arXiv subjects

M. Sauvage

Publications and source records attributed to M. Sauvage.

At least 109 records · Page 6Linked to original sources

arXiv2002

ISOCAM view of the starburst galaxies M82, NGC253, and NGC1808

We present results of mid-infrared 5.0-16.5 micron spectrophotometric imaging of the starburst galaxies M82, NGC253, and NGC1808 from the ISOCAM instrument on board the Infrared Space Observatory. The mid-infrared spectra of the three galaxies are very similar in terms of features present. The > 11 micron continuum attributed to very small dust grains (VSGs) exhibits a large spread in intensity relative to the short-wavelength emission. We find that the 15 micron dust continuum flux density correlates well with the fine-structure [ArII] 6.99 micron line flux and thus provides a good quantitative indicator of the level of star formation activity. By contrast, the 5-11 micron region dominated by emission from polycyclic aromatic hydrocarbons (PAHs) has a nearly invariant shape. Variations in the relative intensities of the PAH features are nevertheless observed, at the 20%-100% level. We illustrate extinction effects on the shape of the mid-infrared spectrum of obscured starbursts, emphasizing the differences depending on the applicable extinction law and the consequences for the interpretation of PAH ratios and extinction estimates. The relative spatial distributions of the PAH, VSG, and [ArII] 6.99 micron emission between the three galaxies exhibit remarkable differences. The < 1 kpc size of the mid-infrared source is much smaller than the optical extent of our sample galaxies and 70%-100% of the IRAS 12 micron flux is recovered within the ISOCAM < 1.5 arcmin squared field of view, indicating that the nuclear starburst dominates the total mid-infrared emission while diffuse light from quiescent disk star formation contributes little.

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arXiv2002

Mid-infrared observations of the ultraluminous galaxies IRAS14348-1447, IRAS19254-7245, and IRAS23128-5919

We present a study of the three ultraluminous infrared galaxies IRAS14348-1447, IRAS19254-7245, and IRAS23128-5919, based on mid-infrared (MIR) spectro-imaging (5-18microns) observations performed with ISOCAM. We find that the MIR emission from each system, which consists of a pair of interacting late type galaxies, is principally confined to the nuclear regions with diameters of 1-2kpc and can account for more than 95% of their IRAS 12micron flux. In each interacting system, the galaxy hosting an active galactic nucleus (AGN) dominates the total spectrum and shows stronger dust continuum (12-16microns) relative to the Unidentified Infrared Band (UIB) emission (6-9microns), suggestive of its enhanced radiation field. The MIR dominant galaxy also exhibits elevated 15micron/Halpha and 15micron/K ratios which trace the high extinction due to the large quantities of molecular gas and dust present in its central regions. Using only diagnostics based on our mid-infrared spectra, we can establish that the Seyfert galaxy IRAS19254-7245 exhibits MIR spectral features of an AGN while the MIR spectrum of the Seyfert (or LINER) member of IRAS23128-5919 is characteristic of dust emission principally heated by star forming regions.

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arXiv2002

Extended mid-infrared emission from VV 114: probing the birth of a ULIRG

We present our 5-16 micron spectro-imaging observations of VV114, an infrared luminous early-stage merger, taken with the ISOCAM camera on-board ISO. We find that only 40% of the mid-infrared (MIR) flux is associated with a compact nuclear region, while the rest of the emission originates from a rather diffuse component extended over several kpc. This is in stark contrast with the very compact MIR starbursts usually seen in luminous infrared galaxies. A secondary peak of MIR emission is associated with an extra-nuclear star forming region which displays the largest Halpha equivalent width in the whole system. Comparing our data with the distribution of the molecular gas and cold dust, as well as with radio observations, it becomes evident that the conversion of molecular gas into stars can be triggered over large areas at the very first stages of an interaction. The presence of a very strong continuum at 5 microns in one of the sources indicates that an enshrouded active galactic nucleus may contribute to 40% of its MIR flux. We finally note that the relative variations in the UV to radio spectral properties between the merging galaxies provide evidence that the extinction-corrected star formation rate of similar objects at high z, such as those detected in optical deep surveys, can not be accurately derived from their rest-frame UV properties.

