arXiv subjects

D. Farrah

Publications and source records attributed to D. Farrah.

At least 73 records · Page 4Linked to original sources

arXiv2017

X-ray bolometric corrections for Compton-thick active galactic nuclei

We present X-ray bolometric correction factors, $κ_{Bol}$ ($\equiv L_{Bol}/L_X$), for Compton-thick (CT) active galactic nuclei (AGN) with the aim of testing AGN torus models, probing orientation effects, and estimating the bolometric output of the most obscured AGN. We adopt bolometric luminosities, $L_{Bol}$, from literature infrared (IR) torus modeling and compile published intrinsic 2--10 keV X-ray luminosities, $L_{X}$, from X-ray torus modeling of NuSTAR data. Our sample consists of 10 local CT AGN where both of these estimates are available. We test for systematic differences in $κ_{Bol}$ values produced when using two widely used IR torus models and two widely used X-ray torus models, finding consistency within the uncertainties. We find that the mean $κ_{Bol}$ of our sample in the range $L_{Bol}\approx10^{42}-10^{45}$ erg/s is log$_{10}κ_{Bol}=1.44\pm0.12$ with an intrinsic scatter of $\sim0.2$ dex, and that our derived $κ_{Bol}$ values are consistent with previously established relationships between $κ_{Bol}$ and $L_{Bol}$ and $κ_{Bol}$ and Eddington ratio. We investigate if $κ_{Bol}$ is dependent on $N_H$ by comparing our results on CT AGN to published results on less-obscured AGN, finding no significant dependence. Since many of our sample are megamaser AGN, known to be viewed edge-on, and furthermore under the assumptions of AGN unification whereby unobscured AGN are viewed face-on, our result implies that the X-ray emitting corona is not strongly anisotropic. Finally, we present $κ_{Bol}$ values for CT AGN identified in X-ray surveys as a function of their observed $L_X$, where an estimate of their intrinsic $L_{X}$ is not available, and redshift, useful for estimating the bolometric output of the most obscured AGN across cosmic time.

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arXiv2017

Far-infrared metallicity diagnostics: Application to local ultraluminous infrared galaxies

The abundance of metals in galaxies is a key parameter which permits to distinguish between different galaxy formation and evolution models. Most of the metallicity determinations are based on optical line ratios. However, the optical spectral range is subject to dust extinction and, for high-z objects (z > 3), some of the lines used in optical metallicity diagnostics are shifted to wavelengths not accessible to ground based observatories. For this reason, we explore metallicity diagnostics using far-infrared (IR) line ratios which can provide a suitable alternative in such situations. To investigate these far-IR line ratios, we modeled the emission of a starburst with the photoionization code CLOUDY. The most sensitive far-IR ratios to measure metallicities are the [OIII]52$μ$m and 88$μ$m to [NIII]57$μ$m ratios. We show that this ratio produces robust metallicities in the presence of an AGN and is insensitive to changes in the age of the ionizing stellar. Another metallicity sensitive ratio is the [OIII]88$μ$m/[NII]122$μ$m ratio, although it depends on the ionization parameter. We propose various mid- and far-IR line ratios to break this dependency. Finally, we apply these far-IR diagnostics to a sample of 19 local ultraluminous IR galaxies (ULIRGs) observed with Herschel and Spitzer. We find that the gas-phase metallicity in these local ULIRGs is in the range 0.7 < Z_gas/Z_sun < 1.5, which corresponds to 8.5 < 12 + log (O/H) < 8.9. The inferred metallicities agree well with previous estimates for local ULIRGs and this confirms that they lie below the local mass-metallicity relation.

