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Register a new userRole of AGNs in the Luminous Infrared Galaxy phase since z ~ 3
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In order to understand the interactions between active galactic nuclei (AGN) and star formation during the evolution of galaxies, we investigate 142 galaxies detected in both X-ray and 70{mu}m observations in the COSMOS (Cosmic Evolution Survey) field. All of our data are obtained from the archive, X-ray point source catalogs from Chandra and XMM-Newton observations; far-infrared 70{mu}m point source catalog from Spitzer-MIPS observations. Although the IRAC [3.6{mu}m]-[4.5{mu}m] vs. [5.8{mu}m]-[8.0{mu}m] colours of our sample indicate that only ~63% of our sources would be classified as AGN, the ratio of the rest-frame 2-10 keV luminosity to the total infrared luminosity (8-1000{mu}m) shows that all of the sample has comparatively higher X-ray luminosity than that expected from pure star-forming galaxies, suggesting the presence of an AGN in all of our sources. From the analysis of the X-ray hardness ratio, we find that sources with both 70{mu}m and X-ray detection tend to have a higher hardness ratio relative to the whole X-ray selected source population, suggesting the presence of more X-ray absorption in the 70{mu}m detected sources. In addition, we find that the observed far-infrared colours of 70{mu}m detected sources with and without X-ray emission are similar, suggesting the far-infrared emission could be mainly powered by star formation.
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We present results on low-resolution mid-infrared (MIR) spectra of 70 infrared-luminous galaxies obtained with the Infrared Spectrograph (IRS) onboard Spitzer. We selected sources from the European Large Area Infrared Survey (ELAIS) with S15 > 0.8 mJy and photometric or spectroscopic z > 1. About half of the sample are QSOs in the optical, while the remaining sources are galaxies, comprising both obscured AGN and starbursts. We classify the spectra using well-known infrared diagnostics, as well as a new one that we propose, into three types of source: those dominated by an unobscured AGN (QSOs), obscured AGN, and starburst-dominated sources. Starbursts concentrate at z ~ 0.6-1.0 favored by the shift of the 7.7-micron PAH band into the selection 15 micron band, while AGN spread over the 0.5 < z < 3.1 range. Star formation rates (SFR) are estimated for individual sources from the luminosity of the PAH features. An estimate of the average PAH luminosity in QSOs and obscured AGN is obtained from the composite spectrum of all sources with reliable redshifts. The estimated mean SFR in the QSOs is 50-100 Mo yr^-1, but the implied FIR luminosity is 3-10 times lower than that obtained from stacking analysis of the FIR photometry, suggesting destruction of the PAH carriers by energetic photons from the AGN. The SFR estimated in obscured AGN is 2-3 times higher than in QSOs of similar MIR luminosity. This discrepancy might not be due to luminosity effects or selection bias alone, but could instead indicate a connection between obscuration and star formation. However, the observed correlation between silicate absorption and the slope of the near- to mid-infrared spectrum is compatible with the obscuration of the AGN emission in these sources being produced in a dust torus.
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.
Ly-alpha emitters (LAEs) are seen everywhere in the redshift domain from local to z~7. Far-infrared (FIR) counterparts of LAEs at different epochs could provide direct clues on dust content, extinction, and spectral energy distribution (SED) for these galaxies. We search for FIR counterparts of LAEs that are optically detected in the GOODS-North field at redshift z~2.2 using data from the Herschel Space Telescope with the Photodetector Array Camera and Spectrometer (PACS). The LAE candidates were isolated via color-magnitude diagram using the medium-band photometry from the ALHAMBRA Survey, ancillary data on GOODS-North, and stellar population models. According to the fitting of these spectral synthesis models and FIR/optical diagnostics, most of them seem to be obscured galaxies whose spectra are AGN-dominated. From the analysis of the optical data, we have observed a fraction of AGN or composite over source total number of ~0.75 in the LAE population at z~2.2, which is marginally consistent with the fraction previously observed at z=2.25 and even at low redshift (0.2<z<0.45), but significantly different from the one observed at redshift ~3, which could be compatible either with a scenario of rapid change in the AGN fraction between the epochs involved or with a non detection of obscured AGN in other z=2-3 LAE samples due to lack of deep FIR observations. We found three robust FIR (PACS) counterparts at z~2.2 in GOODS-North. This demonstrates the possibility of finding dust emission in LAEs even at higher redshifts.
We study the clustering of galaxies as function of luminosity and redshift in the range $0.35 < z < 1.25$ using data from the Advanced Large Homogeneous Area Medium Band Redshift Astronomical (ALHAMBRA) survey. The ALHAMBRA data used in this work cover $2.38 mathrm{deg}^2$ in 7 independent fields, after applying a detailed angular selection mask, with accurate photometric redshifts, $sigma_z lesssim 0.014 (1+z)$, down to $I_{rm AB} < 24$. Given the depth of the survey, we select samples in $B$-band luminosity down to $L^{rm th} simeq 0.16 L^{*}$ at $z = 0.9$. We measure the real-space clustering using the projected correlation function, accounting for photometric redshifts uncertainties. We infer the galaxy bias, and study its evolution with luminosity. We study the effect of sample variance, and confirm earlier results that the COSMOS and ELAIS-N1 fields are dominated by the presence of large structures. For the intermediate and bright samples, $L^{rm med} gtrsim 0.6L^{*}$, we obtain a strong dependence of bias on luminosity, in agreement with previous results at similar redshift. We are able to extend this study to fainter luminosities, where we obtain an almost flat relation, similar to that observed at low redshift. Regarding the evolution of bias with redshift, our results suggest that the different galaxy populations studied reside in haloes covering a range in mass between $log_{10}[M_{rm h}/(h^{-1}mathrm{M}_{odot})] gtrsim 11.5$ for samples with $L^{rm med} simeq 0.3 L^{*}$ and $log_{10}[M_{rm h}/(h^{-1}mathrm{M}_{odot})] gtrsim 13.0$ for samples with $L^{rm med} simeq 2 L^{*}$, with typical occupation numbers in the range of $sim 1 - 3$ galaxies per halo.
We report spectroscopic confirmation and high-resolution infrared imaging of a z=2.79 triply-imaged galaxy behind the Bullet Cluster. This source, a Spitzer-selected luminous infrared galaxy (LIRG), is confirmed via polycyclic aromatic hydrocarbon (PAH) features using the Spitzer Infrared Spectrograph (IRS) and resolved with HST WFC3 imaging. In this galaxy, which with a stellar mass of M*=4e9 Msun is one of the two least massive ones studied with IRS at z>2, we also detect H_2 S(4) and H_2 S(5) pure rotational lines (at 3.1 sigma and 2.1 sigma) - the first detection of these molecular hydrogen lines in a high-redshift galaxy. From the molecular hydrogen lines we infer an excitation temperature T=377+68-84 K. The detection of these lines indicates that the warm molecular gas mass is 6(+36-4)% of the stellar mass and implies the likely existence of a substantial reservoir of cold molecular gas in the galaxy. Future spectral observations at longer wavelengths with facilities like the Herschel Space Observatory, the Large Millimeter Telescope, and the Atacama Pathfinder EXperiment (APEX) thus hold the promise of precisely determining the total molecular gas mass. Given the redshift, and using refined astrometric positions from the high resolution imaging, we also update the magnification estimate and derived fundamental physical properties of this system. The previously published values for total infrared luminosity, star formation rate, and dust temperature are confirmed modulo the revised magnification; however we find that PAH emission is roughly a factor of five stronger than would be predicted by the relations between the total infrared and PAH luminosity reported for SMGs and starbursts in Pope et al. (2008).
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