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arXiv2001

The impact of bars on the mid-infrared dust emission of spiral galaxies: global and circumnuclear properties

We study the mid-infrared properties of a sample of 69 nearby spiral galaxies, selected to avoid Seyfert activity contributing a significant fraction of the central energetics, or strong tidal interaction, and to have normal infrared luminosities. These observations were obtained with ISOCAM, which provides an angular resolution of the order of 10 arcsec (half-power diameter of the point spread function) and low-resolution spectro-imaging information. Between 5 and 18 microns, we mainly observe two dust phases, aromatic infrared bands and very small grains, both out of thermal equilibrium. On this sample, we show that the global F15/F7 colors of galaxies are very uniform, the only increase being found in early-type strongly barred galaxies, consistent with previous IRAS studies. The F15/F7 excesses are unambiguously due to galactic central regions where bar-induced starbursts occur. However, the existence of strongly barred early-type galaxies with normal circumnuclear colors indicates that the relationship between a distortion of the gravitational potential and a central starburst is not straightforward. As the physical processes at work in central regions are in principle identical in barred and unbarred galaxies, and since this is where the mid-infrared activity is mainly located, we investigate the mid-infrared circumnuclear properties of all the galaxies in our sample. We show how surface brightnesses and colors are related to both the available molecular gas content and the mean age of stellar populations contributing to dust heating. Therefore, the star formation history in galactic central regions can be constrained by their position in a color-surface brightness mid-infrared diagram.

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arXiv2001

The relationship between star formation rates and mid-infrared emission in galactic disks

The Halpha and mid-infrared mean disk surface brightnesses are compared in a sample of nearby spirals observed by ISOCAM. This shows that, in spiral disks, dust emission at 7 and 15 microns provides a reasonable star formation tracer. The fact that the 15 to 7 micron flux ratio is nearly constant in various global exciting conditions indicates a common origin, namely the aromatic infrared band carriers, and implies that at these wavelengths, dust emission from the disks of normal galaxies is dominated by photodissociation regions and not by HII regions themselves. We use this newly-found correlation between the mid-infrared and the Halpha line to investigate the nature of the link between the far-infrared (60 and 100 microns) and Halpha. Although the separation of the central regions from the disk is impossible to achieve in the far-infrared, we show that a circumnuclear contribution to the dust emission, having no equivalent counterpart in Halpha, is most likely responsible for the well-known non-linearity between far-infrared and Halpha fluxes in spiral galaxies. We derive a calibration of 7 and 15 micron fluxes in terms of star formation rates from a primary calibration of Halpha in the literature, and also outline the applicability limits of the proposed conversion, which should not be blindly extrapolated to objects whose nature is unknown.

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arXiv2001

An atlas of mid-infrared dust emission in spiral galaxies

We present maps of dust emission at 7 microns and 15 microns/7 microns intensity ratios of selected regions in 43 spiral galaxies observed with ISOCAM. This atlas is a complement to studies based on these observations, dealing with star formation in centers of barred galaxies and in spiral disks. It is accompanied by a detailed description of data reduction and an inventory of generic morphological properties in groups defined according to bar strength and HI gas content.

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arXiv2001

Mid-infrared observations of NGC 1068 with ISOCAM

We report on Mid-Infrared (MIR) observations of the Seyfert 2 galaxy NGC 1068, obtained with ISOCAM in low-resolution spectro-imaging mode. The spatial resolution (around 5 arcsec) allows us to disentangle the circumnuclear starburst regions from the emission of the active galactic nucleus (AGN). The global spatial distribution of the Unidentified Infrared Bands (UIBs) is similar to the cold dust component, traced by the 450 microns emission and the gaseous component obtained from the CO(1-0) map. However, a shift between the maximum of the UIB and 450 microns emission is clearly seen in our maps. The UIBs in the MIR (5-16 microns) originate almost exclusively from the starburst regions in the galactic disk with an emission peaking at the extremity of the stellar/ gaseous bar at a distance of 1 kpc from the AGN. The spectrum of the nucleus is characterized over the whole 5-16 microns range by a strong continuum which can be fitted with a power law of index -1.7. Moreover, the high [NeIII]/[NeII] ratio (>2.5) in the nuclear region argues for a hard radiation field from the AGN. Observations indicate that the AGN in NGC 1068 contributes less than 5% to the total integrated UIB emission even though its hot dust continuum contributes as much as 75% to the total MIR flux. On the contrary, the nuclear contribution to the cold dust emission decreases considerably at submillimeter wavelengths and does not represent more than 25% of the total integrated emission at 450 microns.

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arXiv2000

Mid-infrared studies of the AGB star populations in the Local Group galaxies : the Magellanic Clouds

We present a brief summary of the ISO survey of AGB stars in 8 populous intermediate age clusters of the Magellanic Clouds. Totally, more than 100 AGB and RGB stars were observed at 4.5, 6.7 and 12 $μ$m. We introduce a new method to estimate effective temperatures of oxygen-rich AGB stars, and derive precise bolometric magnitudes and effective temperatures for a large population of AGB objects in both LMC and SMC. The derived stellar parameters are used to construct observed HR diagrams which are employed further to estimate cluster ages using the isochrone fitting. We show that proposed method gives a powerful tool to study AGB star populations within the Local Group galaxies, especially, when backed up with the observations from forthcoming mid-IR astronomical space facilities (IRIS, SIRTF) and large ground-based telescopes (SUBARU, VLT, etc.).

