Searcharxiv⌕ Search

arXiv subjects

Veronique Buat

Publications and source records attributed to Veronique Buat.

48 records · Page 3Linked to original sources
arXiv2006

Effects of dust scattering albedo and 2175 A bump on ultraviolet colours of normal disc galaxies

We discuss dust properties in the interstellar medium (ISM) of nearby normal galaxies, by comparing observations in the ultraviolet (UV) with simulations by a radiative transfer model. The observed UV colours of nearby galaxies show a reddening relative to their expected intrinsic colours. Some authors argued that the Milky Way dust cannot reproduce the reddening because of the prominent 2175 Åabsorption bump. Other authors proposed a reduction mechanism of the bump strength in an {\it attenuation law} derived from the ratio of the observed intensity to the intrinsic one through an age-selective attenuation (i.e., young stars are more attenuated selectively). We newly find that the wavelength dependence of the scattering albedo also has a strong effect on the UV colour; an albedo decreasing toward shorter wavelengths (except for the absorption bump range) produces a significant UV reddening. After comparing the observed UV colours of nearby normal galaxies with those expected from radiative transfer simulations assumed several dust models, we find two sorts of dust suitable for these galaxies: (1) dust with a bump and a smaller albedo for a shorter wavelength (except for the bump range), and (2) dust without any bump but with an almost constant albedo. If very small carbonaceous grains responsible for the common unidentified infrared emission band are also the bump carrier, the former dust is favorable. Finally, we derive mean attenuation laws of various dust models as a function of the UV attenuation, and derive some relations between the UV attenuation and observable/theoretical quantities.

astro-ph↗
arXiv2006

UV and FIR selected star-forming galaxies at z=0: differences and overlaps

We study two samples of local galaxies, one is UV (GALEX) selected and the other FIR (IRAS) selected, to address the question whether UV and FIR surveys see the two sides ('bright' and 'dark') of the star formation of the same population of galaxies or two different populations of star forming galaxies. No significant difference between the L$_{tot}$ ($=L_{60}+L_{FUV}$) luminosity functions of the UV and FIR samples is found. Also, after the correction for the `Malmquist bias' (bias for flux limited samples), the FIR-to-UV ratio v.s. L$_{tot}$ relations of the two samples are consistent with each other. In the range of $9 \la \log(L_{tot}/L_\sun) \la 12$, both can be approximated by a simple linear relation of $\log (L_{60}/L_{FUV})=\log(L_{tot}/L_\sun)-9.66$. These are consistent with the hypothesis that the two samples represent the same population of star forming galaxies, and their well documented differences in L$_{tot}$ and in FIR-to-UV ratio are due only to the selection effect. A comparison between the UV luminosity functions shows marginal evidence for a population of faint UV galaxies missing in the FIR selected sample. The contribution from these 'FIR-quiet' galaxies to the overall UV population is insignificant, given that the K-band luminosity functions (i.e. the stellar mass functions) of the two samples do not show any significant difference.

astro-ph↗
arXiv2005

The evolution of the ultraviolet and infrared luminosity densities in the universe at 0<z<1

The ratio between far-ultraviolet (FUV) and infrared (IR) luminosity densities from z=0 to z=1 is discussed by using the luminosity functions (LFs) of both wavelengths. The FUV LF (z=0-1) based on GALEX has been reported by Arnouts et al. (2005), whilst for the IR LF, we used the IRAS PSCz 60-um LF for the local universe and the Spitzer 15-um LF at higher-z as used by Le Floch et al. (2005). Both luminosity densities show a significant evolutionary trend, but the IR evolves much faster than the FUV. Consequently, the ratio $ρ_{dust}/ρ_{FUV}$ increases toward higher-z, from 4 (local) to 15 (z = 1). It is also shown that more than 70% of the star formation activity in the universe is obscured by dust at 0.5 < z < 1.2.

astro-ph↗
arXiv2005

Star Formation and Dust Attenuation Properties in Galaxies from a Statistical UV-to-FIR Analysis

We study two galaxy samples selected in ultraviolet (UV) and in far-infrared (FIR) for which the spectral energy distributions (SEDs) from the far UV (FUV) to the FIR are available. We compare the observed SEDs to modelled SEDs with several star formation histories (SFHs; decaying star formation rate plus burst) and attenuation laws (power law + 2175 Angstroem bump). The Bayesian method allows to estimate statistically the best parameters by comparing each observed SED to the full set of 82800 models. We reach the conclusion that the UV dust attenuation cannot be estimated correctly from an SED analysis if the FIR information is not used. The deduced dispersion is larger than with the FIR data and the distribution is not symetrically distributed about zero: there is an over-estimation for UV-selected galaxies and an under-estimation for FIR-selected galaxies. The output from the analysis process suggests that UV-selected galaxies have attenuation laws in average similar to the LMC extinction while FIR-selected galaxy attenuation laws more resemble the MW extinction law. The dispersion about the average relation in the Log(Fdust/Ffuv) vs. FUV-NUV diagram (once the main relation with FUV-NUV is accounted for) is explained by two other parameters: the slope of the attenuation law and the instantaneous birthrate parameter b_0 for UV-selected galaxies and the same ones plus the strength of the bump for the FIR-selected galaxies. We propose a recipe to estimate the UV dust attenuation for UV-galaxies only (that should be used whenever the FIR information is not available because the resulting Afuv is poorly defined with an uncertainty of about 0.32): A_{FUV} = 1.4168 (FUV-NUV)^2 + 0.3298 (NUV-I)^2 + 2.1207 (FUV-NUV) + 2.7465 (NUV-I) + 5.8408

