arXiv subjects

Andreas Efstathiou

Publications and source records attributed to Andreas Efstathiou.

At least 37 records · Page 2Linked to original sources

arXiv2020

Solving Inverse Problems for Spectral Energy Distributions with Deep Generative Networks

We propose an end-to-end approach for solving inverse problems for a class of complex astronomical signals, namely Spectral Energy Distributions (SEDs). Our goal is to reconstruct such signals from scarce and/or unreliable measurements. We achieve that by leveraging a learned structural prior in the form of a Deep Generative Network. Similar methods have been tested almost exclusively for images which display useful properties (e.g., locality, periodicity) that are implicitly exploited. However, SEDs lack such properties which make the problem more challenging. We manage to successfully extend the methods to SEDs using a Generative Latent Optimization model trained with significantly fewer and corrupted data.

astro-ph.IM↗

arXiv2020

Star formation and nuclear activity in luminous infrared galaxies: An infrared through radio review

Nearby galaxies offer unique laboratories allowing multi-wavelength spatially resolved studies of the interstellar medium, star formation and nuclear activity across a broad range of physical conditions. In particular, detailed studies of individual local luminous infrared galaxies (LIRGs) are crucial for gaining a better understanding of these processes and for developing and testing models that are used to explain statistical studies of large populations of such galaxies at high redshift for which it is currently impossible to reach a sufficient physical resolution. Here, we provide an overview of the impact of spatially resolved infrared, sub-millimetre and radio observations in the study of the interstellar medium, star formation and active galactic nuclei as well as their interplay in local LIRGs. We also present an overview of the modelling of their spectral energy distributions using state-of-the-art radiative transfer codes. These contribute necessary and powerful 'workhorse' tools for the study of LIRGs (and their more luminous counterparts) at higher redshifts which are unresolved in observations. We describe how spatially-resolved time domain observations have recently opened a new window to study the nuclear activity in LIRGs. We describe in detail the observational characteristics of Arp 299 which is one of the best studied local LIRGs and exemplifies the power of the combination of high-resolution observations at infrared to radio wavelengths together with radiative transfer modelling used to explain the spectral energy distributions of its different components. We summarise the previous achievements obtained using high-spatial resolution observations and provide an outlook into what we can expect to achieve with future facilities.

astro-ph.GA↗

arXiv2020

High-resolution VLA Imaging of Obscured Quasars: Young Radio Jets Caught in a Dense ISM

We present new sub-arcsecond-resolution Karl G. Jansky Very Large Array (VLA) imaging at 10 GHz of 155 ultra-luminous ($L_{\rm bol}\sim10^{11.7-14.2} L_\odot$) and heavily obscured quasars with redshifts $z \sim0.4-3$. The sample was selected to have extremely red mid-infrared (MIR)-optical color ratios based on data from Wide-Field Infrared Survey Explorer (WISE) along with a detection of bright, unresolved radio emission from the NRAO VLA Sky Survey (NVSS) or Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) Survey. Our high-resolution VLA observations have revealed that the majority of the sources in our sample (93 out of 155) are compact on angular scales $<0.2^{\prime \prime}$ ($\leq 1.7$ kpc at $z \sim2$). The radio luminosities, linear extents, and lobe pressures of our sources are similar to young radio active galactic nuclei (AGN; e.g., Gigahertz Peaked Spectrum, GPS, and Compact Steep Spectrum, CSS, sources), but their space density is considerably lower. Application of a simple adiabatic lobe expansion model suggests relatively young dynamical ages ($\sim10^{4-7}$ years), relatively high ambient ISM densities ($\sim1-10^4$ cm$^{-3}$), and modest lobe expansion speeds ($\sim30-10,000$ km s$^{-1}$). Thus, we find our sources to be consistent with a population of newly triggered, young jets caught in a unique evolutionary stage in which they still reside within the dense gas reservoirs of their hosts. Based on their radio luminosity function and dynamical ages, we estimate only $\sim20\%$ of classical large scale FRI/II radio galaxies could have evolved directly from these objects. We speculate that the WISE-NVSS sources might first become GPS or CSS sources, of which some might ultimately evolve into larger radio galaxies.

