Characterizing the UV-to-NIR shape of the dust attenuation curve of IR luminous galaxies up to z ∼ 2
In this work, we investigate the far-ultraviolet (UV) to near-infrared (NIR) shape of the dust attenuation curve of a sample of IR-selected dust obscured (ultra)luminous IR galaxies at z~2. The spectral energy distributions (SEDs) are fitted with Code Investigating GALaxy Emission, a physically motivated spectral-synthesis model based on energy balance. Its flexibility allows us to test a wide range of different analytical prescriptions for the dust attenuation curve, including the well-known Calzetti and Charlot & Fall curves, and modified versions of them. The attenuation curves computed under the assumption of our reference double power-law model are in very good agreement with those derived, in previousworks, with radiative transfer (RT) SED fitting. We investigate the position of our galaxies in the IRX-β diagram and find this to be consistent with greyer slopes, on average, in the UV. We also find evidence for a flattening of the attenuation curve in the NIR with respect to more classical Calzetti-like recipes. This larger NIR attenuation yields larger derived stellar masses from SED fitting, by a median factor of ~1.4 and up to a factor ~10 for the most extreme cases. The star formation rate appears instead to be more dependent on the total amount of attenuation in the galaxy. Our analysis highlights the need for a flexible attenuation curve when reproducing the physical properties of a large variety of objects.
ISO photometry of hyperluminous infrared galaxies: Implications for the origin of their extreme luminosities
We present 7-180μm photometry of a sample of hyperluminous infrared galaxies (HyLIGs) obtained with the photometer and camera mounted on the Infrared Space Observatory (ISO). We have used state-of-the-art' radiative transfer models of obscured starbursts and dusty tori to model their broadband spectral energy distributions (SEDs). We find that IRAS F00235+1024, IRAS F14218+3845 and IRAS F15307+3252 require a combination of starburst and AGN components to explain their mid to far-infrared emission, while for TXS0052+471 a dust torus model alone is sufficient. For IRAS F00235+1024 and IRAS F14218+3845 the starburst component is the predominant contributor whereas for IRAS F15307+3252 the dust torus component dominates. The implied star formation rates (SFR) estimated from the starburst infrared luminosities are dM*,all/dt > 1000M⊙yr-1h50-2 and are amongst the highest SFRs estimated to date. We also demonstrate that the well-known radio-FIR correlation observed for extragalactic sources extends into both higher radio and infrared power than previously investigated. The relation for HyLIGs has a mean q value of 1.94. The results of this study imply that better sampling of the IR SEDs of HyLIGs may reveal that both AGN and starburst components are required to explain their emission from the NIR to the sub-millimetre.
The ISOPHOT 170 micron serendipity sky survey: A plea to FIRST
The ISOPHOT Serendipity Survey utilized the slew time between ISO's pointed observations with strip scanning measurements of the sky in the far-infrared (FIR) at 170μm. The integral 170μm fluxes for compact sources derived from the slews are put on an absolute flux level by using a number of galaxies as calibrator sources observed with ISOPHOT's photometric mapping mode, supplemented by Serendipity Survey observations of two planets and two asteroids with available model fluxes. A first group of 115 well-observed sources with a high signal-to-noise ratio in all four detector pixels having a galaxy association were extracted from the slew data with low (I100μm ≤ 15 MJy/sr) cirrus background. For all but a few galaxies, the 170μm fluxes are determined for the first time, which represents a significant increase in the number of galaxies with measured FIR fluxes beyond the IRAS 100μm limit. The large fraction of sources with a high F170μm/F100μm flux ratio indicates that a very cold (T < 20 K) dust component is present in many galaxies. The typical mass of the coldest dust component is MDust = 107.5 ± 0.5 M⊙, a factor 2 - 10 larger than that derived from IRAS fluxes alone. As a consequence, the gas-to-dust ratios are much closer to the canonical value for the Milky Way. A similar Serendipity Survey with FIRST has the prospects of delivering FIR data with a much higher angular resolution (PACS) or at longer wavelengths (SPIRE) than ISOPHOT, thereby providing either crucial information for the identification of compact sources in confused regions or extending the spectral coverage for a large number of sources and finding rare classes of very cold FIR emitters.
