Emeric Le Floc’h

Black Hole Masses and Eddington Ratios at 0.3 < z < 4

We study the distribution of Eddington luminosity ratios, Lbol/LEdd, of active galactic nuclei (AGNs) discovered in the AGN and Galaxy Evolution Survey (AGES). We combine Hβ, Mg II, and C IV line widths with continuum luminosities to estimate black hole (BH) masses in 407 AGNs, covering the redshift range z ~ 0.3-4 and the bolometric luminosity range Lbol ~ 1045-1047 ergs s-1. The sample consists of X-ray or mid-infrared (24 μm) point sources with optical magnitude R ≤ 21.5 mag and optical emission-line spectra characteristic of AGNs. For the range of luminosity and redshift probed by AGES, the distribution of estimated Eddington ratios is well described as log-normal, with a peak at Lbol/LEdd 1/4 and a dispersion of 0.3 dex. Since additional sources of scatter are minimal, this dispersion must account for contributions from the scatter between estimated and true BH mass and the scatter between estimated and true bolometric luminosity. Therefore, we conclude that (1) neither of these sources of error can contribute more than ~0.3 dex rms, and (2) the true Eddington ratios of optically luminous AGNs are even more sharply peaked. Because the mass estimation errors must be smaller than ~0.3 dex, we can also investigate the distribution of Eddington ratios at fixed BH mass. We show for the first time that the distribution of Eddington ratios at fixed BH mass is peaked, and that the dearth of AGNs at a factor of ~10 below Eddington is real and not an artifact of sample selection. These results provide strong evidence that supermassive BHs gain most of their mass while radiating close to the Eddington limit, and they suggest that the fueling rates in luminous AGNs are ultimately determined by BH self-regulation of the accretion flow rather than galactic-scale dynamical disturbances.

Probing Cosmic Star Formation Using Long Gamma-Ray Bursts: New Constraints from the Spitzer Space Telescope*

We report on IRAC 4.5 μm, IRAC 8.0 μm, and MIPS 24 μm deep observations of 16 gamma-ray burst (GRB) host galaxies performed with the Spitzer Space Telescope, and we investigate in the thermal infrared the presence of evolved stellar populations and dust-enshrouded star-forming activity associated with these objects. Our sample is derived from GRBs that were identified with subarcsecond localization between 1997 and 2001, and only a very small fraction (~20%) of the targeted sources are detected down to f4.5 μm ~ 3.5 μJy and f24 μm ~ 85 μJy (3 σ). This likely argues against a population dominated by massive and strongly starbursting (i.e., SFR 100 M☉ yr-1) galaxies as has been recently suggested from submillimeter/radio and optical studies of similarly selected GRB hosts. Furthermore, we find evidence that some GRBs do not occur in the most infrared luminous regions—hence the most actively star-forming environments—of their host galaxies. Should the GRB hosts be representative of all star-forming galaxies at high redshift, models of infrared galaxy evolution indicate that 50% of GRB hosts should have f24 μm 100 μJy. Unless the identification of GRBs prior to 2001 was prone to strong selection effects biasing our sample against dusty galaxies, we infer in this context that the GRBs identified with the current techniques cannot be directly used as unbiased probes of the global and integrated star formation history of the universe.

The 1 < z < 5 Infrared Luminosity Function of Type I Quasars

We determine the rest-frame 8 μm luminosity function of type I quasars over the redshift range 1 < z < 5. Our sample consists of 292 24 μm sources brighter than 1 mJy selected from 7.17 deg2 of the Spitzer Space Telescope MIPS survey of the NOAO Deep Wide-Field Survey Bootes field. The AGN and Galaxy Evolution Survey (AGES) has measured redshifts for 270 of the R < 21.7 sources, and we estimate that the contamination of the remaining 22 sources by stars and galaxies is low. We are able to select quasars missed by ultraviolet excess quasar surveys, including reddened type I quasars and 2.2 < z < 3.0 quasars with optical colors similar to main-sequence stars. We find that reddened type I quasars comprise ~20% of the type I quasar population. Nonetheless, the shape, normalization, and evolution of the rest-frame 8 μm luminosity function are comparable to those of quasars selected from optical surveys. The 8 μm luminosity function of type I quasars is well approximated by a power law with index -2.75 ± 0.14. We directly measure the peak of the quasar space density to be at z = 2.6 ± 0.3.