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arXiv2019

HELP: A catalogue of 170 million objects, selected at 0.36-4.5 $μ$m, from 1270 deg.$^{2}$ of prime extragalactic fields

We present an optical to near-infrared selected astronomical catalogue covering 1270 deg.$^2$. This is the first attempt to systematically combine data from 23 of the premier extragalactic survey fields - the product of a vast investment of telescope time. The fields are those imaged by the Herschel Space Observatory which form the Herschel Extragalactic Legacy Project (HELP). Our catalogue of 170 million objects is constructed by a positional cross match of 51 public surveys. This high resolution optical, near-infrared, and mid-infrared catalogue is designed for photometric redshift estimation, extraction of fluxes in lower resolution far-infrared maps, and spectral energy distribution modelling. It collates, standardises, and provides value added derived quantities including corrected aperture magnitudes and astrometry correction over the Herschel extragalactic wide fields for the first time. $grizy$ fluxes are available on all fields with $g$ band data reaching $5σ$ point-source depths in a 2 arcsec aperture of 23.5, 24.4, and 24.6 (AB) mag at the 25th, 50th, and 75th percentiles, by area covered, across all HELP fields. It has $K$ or $K_s$ coverage over 1146 deg.$^2$ with depth percentiles of 20.2, 20.4, and 21.0 mag respectively. The IRAC Ch 1 band is available over 273 deg.$^2$ with depth percentiles of 17.7, 21.4, and 22.2 mag respectively. This paper defines the "masterlist" objects for the first data release (DR1) of HELP. This large sample of standardised total and corrected aperture fluxes, uniform quality flags, and completeness measures provides large well understood statistical samples over the full Herschel extragalactic sky.

astro-ph.GA↗

arXiv2019

The mid-infrared and CO gas properties of an extreme star-forming FeLoBAL quasar

We present a detailed study of a high-redshift iron low-ionization broad absorption line (FeLoBAL) quasar (SDSS1214 at $z = 1.046$), including new interferometric $^{12}$CO $J$=2-1 observations, optical through far-infrared photometry, and mid-infrared spectroscopy. The CO line is well-fit by a single Gaussian centered 40 kms$^{-1}$ away from the systemic velocity and implies a total molecular gas mass of $M_\textrm{gas} = 7.3 \times 10^{10} \textrm{M}_\odot$. The infrared SED requires three components: an active galactic nucleus (AGN) torus, an AGN polar dust component, and a starburst. The starburst dominates the infrared emission with a luminosity of log($L_\textrm{SB}[\textrm{L}_\odot]) = 12.91^{+0.02}_{-0.02}$, implying a star formation rate of about 2000 $\textrm{M}_{\odot}$yr$^{-1}$, the highest known among FeLoBAL quasars. The AGN torus and polar dust components are less luminous, at log($L_\textrm{AGN}[\textrm{L}_\odot]) = 12.36^{+0.14}_{-0.15}$ and log($L_\textrm{dust}[\textrm{L}_\odot]) = 11.75^{+0.26}_{-0.46}$, respectively. If all of the molecular gas is used to fuel the ongoing star formation, then the lower limit on the subsequent duration of the starburst is 40 Myr. We do not find conclusive evidence that the AGN is affecting the CO gas reservoir. The properties of SDSS1214 are consistent with it representing the endpoint of an obscured starburst transitioning through a LoBAL phase to that of a classical quasar.