Self-consistent two-phase AGN torus models
We assume that dust near active galactic nuclei (AGNs) is distributed in a torus-like geometry, which can be described as a clumpy medium or a homogeneous disk, or as a combination of the two (i.e. a two-phase medium). The dust particles considered are fluffy and have higher submillimeter emissivities than grains in the diffuse interstellar medium. 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 AGNs, 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 10 μm 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 in the library cover the observed range of mid-infrared colors of known AGNs. 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 that pure AGN models fit the SED without needing to postulate starburst activity.
The European Large Area ISO Survey -- VI. Discovery of a new hyperluminous infrared galaxy
We report the discovery of the first hyperluminous infrared galaxy (HyLIG) in the course of the European Large Area ISO Survey (ELAIS). This object has been detected by ISO at 6.7, 15 and 90 μm, and is found to be a broad-line, radio-quiet quasar at a redshift z = 1.099. From a detailed multicomponent model fit of the spectral energy distribution, we derive a total IR luminosity LIR (1-1000 μm) ≈ 1.0 × 1013 h-265 L⊙ (0 = 0.5), and discuss the possible existence of a starburst contributing to the far-IR output. Observations to date present no evidence for lens magnification. This galaxy is one of the very few HyLIGs with a X-ray detection. On the basis of its soft X-ray properties, we suggest that this broad-line object may be the face-on analogue of narrow-line, Seyfert-like HyLIGs.
Feedback and Feeding in the Context of Galaxy Evolution with SPICA : Direct Characterisation of Molecular Outflows and Inflows
A far-infrared observatory such as the SPace Infrared telescope for Cosmology and Astrophysics, with its unprecedented spectroscopic sensitivity, would unveil the role of feedback in galaxy evolution during the last ∼10 Gyr of the Universe (z = 1.5–2), through the use of far- and mid-infrared molecular and ionic fine structure lines that trace outflowing and infalling gas. Outflowing gas is identified in the far-infrared through P-Cygni line shapes and absorption blueshifted wings in molecular lines with high dipolar moments, and through emission line wings of fine-structure lines of ionised gas. We quantify the detectability of galaxy-scale massive molecular and ionised outflows as a function of redshift in AGN-dominated, starburst-dominated, and main-sequence galaxies, explore the detectability of metal-rich inflows in the local Universe, and describe the most significant synergies with other current and future observatories that will measure feedback in galaxies via complementary tracers at other wavelengths.
A model for the infrared continuum spectrum of NGC 1068
We present a model for the nuclear infrared (IR) continuum spectrum of the Seyfert galaxy NGC 1068. The torus emission is modelled in terms of the tapered disc models of Efstathiou & Rowan-Robinson, which give a good fit to the global infrared properties of active galactic nuclei. The models include the effects of a distribution of grain species and sizes and multiple scattering from dust. Our analysis is constrained by the inclination of the torus predicted by optical spectropolarimetry. We assume in particular that our line of sight is inclined to the axis of symmetry by about 35°, and that the half-opening angle of the cone is 30°. We find that the torus emission alone cannot account for the whole of the IR continuum spectrum. While this is in agreement with recent mid-IR imaging observations, which show that up to 60 per cent of the flux is not originating from the torus, our model suggests that the difference between the observed and predicted torus emission is actually much greater at near-IR wavelengths. We attribute this excess IR emission to a component of optically thin dust [Av=0.1-0.5 mag) located in the ionization cone between the BLR and the NLR. This dust must be distributed as r-2 in order to produce the required spectrum. Flatter density distributions peak at longer density distributions peak at longer wavelengths and also produce a strong emission feature at 10 µm, contrary to observations. Even with an r-2 distribution, the grain mixture in the cone needs to be modified in order to suppress further the silicate emission feature. We suggest that this may be due to either destruction of silicate grains by shocks or the clumping of NLR dust. In addition, our model requires that the flux radiated by the central source towards the cone is at least a factor of 6 higher than that directed towards the bulk of the torus, which is naturally explained if the central source is an accretion disc. This conclusion depends mainly on the assumed inclination and opening angle of the torus, but is rather insensitive to other geometrical parameters.
