arXiv subjects

D. Elbaz

Publications and source records attributed to D. Elbaz.

At least 127 records · Page 7Linked to original sources

arXiv2012

PACS-Herschel FIR detections of Lyman-alpha emitters at 2.0<z<3.5

In this work we analyze the physical properties of a sample of 56 spectroscopically selected star-forming (SF) Ly$α$ emitting galaxies at 2.0$\lesssim$z$\lesssim$3.5 using both a spectral energy distribution (SED) fitting procedure from rest-frame UV to mid-IR and direct 160$μ$m observations taken with the Photodetector Array Camera & Spectrometer (PACS) instrument onboard \emph{Herschel Space Observatory}. We define LAEs as those Ly$α$ emitting galaxies whose rest-frame Ly$α$ equivalent widths (Ly$α$ EW$_{rest-frame}$) are above 20Å, the typical threshold in narrow-band searches. Ly$α$ emitting galaxies with Ly$α$ EW$_{rest-frame}$ are called non-LAEs. As a result of an individual SED fitting for each object, we find that the studied sample of LAEs contains galaxies with ages mostly below 100Myr and a wide variety of dust attenuations, SFRs, and stellar masses. The heterogeneity in the physical properties is also seen in the morphology, ranging from bulge-like galaxies to highly clumpy systems. In this way, we find that LAEs at 2.0$\lesssim$z$\lesssim$3.5 are very diverse, and do not have a bimodal nature, as suggested in previous works. Furthermore, the main difference between LAEs and non-LAEs is their dust attenuation, because LAEs are not as dusty as non-LAEs. On the FIR side, four galaxies of the sample (two LAEs and two non-LAEs) have PACS-FIR counterparts. Their total IR luminosity place all of them in the ULIRG regime and are all dusty objects, with A$_{1200}$$\gtrsim$4mag. This is an indication from direct FIR measurements that dust and Ly$α$ emission are not mutually exclusive. This population of red and dusty LAEs is not seen at z$\sim$0.3, suggesting an evolution with redshift of the IR nature of galaxies selected via their Ly$α$ emission.

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arXiv2012

A Herschel view of the far-infrared properties of submillimetre galaxies

We study a sample of 61 submillimetre galaxies (SMGs) selected from ground-based surveys, with known spectroscopic redshifts and observed with Herschel as part of the PACS Evolutionary Probe (PEP) and the Herschel Multi-tiered Extragalactic Survey (HerMES) key programmes. We use the broad far-infrared wavelength coverage (100-600um) provided by the combination of PACS and SPIRE observations. Using a power-law temperature distribution model to derive infrared luminosities and dust temperatures, we measure a dust emissivity spectral index for SMGs of beta=2.0+/-0.2. Our results unveil the diversity of the SMG population. Some SMGs exhibit extreme infrared luminosities of ~10^13 Lsun and relatively warm dust components, while others are fainter (~10^12 Lsun) and are biased towards cold dust temperatures. The extreme infrared luminosities of some SMGs (LIR>10^12.7 Lsun, 26/61 systems) imply SFRs of >500Msun yr^-1. Such high SFRs are difficult to reconcile with a secular mode of star formation, and may instead correspond to a merger-driven stage in the evolution of these galaxies. Another observational argument in favour of this scenario is the presence of dust temperatures warmer than that of SMGs of lower luminosities (~40K as opposed to ~25K), consistent with observations of local ULIRGs triggered by major mergers and with results from hydrodynamic simulations of major mergers combined with radiative transfer calculations. Luminous SMGs are also offset from normal star-forming galaxies in the stellar mass-SFR plane, suggesting that they are undergoing starburst events with short duty cycles, compatible with the major merger scenario. On the other hand, a significant fraction of the low infrared luminosity SMGs have cold dust temperatures, are located close to the main sequence of star formation, and thus might be evolving through a secular mode of star formation. [abridged]

