arXiv subjects

V. Buat

Publications and source records attributed to V. Buat.

198 records · Page 11Linked to original sources

arXiv2004

Panoramic GALEX FUV and NUV imaging of M31 and M33

We present Galaxy Evolution Explorer (GALEX) far-UV and near-UV mosaic observations covering the entirety of M31 and M33. For both targets, we measure the decline of surface brightness (in FUV and NUV) and changes in FUV--NUV color as a function of galactocentric radius. These UV radial profiles are compared to the distribution of ionized gas traced by H-alpha emission. We find that the extent of the UV emission, in both targets, is greater than the extent of the observed HII regions and diffuse ionized gas. We determine the ultraviolet diffuse fraction in M33 using our FUV observations and compare it to the H-alpha diffuse fraction obtained from wide-field narrow-band imaging. The FUV diffuse fraction appears to be remarkably constant near 0.65 over a large range in galactocentric radius, with departures to higher values in circumnuclear regions and, most notably, at the limit of the H-alpha disk. We suggest that the increase in FUV diffuse fraction at large galactocentric radii could indicate that a substantial portion of the diffuse emission beyond this point is not generated in situ but rather scattered from dust, after originating in the vicinity of the disk's outermost HII regions. Radial variation of the H-alpha diffuse fraction was also measured. We found the H-alpha diffuse fraction generally near 0.4 but rising toward the galaxy center, up to 0.6. We made no attempt to correct our diffuse fraction measurements for position-dependent extinction, so the quoted values are best interpreted as upper limits given the plausibly higher extinction for stellar clusters relative to their surroundings.

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arXiv2004

The radial extinction profiles of late-type galaxies

We have used UV (FOCA) and FIR (IRAS) images of six nearby late type galaxies to study the radial variation of the UV extinction (deduced from the FIR/UV ratio). We compare the UV extinction gradient with other extinction indicators (Balmer decrement) and search for a relation between the extinction, the metallicity and the gas surface density among our galaxies. We detect in our small sample a clear relation between extinction and metallicity. These observed relations are used to calibrate an empirical recipe useful for extinction correction in the UV, visible and near-infrared images of late type galaxies.

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arXiv2004

UV and FIR selected samples of galaxies in the local Universe. Dust extinction and star formation rates

We have built two samples of galaxies selected at 0.2micron (hereafter UV) and 60micron (hereafter FIR) covering a sky area of 35.36 deg^2. The UV selected sample contains 25 galaxies brighter than AB_0.2=17mag. All of them, but one elliptical, are detected at 60micron with a flux density larger or equal to 0.2Jy. The UV counts are significantly lower than the euclidean extrapolation towards brighter fluxes of previous determinations. The FIR selected sample contains 42 galaxies brighter than f_60=0.6Jy. Excepting four galaxies, all of them have a UV counterpart at the limiting magnitude AB_0.2=20.3mag. The mean extinction derived from the analysis of the FIR to UV flux ratio is ~1mag for the UV selected sample and ~2mag for the FIR selected one. For each sample we compare several indicators of the recent star formation rate (SFR) based on the FIR and/or the UV emissions and we find linear relationships with slopes close to unity, meaning that no trend with the SFR exists when converting between each other. Various absolute calibrations for both samples are discussed in this paper. A positive correlation between extinction and SFR is found when both samples are considered together although with a considerable scatter. A similar result is obtained when using the SFR normalized to the optical surface of the galaxies.

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arXiv2003

The ultraviolet extragalactic background light: dust extinction and the evolution of the cosmic star formation rate from z=0 to 0.6

We show that the accumulated light of galaxies in the ultraviolet can be evaluated from their luminosity density as a function of the evolution of the cosmic star formation rate and dust extinction properties. Constraints on the evolution rate are expected in future. Data available at the moment are consistent with an evolution rate at low z steeper than (1+z)^(3.5). A shallower rate remains possible if the luminosity-weighted dust extinction at 2000 A, as suggested by some data, is lower than about 1.2.

