysis. We focus on the four galaxy groups (σ1D = 303-580 km s–1) in supergroup 1120-12 that will merge to form a galaxy cluster comparable in mass to Coma. We find that (1) the fraction of supergroup galaxies with SFRIR ≥ 3 M sun yr–1 is 4 times higher than in the cluster (32% ± 5% versus 7% ± 2%); (2) the supergroups infrared luminosity function confirms that it has a higher density of IR members compared to the cluster and includes bright IR sources (log(L IR)[erg s–1] >45) not found in galaxy clusters at z lsim 0.35; and (3) there is a strong trend of decreasing 24 μm fraction with increasing galaxy density, i.e., an infrared-density relation, not observed in the cluster. These dramatic differences are surprising because the early-type fraction in the supergroup is already as high as in clusters, i.e., the timescales for morphological transformation cannot be strongly coupled to when the star formation is completely quenched. The supergroup has a significant fraction (~17%) of luminous, low-mass (10.0 < log(M *)[M sun] < 10.6), SFRIR ≥ 3 M sun yr–1 members that are outside the group cores (R proj ≥ 0.5 Mpc); once their star formation is quenched, most will evolve into faint red galaxies. Our analysis indicates that the supergroups 24 μm population also differs from that in the field: (1) despite the supergroup having twice the fraction of E/S0s as the field, the fraction of SFRIR ≥ 3 M sun yr–1 galaxies is comparable in both environments, and (2) the supergroups IR luminosity function has a higher L*IR than that previously measured for the field.

Infrared luminosity function of the Coma Cluster

Using mid-IR and optical data, we deduce the total infrared (IR) luminosities of galaxies in the Coma Cluster and present their IR luminosity function (LF). The shape of the overall Coma IR LF does not show significant differences from the IR LFs of the general field, which indicates the general independence of global galaxy star formation from environment up to densities ~40 times greater than in the field (we cannot test such independence above LIR ≈ 1044 ergs s-1). However, a shallower faint-end slope and a smaller L are found in the core region (where the densities are still higher) compared to the outskirt region of the cluster, and most of the brightest IR galaxies are found outside the core region. The IR LF in the NGC 4839 group region does not show any unique characteristics. By integrating the IR LF, we find a total star formation rate in the cluster of about 97.0 M☉ yr-1. We also studied the contributions of early- and late-type galaxies to the IR LF. The late-type galaxies dominate the bright end of the LF, and the early-type galaxies, although only making up a small portion (≈15%) of the total IR emission of the cluster, contribute greatly to the number counts of the LF at LIR < 1043 ergs s-1.

The Late Stellar Assembly of Massive Cluster Galaxies via Major Merging

We present multiwavelength observations of the brightest galaxies in four X-ray-luminous groups at z ~ 0.37 that will merge to form a cluster comparable in mass to Coma. Ordered by increasing stellar mass, the four brightest group galaxies (BGGs) present a time sequence where BGG-1, 2, and 3 are in merging systems and BGG-4 is a massive remnant (M* = 6.7 × 1011 M☉). BGG-1 and 2 have bright, gravitationally bound companions and BGG-3 has two nuclei separated by only 2.5 kpc; thus, merging at z 3 Gyr, and their tight scatter in (B − V) color (σBV = 0.032) confirms that they formed the bulk of their stars at z > 0.9. Optical spectroscopy shows no signs of recent (< 1.5 Gyr) or ongoing star formation. Only two BGGs are weakly detected at 24 μm, and X-ray and optical data indicate that the emission in BGG-2 is due to an AGN. All four BGGs and their companions are early-type (bulge-dominated) galaxies, and they are embedded in diffuse stellar envelopes up to ~140 kpc across. The four BGG systems must evolve into the massive, red, early-type galaxies dominating local clusters. Our results show that (1) massive galaxies in groups and clusters form via dissipationless merging and (2) the group environment is critical for this process.

Far-Infrared Characterization of an Ultraluminous Starburst Associated with a Massively Accreting Black Hole at z = 1.15

As part of the All-Wavelength Extended Groth Strip International Survey (AEGIS), we describe the panchromatic characterization of an X-ray–luminous active galactic nucleus (AGN) in a merging galaxy at z = 1.15. This object is detected at infrared (8, 24, 70, and 160 μm), submillimeter (850 μm), and radio wavelengths, from which we derive a bolometric luminosity Lbol ∼ 9 x 10^12 L☉. We find that the AGN clearly dominates the hot dust emission below 40 μm but its total energetic power inferred from the hard X-rays is substantially less than the bolometric output of the system. About 50% of the infrared luminosity is indeed produced by a cold dust component that probably originates from enshrouded star formation in the host galaxy. In the context of a coeval growth of stellar bulges and massive black holes, this source might represent a “transition” object, sharing properties with both quasars and luminous starbursts. Study of such composite galaxies will help address how the star formation and disk-accretion phenomena may have regulated each other at high redshift and how this coordination may have participated in the buildup of the relationship observed locally between the masses of black holes and stellar spheroids.

The imacs cluster building survey. I. description of the survey and analysis methods

The IMACS Cluster Building Survey uses the wide field spectroscopic capabilities of the IMACS spectrograph on the 6.5 m Baade Telescope to survey the large-scale environment surrounding rich intermediate-redshift clusters of galaxies. The goal is to understand the processes which may be transforming star-forming field galaxies into quiescent cluster members as groups and individual galaxies fall into the cluster from the surrounding supercluster. This first paper describes the survey: the data taking and reduction methods. We provide new calibrations of star formation rates (SFRs) derived from optical and infrared spectroscopy and photometry. We demonstrate that there is a tight relation between the observed SFR per unit B luminosity, and the ratio of the extinctions of the stellar continuum and the optical emission lines. With this, we can obtain accurate extinction-corrected colors of galaxies. Using these colors as well as other spectral measures, we determine new criteria for the existence of ongoing and recent starbursts in galaxies.