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arXiv2011

The Herschel Multi-Tiered Extragalactic Survey: SPIRE-mm Photometric Redshifts

We investigate the potential of submm-mm and submm-mm-radio photometric redshifts using a sample of mm-selected sources as seen at 250, 350 and 500 μm by the SPIRE instrument on Herschel. From a sample of 63 previously identified mm-sources with reliable radio identifications in the GOODS-N and Lockman Hole North fields 46 (73 per cent) are found to have detections in at least one SPIRE band. We explore the observed submm/mm colour evolution with redshift, finding that the colours of mm-sources are adequately described by a modified blackbody with constant optical depth τ = (ν/ν0)^β where β = +1.8 and ν0 = c/100 μm. We find a tight correlation between dust temperature and IR luminosity. Using a single model of the dust temperature and IR luminosity relation we derive photometric redshift estimates for the 46 SPIRE detected mm-sources. Testing against the 22 sources with known spectroscopic, or good quality optical/near-IR photometric, redshifts we find submm/mm photometric redshifts offer a redshift accuracy of |z|/(1+z) = 0.16 (< |z| >= 0.51). Including constraints from the radio-far IR correlation the accuracy is improved to |z|/(1 + z) = 0.15 (< |z| >= 0.45). We estimate the redshift distribution of mm-selected sources finding a significant excess at z > 3 when compared to ~ 850 μm selected samples.

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arXiv2011

A view of the narrow-line region in the infrared: active galactic nuclei with resolved fine-structure lines in the Spitzer archive

We queried the Spitzer archive for high-resolution observations with the Infrared Spectrograph of optically selected active galactic nuclei (AGN) for the purpose of identifying sources with resolved fine-structure lines that would enable studies of the narrow-line region (NLR) at mid-infrared wavelengths. By combining 298 Spitzer spectra with 6 Infrared Space Observatory spectra, we present kinematic information of the NLR for 81 z<=0.3 AGN. We used the [NeV], [OIV], [NeIII], and [SIV] lines, whose fluxes correlate well with each other, to probe gas photoionized by the AGN. We found that the widths of the lines are, on average, increasing with the ionization potential of the species that emit them. No correlation of the line width with the critical density of the corresponding transition was found. The velocity dispersion of the gas, sigma, is systematically higher than that of the stars, sigma_*, in the AGN host galaxy, and it scales with the mass of the central black hole, M_BH. Further correlations between the line widths and luminosities L, and between L and M_BH, are suggestive of a three dimensional plane connecting log(M_BH) to a linear combination of log(sigma) and log(L). Such a plane can be understood within the context of gas motions that are driven by AGN feedback mechanisms, or virialized gas motions with a power-law dependence of the NLR radius on the AGN luminosity. The M_BH estimates obtained for 35 type 2 AGN from this plane are consistent with those obtained from the M_BH-sigma_* relation.

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arXiv2011

Euclid Definition Study Report

Euclid is a space-based survey mission from the European Space Agency designed to understand the origin of the Universe's accelerating expansion. It will use cosmological probes to investigate the nature of dark energy, dark matter and gravity by tracking their observational signatures on the geometry of the universe and on the cosmic history of structure formation. The mission is optimised for two independent primary cosmological probes: Weak gravitational Lensing (WL) and Baryonic Acoustic Oscillations (BAO). The Euclid payload consists of a 1.2 m Korsch telescope designed to provide a large field of view. It carries two instruments with a common field-of-view of ~0.54 deg2: the visual imager (VIS) and the near infrared instrument (NISP) which contains a slitless spectrometer and a three bands photometer. The Euclid wide survey will cover 15,000 deg2 of the extragalactic sky and is complemented by two 20 deg2 deep fields. For WL, Euclid measures the shapes of 30-40 resolved galaxies per arcmin2 in one broad visible R+I+Z band (550-920 nm). The photometric redshifts for these galaxies reach a precision of dz/(1+z) < 0.05. They are derived from three additional Euclid NIR bands (Y, J, H in the range 0.92-2.0 micron), complemented by ground based photometry in visible bands derived from public data or through engaged collaborations. The BAO are determined from a spectroscopic survey with a redshift accuracy dz/(1+z) =0.001. The slitless spectrometer, with spectral resolution ~250, predominantly detects Ha emission line galaxies. Euclid is a Medium Class mission of the ESA Cosmic Vision 2015-2025 programme, with a foreseen launch date in 2019. This report (also known as the Euclid Red Book) describes the outcome of the Phase A study.