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arXiv2017

The SCUBA-2 Cosmology Legacy Survey: Multi-wavelength Properties of ALMA-identified Submillimeter Galaxies in UKIDSS-UDS

We present a multi-wavelength analysis of 52 sub-millimeter galaxies (SMGs), identified using ALMA 870$μ$m continuum imaging in a pilot program to precisely locate bright SCUBA2-selected sub-mm sources in the UKIDSS Ultra Deep Survey (UDS) field. Using the available deep (especially near-infrared), panoramic imaging of the UDS field at optical-to-radio wavelengths we characterize key properties of the SMG population. The median photometric redshift of the bright ALMA/SCUBA-2 UDS (AS2UDS) SMGs that are detected in a sufficient number of wavebands to derive a robust photometric redshift is $z$=2.65$\pm$0.13. However, similar to previous studies, 27% of the SMGs are too faint at optical-to-near-infrared wavelengths to derive a reliable photometric redshift. Assuming that these SMGs lie at z$\gtrsim$3 raises the median redshift of the full sample to $z$=2.9$\pm$0.2. A subset of 23, unlensed, bright AS2UDS SMGs have sizes measured from resolved imaging of their rest-frame far-infrared emission. We show that the extent and luminosity of the far-infrared emission are consistent with the dust emission arising from regions that are optically thick, on average, at a wavelength of $λ_0$$\ge$75$μ$m (1-$σ$ dispersion of 55-90$μ$m). Using the dust masses derived from our optically-thick spectral energy distribution models we determine that these galaxies have a median hydrogen column density of $N_{H}$=9.8$_{-0.7}^{+1.4}$$\times$10$^{23}$cm$^{-2}$, or a corresponding median $V$-band obscuration of $A_\mathrm{v}$=540$^{+80}_{-40}$mag, averaged along the line of sight to the source of their restframe $\sim$200$μ$m emission. We discuss the implications of this extreme attenuation by dust for the multiwavelength study of dusty starbursts and reddening-sensitive tracers of star formation.

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arXiv2017

Molecular outflows in local ULIRGs: energetics from multi-transition OH analysis

We report on the energetics of molecular outflows in 14 local Ultraluminous Infrared Galaxies (ULIRGs) that show unambiguous outflow signatures (P-Cygni profiles or high-velocity absorption wings) in the far-infrared lines of OH measured with the Herschel/PACS spectrometer. Detection of both ground-state (at 119 and 79 um) and one or more radiatively-excited (at 65 and 84 um) lines allows us to model the nuclear gas (<~300 pc) as well as the more extended components using spherically symmetric radiative transfer models. The highest molecular outflow velocities are found in buried sources, in which slower but massive expansion of the nuclear gas is also observed. With the exception of a few outliers, the outflows have momentum fluxes of (2-5)xL_IR/c and mechanical luminosities of (0.1-0.3)% of L_IR. The moderate momentum boosts in these sources (<~3) suggest that the outflows are mostly momentum-driven by the combined effects of AGN and nuclear starbursts, as a result of radiation pressure, winds, and supernovae remnants. In some sources (~20%), however, powerful (10^{10.5-11} Lsun) AGN feedback and (partially) energy-conserving phases are required, with momentum boosts in the range 3-20. These outflows appear to be stochastic strong-AGN feedback events that occur throughout the merging process. In a few sources, the outflow activity in the innermost regions has subsided in the last ~1 Myr. While OH traces the molecular outflows at sub-kpc scales, comparison of the masses traced by OH with those previously inferred from tracers of more extended outflowing gas suggests that most mass is loaded (with loading factors of Mdot/SFR=1-10) from the central galactic cores (a few x 100 pc). Outflow depletion timescales are <10^8 yr, shorter than the gas consumption timescales by factors of 1.1-15, and are anti-correlated with the AGN luminosity.

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arXiv2017

A Long Look At MCG-5-23-16 With NuSTAR: I- Relativistic Reflection And Coronal Properties

MCG-5-23-16 was targeted in early 2015 with a half mega-seconds observing campaign using NuSTAR. Here we present the spectral analysis of these datasets along with an earlier observation and study the relativistic reflection and the primary coronal source. The data show strong reflection features in the form of both narrow and broad iron lines plus a Compton reflection hump. A cutoff energy is significantly detected in all exposures. The shape of the reflection spectrum does not change in the two years spanned by the observations, suggesting a stable geometry. A strong positive correlation is found between the cutoff energy and both the hard X-ray flux and spectral index. The measurements imply that the coronal plasma is not at the runaway electron-positron pair limit, and instead contains mostly electrons. The observed variability in the coronal properties is driven by a variable optical depth. A constant heating to cooling ratio is measured implying that there is a feedback mechanism in which a significant fraction of the photons cooling the corona are due to reprocessed hard X-rays.