astro-ph↗
arXiv2004

The Star Formation Rate Function of the Local Universe

We have derived the bivariate luminosity function for the far ultraviolet (1530Angstroms) and far infrared (60 microns). We used matched GALEX and IRAS data, and redshifts from NED and PSC-z. We have derived a total star formation luminosity function phi(L_{tot}), with L_{tot} = L_{FUV}+L_{FIR}. Using these, we determined the cosmic ``star formation rate'' function and density for the local universe. The total SFR function is fit very well by a log-normal distribution over five decades of luminosity. We find that the bivariate luminosity function phi(L_{FUV},L_{FIR}) shows a bimodal behavior, with L_{FIR} tracking L_{FUV} for L_{TOT}< 10^10 L_sun, and L_{FUV} saturating at 10^10 L_sun, while L_{TOT} L_{FIR} for higher luminosities. We also calculate the SFR density and compare it to other measurements.

astro-ph↗
arXiv2004

Spectro-Morphology of Galaxies

We present a quantitative method to classify galaxies, based on multi-wavelength data and elaborated from the properties of nearby galaxies. Our objective is to define an evolutionary method that can be used for low and high redshift objects. We estimate the concentration of light (C) at the galaxy center and the 180 degree-rotational asymmetry (A), computed at several wavelengths, from ultraviolet (UV) to I-band. The variation of the indices of concentration and asymmetry with the wavelength reflects the proportion and the distribution of young and old stellar populations in galaxies. In general C is found to decrease from optical to UV, and A is found to increase from optical to UV: the patchy appearance of galaxies in UV with no bulge is often very different from their counterpart at optical wavelengths, with prominent bulges and more regular disks. The variation of C and A with the wavelength is quantified. By this way, we are able to distinguish five types of galaxies that we call spectro-morphological types: compact, ringed, spiral, irregular and central-starburst galaxies, which can be differentiated by the repartition of their stellar populations. We discuss in detail the morphology of galaxies of the sample, and describe the morphological characteristics of each spectro-morphological type. We apply spectro-morphology to three objects at a redshift z=1 in the Hubble Deep Field North, that gives encouraging results for applications to large samples of high-redshift galaxies. This method of morphological classification could be used to study the evolution of the morphology with the redshift and is expected to bring observational constraints on scenarios of galaxy evolution.

astro-ph↗
arXiv2003

Star formation rate in galaxies from UV, IR, and H-alpha estimators

Infrared (IR) luminosity of galaxies originating from dust emission can be used as an indicator of the star formation rate (SFR). Inoue et al. (2000, IHK) have derived a formula for the conversion from IR luminosity to SFR by using the following three quantities: the fraction of Lyman continuum luminosity absorbed by gas (f), the fraction of UV luminosity absorbed by dust (epsilon), and the fraction of dust heating from old (>10^8 yr) stellar populations (eta). We develop a method to estimate those three quantities based on the idea that the various way of SFR estimates should return the same SFR. After applying our method to samples of galaxies, the following results are obtained. First, our method is applied to star-forming galaxies, finding that f~0.6, epsilon~0.5, and eta~0.4 as representative values. Next, we apply the method to a starburst sample, which shows larger extinction than the star-forming galaxy sample. With the aid of f, epsilon, and eta, we estimate reliable SFRs. Moreover, the H-alpha luminosity, if the H-alpha extinction is corrected by using the Balmer decrement, is suitable for a statistical analysis of SFR, because the same correction factor for the Lyman continuum extinction is applicable to both normal and starburst galaxies over all the range of SFR. The metallicity dependence of f and epsilon is also tested: Only the latter proves to have a correlation with metallicity. As an extension of our result, we show that all UV, H-alpha, and IR comoving luminosity densities at z=0 give a consistent SFR (~ 3x10^{-2}h M_sun/Mpc^3). Useful formulae for SFR estimate are listed.

astro-ph↗
arXiv2001

The ultraviolet visibility and quantitative morphology of galactic disks at low and high redshift