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arXiv2011

The evolution of the star formation activity per halo mass up to redshift ~ 1.6 as seen by Herschel

Star formation in massive galaxies is quenched at some point during hierarchical mass assembly. To understand where and when the quenching processes takes place, we study the evolution of the total star formation rate per unit total halo mass (Σ(SFR/M)) in three different mass scales: low mass halos (field galaxies), groups, and clusters, up to a redshift ~1.6. We use deep far-infrared PACS data at 100 and 160 um to accurately estimate the total star formation rate of the Luminous Infrared Galaxy population of 9 clusters with mass ~10^{15} M_{\odot}, and 9 groups/poor clusters with mass ~ 5 x 10^{13} M_{\odot}. Estimates of the field Σ(SFR/M) are derived from the literature, by dividing the star formation rate density by the mean comoving matter density of the universe. The field Σ(SFR/M) increases with redshift up to z~1 and it is constant thereafter. The evolution of the Σ(SFR/M)-z relation in galaxy systems is much faster than in the field. Up to redshift z~0.2, the field has a higher Σ(SFR/M) than galaxy groups and galaxy clusters. At higher redshifts, galaxy groups and the field have similar Σ(SFR/M), while massive clusters have significantly lower Σ(SFR/M) than both groups and the field. There is a hint of a reversal of the SFR activity vs. environment at z~1.6, where the group Σ(SFR/M) lies above the field Σ(SFR/M)-z relation. We discuss possible interpretations of our results in terms of the processes of downsizing, and star-formation quenching.

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arXiv2011

Herschel observations of Cen A: stellar heating of two extragalactic dust clouds

We present the first results of a multi-wavelength survey, incoporating Herschel-SPIRE, Spitzer, GALEX and ATCA observations, of a 1 deg x 1 deg field centred on Centaurus A. As well as detecting the inner lobes of the active galactic nucleus (AGN) jet and counterjet, we have found two clouds, bright at sub-mm wavelengths, ~15 kpc from the centre of Cen A that are co-aligned with the jets. Flux measurements at Herschel wavelengths have proved vital in constraining fits to the Spectral Energy Distributions (SEDs). The clouds are well fit by a single-temperature, modified blackbody spectrum (beta=2) indicating that we are looking at two cold dust clouds on the outskirts of Cen A. The temperature and masses of the clouds are: T_{north} = 12.6^{+1.1}_{-1.2} K, T_{south} = 15.1^{+1.7}_{-1.6} K; log(M_{north} / M_o) = 5.8^{+0.2}_{-0.2}, log(M_{south} / M_o) = 5.6^{+0.2}_{-0.2} and the gas-dust ratio for both clouds is ~100. The measured values for the northern dust cloud are consistent with previous measurements from ISO while the southern cloud is a new sub-mm detection. The two dust clouds are located at the termini of the partial HI ring that surrounds Cen A which is also where the gas column density peaks... abridged

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arXiv2011

GOODS-Herschel: an infrared main sequence for star-forming galaxies

We present the deepest far-IR observations obtained with Herschel and examine the 3-500um SEDs of galaxies at 0 3x10^10 Lsun kpc^-2) and a high specific SFR (i.e., SBs). The rest-frame, UV-2700A size of these distant SBs is typically half that of MS galaxies, supporting the correlation between star formation density and SB activity that is measured for the local sample. Locally, (U)LIRGs are systematically in the SB mode, whereas most distant (U)LIRGs form stars in the "normal" MS mode. This confusion between two modes of star formation is the cause of the so-called "mid-IR excess" population of galaxies found at z>1.5 by previous studies. MS galaxies have strong PAH emission line features, a broad far-IR bump resulting from a combination of dust temperatures (Tdust~15-50 K), and an effective Tdust~31 K, as derived from the peak wavelength of their IR SED. Galaxies in the SB regime instead exhibit weak PAH EW and a sharper far-IR bump with an effective Tdust~40 K. Finally, we present evidence that the mid-to-far IR emission of X-ray AGNs is predominantly produced by star formation and that candidate dusty AGNs with a power-law emission in the mid-IR systematically occur in compact, dusty SBs. After correcting for the effect of SBs on IR8, we identify new candidates for extremely obscured AGNs.