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arXiv2017

Evolution of cosmic star formation in the SCUBA-2 Cosmology Legacy Survey

We present a new exploration of the cosmic star-formation history and dust obscuration in massive galaxies at redshifts $0.5< z<6$. We utilize the deepest 450 and 850$μ$m imaging from SCUBA-2 CLS, covering 230arcmin$^2$ in the AEGIS, COSMOS and UDS fields, together with 100-250$μ$m imaging from Herschel. We demonstrate the capability of the T-PHOT deconfusion code to reach below the confusion limit, using multi-wavelength prior catalogues from CANDELS/3D-HST. By combining IR and UV data, we measure the relationship between total star-formation rate (SFR) and stellar mass up to $z\sim5$, indicating that UV-derived dust corrections underestimate the SFR in massive galaxies. We investigate the relationship between obscuration and the UV slope (the IRX-$β$ relation) in our sample, which is similar to that of low-redshift starburst galaxies, although it deviates at high stellar masses. Our data provide new measurements of the total SFR density (SFRD) in $M_\ast>10^{10}M_\odot$ galaxies at $0.5 10$. One third of this is accounted for by 450$μ$m-detected sources, while one fifth is attributed to UV-luminous sources (brighter than $L^\ast_{UV}$), although even these are largely obscured. By extrapolating our results to include all stellar masses, we estimate a total SFRD that is in good agreement with previous results from IR and UV data at $z\lesssim3$, and from UV-only data at $z\sim5$. The cosmic star-formation history undergoes a transition at $z\sim3-4$, as predominantly unobscured growth in the early Universe is overtaken by obscured star formation, driven by the build-up of the most massive galaxies during the peak of cosmic assembly.

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arXiv2017

A New Compton-thick AGN in our Cosmic Backyard: Unveiling the Buried Nucleus in NGC 1448 with NuSTAR

NGC 1448 is one of the nearest luminous galaxies ($L_{8-1000μm} >$ 10$^{9} L_{\odot}$) to ours ($z$ $=$ 0.00390), and yet the active galactic nucleus (AGN) it hosts was only recently discovered, in 2009. In this paper, we present an analysis of the nuclear source across three wavebands: mid-infrared (MIR) continuum, optical, and X-rays. We observed the source with the Nuclear Spectroscopic Telescope Array (NuSTAR), and combined this data with archival Chandra data to perform broadband X-ray spectral fitting ($\approx$0.5-40 keV) of the AGN for the first time. Our X-ray spectral analysis reveals that the AGN is buried under a Compton-thick (CT) column of obscuring gas along our line-of-sight, with a column density of $N_{\rm H}$(los) $\gtrsim$ 2.5 $\times$ 10$^{24}$ cm$^{-2}$. The best-fitting torus models measured an intrinsic 2-10 keV luminosity of $L_{2-10\rm{,int}}$ $=$ (3.5-7.6) $\times$ 10$^{40}$ erg s$^{-1}$, making NGC 1448 one of the lowest luminosity CTAGNs known. In addition to the NuSTAR observation, we also performed optical spectroscopy for the nucleus in this edge-on galaxy using the European Southern Observatory New Technology Telescope. We re-classify the optical nuclear spectrum as a Seyfert on the basis of the Baldwin-Philips-Terlevich diagnostic diagrams, thus identifying the AGN at optical wavelengths for the first time. We also present high spatial resolution MIR observations of NGC 1448 with Gemini/T-ReCS, in which a compact nucleus is clearly detected. The absorption-corrected 2-10 keV luminosity measured from our X-ray spectral analysis agrees with that predicted from the optical [OIII]$λ$5007Å emission line and the MIR 12$μ$m continuum, further supporting the CT nature of the AGN.