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arXiv2003

Submillimeter Emission from Water in the W3 Region

We have mapped the submillimeter emission from the 1(10)-1(01) transition of ortho-water in the W3 star-forming region. A 5'x5' map of the W3 IRS4 and W3 IRS5 region reveals strong water lines at half the positions in the map. The relative strength of the Odin lines compared to previous observations by SWAS suggests that we are seeing water emission from an extended region. Across much of the map the lines are double-peaked, with an absorption feature at -39 km/s; however, some positions in the map show a single strong line at -43 km/s. We interpret the double-peaked lines as arising from optically thick, self-absorbed water emission near the W3 IRS5, while the narrower blue-shifted lines originate in emission near W3 IRS4. In this model, the unusual appearance of the spectral lines across the map results from a coincidental agreement in velocity between the emission near W3 IRS4 and the blue peak of the more complex lines near W3 IRS5. The strength of the water lines near W3 IRS4 suggests we may be seeing water emission enhanced in a photon-dominated region.

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arXiv2003

First detection of NH3 (1,0 - 0,0) from a low mass cloud core: On the low ammonia abundance of the rho Oph A core

Odin has successfully observed the molecular core rho Oph A in the 572.5 GHz rotational ground state line of ammonia, NH3 (J,K = 1,0 - 0,0). The interpretation of this result makes use of complementary molecular line data obtained from the ground (C17O and CH3OH) as part of the Odin preparatory work. Comparison of these observations with theoretical model calculations of line excitation and transfer yields a quite ordinary abundance of methanol, X(CH3OH) = 3e-9. Unless NH3 is not entirely segregated from C17O and CH3OH, ammonia is found to be significantly underabundant with respect to typical dense core values, viz. X(NH3) = 8e-10.

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arXiv2003

First NH3 detection of the Orion Bar

Odin has successfully observed three regions in the Orion A cloud, i.e. Ori KL, Ori S and the Orion Bar, in the 572.5 GHz rotational ground state line of ammonia, ortho-NH3 (J,K) = (1,0) -> (0,0), and the result for the Orion Bar represents the first detection in an ammonia line. Several velocity components are present in the data. Specifically, the observed line profile from the Orion Bar can be decomposed into two components, which are in agreement with observations in high-J CO lines by Wilson et al. 2001. Using the source model for the Orion Bar by these authors, our Odin observation implies a total ammonia abundance of NH3/H2 = 5E-9.

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arXiv2002

The UV (GALEX) and FIR (ASTRO-F) All Sky Surveys: the measure of the dust extinction in the local universe

Before the end of 2002 will be launched the GALEX satellite (a NASA/SMEX project) which will observe all the sky in Ultraviolet (UV) through filters at 1500 and 2300 A down to m(AB) 21. In 2004 will be launched the ASTRO-F satellite which will perform an all sky survey at Far-Infrared (FIR) wavelengths. The cross-correlation of both surveys will lead to very large samples of galaxies for which FIR and UV fluxes will be available. Using the FIR to UV flux ratio as a quantitative tracer of the dust extinction we will be able to measure the extinction in the nearby universe (z<0.2) and to perform a statistically significant analysis of the extinction as a function of galactic properties. Of particular interest is the construction of pure FIR and UV selected samples for which the extinction will be measured as templates for the observation of high redshift galaxies.

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arXiv2002

Spectral Energy Distributions of starburst galaxies in the 900-1200 A range

We present the 970-1175 A spectral energy distributions (SEDs) of 12 starburst galaxies observed with the Far Ultraviolet Spectroscopic Explorer FUSE. We take benefit of the high spectral resolution of FUSE to estimate a continuum as much as possible unaffected by the interstellar lines. The continuum is rather flat with, in few cases, a decrease at lambda <~1050 A, the amplitude of which being correlated with various indicators of the dust extinction. The far-UV SEDs are compared with synthetic population models. The galaxies with almost no extinction have a SED consistent with an on-going star formation over some Myrs. We derive a mean dust attenuation law in the wavelength range 965-1140 A by comparing the SED of obscured galaxies to an empirical dust-free SED. The extinction is nearly constant longward of 1040 A but rises at shorter wavelengths. We compare our results with other studies of the extinction for galaxies and stars in this wavelength range.

