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I. Perez-Fournon

Publications and source records attributed to I. Perez-Fournon.

At least 73 records · Page 4Linked to original sources

Herschel photometric observations of the low metallicity dwarf galaxy NGC 1705

We present Herschel SPIRE and PACS photometeric observations of the low metallicity (Z ~ 0.35 solar) nearby dwarf galaxy, NGC 1705, in six wavelength bands as part of the Dwarf Galaxy Survey guaranteed time Herschel Key Program. We confirm the presence of two dominant circumnuclear IR-bright regions surrounding the central super star cluster that had been previously noted at mid-IR wavelengths and in the sub-mm by LABOCA. On constructing a global spectral energy distribution using the SPIRE and PACS photometry, in conjunction with archival IR measurements, we note the presence of an excess at sub-mm wavelengths. This excess suggests the presence of a significant cold dust component within NGC 1705 and was modeled as an additional cold component in the SED. Although alternative explanations for the sub-mm excess beyond 350 microns, such as changes to the dust emissivity cannot be ruled out, the most likely explanation for the observed submillimetre excess is that of an additional cold dust component.

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First results from HerMES on the evolution of the submillimetre luminosity function

We have carried out two extremely deep surveys with SPIRE, one of the two cameras on Herschel, at 250 microns, close to the peak of the far-infrared background. We have used the results to investigate the evolution of the rest-frame 250-micron luminosity function out to z=2. We find evidence for strong evolution out to a redshift of around 1 but evidence for at most weak evolution beyond this redshift. Our results suggest that a significant part of the stars and metals in the Universe today were formed at z<1.4 in spiral galaxies.

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HerMES: Far-infrared properties of known AGN in the HerMES fields

Nuclear and starburst activity are known to often occur concomitantly. Herschel-SPIRE provides sampling of the FIR SEDs of type 1 and type 2 AGN, allowing for the separation between the hot dust (torus) and cold dust (starburst) emission. We study large samples of spectroscopically confirmed type 1 and type 2 AGN lying within the Herschel Multi-tiered Extragalactic Survey (HerMES) fields observed during the science demonstration phase, aiming to understand their FIR colour distributions and constrain their starburst contributions. We find that one third of the spectroscopically confirmed AGN in the HerMES fields have 5-sigma detections at 250um, in agreement with previous (sub)mm AGN studies. Their combined Spitzer-MIPS and Herschel-SPIRE colours - specifically S(250)/S(70) vs. S(70)/S(24) - quite clearly separate them from the non-AGN, star-forming galaxy population, as their 24-um flux is dominated by the hot torus emission. However, their SPIRE colours alone do not differ from those of non-AGN galaxies. SED fitting shows that all those AGN need a starburst component to fully account for their FIR emission. For objects at z > 2, we find a correlation between the infrared luminosity attributed to the starburst component, L(SB), and the AGN accretion luminosity, L(acc), with L(SB) propto L(acc)^0.35. Type 2 AGN detected at 250um show on average higher L(SB) than type 1 objects but their number is still too low to establish whether this trend indicates stronger star-formation activity.

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HerMES: The SPIRE confusion limit

We report on the sensitivity of SPIRE photometers on the Herschel Space Observatory. Specifically, we measure the confusion noise from observations taken during the Science Demonstration Phase of the Herschel Multi-tiered Extragalactic Survey. Confusion noise is defined to be the spatial variation of the sky intensity in the limit of infinite integration time, and is found to be consistent among the different fields in our survey at the level of 5.8, 6.3 and 6.8 mJy/beam at 250, 350 and 500 microns, respectively. These results, together with the measured instrument noise, may be used to estimate the integration time required for confusion-limited maps, and provide a noise estimate for maps obtained by SPIRE.

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HerMES: SPIRE galaxy number counts at 250, 350 and 500 microns

Emission at far-infrared wavelengths makes up a significant fraction of the total light detected from galaxies over the age of Universe. Herschel provides an opportunity for studying galaxies at the peak wavelength of their emission. Our aim is to provide a benchmark for models of galaxy population evolution and to test pre-existing models of galaxies. With the Herschel Multi-tiered Extra-galactic survey, HerMES, we have observed a number of fields of different areas and sensitivity using the SPIRE instrument on Herschel. We have determined the number counts of galaxies down to ~20 mJy. Our constraints from directly counting galaxies are consistent with, though more precise than, estimates from the BLAST fluctuation analysis. We have found a steep rise in the Euclidean normalised counts at <100 mJy. We have directly resolved 15% of the infrared extra-galactic background at the wavelength near where it peaks.