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arXiv2016

Broadband X-ray spectral analysis of the Seyfert 1 galaxy GRS 1734-292

We discuss the broadband X-ray spectrum of GRS 1734-292 obtained from non-simultaneous XMM-Newton and NuSTAR observations, performed in 2009 and 2014, respectively. GRS1734-292 is a Seyfert 1 galaxy, located near the Galactic plane at $z=0.0214$. The NuSTAR spectrum ($3-80$ keV) is dominated by a primary power-law continuum with $Γ=1.65 \pm 0.05$ and a high-energy cutoff $E_c=53^{+11}_{-8}$ keV, one of the lowest measured by NuSTAR in a Seyfert galaxy. Comptonization models show a temperature of the coronal plasma of $kT_e=11.9^{+1.2}_{-0.9}$ keV and an optical depth, assuming a slab geometry, $τ=2.98^{+0.16}_{-0.19}$ or a similar temperature and $τ=6.7^{+0.3}_{-0.4}$ assuming a spherical geometry. The 2009 XMM-Newton spectrum is well described by a flatter intrinsic continuum ($Γ=1.47^{+0.07}_{-0.03}$) and one absorption line due to Fe\textsc{XXV} K$α$ produced by a warm absorber. Both data sets show a modest iron K$α$ emission line at $6.4$ keV and the associated Compton reflection, due to reprocessing from neutral circumnuclear material.

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arXiv2016

NuSTAR observations of WISE J1036+0449, a Galaxy at z$\sim1$ obscured by hot dust

Hot, Dust-Obscured Galaxies (Hot DOGs), selected from the WISE all sky infrared survey, host some of the most powerful Active Galactic Nuclei (AGN) known, and might represent an important stage in the evolution of galaxies. Most known Hot DOGs are at $z> 1.5$, due in part to a strong bias against identifying them at lower redshift related to the selection criteria. We present a new selection method that identifies 153 Hot DOG candidates at $z\sim 1$, where they are significantly brighter and easier to study. We validate this approach by measuring a redshift $z=1.009$, and an SED similar to higher redshift Hot DOGs for one of these objects, WISE J1036+0449 ($L_{\rm\,Bol}\simeq 8\times 10^{46}\rm\,erg\,s^{-1}$), using data from Keck/LRIS and NIRSPEC, SDSS, and CSO. We find evidence of a broadened component in MgII, which, if due to the gravitational potential of the supermassive black hole, would imply a black hole mass of $M_{\rm\,BH}\simeq 2 \times 10^8 M_{\odot}$, and an Eddington ratio of $λ_{\rm\,Edd}\simeq 2.7$. WISE J1036+0449 is the first Hot DOG detected by NuSTAR, and the observations show that the source is heavily obscured, with a column density of $N_{\rm\,H}\simeq(2-15)\times10^{23}\rm\,cm^{-2}$. The source has an intrinsic 2-10 keV luminosity of $\sim 6\times 10^{44}\rm\,erg\,s^{-1}$, a value significantly lower than that expected from the mid-infrared/X-ray correlation. We also find that the other Hot DOGs observed by X-ray facilities show a similar deficiency of X-ray flux. We discuss the origin of the X-ray weakness and the absorption properties of Hot DOGs. Hot DOGs at $z\lesssim1$ could be excellent laboratories to probe the characteristics of the accretion flow and of the X-ray emitting plasma at extreme values of the Eddington ratio.

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arXiv2016

IC 3639 - A new bona fide Compton thick AGN unveiled by NuSTAR

We analyse high-quality NuSTAR observations of the local (z = 0.011) Seyfert 2 active galactic nucleus (AGN) IC 3639, in conjunction with archival Suzaku and Chandra data. This provides the first broadband X-ray spectral analysis of the source, spanning nearly two decades in energy (0.5-30 keV). Previous X-ray observations of the source below 10 keV indicated strong reflection/obscuration on the basis of a pronounced iron fluorescence line at 6.4 keV. The hard X-ray energy coverage of NuSTAR, together with self-consistent toroidal reprocessing models, enables direct broadband constraints on the obscuring column density of the source. We find the source to be heavily Compton-thick (CTK) with an obscuring column in excess of $3.6\times10^{24}$ cm$^{-2}$, unconstrained at the upper end. We further find an intrinsic 2-10 keV luminosity of $\textrm{log}_{10}(L_{\textrm{2-10 keV}} \textrm{[erg s}^{-1}]) = 43.4^{+0.6}_{-1.1}$ to 90% confidence, almost 400 times the observed flux, and consistent with various multi-wavelength diagnostics. Such a high intrinsic to observed flux ratio in addition to an Fe-K$α$ fluorescence line equivalent width exceeding 2 keV is extreme amongst known bona fide CTK AGN, which we suggest are both due to the high level of obscuration present around IC 3639. Our study demonstrates that broadband spectroscopic modelling with NuSTAR enables large corrections for obscuration to be carried out robustly, and emphasises the need for improved modelling of AGN tori showing intense iron fluorescence.