Infrared Spectral Energy Distributions of z ~ 0.7 Star-forming Galaxies

We analyze the infrared (IR) spectral energy distributions (SEDs) for 10 μm < λrest < 100 μm for ~600 galaxies at z ~ 0.7 in the extended Chandra Deep Field South by stacking their Spitzer 24, 70, and 160 μm images. We place interesting constraints on the average IR SED shape in two bins: the brightest 25% of z ~ 0.7 galaxies detected at 24 μm, and the remaining 75% of individually detected galaxies. Galaxies without individual detections at 24 μm were not well detected at 70 and 160 μm even through stacking. We find that the average IR SEDs of z ~ 0.7 star-forming galaxies fall within the diversity of z ~ 0 templates. While dust obscuration LIR/LUV seems to be only a function of star formation rate (SFR; ~LIR + LUV), not of redshift, the dust temperature of star-forming galaxies (with SFR ~ 10 M☉ yr-1 ) at a given IR luminosity was lower at z ~ 0.7 than today. We suggest an interpretation of this phenomenology in terms of dust geometry: intensely star-forming galaxies at z ~ 0 are typically interacting, and host dense centrally concentrated bursts of star formation and warm dust temperatures. At z ~ 0.7, the bulk of intensely star-forming galaxies are relatively undisturbed spirals and irregulars, and we postulate that they have large amounts of widespread lower density star formation, yielding lower dust temperatures for a given IR luminosity. We recommend which IR SEDs are most suitable for modeling intermediate-redshift galaxies with different SFRs.

Detecting Faint Galaxies by Stacking at 24 μm

We stack Spitzer 24 μm images for ~7000 galaxies with 0.1 ≤ z < 1 in the Chandra Deep Field South to probe the thermal dust emission in low-luminosity galaxies over this redshift range. Through stacking, we can detect mean 24 μm fluxes that are more than an order of magnitude below the individual detection limit. We find that the correlations for low- and moderate-luminosity galaxies between the average LIR/LUV and rest-frame B-band luminosity, and between the star formation rate (SFR) and LIR/LUV, are similar to those in the local universe. This verifies that oft-used assumption in deep UV/optical surveys that the dust obscuration-SFR relation for galaxies with SFR ≤ 20 M☉ yr-1 varies little with epoch. We have used this relation to derive the cosmic IR luminosity density from z = 1 to z = 0.1. The results also demonstrate directly that little of the bolometric luminosity of the galaxy population arises from the faint end of the luminosity function, indicating a relatively flat faint-end slope of the IR luminosity function with a power-law index of 1.2 ± 0.3.

An MMT Hectospec Redshift Survey of 24 μm Sources in the Spitzer First Look Survey

We present a spectroscopic survey using the MMT Hectospec fiber spectrograph of 24 μm sources selected with the Spitzer Space Telescope in the Spitzer First Look Survey. We report 1296 new redshifts for 24 μm sources, including 599 with fν(24 μm) ≥ 1 mJy. Combined with 291 additional redshifts for sources from the Sloan Digital Sky Survey (SDSS), our observing program was highly efficient and is ~90% complete for i ≤ 21 mag and fν(24 μm) ≥ 1 mJy and 35% complete for i ≤ 20.5 mag and 0.3 mJy ≤ fν(24 μm) < 1.0 mJy. Our Hectospec survey includes 1078 and 168 objects spectroscopically classified as galaxies and QSOs, respectively. Combining the Hectospec and SDSS samples, we find 24 μm-selected galaxies to zgal ≤ 0.98 and QSOs to zQSO ≤ 3.6, with mean redshifts of zgal = 0.27 and zQSO = 1.1. As part of this publication, we include the redshift catalogs and the reduced spectra; these are also available through the NASA/IPAC Infrared Science Archive.