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arXiv2002

Star formation and dust extinction in nearby star forming and starburst galaxies

We study the star formation rate and dust extinction properties of a sample of nearby star forming galaxies as derived from Halpha and UV (2000 A) observations and we compare them to those of a sample of starburst galaxies. The dust extinction in Halpha is estimated from the Balmer decrement and the extinction in UV using the FIR to UV flux ratio or the attenuation law for starburst galaxies of Calzetti et al. The Halpha and UV emissions are strongly correlated with a very low scatter for the star forming objects and with a much higher scatter for the starburst galaxies. The Halpha to UV flux ratio is found larger by a factor ~ 2 for the starburst galaxies. We compare both samples with a purely UV selected sample of galaxies and we conclude that the mean Halpha and UV properties of nearby star forming galaxies are more representative of UV selected galaxies than starburst galaxies. We emphasize that the Halpha to UV flux ratio is strongly dependent on the dust extinction: the positive correlation found between F{Halpha}/F{UV}$ and F{FIR}/F{UV} vanishes when the Halpha and UV flux are corrected for dust extinction. The Halpha to UV flux ratios converted into star formation rate and combined with the Balmer decrement measurements are tentatively used to estimate the dust extinction in UV.

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arXiv2001

The FALCON concept: multi-object spectroscopy combined with MCAO in near-IR

A large fraction of the present-day stellar mass was formed between z=0.5 and z~3 and our understanding of the formation mechanisms at work at these epochs requires both high spatial and high spectral resolution: one shall simultaneously} obtain images of objects with typical sizes as small as 1-2kpc(~0''.1), while achieving 20-50 km/s (R >= 5000) spectral resolution. The obvious instrumental solution to adopt in order to tackle the science goal is therefore a combination of multi-object 3D spectrograph with multi-conjugate adaptive optics in large fields. A partial, but still competitive correction shall be prefered, over a much wider field of view. This can be done by estimating the turbulent volume from sets of natural guide stars, by optimizing the correction to several and discrete small areas of few arcsec2 selected in a large field (Nasmyth field of 25 arcmin) and by correcting up to the 6th, and eventually, up to the 60th Zernike modes. Simulations on real extragalactic fields, show that for most sources (>80%), the recovered resolution could reach 0".15-0".25 in the J and H bands. Detection of point-like objects is improved by factors from 3 to >10, when compared with an instrument without adaptive correction. The proposed instrument concept, FALCON, is equiped with deployable mini-integral field units (IFUs), achieving spectral resolutions between R=5000 and 20000. Its multiplex capability, combined with high spatial and spectral resolution characteristics, is a natural ground based complement to the next generation of space telescopes.

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arXiv2001

Constraints on the Lyman continuum radiation from galaxies: first results with FUSE on Mrk 54

We present Far Ultraviolet Spectroscopic Explorer observations of the star-forming galaxy Mrk 54 at z = 0.0448. The Lyman continuum radiation is not detected above the HI absorption edge in our Galaxy. An upper limit is evaluated by comparison with the background measured in regions of the detector adjacent to the observed spectrum. A spectral window of 16 A, reasonably free of additional HI Lyman series line absorption is used. No correction is needed for molecular hydrogen absorption in our Galaxy but a foreground extinction of 0.29 mag is accounted for. An upper limit of 6.15 10^{-16} erg/cm^2/s/A is obtained for the flux at ~ 900 A in the rest frame of Mrk 54. By comparison with the number of ionizing photons derived from the H-alpha flux, this limit translates into an upper limit of f_esc < 0.062 for the fraction of Lyman continuum photons that escape the galaxy without being absorbed by interstellar material. This limit compares with the limits obtained in three other nearby galaxies and is compatible with the escape fractions predicted by models. The upper limits obtained in nearby galaxies contrasts with the detection of Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z ~ 3.4. The difficulties and implications of a comparison are discussed.

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arXiv1999

Far infrared and Ultraviolet emissions of individual galaxies at z=0: selection effects on the estimate of the dust extinction

We have cross-correlated Far Infrared (IRAS) and UV (FOCA) observations of galaxies to construct a sample of FIR selected galaxies with a UV observation at 0.2 microns. The FIR and UV properties of this sample are compared to the mean properties of the local Universe deduced from the luminosity distributions at both wavelengths. Almost all the galaxies of our sample have a FIR to UV flux ratio larger than the ratio of the FIR and UV luminosity densities, this effect becoming worse as the galaxies become brighter: the increase of the UV (0.2 microns) extinction is about 0.5 mag per decade of FIR (60 microns) luminosity. Quantitative star formation rates are estimated by adding the contribution of the FIR and UV emissions. They are found consistent with the corrections for extinction deduced from the FIR to UV flux ratio. A total local volume-average star formation rate is calculated by summing the contribution of the FIR and UV wavelengths bands. Each band contributes for an almost similar amount to the total star formation rate with rho_{SFR} = 0.03 +/- 0.01 h M solar/yr/Mpc^3 at z=0. This is equivalent to a global extinction of 0.75 mag to apply to the local luminosity density at 0.2 microns. The trend of a larger FIR to UV flux ratio for a larger FIR luminosity found for our sample of nearby galaxies is extended and amplified toward the very large FIR luminosities when we consider the galaxies detected by ISOCAM in a CFRS field and the Ultra Luminous Infrared Galaxies at low and high redshift. A UV extinction is tentatively estimated for these objects.