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arXiv2016

The SCUBA-2 Cosmology Legacy Survey: The EGS deep field I - Deep number counts and the redshift distribution of the recovered Cosmic Infrared Background at 450 and 850 um

We present deep observations at 450 um and 850 um in the Extended Groth Strip field taken with the SCUBA-2 camera mounted on the James Clerk Maxwell Telescope as part of the deep SCUBA-2 Cosmology Legacy Survey (S2CLS), achieving a central instrumental depth of $σ_{450}=1.2$ mJy/beam and $σ_{850}=0.2$ mJy/beam. We detect 57 sources at 450 um and 90 at 850 um with S/N > 3.5 over ~70 sq. arcmin. From these detections we derive the number counts at flux densities $S_{450}>4.0$ mJy and $S_{850}>0.9$ mJy, which represent the deepest number counts at these wavelengths derived using directly extracted sources from only blank-field observations with a single-dish telescope. Our measurements smoothly connect the gap between previous shallower blank-field single-dish observations and deep interferometric ALMA results. We estimate the contribution of our SCUBA-2 detected galaxies to the cosmic infrared background (CIB), as well as the contribution of 24 um-selected galaxies through a stacking technique, which add a total of $0.26\pm0.03$ and $0.07\pm0.01$ MJy/sr, at 450 um and 850 um, respectively. These surface brightnesses correspond to $60\pm20$ and $50\pm20$ per cent of the total CIB measurements, where the errors are dominated by those of the total CIB. Using the photometric redshifts of the 24 um-selected sample and the redshift distributions of the submillimetre galaxies, we find that the redshift distribution of the recovered CIB is different at each wavelength, with a peak at $z\sim1$ for 450 um and at $z\sim2$ for 850um, consistent with previous observations and theoretical models.

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arXiv2016

On the redshift distribution and physical properties of ACT-selected DSFGs

We present multi-wavelength detections of nine candidate gravitationally-lensed dusty star-forming galaxies (DSFGs) selected at 218GHz (1.4mm) from the ACT equatorial survey. Among the brightest ACT sources, these represent the subset of the total ACT sample lying in Herschel SPIRE fields, and all nine of the 218GHz detections were found to have bright Herschel counterparts. By fitting their spectral energy distributions (SEDs) with a modified blackbody model with power-law temperature distribution, we find the sample has a median redshift of $z=4.1^{+1.1}_{-1.0}$ (68 per cent confidence interval), as expected for 218GHz selection, and an apparent total infrared luminosity of $\log_{10}(μL_{\rm IR}/{\rm L}_\odot) = 13.86^{+0.33}_{-0.30}$, which suggests that they are either strongly lensed sources or unresolved collections of unlensed DSFGs. The effective apparent diameter of the sample is $\sqrtμd= 4.2^{+1.7}_{-1.0}$kpc, further evidence of strong lensing or multiplicity, since the typical diameter of dusty star-forming galaxies is $1.0$--$2.5$ kpc. We emphasize that the effective apparent diameter derives from SED modelling without the assumption of optically thin dust (as opposed to image morphology). We find that the sources have substantial optical depth ($τ= 4.2^{+3.7}_{-1.9}$) to dust around the peak in the modified blackbody spectrum ($λ_{\rm obs} \le 500$ $μ$m), a result that is robust to model choice.