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The Herschel Space Observatory view of dust in M81

We use Herschel Space Observatory data to place observational constraints on the peak and Rayleigh-Jeans slope of dust emission observed at 70-500 microns in the nearby spiral galaxy M81. We find that the ratios of wave bands between 160 and 500 microns are primarily dependent on radius but that the ratio of 70 to 160 micron emission shows no clear dependence on surface brightness or radius. These results along with analyses of the spectral energy distributions imply that the 160-500 micron emission traces 15-30 K dust heated by evolved stars in the bulge and disc whereas the 70 micron emission includes dust heated by the active galactic nucleus and young stars in star forming regions.

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The central region of spiral galaxies as seen by Herschel. M81, M99 and M100

With appropriate spatial resolution, images of spiral galaxies in thermal infrared (~10 micron and beyond) often reveal a bright central component, distinct from the stellar bulge, superimposed on a disk with prominent spiral arms. ISO and Spitzer studies have shown that much of the scatter in the mid-infrared colors of spiral galaxies is related to changes in the relative importance of these two components, rather than to other modifications, such as the morphological type or star formation rate, that affect the properties of the galaxy as a whole. With the Herschel imaging capability from 70 to 500 micron, we revisit this two-component approach at longer wavelengths, to see if it still provides a working description of the brightness distribution of galaxies, and to determine its implications on the interpretation of global far-infrared properties of galaxies.

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Herschel photometric observations of the nearby low metallicity irregular galaxy NGC 6822

We present the first Herschel PACS and SPIRE images of the low-metallicity galaxy NGC6822 observed from 70 to 500 mu and clearly resolve the HII regions with PACS and SPIRE. We find that the ratio 250/500 is dependent on the 24 mu surface brightness in NGC6822, which would locally link the heating processes of the coldest phases of dust in the ISM to the star formation activity. We model the SEDs of some regions HII regions and less active regions across the galaxy and find that the SEDs of HII regions show warmer ranges of dust temperatures. We derive very high dust masses when graphite is used in our model to describe carbon dust. Using amorphous carbon, instead, requires less dust mass to account for submm emission due to its lower emissivity properties. This indicates that SED models including Herschel constraints may require different dust properties than commonly used.

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The dust morphology of the elliptical Galaxy M86 with SPIRE

We present Herschel-SPIRE observations at 250-500um of the giant elliptical galaxy M86 and examine the distribution of the resolved cold dust emission and its relation with other galactic tracers. The SPIRE images reveal three dust components: emission from the central region; a dust lane extending north-south; and a bright emission feature 10kpc to the south-east. We estimate that approximately 10^6 solar masses of dust is spatially coincident with atomic and ionized hydrogen, originating from stripped material from the nearby spiral NGC4438 due to recent tidal interactions with M86. The gas-to-dust ratio of the cold gas component ranges from ~20-80. We discuss the different heating mechanisms for the dust features.

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FIR colours and SEDs of nearby galaxies observed with Herschel

We present infrared colours (in the 25-500 mic spectral range) and UV to radio continuum spectral energy distributions of a sample of 51 nearby galaxies observed with SPIRE on Herschel. The observed sample includes all morphological classes, from quiescent ellipticals to active starbursts. Active galaxies have warmer colour temperatures than normal spirals. In ellipticals hosting a radio galaxy, the far-infrared (FIR) emission is dominated bynthe synchrotron nuclear emission. The colour temperature of the cold dust is higher in quiescent E-S0a than in star-forming systems probably because of the different nature of their dust heating sources (evolved stellar populations, X-ray, fast electrons) and dust grain properties. In contrast to the colour temperature of the warm dust, the f350/f500 index sensitive to the cold dust decreases with star formation and increases with metallicity, suggesting an overabundance of cold dust or an emissivity parameter beta<2 in low metallicity, active systems.