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arXiv2011

GOODS-Herschel: the impact of galaxy-galaxy interactions on the far-infrared properties of galaxies

We study the impact of galaxy-galaxy interactions on the FIR properties of galaxies and its evolution at 0 99%. However, the SFR and the SSFR decrease or do not change much as it approaches an early-type neighbor. The bifurcations of SFRs and SSFRs depending on the neighbor's morphology seem to occur at R_n~0.5r_vir,nei, which is supported by K-S and MC tests with a significance level of >98%. For all redshift bins, the SSFRs of late-type galaxies interacting with late-type neighbors are increased by factors of about 1.8+/-0.7 and 4.0+/-1.2 compared to those of non-interacting galaxies when the pair separation is smaller than 0.5r_vir,nei and 0.1r_vir,nei, respectively. The Tdust of both local and high-z late-type galaxies strongly interacting with late-type neighbors (i.e. R_n < 0.1r_vir,nei) appears to be higher than that of non-interacting galaxies with a significance level of 96-99%. However, Tdust of local late-type galaxies strongly interacting with early-type neighbors seems to be lower than or similar to that of non-interacting galaxies. Our results suggest that galaxy-galaxy interactions and mergers have been strongly affecting the SFR and the dust properties of star-forming galaxies over at least 8 billion years.

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arXiv2011

GOODS-Herschel: The far-infrared view of star formation in AGN host galaxies since z~3

We present a study of the infrared properties of X-ray selected, moderate luminosity (Lx=10^{42}-10^{44}ergs/s) active galactic nuclei (AGNs) up to z~3, to explore the links between star formation in galaxies and accretion onto their central black holes. We use 100um and 160um fluxes from GOODS-Herschel -the deepest survey yet undertaken by the Herschel telescope- and show that in >94 per cent of cases these fluxes are dominated by the host. We find no evidence of any correlation between the X-ray and infrared luminosities of moderate AGNs at any redshift, suggesting that star-formation is decoupled from nuclear (AGN) activity. The star formation rates of AGN hosts increase strongly with redshift; by a factor of 43 from z<0.1 to z=2-3 for AGNs with the same X-ray luminosities. This increase is consistent with the factor of 25-50 increase in the specific star formation rates (SSFRs) of normal, star-forming (main-sequence) galaxies. Indeed, the average SSFRs of AGN hosts are only marginally (20 per cent) lower than those of main-sequence galaxies, with this small deficit being due to a fraction of AGNs residing in quiescent (low-SSFR) galaxies. We estimate 79+/-10 per cent of moderate AGNs are hosted in main-sequence galaxies, 15+/-7 per cent in quiescent galaxies and <10 per cent in strongly starbursting galaxies. The fractions of all main sequence galaxies at z<2 experiencing a period of moderate nuclear activity is strongly dependent on galaxy stellar mass (Mstars); rising from a few per cent at Mstars~10^{10}Msun to >20 per cent at Mstars>10^{11}Msun. Our results indicate that it is galaxy stellar mass that is most important in dictating whether a galaxy hosts a moderate luminosity AGN. We argue that the majority of moderate nuclear activity is fuelled by internal mechanisms rather than violent mergers, suggesting that disk instabilities could be an important AGN feeding mechanism.