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arXiv2000

ISO observations of the planetary nebula Lindsay 305 in the Small Magellanic Cloud

We present ISO (Infrared Space Observatory) observations of the planetary nebula Lindsay 305 (L 305) in the Small Magellanic Cloud. L 305 is very prominent in the ISOCAM frames at 6.75 and 11.5 $μ$m, although it is under the detection limit at 4.5 $μ$m. The obtained spectral energy distribution shows a strong mid-IR excess, which, depending on the amount of energy radiated at wavelengths longer than 11.5 $μ$m, may be as large as $\sim 1500 L_{\odot}$. However, since an accurate estimate of the total nebular luminosity is not available up to date, the evolutionary status of L 305 can not yet be constrained precisely.

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arXiv2000

Mid-infrared diagnostics to distinguish AGNs from starbursts

We present new mid-infrared (MIR) diagnostics to distinguish emission of active galactic nuclei (AGN) from that originating in starburst regions. Our method uses empirical spectroscopic criteria based on the fact that MIR emission from star forming or active galaxies arises mostly from HII regions, photo-dissociation regions (PDRs) and AGNs. The analysis of the strength of the 6.2micron Unidentified Infrared Band (UIB) and the MIR continuum shows that UIBs are very faint or absent in regions harboring the intense and hard radiation fields of AGNs and pure HII regions, where the UIB carriers could be destroyed. The MIR signature of AGNs is the presence of an important continuum in the 3-10micron band which originates from very hot dust heated by the intense AGN radiation field. Using these two distinct spectral properties found in our MIR templates, we build diagnostic diagrams which provide quantitative estimates of the AGN, PDR and HII region contribution in a given MIR spectrum. This new MIR classification can be used to reveal the presence of AGNs highly obscured by large columns of dust.

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arXiv2000

Spatial resolution bias in the mid-infrared Starburst/AGN classification

We present the effects of limited spatial resolution to the observed mid-infrared (MIR) spectrum of an active galactic nucleus (AGN) surrounded by a disk with massive star forming regions. Using MIR observations of the face-on nearby Seyfert 1 galaxy NGC 6814, we vary the observing aperture and examine the evolution of the observed AGN/starburst fraction with our MIR diagnostic. We show that the spatial resolution of ISOCAM is sufficient to disentangle AGN from starburst features in nuclear regions of nearby galaxies (D<50Mpc). However, with the exception of a few ultra-luminous galaxies, dilution effects hide completely the AGN contribution in more distant galaxies.

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arXiv1999

New infrared object in the field of the SMC cluster NGC 330

We report ISO (Infrared Space Observatory) observations of a new infrared source discovered in the vicinity of the young populous cluster NGC 330 in the Small Magellanic Cloud. The object was observed with ISOCAM at 4.5, 6.75 and 11.5 $μ$m and shows a prominent mid-infrared excess, indicating the presence of a dust shell. The available observations of the optical counterpart together with the mid-infrared ISOCAM data suggest that this object is most likely a post-AGB star, or a Be supergiant. Cluster membership and candidate evolutionary scenarios are discussed briefly.

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arXiv1999

The ISOCAM/LW Detector Dark Current Behaviour

We describe the calibration, measurements and data reduction, of the dark current of the ISOCAM/LW detector. We point-out the existence of two significant drifts of the LW dark-current, one throughout the ISO mission, on a timescale of days, another within each single revolution, on a timescale of hours. We also show the existence of a dependence of the dark current on the temperature of the ISOCAM detector. By characterizing all these effects through polynomial fittings, we build a model for the LW calibration dark, that depends on the epoch of observation (parametrized with the revolution number and the time elapsed in that given revolution since the activation) and on the temperature of the ISOCAM detector. The model parameters are tuned for each of ISOCAM/LW pixel. We show that the modelling is very effective in taking into account the dark-current variations and allows a much cleaner dark subtraction than using a brute average of several calibration dark images. The residuals of the LW model-dark subtraction are, on average, similar to the pre-launch expectation.

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arXiv1999

A Mid-Infrared and Far-Infrared View of Galaxies

From the disk of normal galaxies to the nucleus of prototype active sources, we review the wealth of results and new understanding provided by recent infrared probes and, in particular, the four instruments on-board of ISO.