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SPIRE imaging of M82: cool dust in the wind and tidal streams

M82 is a unique representative of a whole class of galaxies, starbursts with superwinds, in the Very Nearby Galaxy Survey with Herschel. In addition, its interaction with the M81 group has stripped a significant portion of its interstellar medium from its disk. SPIRE maps now afford better characterization of the far-infrared emission from cool dust outside the disk, and sketch a far more complete picture of its mass distribution and energetics than previously possible. They show emission coincident in projection with the starburst wind and in a large halo, much more extended than the PAH band emission seen with Spitzer. Some complex substructures coincide with the brightest PAH filaments, and others with tidal streams seen in atomic hydrogen. We subtract the far-infrared emission of the starburst and underlying disk from the maps, and derive spatially-resolved far-infrared colors for the wind and halo. We interpret the results in terms of dust mass, dust temperature, and global physical conditions. In particular, we examine variations in the dust physical properties as a function of distance from the center and the wind polar axis, and conclude that more than two thirds of the extraplanar dust has been removed by tidal interaction, and not entrained by the starburst wind.

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Herschel and SCUBA-2 imaging and spectroscopy of a bright, lensed submillimetre galaxy at z = 2.3

We present a detailed analysis of the far-IR properties of the bright, lensed, z = 2.3, SMG, SMM J2135-0102, using new observations with Herschel, SCUBA-2 and the VLA. These data allow us to constrain the galaxy's SED and show that it has an intrinsic rest-frame 8-1000um luminosity, L(bol), of (2.3 +/- 0.2) x 10^12 L(sun) and a likely SFR of ~400 M(sun)/yr. The galaxy sits on the far-IR/radio correlation for far-IR-selected galaxies. At ~>70um, the SED can be described adequately by dust components with T(d) ~ 30 and 60K. Using SPIRE's Fourier Transform Spectrometer we report a detection of the [CII] 158um cooling line. If the [CII], CO and far-IR continuum arise in photo-dissociation regions, we derive a characteristic gas density, n ~ 10^3 cm^-3, and a far-UV radiation field, G_0, 10^3x stronger than the Milky Way. L([CII])/L(bol) is significantly higher than in local ULIRGs but similar to the values found in local star-forming galaxies and starburst nuclei. This is consistent with SMM J2135-0102 being powered by starburst clumps distributed across ~2 kpc, evidence that SMGs are not simply scaled-up ULIRGs. Our results show that SPIRE's FTS has the ability to measure the redshifts of distant, obscured galaxies via the blind detection of atomic cooling lines, but it will not be competitive with ground-based CO-line searches. It will, however, allow detailed study of the integrated properties of high-redshift galaxies, as well as the chemistry of their ISM, once more suitably bright candidates have been found.

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The Herschel Reference Survey

The Herschel Reference Survey is a guaranteed time Herschel key project and will be a benchmark study of dust in the nearby universe. The survey will complement a number of other Herschel key projects including large cosmological surveys that trace dust in the distant universe. We will use Herschel to produce images of a statistically-complete sample of 323 galaxies at 250, 350 and 500 micron. The sample is volume-limited, containing sources with distances between 15 and 25 Mpc and flux limits in the K-band to minimize the selection effects associated with dust and with young high-mass stars and to introduce a selection in stellar mass. The sample spans the whole range of morphological types (ellipticals to late-type spirals) and environments (from the field to the centre of the Virgo Cluster) and as such will be useful for other purposes than our own. We plan to use the survey to investigate (i) the dust content of galaxies as a function of Hubble type, stellar mass and environment, (ii) the connection between the dust content and composition and the other phases of the interstellar medium and (iii) the origin and evolution of dust in galaxies. In this paper, we describe the goals of the survey, the details of the sample and some of the auxiliary observing programs that we have started to collect complementary data. We also use the available multi-frequency data to carry out an analysis of the statistical properties of the sample.