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arXiv2016

The Phoenix galaxy as seen by NuSTAR

Aims. We study the long-term variability of the well-known Seyfert 2 galaxy Mrk 1210 (a.k.a. UGC 4203, or the Phoenix galaxy). Methods. The source was observed by many X-ray facilities in the last 20 years. Here we present a NuSTAR observation and put the results in context of previously published observations. Results. NuSTAR observed Mrk 1210 in 2012 for 15.4 ks. The source showed Compton-thin obscuration similar to that observed by Chandra, Suzaku, BeppoSAX and XMM-Newton over the past two decades, but different from the first observation by ASCA in 1995, in which the active nucleus was caught in a low flux state - or obscured by Compton-thick matter, with a reflection-dominated spectrum. Thanks to the high-quality hard X-ray spectrum obtained with NuSTAR and exploiting the long-term spectral coverage spanning 16.9 years, we can precisely disentangle the transmission and reflection components and put constraints on both the intrinsic long-term variability and hidden nucleus scenarios. In the former case, the distance between the reflector and the source must be at least ~ 2 pc, while in the latter one the eclipsing cloud may be identified with a water maser-emitting clump.

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arXiv2016

ALMA observations of Lyman-alpha Blob 1: halo sub-structure illuminated from within

We present new Atacama Large Millimeter/Submillimeter Array (ALMA) 850um continuum observations of the original Lyman-alpha Blob (LAB) in the SSA22 field at z=3.1 (SSA22-LAB01). The ALMA map resolves the previously identified submillimeter source into three components with total flux density S_850 = 1.68+/-0.06 mJy, corresponding to a star formation rate of ~150 M_sun/yr. The submillimeter sources are associated with several faint (m~27 mag) rest-frame ultraviolet sources identified in Hubble Space Telescope Imaging Spectrograph (STIS) clear filter imaging (~5850A). One of these companions is spectroscopically confirmed with Keck MOSFIRE to lie within 20 projected kpc and 250 km/s of one of the ALMA components. We postulate that some of these STIS sources represent a population of low-mass star-forming satellites surrounding the central submillimeter sources, potentially contributing to their growth and activity through accretion. Using a high resolution cosmological zoom simulation of a 10^13 M_sun halo at z=3, including stellar, dust and Ly-alpha radiative transfer, we can model the ALMA+STIS observations and demonstrate that Ly-alpha photons escaping from the central submillimeter sources are expected to resonantly scatter in neutral hydrogen, the majority of which is predicted to be associated with halo substructure. We show how this process gives rise to extended Ly-alpha emission with similar surface brightness and morphology to observed giant LABs.

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arXiv2016

HerMES: A search for high-redshift dusty galaxies in the HerMES Large Mode Survey - Catalogue, number counts and early results

Selecting sources with rising flux densities towards longer wavelengths from Herschel/SPIRE maps is an efficient way to produce a catalogue rich in high-redshift (z > 4) dusty star-forming galaxies. The effectiveness of this approach has already been confirmed by spectroscopic follow-up observations, but the previously available catalogues made this way are limited by small survey areas. Here we apply a map-based search method to 274 deg$^2$ of the HerMES Large Mode Survey (HeLMS) and create a catalogue of 477 objects with SPIRE flux densities $S_{500} > S_{350} >S_{250}$ and a $5 σ$ cut-off $S_{500} > $ 52 mJy. From this catalogue we determine that the total number of these "red" sources is at least an order of magnitude higher than predicted by galaxy evolution models. These results are in agreement with previous findings in smaller HerMES fields; however, due to our significantly larger sample size we are also able to investigate the shape of the red source counts for the first time. We have obtained spectroscopic redshift measurements for two of our sources using the Atacama Large Millimeter/submillimeter Array (ALMA). The redshifts z = 5.1 and z = 3.8 confirm that with our selection method we can indeed find high-redshift dusty star-forming galaxies.