Infrared luminosity functions of AKARI Sloan Digital Sky Survey galaxies

By cross-correlating the AKARI all-sky survey in six infrared (IR) bands (9, 18, 65, 90, 140 and 160 μm) with the Sloan Digital Sky Survey (SDSS) galaxies, we identified 2357 IR galaxies with a spectroscopic redshift. This is not just one of the largest samples of local IR galaxies, but AKARI provides crucial far-IR (FIR) bands for accurately measuring the galaxy spectral energy distribution (SED) across the peak of the dust emission at > 100 μ m. By fitting modern IR SED models to the AKARI photometry, we measured the total infrared luminosity (L_(IR)) of individual galaxies. Using this L_(IR), we constructed the luminosity functions (LF) of IR galaxies at a median redshift of z= 0.031. The LF agrees well with that at z= 0.0082 (the Revised Bright Galaxy Sample), showing smooth and continuous evolution towards higher redshift LFs measured in the AKARI North Ecliptic Pole (NEP) deep field. By integrating the IR LF weighted by L_(IR), we measured the local cosmic IR luminosity density of Ω_(IR_= (3.8^(+5.8)_(−1.2)) × 10^8 L_⊙ Mpc^(−3). We separate galaxies into active galactic nuclei (AGN), star-forming galaxies (SFG) and composite by using the [N ii]/Hα versus [O iii]/Hβ line ratios. The fraction of AGN shows a continuous increase with increasing L_(IR) from 25 to 90 per cent at 9 10^(11) L_⊙, coinciding with the break of both the SFG and AGN IR LFs. At L_(IR)≤ 10^(11) L_⊙, SFG dominates IR LFs. Only 1.1 ± 0.1 per cent of Ω_(IR) is produced by luminous infrared galaxies (L_(IR) > 10^(11) L_⊙), and only 0.03 ± 0.01 per cent by ultraluminous infrared galaxies (L_(IR) > 10^(12) L_⊙) in the local Universe. Compared with high-redshift results from the AKARI NEP deep survey, we observed a strong evolution of Ω^(SFG)IR^∝ (1 +z)^(4.1±0.4) and Ω^(AGN)IR^∝ (1+z)^(4.1±0.5). Our results show that all of our measured quantities (IR LFs, L^*, Ω^(AGN)IR, Ω^(SFG)IR) show smooth and steady increase from lower redshift (the Revised Bright Galaxy Sample) to higher redshift (the AKARI NEP deep survey).

DUST PROPERTIES AND STAR FORMATION RATES IN STAR-FORMING DWARF GALAXIES

We have used the Spitzer Space Telescope to study the dust properties of a sample of star-forming dwarf galaxies. The differences in the mid-infrared spectral energy distributions for these galaxies, which, in general, are low-metallicity systems, indicate differences in the physical properties, heating, and/or distribution of the dust. Specifically, these galaxies have more hot dust and/or very small grains and less PAH emission than either spiral or higher luminosity starburst galaxies. As has been shown in previous studies, there is a gradual decrease in PAH emission as a function of metallicity. Because much of the energy from star formation in galaxies is reradiated in the mid-infrared, star formation rate indicators based on both line and continuum measurements in this wavelength range are coming into more common usage. We show that the variations in the interstellar medium properties of galaxies in our sample, as measured in the mid-infrared, result in over an order of magnitude spread in the computed star formation rates.

First Constraints on Source Counts at 350 μm

We have imaged a ~6 arcmin2 region in the Bootes Deep Field using the 350 μm-optimized second-generation Submillimeter High Angular Resolution Camera (SHARC II), achieving a peak 1 σ sensitivity of ~5 mJy. We detect three sources above 3 σ, and determine a spurious source detection rate of 1.09 in our maps. In the absence of 5 σ detections, we rely on deep 24 μm and 20 cm imaging to deduce which sources are most likely to be genuine, giving two real sources. From this we derive an integral source count of 0.84 sources arcmin-2 at S > 13 mJy, which is consistent with 350 μm source count models that have an IR-luminous galaxy population evolving with redshift. We use these constraints to consider the future for ground-based short-submillimeter surveys.

The galaxy density environment of gamma-ray burst host galaxies

We analyze cross-correlation functions between Gamma-Ray Burst (GRB) hosts and surrounding galaxies. We have used data obtained with the Very Large Telescope at Cerro Paranal (Chile), as well as public Hubble Space Telescope data. Our results indicate that Gamma-Ray Burst host galaxies do not reside in high galaxy density environments. Moreover, the host-galaxy cross-correlations show a relatively low amplitude. Our results are in agreement with the cross-correlation function between star-forming galaxies and surrounding objects in the HDF-N.