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arXiv2018

AKARI and IRAS: From Beam Corrections to SEDs

There is significant scientific value to be gained from combining AKARI fluxes with data at other far-IR wavelengths from the IRAS and Herschel missions. To be able to do this we must ensure that there are no systematic differences between the datasets that need to be corrected before the fluxes are compatible with each other. One such systematic effect identified in the BSCv1 data is the issue of beam corrections. We determine these for the BSCv2 data by correlating ratios of appropriate IRAS and AKARI bands with the difference in 2MASS J band extended and point source magnitudes for sources cross matched between the IRAS FSC, AKARI BSCv2 and 2MASS catalogs. We find significant correlations (p<< 10^-13) indicating that beam corrections are necessary in the 65 and 90 micron bands. We then use these corrected fluxes to supplement existing data in spectral energy distribution (SED) fits for ultraluminous infrared galaxies in the HERUS survey. The addition of AKARI fluxes makes little difference to the results of simple (T,beta) fits to the SEDs of these sources, though there is a general decrease in reduced chi^2 values. The utility of the extra AKARI data, however, is in allowing physically more realistic SED models with more parameters to be fit to the data. We also extend our analysis of beam correction issues in the AKARI data by examining the Herschel Reference Sample, which have Herschel photometry from 100 to 500 microns and which are more spatially extended than the HERUS ULIRGs. Thirty-four of the HRS sources have good Herschel SEDs and matching data from AKARI. This investigation finds that our simple 2MASS-based beam correction scheme is inadequate for these larger and more complex sources. There are also indications that additional beam corrections at 140 and 160 microns are needed for these sources, extended on scales >1 arcminute.

astro-ph.GA↗

arXiv2018

The emission and distribution of dust of the torus of NGC 1068

We present observations of NGC 1068 covering the $19.7-53.0$ $μ$m wavelength range using FORCAST and HAWC+ onboard SOFIA. Using these observations, high-angular resolution infrared (IR) and sub-mm observations, we find an observational turn-over of the torus emission in the $30-40$ $μ$m wavelength range with a characteristic temperature of $70-100$ K. This component is clearly different from the diffuse extended emission in the narrow line and star formation regions at 10-100 $μ$m within the central 700 pc. We compute $2.2-432$ $μ$m 2D images using the best inferred \textsc{clumpy} torus model based on several nuclear spectral energy distribution (SED) coverages. We find that when $1-20$ $μ$m SED is used, the inferred result gives a small torus size ($<4$ pc radius) and a steep radial dust distribution. The computed torus using the $1-432$ $μ$m SED provides comparable torus sizes, $5.1^{+0.4}_{-0.4}$ pc radius, and morphology to the recently resolved 432 $μ$m ALMA observations. This result indicates that the $1-20$ $μ$m wavelength range is not able to probe the full extent of the torus. The characterization of the turn-over emission of the torus using the $30-60$ $μ$m wavelength range is sensitive to the detection of cold dust in the torus. The morphology of the dust emission in our 2D image at 432 $μ$m is spatially coincident with the cloud distribution, while the morphology of the emission in the $1-20$ $μ$m wavelength range shows an elongated morphology perpendicular to the cloud distribution. We find that our 2D \textsc{clumpy} torus image at 12 $μ$m can produce comparable results to those observed using IR interferometry.

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arXiv2017

Star formation and AGN activity in a sample of local Luminous Infrared Galaxies through multi-wavelength characterization

Nuclear starbursts and AGN activity are the main heating processes in luminous infrared galaxies (LIRGs) and their relationship is fundamental to understand galaxy evolution. In this paper, we study the star-formation and AGN activity of a sample of 11 local LIRGs imaged with subarcsecond angular resolution at radio (8.4GHz) and near-infrared ($2.2μ$m) wavelengths. This allows us to characterize the central kpc of these galaxies with a spatial resolution of $\simeq100$pc. In general, we find a good spatial correlation between the radio and the near-IR emission, although radio emission tends to be more concentrated in the nuclear regions. Additionally, we use an MCMC code to model their multi-wavelength spectral energy distribution (SED) using template libraries of starburst, AGN and spheroidal/cirrus models, determining the luminosity contribution of each component, and finding that all sources in our sample are starburst-dominated, except for NGC6926 with an AGN contribution of $\simeq64$\%. Our sources show high star formation rates ($40$ to $167M_\odot\mathrm{yr}^{-1}$), supernova rates (0.4 to $2.0\mathrm{SN}\mathrm{yr}^{-1}$), and similar starburst ages (13 to $29\mathrm{Myr}$), except for the young starburst (9Myr) in NGC6926. A comparison of our derived star-forming parameters with estimates obtained from different IR and radio tracers shows an overall consistency among the different star formation tracers. AGN tracers based on mid-IR, high-ionization line ratios also show an overall agreement with our SED model fit estimates for the AGN. Finally, we use our wide-band VLA observations to determine pixel-by-pixel radio spectral indices for all galaxies in our sample, finding a typical median value ($α\simeq-0.8$) for synchrotron-powered LIRGs.