We used ultraviolet (200 nm) images of the local spiral galaxies M33, M51, M81, M100, M101 to compute morphological parameters of galactic disks at this wavelength : half-light radius $r_{hl}$, surface brightness distributions, asymmetries ($A$) and concentrations ($C_A$). The visibility and the evolution of the morphological parameters are studied as a function of the redshift. The main results are : local spiral galaxies would be hardly observed and classified if projected at high redshifts (z $\ge$ 1) unless a strong luminosity evolution is assumed. Consequently, the non-detection of large galactic disks cannot be used without caution as a constraint on the evolution of galatic disks. Spiral galaxies observed in ultraviolet appear more irregular since the contribution from the young stellar population becomes predominent. When these galaxies are put in a (log $A$ vs. log $C_A$) diagram, they move to the irregul ar sector defined at visible wavelengths. Moreover, the log $A$ parameter is degenerate and cannot be used for an efficient classification of morphological ultraviolet types. The analysis of high redshift galaxies cannot be carried out in a reliable way so far and a multi-wavelength approach is required if one does not want to misinterpret the data.

astro-ph↗
arXiv2001

Globular cluster systems II: On the formation of old globular clusters and their sites of formation

We studied the metal-poor globular cluster (GC) populations of a large variety of galaxies (47 galaxies spanning about 10mag in absolute brightness) and compared their mean [Fe/H] with the properties of the host galaxies. The mean [Fe/H] of the systems lie in the -1.65<[Fe/H]<-1.20 range (74% of the population). Using only GC systems with more than 6 objects detected, 85% of the population lie within -1.65<[Fe/H]<-1.20. The relation between the mean [Fe/H] of the metal-poor GC systems and the Mv of their host galaxies presents a very low slope which includes zero. An analysis of the correlation of the mean [Fe/H] with other galaxy properties also leads to the conclusion that no strong correlation exists. The lack of correlation suggests a formation of all metal-poor GC in similar gas fragments. A weak correlation might exist between mean [Fe/H] of the metal-poor GC and host galaxy metallicity. This would imply that some fragments in which metal-poor GC formed were already embedded in the larger dark matter halo of the final galaxy (as oppose to being independent satellites that were accreted later). Our result suggests a homogeneous formation of metal-poor GC in all galaxies, in typical fragments of masses around 10^9-10^10 solar masses with very similar metallicities, compatible with hierarchical formation scenarios for galaxies. We compared the mean [Fe/H] of the metal-poor GC populations with the typical metallicities of high-z objects. If we add the constraint that GC need a high column density of gas to form, DLAs are the most likely sites for the formation of metal-poor GC populations.

astro-ph↗
arXiv2000

The Formation of Galaxies, the Formation of Old Globular Clusters and the Link with High-Redshift Objects

In this paper, we are exploring the properties of old, metal-poor globular clusters in galaxies. We investigate whether their properties are related to the properties of their host galaxies, and whether we can constrain their formation. The main result is that the mean metallicities of old GC systems are found to lie in a narrow range -1.7 < [Fe/H] < -1.1 (80 % of the population). Moreover, no correlations are found between the mean metallicities and other galaxy properties which implies a GC formation independent of the host galaxies. Further, we try to identify the sites of old, metal-poor GC formation, with any currently known high redshift objects. We find that the metalicities of damped Ly$α$ systems in the redshift range 1.6 < z < 4 are consistent with our GC metalicities, which suggests that these high-density neutral gas objects may be the progenitors of the old, metal-poor globular clusters.

astro-ph↗
arXiv1998

The Observation of the Nearby Universe in UV and in FIR: an evidence for a moderate extinction in present day star forming galaxies

We study the FIR and UV-visible properties of nearby star forming galaxies. This comparison is performed using the local luminosity functions at UV and FIR wavelengths and on individual starburst galaxies for which photometric data from UV to NIR and FIR are available. The comparison of the FIR and UV local luminosity functions argues for a moderate extinction in nearby disk galaxies. For a sample of 22 starburst galaxies, it is found that the UV(912-3650AA), the visible(3600-12500 AA) and the NIR(12500-22000 AA) wavelength range contribute 30%, 50% and 20% respectively to the total emerging stellar emission. The mean ratio of the dust to bolometric luminosity of these galaxies is 0.37+/-0.22 similar to the ratio found for normal spiral galaxies. The mean extinction at 2000AA is found to be ~1.2mag although with a large dispersion. The conversion factor of the stellar emission into dust emission is found to correlate with the luminosity of the galaxies, brighter galaxies having a higher conversion factor. We conclude that a very large conversion of the stellar light into dust emission can no longer be assumed as a general property of starburst galaxies at least in the local Universe. We compare the UV properties of our local starburst galaxies to those of high redshift galaxies. The larger extinction found in the distant galaxies is consistent with the trend we find for the nearby starburst galaxies namely the brighter the galaxies the lower the escape fraction of stellar light.

astro-ph↗