Galaxy evolution studies with the SPace IR telescope for cosmology and astrophysics (SPICA): The power of IR spectroscopy
IR spectroscopy in the range 12–230 μm with the SPace IR telescope for Cosmology and Astrophysics (SPICA) will reveal the physical processes governing the formation and evolution of galaxies and black holes through cosmic time, bridging the gap between the James Webb Space Telescope and the upcoming Extremely Large Telescopes at shorter wavelengths and the Atacama Large Millimeter Array at longer wavelengths. The SPICA, with its 2.5-m telescope actively cooled to below 8 K, will obtain the first spectroscopic determination, in the mid-IR rest-frame, of both the star-formation rate and black hole accretion rate histories of galaxies, reaching lookback times of 12 Gyr, for large statistically significant samples. Densities, temperatures, radiation fields, and gas-phase metallicities will be measured in dust-obscured galaxies and active galactic nuclei, sampling a large range in mass and luminosity, from faint local dwarf galaxies to luminous quasars in the distant Universe. Active galactic nuclei and starburst feedback and feeding mechanisms in distant galaxies will be uncovered through detailed measurements of molecular and atomic line profiles. The SPICA’s large-area deep spectrophotometric surveys will provide mid-IR spectra and continuum fluxes for unbiased samples of tens of thousands of galaxies, out to redshifts of z ∼ 6.
Observations of hyperluminous infrared galaxies with theInfrared Space Observatory: implications for the origin of their extreme luminosities
We present 7-180 μm photometry of a sample of hyperluminous infrared galaxies (HyLIGs) obtained with the photometer and camera mounted on the Infrared Space Observatory. We have used radiative transfer models of obscured starbursts and dusty torii to model their spectral energy distributions (SEDs). We find that IRAS F00235+1024, IRAS F14218+3845 and IRAS F15307+3252 require a combination of starburst and active galactic nuclei (AGN) components to explain their mid-to far-infrared (FIR) emission, while for TXS 0052+471 a dust torus AGN model alone is sufficient. For IRAS F00235+1024 and IRAS F14218+3845 the starburst component is the predominant contributor, whereas for IRAS F15307+3252 the dust torus component dominates. The implied star formation rates (SFRs) for these three sources estimated from their infrared luminosities are M*,all > 3000 M⊙ yr-1 h-250 and are amongst the highest SFRs estimated to date. We also demonstrate that the well-known radio-FIR correlation extends into both higher radio and infrared power than previously investigated. The relation for HyLIGs has a mean q value of 1.94. The results of this study imply that better sampling of the infrared spectral energy distributions of HyLIGs may reveal that both AGN and starburst components are required to explain all the emission from the near-infrared to the submillimetre.
The ISOPHOT 170μm serendipity survey: I. Compact sources with galaxy associations
The first set of compact sources observed in the ISOPHOT 170 μm Serendipity Survey is presented. From the slew data with low (I100μm ≤ 15 MJy/sr) cirrus background, 115 well-observed sources with a high signal-to-noise ratio in all detector pixels having a galaxy association were extracted. Of the galaxies with known optical morphologies, the vast majority are classified as spirals, barred spirals, or irregulars. The 170 μm fluxes measured from the Serendipity slews have been put on an absolute flux level by using calibration sources observed additionally with the photometric mapping mode of ISOPHOT. For all but a few galaxies, the 170 μm fluxes are determined for the first time, which represents a significant increase in the number of galaxies with measured Far-Infrared (FIR) fluxes beyond the IRAS 100 μm limit. The 170 μm fluxes cover the range 2 ≲ F170μm ≲ 100 Jy. Formulae for the integrated FIR fluxes F40-220μm and the total infrared fluxes F1-1000/μm incorporating the new 170 μm fluxes are provided. The large fraction of sources with a high F170μm/F100μm flux ratio indicates that a cold (T/Dust ≲ 20 K) dust component is present in many galaxies. The detection of such a cold dust component is crucial for the determination of the total dust mass in galaxies, and, in cases with a large F170μm/F100μm flux ratio, increases the dust mass by a significant factor. The typical mass of the coldest dust component is MDust = 107.5 ±0.5 M⊙, a factor 2-10 larger than that derived from IRAS fluxes alone. As a consequence, the majority of the derived gas-to-dust ratios are much closer to the canonical value of ≈ 160 for the Milky Way. By relaxing the selection cri-teria, it is expected that the Serendipity Survey will eventually lead to a catalog of 170 μm fluxes for ≈ 1000 galaxies.