astro-ph.GA↗

arXiv2017

The role of the most luminous, obscured AGN in galaxy assembly at z~2

We present HST WFC3 F160W imaging and infrared spectral energy distributions for twelve extremely luminous, obscured AGN at $1.8<z<2.7$, selected via "Hot, Dust Obscured" mid-infrared colors. Their infrared luminosities span $2-15\times10^{13}$L$_{\odot}$, making them among the most luminous objects in the Universe at $z\sim2$. In all cases the infrared emission is consistent with arising at least in most part from AGN activity. The AGN fractional luminosities are higher than those in either sub-millimeter galaxies, or AGN selected via other mid-infrared criteria. Adopting the $G$, M$_{20}$ and $A$ morphological parameters, together with traditional classification boundaries, infers that three quarters of the sample as mergers. Our sample do not, however, show any correlation between the considered morphological parameters and either infrared luminosity or AGN fractional luminosity. Moreover, their asymmetries and effective radii are distributed identically to those of massive galaxies at $z\sim2$. We conclude that our sample is not preferentially associated with mergers, though a significant merger fraction is still plausible. Instead, we propose that our sample are examples of the massive galaxy population at $z\sim2$ that harbor a briefly luminous, "flickering" AGN, and in which the $G$ and M$_{20}$ values have been perturbed, due to either the AGN, and/or the earliest formation stages of a bulge in an inside-out manner. Furthermore, we find that the mass assembly of the central black holes in our sample leads the mass assembly of any bulge component. Finally, we speculate that our sample represent a small fraction of the immediate antecedents of compact star-forming galaxies at $z\sim2$.

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arXiv2017

Unveiling the AGN in IC 883: discovery of a parsec-scale radio jet

IC883 is a luminous infrared galaxy (LIRG) classified as a starburst-active galactic nucleus (AGN) composite. In a previous study we detected a low-luminosity AGN (LLAGN) radio candidate. Here we report on our radio follow-up at three frequencies which provides direct and unequivocal evidence of the AGN activity in IC883. Our analysis of archival X-ray data, together with the detection of a transient radio source with luminosity typical of bright supernovae, give further evidence of the ongoing star formation activity, which dominates the energetics of the system. At sub-parsec scales, the radio nucleus has a core-jet morphology with the jet being a newly ejected component showing a subluminal proper motion of 0.6c-1c. The AGN contributes less than two per cent of the total IR luminosity of the system. The corresponding Eddington factor is ~1E-3, suggesting this is a low-accretion rate engine, as often found in LLAGNs. However, its high bolometric luminosity (~10E44erg/s) agrees better with a normal AGN. This apparent discrepancy may just be an indication of the transition nature of the nucleus from a system dominated by star-formation, to an AGN-dominated system. The nucleus has a strongly inverted spectrum and a turnover at ~4.4GHz, thus qualifying as a candidate for the least luminous (L_5.0GHz ~ 6.3E28erg/s/Hz) and one of the youngest (~3000yr) gigahertz-peaked spectrum (GPS) sources. If the GPS origin for the IC883 nucleus is confirmed, then advanced mergers in the LIRG category are potentially key environments to unveil the evolution of GPS sources into more powerful radio galaxies.