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arXiv2016

The SCUBA-2 Cosmology Legacy Survey: 850um maps, catalogues and number counts

We present a catalogue of nearly 3,000 submillimetre sources detected at 850um over ~5 square degrees surveyed as part of the James Clerk Maxwell Telescope (JCMT) SCUBA-2 Cosmology Legacy Survey (S2CLS). This is the largest survey of its kind at 850um, probing a meaningful cosmic volume at the peak of star formation activity and increasing the sample size of submillimetre galaxies selected at 850um by an order of magnitude. We describe the wide 850um survey component of S2CLS, which covers the key extragalactic survey fields: UKIDSS-UDS, COSMOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22 and GOODS-North. The average 1-sigma depth of S2CLS is 1.2 mJy/beam, approaching the SCUBA-2 850um confusion limit, which we determine to be ~0.8 mJy/beam. We measure the single dish 850um number counts to unprecedented accuracy, reducing the Poisson errors on the differential counts to approximately 4% at S_850~3mJy. With several independent fields, we investigate field-to-field variance, finding that the number counts on 0.5-1 degree scales are generally within 50% of the S2CLS mean for S_850>3mJy, with scatter consistent with the Poisson and estimated cosmic variance uncertainties, although there is a marginal (2-sigma) density enhancement in the GOODS-North field. The observed number counts are in reasonable agreement with recent phenomenological and semi-analytic models. Finally, the large solid angle of S2CLS allows us to measure the bright-end counts: at S_850>10mJy there are approximately ten sources per square degree, and we detect the distinctive up-turn in the number counts indicative of the detection of local sources of 850um emission and strongly lensed high-redshift galaxies. Here we describe the data collection and reduction procedures and present calibrated maps and a catalogue of sources; these are made publicly available.

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arXiv2016

The faint end of the 250 micron luminosity function at z < 0.5

Aims. We aim to study the 250 micron luminosity function (LF) down to much fainter luminosities than achieved by previous efforts. Methods. We developed a modified stacking method to reconstruct the 250 micron LF using optically selected galaxies from the SDSS survey and Herschel maps of the GAMA equatorial fields and Stripe 82. Our stacking method not only recovers the mean 250 micron luminosities of galaxies that are too faint to be individually detected, but also their underlying distribution functions. Results. We find very good agreement with previous measurements in the overlapping luminosity range. More importantly, we are able to derive the LF down to much fainter luminosities (around 25 times fainter) than achieved by previous studies. We find strong positive luminosity evolution \propto (1 + z)^4.89\pm1.07 and moderate negative density evolution \propto (1 + z)^-1.02\pm0.54 over the redshift range z=[0.02, 0.5].

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arXiv2016

A growth-rate indicator for Compton-thick active galactic nuclei

Due to their heavily obscured central engines, the growth rate of Compton-thick (CT) active galactic nuclei (AGN) is difficult to measure. A statistically significant correlation between the Eddington ratio, λ$_{Edd}$, and the X-ray power-law index, Γ, observed in unobscured AGN offers an estimate of their growth rate from X-ray spectroscopy (albeit with large scatter). However, since X-rays undergo reprocessing by Compton scattering and photoelectric absorption when the line-of-sight to the central engine is heavily obscured, the recovery of the intrinsic Γ is challenging. Here we study a sample of local, predominantly Compton-thick megamaser AGN, where the black hole mass, and thus Eddington luminosity, are well known. We compile results on X-ray spectral fitting of these sources with sensitive high-energy (E> 10 keV) NuSTAR data, where X-ray torus models which take into account the reprocessing effects have been used to recover the intrinsic Γ values and X-ray luminosities, L$_X$. With a simple bolometric correction to L$_X$ to calculate λ$_{Edd}$, we find a statistically significant correlation between Γ and λ$_{Edd}$ (p = 0.007). A linear fit to the data yields Γ = (0.41$\pm$0.18)log$_{10}$λ$_{Edd}$+(2.38$\pm$ 0.20), which is statistically consistent with results for unobscured AGN. This result implies that torus modeling successfully recovers the intrinsic AGN parameters. Since the megamasers have low-mass black holes (M$_{BH}\approx10^6-10^7$ M$_{sol}$) and are highly inclined, our results extend the Γ-λ$_{Edd}$ relationship to lower masses and argue against strong orientation effects in the corona, in support of AGN unification. Finally this result supports the use of Γ as a growth-rate indicator for accreting black holes, even for Compton-thick AGN.

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