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arXiv2011

FIR measurements of Ly-$α$ emitters at z$\lesssim$1.0: dust attenuation from PACS-\emph{Herschel}

One remaining open question regarding the physical properties of Ly$α$ emitters (LAEs) is their dust content and its evolution with redshift. The variety of results is large and with those reported by now is difficult to establish clear relations between dust, other fundamental parameters of galaxies (star-formation rate, metallicity or age) and redshift. In this Letter, we report \emph{Herschel} PACS-100$μ$m, PACS-160$μ$m and \emph{Spitzer} MIPS-24$μ$m detections of a sample of spectroscopically GALEX-selected LAEs at z$\sim$0.3 and $\sim$1.0. Five out of ten and one out of two LAEs are detected in, at least, one PACS band at z$\sim$0.3 and $\sim$1.0, respectively. These measurements have a great importance given that they allow us to quantify, for the first time, the dust content of LAEs from direct FIR observations. MIPS-24$μ$m detections allow us to determine IR properties of the PACS-undetected LAEs. We obtain that mid-IR/FIR detected star-forming (SF) LAEs at z$\sim$0.3 have dust content within 0.75$\lesssim$ $A_{1200Å}$ $\lesssim$2.0, with a median value of A$_{1200\textrmÅ}$$\sim$1.1. This range broadens out to 0.75$\lesssim$ $A_{1200Å}$ $\lesssim$2.5 when considering those LAEs at z$\sim$1.0. Only one SF LAE is undetected both in MIPS-24$μ$m and PACS, with $A_{1200Å}$ $\lesssim$0.75. These results seem to be larger than those reported for high-redshift LAEs and, therefore, although an evolutionary trend is not clearly seen, it could point out that low-redshift LAEs are dustier than high-redshift ones. However, the diverse methods used could introduce a systematic offset in the results.

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arXiv2011

GOODS-Herschel A population of 24micron dropout sources at z < 2

Using extremely deep PACS 100- and 160um Herschel data from the GOODS-Herschel program, we identify 21 infrared bright galaxies previously missed in the deepest 24um surveys performed by MIPS. These MIPS dropouts are predominantly found in two redshift bins, centred at z ~0.4 and ~1.3. Their S_100/S_24 flux density ratios are similar to those of local LIRGs and ULIRGs, whose silicate absorption features at 18um (at z ~ 0.4) and 9.7um (at z ~ 1.3) are shifted into the 24um MIPS band at these redshifts. The high-z sub-sample consists of 11 infrared luminous sources, accounting for ~2% of the whole GOODS-Herschel sample and putting strong upper limits on the fraction of LIRGs/ULIRGs at 1.0 43 colour cut selects galaxies with a redshift distribution similar to that of the MIPS dropouts and when combined with a second colour cut, S_16/S_8 > 4, isolates sources at 1.0 < z < 1.7. We show that these sources have elevated specific star formation rates (sSFR) compared to main sequence galaxies at these redshifts and are likely to be compact starbursts with moderate/strong 9.7um silicate absorption features in their mid-IR spectra. Herschel data reveal that their infrared luminosities extrapolated from the 24um flux density are underestimated, on average, by a factor of ~3. These silicate break galaxies account for 16% (8%) of the ULIRG (LIRG) population in the GOODS fields, indicating a lower limit in their space density of 2.0 \times 10^(-5) Mpc^(-3). Finally, we provide estimates of the fraction of z < 2 MIPS dropout sources as a function of the 24-, 100-, 160-, 250- and 350um sensitivity limits, and conclude that previous predictions of a population of silicate break galaxies missed by the major 24um extragalactic surveys have been overestimated.

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arXiv2011

The lesser role of starbursts for star formation at z=2

Two main modes of star formation are know to control the growth of galaxies: a relatively steady one in disk-like galaxies, defining a tight star formation rate (SFR)-stellar mass sequence, and a starburst mode in outliers to such a sequence which is generally interpreted as driven by merging. Such starburst galaxies are rare but have much higher SFRs, and it is of interest to establish the relative importance of these two modes. PACS/Herschel observations over the whole COSMOS and GOODS-South fields, in conjunction with previous optical/near-IR data, have allowed us to accurately quantify for the first time the relative contribution of the two modes to the global SFR density in the redshift interval 1.5 1000M(sun)/yr, off-sequence sources significantly contribute to the SFR density (46+/-20%). We conclude that merger-driven starbursts play a relatively minor role for the formation of stars in galaxies, whereas they may represent a critical phase towards the quenching of star formation and morphological transformation in galaxies.