astro-ph.GA↗

arXiv2016

HERUS: A CO Atlas from SPIRE Spectroscopy of local ULIRGs

We present the Herschel SPIRE Fourier Transform Spectroscopy (FTS) atlas for a complete flux limited sample of local Ultra-Luminous Infra-Red Galaxies as part of the HERschel ULIRG Survey (HERUS). The data reduction is described in detail and was optimized for faint FTS sources with particular care being taken with the subtraction of the background which dominates the continuum shape of the spectra. Special treatment in the data reduction has been given to any observation suffering from artefacts in the data caused by anomalous instrumental effects to improve the final spectra. Complete spectra are shown covering $200 - 671μ$m with photometry in the SPIRE bands at 250$μ$m, 350$μ$m and 500$μ$m. The spectra include near complete CO ladders for over half of our sample, as well as fine structure lines from [CI] 370 $μ$m, [CI] 609 $μ$m, and [NII] 205 $μ$m. We also detect H$_{2}$O lines in several objects. We construct CO Spectral Line Energy Distributions (SLEDs) for the sample, and compare their slopes with the far-infrared colours and luminosities. We show that the CO SLEDs of ULIRGs can be broadly grouped into three classes based on their excitation. We find that the mid-J (5$<$J$<$8) lines are better correlated with the total far-infrared luminosity, suggesting that the warm gas component is closely linked to recent star-formation. The higher J transitions do not linearly correlate with the far-infrared luminosity, consistent with them originating in hotter, denser gas unconnected to the current star-formation. {\bf We conclude that in most cases more than one temperature components are required to model the CO SLEDs.}

astro-ph.GA↗

arXiv2016

Star Formation in Luminous Quasars at 2<z<3

We investigate the relation between star formation rates ($\dot{M}_{s}$) and AGN properties in optically selected type 1 quasars at $2<z<3$ using data from Herschel and the SDSS. We find that $\dot{\rm{M}}_s$ remains approximately constant with redshift, at $300\pm100~\rm{M}_{\odot}$yr$^{-1}$. Conversely, $\dot{\rm{M}}_s$ increases with AGN luminosity, up to a maximum of $\sim600~\rm{M}_{\odot}$yr$^{-1}$, and with CIV FWHM. In context with previous results, this is consistent with a relation between $\dot{\rm{M}}_s$ and black hole accretion rate ($\dot{\rm{M}}_{bh}$) existing in only parts of the $z-\dot{\rm{M}}_{s}-\dot{\rm{M}}_{bh}$ plane, dependent on the free gas fraction, the trigger for activity, and the processes that may quench star formation. The relations between $\dot{\rm{M}}_s$ and both AGN luminosity and CIV FWHM are consistent with star formation rates in quasars scaling with black hole mass, though we cannot rule out a separate relation with black hole accretion rate. Star formation rates are observed to decline with increasing CIV equivalent width. This decline can be partially explained via the Baldwin effect, but may have an additional contribution from one or more of three factors; $M_i$ is not a linear tracer of L$_{2500}$, the Baldwin effect changes form at high AGN luminosities, and high CIV EW values signpost a change in the relation between $\dot{\rm{M}}_s$ and $\dot{\rm{M}}_{bh}$. Finally, there is no strong relation between $\dot{\rm{M}}_s$ and Eddington ratio, or the asymmetry of the CIV line. The former suggests that star formation rates do not scale with how efficiently the black hole is accreting, while the latter is consistent with CIV asymmetries arising from orientation effects.

astro-ph.GA↗

arXiv2015

Radio Jet Feedback and Star Formation in Heavily Obscured Quasars at Redshifts ~0.3-3, I: ALMA Observations