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arXiv1999

Far infrared and Far ultraviolet emissions of galaxies: luminosity functions and selection effects. Implications for the future NGST and ALMA observations

The FIR(60 microns) and UV (0.2 microns) emissions of individual star forming galaxies are compared to the mean properties of the local Universe. Almost all the galaxies exhibit a FIR to UV flux ratio larger than the ratio of the FIR and UV luminosity densities. It is likely to be due to the contribution of low luminosity galaxies to the UV luminosity density: these galaxies are deficient in any survey. As the galaxies become brighter in FIR their FIR to UV flux ratio increases: (around 0.2 microns) it implies an increase of 0.5 mag of extinction (around 0.2 microns) per decade of FIR (60 microns) luminosity. This trend is extended and amplified in Ultra Luminous Infrared Galaxies observed at low and high redshift. The Lyman Break galaxies detected by their U dropout in the HDF seem to have less extreme properties more compatible with the trend found in the nearby star forming galaxies. The detection limits of the NGST and of the future large millimeter array (ALMA) in terms of FIR to UV ratios are presented.

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arXiv1999

The globular cluster system around the low-luminosity S0 galaxy NGC 7457

We investigate the globular cluster system around the low-luminosity S0 galaxy NGC 7457 with deep B, V, I photometry. From the V photometry the total number of globular clusters around the galaxy is estimated to be 178 +- 75. The specific frequency is estimated to be S_N = 2.7 +- 1.1 adopting M_V = -19.55 mag for the galaxy. We select 89 globular cluster candidates on the basis of their colors (B-I, V-I and B-V). The B-I and V-I color distributions are found unimodal with a mean metallicity [Fe/H] ~ -1. dex. Nevertheless the width of the B-I color distribution is found larger than that expected for a single population of globular clusters and is consistent with the presence of more than one population. If the latter is the case, the two peaks would not be detected in B-I because of the small statistics and/or a small difference in the mean metallicity of the two populations. No metal poor globular clusters similar to those of the Milky Way halo are detected around NGC 7457.

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arXiv1998

Simulated NGST Observations

This paper presents the results of simulated NGST observations. The goal of this work is to study the detectability of very high redshift galaxies and to try to characterize Wide Deep-Field observations. The main points made in this paper are 1) high-z galaxies observable by NGST would be similar to local starburst galaxies 2) even if they exist, large spiral galaxies similar to local ones would not be observed by a NGST-like telescope and 3) the density of high-z galaxies detected so far is apparently moderate.

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arXiv1998

Multi-Object Spectroscopy in Space

From the scientific objectives of the Next Generation Space Telescope, this paper tries to constrain the design of the NGST Multi-Object Spectrograph. Several technical alternatives are presented that could address the requirements of the Design Reference Mission.

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arXiv1994

Are Spiral Disks Really Opaque ?

We compare the ultra-violet, optical, and far-infrared emission for a sample of 135 spiral galaxies in order to address the widely debated problem concerning the opacity of spiral disks. We find that the re-radiation of the dust, estimated from the far-infrared emission, is on average only 31$\pm 1$ percent of the bolometric luminosity of a spiral galaxy, indicating that less than one third of the stellar radiation is absorbed and then re-radiated by dust in a spiral disk. Applying a radiation transfer model which assumes a `Sandwich' configuration for the spiral disk, and fully takes into account the effect of scattering, we find for our sample a median of the face-on blue (4400Å) optical depth $\rm τ_B=0.49$ and the mean $\rm <τ_B> =0.60\pm 0.04$, indicating that most spiral galaxies in our sample are {\it not} opaque for blue light ($\rm τ_B <1$).

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