arXiv subjects

J. Donas

Publications and source records attributed to J. Donas.

42 records · Page 3Linked to original sources

arXiv2002

The far-ultraviolet emission of early-type galaxies

We have assembled a UV-flux selected sample of 82 early-type galaxies and collected additional information at other wavelengths. These data confirm a large spread of the UV-V color in the range 2 to 5. The spread in UV-V is accompanied by a spread in B-V that is mainly attributed to the range of morphological types and luminosities. A large fraction of the objects have red colors, UV-V = 4 +-0.4, corresponding to a weak UV-upturn as observed with IUE. If the current interpretation for the UV emission from early-type galaxies is applicable to our sample, the PAGB (Post-Asymptotic Giant Branch) tracks are the most common evolution path for the low-mass stars responsible for the UV emission. A small number of very blue (UV-V < 1.4) objects have been found that can be reasonably interpreted as harbouring some low level of star formation. In contrast with a previous sample based on IUE observations, no correlation is found between the UV-V color and the Mg_2 spectral line index; possible explanations are reviewed. The potential of a more extended UV survey like GALEX is briefly presented.

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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

An Ultraviolet-Selected Galaxy Redshift Survey - II: The Physical Nature of Star Formation in an Enlarged Sample

We present further spectroscopic observations for a sample of galaxies selected in the vacuum ultraviolet (UV) at 2000 Åfrom the FOCA balloon-borne imaging camera of Milliard et al. (1992). This work represents an extension of the initial study of Treyer et al. (1998). Our enlarged catalogue contains 433 sources; 273 of these are galaxies, nearly all with redshifts z=0-0.4. Nebular emission line measurements are available for 216 galaxies, allowing us to address issues of reddening and metallicity. The UV and Halpha luminosity functions strengthen our earlier assertions that the local volume-averaged star formation rate is higher than indicated from earlier surveys. Moreover, internally within our sample, we do not find a steep rise in the UV luminosity density with redshift over 0<z<0.4. Our data is more consistent with a modest evolutionary trend as suggested by recent redshift survey results. We find no evidence for a significant number of AGN in our sample. We find the UV flux indicates a consistently higher mean star formation rate than that implied by the Halpha luminosity for typical constant or declining star formation histories. Following Glazebrook et al. (1999), we interpret this discrepancy in terms of a starburst model for our UV-luminous sources. Whilst we can explain most of our observations in this way, there remains a small population with extreme UV-optical colours which cannot be understood.

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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

An Ultraviolet-Selected Galaxy Redshift Survey: New Estimates of the Local Star Formation Rate

We present the first results of an ongoing spectroscopic survey of galaxies selected in the rest-frame ultraviolet (UV). The redshift distribution extends over 0<z<0.5 and a high fraction of the sources show intense nebular emission lines and UV-optical colours bluer than normal Hubble sequence galaxies. Although our sample is currently small, we derive a rest-frame UV luminosity function with a steep faint end slope consistent with that found for late-type galaxies in optical samples. However, the integrated luminosity density derived implies a volume-averaged star formation rate higher than other recent estimates, assuming a normal initial mass function. If representative of other UV fields, our data implies the local abundance of star-forming galaxies may have been underestimated and consequently claims for strong evolution in the global star formation rate in the range 0<z<1 overstated.

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arXiv1997

An Ultraviolet Redshift Survey: A New Estimate of the Local Star Formation Rate

We present the first results of an ongoing spectroscopic survey of galaxies selected in the rest-frame ultraviolet. The source catalogue was constructed from a flux-limited sample of stars, galaxies and QSOs imaged at 2000 Åwith the FOCA balloon-borne camera (Milliard et al. 1992). The redshift distribution obtained for 45 galaxies spans 0<z<0.3 and a high fraction of the spectra show intense nebular emission lines and ultraviolet-optical colors considerably bluer than can be accounted for via normal Hubble sequence galaxies. From the rest-frame ultraviolet galaxy luminosity function, and adopting a normal IMF, we use the integrated UV light to our survey limit to estimate the local volume averaged star formation rate. We find a value significantly larger than recent estimates and argue this must be a lower limit to the true value. Our results suggest the local abundance of star-forming galaxies has been underestimated in surveys based on optical data.

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