We present ALMA 870 micron (345 GHz) data for 49 high redshift (0.47<z<2.85), luminous (11.7 < log L(bol) (Lsun) < 14.2) radio-powerful AGN, obtained to constrain cool dust emission from starbursts concurrent with highly obscured radiative-mode black hole (BH) accretion in massive galaxies which possess a small radio jet. The sample was selected from WISE with extremely steep (red) mid-infrared (MIR) colors and with compact radio emission from NVSS/FIRST. Twenty-six sources are detected at 870 microns, and we find that the sample has large mid- to far-infrared luminosity ratios consistent with a dominant and highly obscured quasar. The rest-frame 3 GHz radio powers are 24.7 < log P3.0 GHz (W/Hz) < 27.3, and all sources are radio-intermediate or radio-loud. BH mass estimates are 7.7 < log M(BH) (Msun) < 10.2. The rest frame 1-5 um SEDs are very similar to the "Hot DOGs" (Hot Dust Obscured Galaxies), and steeper (redder) than almost any other known extragalactic sources. ISM masses estimated for the ALMA detected sources are 9.9 < log M(ISM) (Msun) < 11.75 assuming a dust temperature of 30K. The cool dust emission is consistent with star formation rates (SFRs) reaching several thousand Msun/yr, depending on the assumed dust temperature, however we cannot rule out the alternative that the AGN powers all the emission in some cases. Our best constrained source has radiative transfer solutions with ~ equal contributions from an obscured AGN and a young (10-15 Myr) compact starburst.

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arXiv2015

Self-consistent 2-phase AGN torus models: SED library for observers

We assume that dust near active galactic nuclei (AGN) is distributed in a torus-like geometry, which may be described by a clumpy medium or a homogeneous disk or as a combination of the two (i.e. a 2-phase medium). The dust particles considered are fluffy and have higher submillimeter emissivities than grains in the diffuse ISM. The dust-photon interaction is treated in a fully self-consistent three dimensional radiative transfer code. We provide an AGN library of spectral energy distributions (SEDs). Its purpose is to quickly obtain estimates of the basic parameters of the AGN, such as the intrinsic luminosity of the central source, the viewing angle, the inner radius, the volume filling factor and optical depth of the clouds, and the optical depth of the disk midplane, and to predict the flux at yet unobserved wavelengths. The procedure is simple and consists of finding an element in the library that matches the observations. We discuss the general properties of the models and in particular the 10mic. silicate band. The AGN library accounts well for the observed scatter of the feature strengths and wavelengths of the peak emission. AGN extinction curves are discussed and we find that there is no direct one-to-one link between the observed extinction and the wavelength dependence of the dust cross sections. We show that objects of the library cover the observed range of mid IR colors of known AGN. The validity of the approach is demonstrated by matching the SEDs of a number of representative objects: Four Seyferts and two quasars for which we present new Herschel photometry, two radio galaxies, and one hyperluminous infrared galaxy. Strikingly, for the five luminous objects we find pure AGN models fit the SED without a need to postulate starburst activity.

astro-ph.GA↗

arXiv2014

A multi-wavelength view of the central kiloparsec region in the Luminous Infrared Galaxy NGC1614

The Luminous Infrared Galaxy NGC1614 hosts a prominent circumnuclear ring of star formation. However, the nature of the dominant emitting mechanism in its central ~100 pc is still under debate. We present sub-arcsecond angular resolution radio, mid-infrared, Pa-alpha, optical, and X-ray observations of NGC1614, aimed at studying in detail both the circumnuclear ring and the nuclear region. The 8.4 GHz continuum emission traced by the Very Large Array (VLA) and the Gemini/T-ReCS 8.7 micron emission, as well as the Pa-alpha line emission, show remarkable morphological similarities within the star-forming ring, suggesting that the underlying emission mechanisms are tightly related. We used an HST/NICMOS Pa-alpha map of similar resolution to our radio maps to disentangle the thermal free-free and non-thermal synchrotron radio emission, from which we obtained the intrinsic synchrotron power-law for each individual region within the central kpc of NGC1614. The radio ring surrounds a relatively faint, steep-spectrum source at the very center of the galaxy, suggesting that the central source is not powered by an AGN, but rather by a compact (r < 90 pc) starburst. Chandra X-ray data also show that the central kpc region is dominated by starburst activity, without requiring the existence of an AGN. We also used publicly available infrared data to model-fit the spectral energy distribution of both the starburst ring and a putative AGN in NGC1614. In summary, we conclude that there is no need to invoke an AGN to explain the observed bolometric properties of the galaxy.