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arXiv2011

GOODS-Herschel Measurements of the Dust Attenuation of Typical Star-Forming Galaxies at High Redshift: Observations of UV-Selected Galaxies at z~2

We take advantage of the sensitivity and resolution of Herschel at 100 and 160 micron to directly image the thermal dust emission and investigate the infrared luminosities, L(IR), and dust obscuration of typical star-forming (L*) galaxies at high redshift. Our sample consists of 146 UV-selected galaxies with spectroscopic redshifts 1.5 1e10 Lsun at z~2 are luminous infrared galaxies (LIRGs) with a median L(IR)=(2.2+/-0.3)e11 Lsun. Typical galaxies at 1.5<z<2.6 have a median dust obscuration L(IR)/L(UV) = 7.1+/-1.1, which corresponds to a dust correction factor, required to recover the bolometric star formation rate (SFR) from the unobscured UV SFR, of 5.2+/-0.6. This result is similar to that inferred from previous investigations of the UV, H-alpha, 24 micron, radio, and X-ray properties of the same galaxies studied here. Stacking in bins of UV slope implies that L* galaxies with redder spectral slopes are also dustier, and that the correlation between UV slope and dustiness is similar to that found for local starburst galaxies. Hence, the rest-frame 30 and 50 micron fluxes validate on average the use of the local UV attenuation curve to recover the dust attenuation of typical star-forming galaxies at high redshift. In the simplest interpretation, the agreement between the local and high redshift UV attenuation curves suggests a similarity in the dust production and stellar and dust geometries of starburst galaxies over the last 10 billion years.

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arXiv2011

GOODS-HERSCHEL: evidence for a UV extinction bump in galaxies at z > 1

Dust attenuation curves in external galaxies are useful to study their dust properties as well as to interpret their intrinsic spectral energy distributions. In particular the presence or absence of a UV bump at 2175 A remains an open issue which has consequences on the interpretation of broad band colours of distant galaxies. We study the dust attenuation curve in the UV range at z >1. In particular we search for the presence of a UV bump. We use deep photometric data of the CDFS obtained with intermediate and broad band filters by the MUSYC project to sample the UV rest-frame of galaxies with 1<z <2. Herschel/PACS and Spitzer/MIPS data are used to measure the dust emission. 30 galaxies were selected with high S/N in all bands. Their SEDs from the UV to the far-IR are fitted using the CIGALE code and the characteristics of the dust attenuation curve are obtained. The mean dust attenuation curve we derive exhibits a UV bump at 2175A whose amplitude corresponds to 35 % (76%) that of the MW (LMC2 supershell) extinction curve. An analytical expression of the average attenuation curve is given, it is found slightly steeper than the Calzetti et al. one, although at a 1 sigma level. Our galaxy sample is used to study the derivation of the slopes of the UV continuum from broad band colours, including the GALEX FUV-NUV colour. Systematic errors induced by the presence of the bump are quantified. We compare dust attenuation factors measured with CIGALE to the slope of the UV continuum and find that there is a large scatter around the relation valid for local starbursts (0.7 mag). The uncertainties on the determination of the UV slope lead to an extra systematic error of the order of 0.3 to 0.7 mag on dust attenuation when a filter overlaps the UV bump.