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Resolved Dust Emission in a Quasar at z=3.65

We present submillimetre observations of the z=3.653 quasar SDSS160705+533558 together with data in the optical and infrared. The object is unusually bright in the far-IR and submm with an IR luminosity of ~10^14 L_sun. We ascribe this luminosity to a combination of AGN and starburst emission, with the starburst forming stars at a rate of a few thousand solar masses per year. Submillimetre Array (SMA) imaging observations with a resolution ~1" show that the submm (850 micron) emission is extended on scales of 10--35kpc and is offset from the optical position by ~10 kpc. This morphology is dissimilar to that found in submm galaxies, which are generally un- or marginally resolved on arcsecond scales, or submm-luminous AGN where the AGN lies at the peak of the submm or molecular emission. The simplest explanation is that the object is in the early stages of a merger between a gas rich galaxy, which hosts the starburst, and a gas-poor AGN-host galaxy, which is responsible for the quasar emission. It is also possible that jet induced star formation might contribute to the unusual morphology.

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Mid-infrared spectroscopy of infrared-luminous galaxies at z~0.5-3

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.

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MAMBO 1.2mm observations of luminous starbursts at z~2 in the SWIRE fields

We report on--off pointed MAMBO observations at 1.2 mm of 61 Spitzer-selected star-forming galaxies from the SWIRE survey. The sources are selected on the basis of bright 24um fluxes (f_24um>0.4mJy) and of stellar dominated near-infrared spectral energy distributions in order to favor z~2 starburst galaxies. The average 1.2mm flux for the whole sample is 1.5+/-0.2 mJy. Our analysis focuses on 29 sources in the Lockman Hole field where the average 1.2mm flux (1.9+/-0.3 mJy) is higher than in other fields (1.1+/-0.2 mJy). The analysis of the sources multi-wavelength spectral energy distributions indicates that they are starburst galaxies with far-infrared luminosities ~10^12-10^13.3 Lsun, and stellar masses of ~0.2-6 x10^11 M_sun. Compared to sub-millimeter selected galaxies (SMGs), the SWIRE-MAMBO sources are among those with the largest 24um/millimeter flux ratios. The origin of such large ratios is investigated by comparing the average mid-infrared spectra and the stacked far-infrared spectral energy distributions of the SWIRE-MAMBO sources and of SMGs. The mid-infrared spectra exhibit strong PAH features, and a warm dust continuum. The warm dust continuum contributes to ~34% of the mid-infrared emission, and is likely associated with an AGN component. This constribution is consistent with what is found in SMGs. The large 24um/1.2mm flux ratios are thus not due to AGN emission, but rather to enhanced PAH emission compared to SMGs. The analysis of the stacked far-infrared fluxes yields warmer dust temperatures than typically observed in SMGs. Our selection favors warm ultra-luminous infrared sources at high-z, a class of objects that is rarely found in SMG samples. Our sample is the largest Spitzer-selected sample detected at millimeter wavelengths currently available.

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Properties of dusty tori in AGN: I. The Case of SWIRE/SDSS Quasars

We derive the properties of dusty tori in Active Galactic Nuclei (AGN) from the comparison of observed Spectral Energy Distributions (SEDs) of SDSS quasars and a precomputed grid of torus models. The observed SEDs comprise SDSS photometry, 2MASS J, H, and K data, whenever available and mid-Infrared (MIR) data from the Spitzer Wide-area InfraRed Extragalactic (SWIRE) Survey. The adopted model is that of Fritz et al., 2006. The fit is performed by standard chi^2 minimisation, the model however can be multi-component comprising a stellar and a starburst components, whenever necessary. Models with low equatorial optical depth, tau_9.7, were allowed as well as ``traditional'' models with tau_9.7 > 1.0, corresponding to A_V > 22 and the results were compared. Fits using high optical depth tori models only produced dust more compactly distributed than in the configuration where all tau_9.7 models were permitted. Tori with decreasing dust density with the distance from the centre were favoured while there was no clear preference for models with or without angular variation of the dust density. The computed outer radii of the tori are of some tens of parsecs large but can reach, in a few cases, a few hundreds of parsecs. The mass of dust, M_Dust, and infrared luminosity, L_IR, integrated in the wavelength range between 1 and 1000 micron, do not show significant variations with redshift, once the observational biases are taken into account. Objects with 70 micron detections, representing 25% of the sample, are studied separately and the starburst contribution (whenever present) to the IR luminosity can reach, in the most extreme but very few cases, 80%.

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The XMM-Newton Serendipitous Survey V. Optical identification of the XMM-Newton Medium sensitivity Survey (XMS)

We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited. We report on the optical identification of the XMS samples, complete to 85-95%. At the intermediate flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (~35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN.

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