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arXiv2013

Far-infrared Fine-Structure Line Diagnostics of Ultraluminous Infrared Galaxies

We present Herschel observations of six fine-structure lines in 25 Ultraluminous Infrared Galaxies at z<0.27. The lines, [O III]52, [N III]57, [O I]63, [N II]122, [O I]145, and [C II]158, are mostly single gaussians with widths <600 km s-1 and luminosities of 10^7 - 10^9 Solar. There are deficits in the [O I]63/L_IR, [N II]/L_IR, [O I]145/L_IR, and [C II]/L_IR ratios compared to lower luminosity systems. The majority of the line deficits are consistent with dustier H II regions, but part of the [C II] deficit may arise from an additional mechanism, plausibly charged dust grains. This is consistent with some of the [C II] originating from PDRs or the ISM. We derive relations between far-IR line luminosities and both IR luminosity and star formation rate. We find that [N II] and both [O I] lines are good tracers of IR luminosity and star formation rate. In contrast, [C II] is a poor tracer of IR luminosity and star formation rate, and does not improve as a tracer of either quantity if the [C II] deficit is accounted for. The continuum luminosity densities also correlate with IR luminosity and star formation rate. We derive ranges for the gas density and ultraviolet radiation intensity of 10^1 < n < 10^2.5 and 10^2.2 < G_0 < 10^3.6, respectively. These ranges depend on optical type, the importance of star formation, and merger stage. We do not find relationships between far-IR line properties and several other parameters; AGN activity, merger stage, mid-IR excitation, and SMBH mass. We conclude that these far-IR lines arise from gas heated by starlight, and that they are not strongly influenced by AGN activity.

astro-ph.CO↗

arXiv2012

Principal Component Analysis and Radiative Transfer modelling of Spitzer IRS Spectra of Ultra Luminous Infrared Galaxies

The mid-infrared spectra of ultraluminous infrared galaxies (ULIRGs) contain a variety of spectral features that can be used as diagnostics to characterise the spectra. However, such diagnostics are biased by our prior prejudices on the origin of the features. Moreover, by using only part of the spectrum they do not utilise the full information content of the spectra. Blind statistical techniques such as principal component analysis (PCA) consider the whole spectrum, find correlated features and separate them out into distinct components. We further investigate the principal components (PCs) of ULIRGs derived in Wang et al.(2011). We quantitatively show that five PCs is optimal for describing the IRS spectra. These five components (PC1-PC5) and the mean spectrum provide a template basis set that reproduces spectra of all z<0.35 ULIRGs within the noise. For comparison, the spectra are also modelled with a combination of radiative transfer models of both starbursts and the dusty torus surrounding active galactic nuclei. The five PCs typically provide better fits than the models. We argue that the radiative transfer models require a colder dust component and have difficulty in modelling strong PAH features. Aided by the models we also interpret the physical processes that the principal components represent. The third principal component is shown to indicate the nature of the dominant power source, while PC1 is related to the inclination of the AGN torus. Finally, we use the 5 PCs to define a new classification scheme using 5D Gaussian mixtures modelling and trained on widely used optical classifications. The five PCs, average spectra for the four classifications and the code to classify objects are made available at: http://www.phys.susx.ac.uk/~pdh21/PCA/

astro-ph.CO↗

arXiv2012

A new model for the infrared emission of IRAS F10214+4724

We present a new model for the infrared emission of the high redshift hyperluminous infrared galaxy IRAS F10214+4724 which takes into account recent photometric data from Spitzer and Herschel that sample the peak of its spectral energy distribution. We first demonstrate that the combination of the AGN tapered disc and starburst models of Efstathiou and coworkers, while able to give an excellent fit to the average spectrum of type 2 AGN measured by Spitzer, fails to match the spectral energy distribution of IRAS F10214+4724. This is mainly due to the fact that the nuSnu distribution of the galaxy falls very steeply with increasing frequency (a characteristic of heavy absorption by dust) but shows a silicate feature in emission. We propose a model that assumes two components of emission: clouds that are associated with the narrow-line region and a highly obscured starburst. The emission from the clouds must suffer significantly stronger gravitational lensing compared to the emission from the torus to explain the observed spectral energy distribution.

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