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arXiv2011

PACS Evolutionary Probe (PEP) - A Herschel Key Program

Deep far-infrared photometric surveys studying galaxy evolution and the nature of the cosmic infrared background are a key strength of the Herschel mission. We describe the scientific motivation for the PACS Evolutionary Probe (PEP) guaranteed time key program and its role in the complement of Herschel surveys, and the field selection which includes popular multiwavelength fields such as GOODS, COSMOS, Lockman Hole, ECDFS, EGS. We provide an account of the observing strategies and data reduction methods used. An overview of first science results illustrates the potential of PEP in providing calorimetric star formation rates for high redshift galaxy populations, thus testing and superseeding previous extrapolations from other wavelengths, and enabling a wide range of galaxy evolution studies.

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arXiv2011

Building the cosmic infrared background brick by brick with Herschel/PEP

The cosmic infrared background (CIB) includes roughly half of the energy radiated by all galaxies at all wavelengths across cosmic time, as observed at the present epoch. The PACS Evolutionary Probe (PEP) survey is exploited here to study the CIB and its redshift differential, at 70, 100 and 160 micron, where the background peaks. Combining PACS observations of the GOODS-S, GOODS-N, Lockman Hole and COSMOS areas, we define number counts spanning over more than two orders of magnitude in flux: from ~1 mJy to few hundreds mJy. Stacking of 24 micron sources and P(D) statistics extend the analysis down to ~0.2 mJy. Taking advantage of the wealth of ancillary data in PEP fields, differential number counts and CIB are studied up to z=5. Based on these counts, we discuss the effects of confusion on PACS blank field observations and provide confusion limits for the three bands considered. The total CIB surface brightness emitted above PEP 3 sigma flux limits is 4.52 +/- 1.18, 8.35 +/- 0.95 and 9.49 +/- 0.59 [nW/m2/sr] at 70, 100, and 160 micron, respectively. These values correspond to 58 +/- 7% and 74 +/- 5% of the COBE/DIRBE CIB direct measurements at 100 and 160 micron. Employing the P(D) analysis, these fractions increase to ~65% and ~89%. More than half of the resolved CIB was emitted at redshift z<=1. The 50%-light redshifts lie at z=0.58, 0.67 and 0.73 at the three PACS wavelengths. The distribution moves towards earlier epochs at longer wavelengths: while the 70 micron CIB is mainly produced by z<=1.0 objects, the contribution of z>1.0 sources reaches 50% at 160 micron. Most of the CIB resolved in the three PACS bands was emitted by galaxies with infrared luminosities in the range 1e11-1e12 L(sun).

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arXiv2011

X-ray observations of highly obscured 9.7 micron sources: an efficient method for selecting Compton-thick AGN ?

Spitzer/IRS has revealed many sources with very deep Si features at 9.7micron (tau>1). We set out to investigate whether a strong Si absorption feature is a good indicator for the presence of a heavily obscured AGN. We compile X-ray spectroscopic observations available in the literature on the optically-thick,tau(9.7)>1 sources from the IRAS Seyfert sample. We find that the majority of the high-tau optically confirmed Seyferts (6/9) in this sample are probably CT. Thus we provide direct evidence for a connection between mid-IR optically-thick galaxies and CT AGN, with the success rate being close to 70% in the local Universe. This is at least comparable, if not better, than other rates obtained with photometric information in the mid to far-IR, or even mid-IR to Xray. However, this technique cannot provide complete CT AGN samples,ie there are many CT AGN which do not show significant Si absorption, with the most notable example being N1068. Having assessed the validity of the high 9.7micron technique locally, we attempt to construct a sample of candidate CT AGN at higher redshifts. We compile a sample of 7 high-tau sources in the GOODS and 5 in the Spitzer FLS. All these have been selected to have no PAH features EW(6.2)<0.3 in order to maximize the probability that they are AGN. 6 out of 7 sources in the GOODS have been detected in X-rays, while for the five FLS sources only X-ray flux upper limits are available. The high X-ray luminosities of the detected GOODS sources corroborates that these are AGN. For FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources. SED fitting can support the presence of an AGN in the vast majority of sources. We cannot derive useful X-ray spectroscopy constraints on whether these are CT. However, the low LX/L6 ratios, suggest that at least 4 out of the 6 detected sources in GOODS may be associated with CT AGN.

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