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arXiv1999

Mid-IR and Far-UV observations of the star-forming ring of M 31

We present mid-IR images of a 15'x15' field in the south-west part of the Andromeda galaxy M 31 obtained with the ISOCAM camera on board ISO. We also present a 20" resolution far-UV image of a larger field at 200 nm obtained with the balloon-borne telescope FOCA 1000. These images are compared with HI, CO(1-0) and Halpha maps. The mid-IR emission as seen through filters at 7 and 15 mic is extremely well correlated with the distribution of neutral gas as shown by the HI and CO(1-0) maps, while the correlation is poorer with the distribution of the ionized gas seen through its Halpha emission. There is some correlation with the UV radiation, but it appears that the contribution of UV photons to the excitation of the carriers of the mid-IR emission is not dominant in most of M 31. The spectro-imaging observations of Cesarsky et al. (1998) show that the mid-IR spectra of several regions of M 31 are dominated by a strong emission band at 11.3 mic while emission in the other classical Infrared Bands at 6.2, 7.7 and 8.6 mic is faint or absent. More precisely, we find that the mid-IR spectral variations are not clearly related to the UV radiation field. The present observations have important consequences on our understanding of excitation of the interstellar mid-IR emission. In particular, we conclude that like for M 31, excitation in the Galactic cirruses may not be dominated by UV photons but rather by another mechanism which remains to be identified (visible photons ?). The UV excitation appears to become important when the UV radiation density is of the order of twice that near the Sun.

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arXiv1998

Dust in an extremely metal-poor galaxy: mid-infrared observations of SBS 0335-052

The metal deficient (Z = Z_sun/41) Blue Compact Dwarf Galaxy (BCD) SBS 0335-052 was observed with ISOCAM between 5 and 17 mic. With a L_12mic/L_B ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a low-metallicity object. The mid-infrared spectrum shows no sign of the Unidentified Infrared Bands, which we interpret as an effect of the destruction of their carriers by the very high UV energy density in SBS 0335-052. The spectral energy distribution (SED) is dominated by a very strong continuum which makes the ionic lines of [SIV] and [NeIII] very weak. From 5 to 17 mic, the SED can be fitted with a grey-body spectrum, modified by an extinction law similar to that observed toward the Galactic Center, with an optical depth of A_V~19-21 mag. Such a large optical depth implies that a large fraction (as much as ~ 75%) of the current star-formation activity in SBS 0335-052 is hidden by dust with a mass between 3x10^3 M_sun and 5x10^5 M_sun. Silicate grains are present as silicate extinction bands at 9.7 and 18 mic can account for the unusual shape of the MIR spectrum of SBS 0335-052. It is remarkable that such a nearly primordial environment contains as much dust as galaxies which are 10 times more metal-rich. If the hidden star formation in SBS 0335-052 is typical of young galaxies at high redshifts, then the cosmic star formation rate derived from UV/optical fluxes would be underestimated.

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arXiv1998

A barred spiral at the centre of the giant elliptical radio galaxy Centaurus A

We report observations at mid-infrared and sub-millimeter wavelengths of Centaurus A (CenA, NGC 5128), the giant elliptical galaxy that harbors the closest radio loud Active Galactic Nucleus (AGN) to Earth. The dust emission from the deep interior of CenA reveals a bisymmetric structure with a diameter of 5' (5 kpc), centred at the AGN. This structure is remarkably similar to that of a barred spiral galaxy, with the bar lying in a plane that is tilted ~18 degrees from the line of sight. The true nature of the distribution of dust in the inner regions of CenA is noticeably displaced from the more chaotic and widespread optical obscuration. The barred spiral is a quasi-stable structure formed at the center of the giant elliptical from the tidal debris of a gas-rich object(s) accreted in the past 10^9 years. The total size and mass of interstellar gas in the barred spiral at the center of CenA is comparable to the small Local Group spiral galaxy Messier 33. The observation of this remarkable structure opens the more general question on whether the dusty hosts of giant radio galaxies like CenA, are "symbiotic" galaxies composed of a barred spiral inside an elliptical, where the bar serves to funnel gas toward the AGN.

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arXiv1998

Dust in the Wheel: The Cartwheel Galaxy in the Mid-IR

We present mid-infrared images at $6.7 μm$ and $15 μm$ of ``The Cartwheel'' (AM~0035-33), the prototypical collisional ring galaxy. The observations, taken with ISOCAM, reveal the distribution of hot dust in the galaxy and its two companions in the north-east. The intensity of the mid-IR emission from the outer star forming ring of the Cartwheel shows considerable azimuthal variation and peaks at the most active H$α$ region of the ring. The 15 $μm$ to 6.7 $μm$ flux ratio of 5.2 is the highest among all the galaxies of our sample. A surprising result of our observations is the discovery of significant emission from the inner regions of the galaxy, including the inner ring, spokes and nucleus, where previously only low level H$α$ emission had been reported. At 6.7 $μm$, this emission is stronger than the one from the outer star forming ring, and at 15 $μm$, it represents 40% of the emission from the outer ring. The H$α$ to mid-IR flux ratios from the inner regions are consistent with the heating of grains from weak star formation activity.

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