Aims.We present FUSE spectra of the giant H ii region NGC 604 in the spiral galaxy M 33. Chemical... more Aims.We present FUSE spectra of the giant H ii region NGC 604 in the spiral galaxy M 33. Chemical abundances are tentatively derived from far-UV absorption lines and compared to those derived from optical emission lines. Methods: .Absorption lines from neutral hydrogen and heavy elements were observed against the continuum provided by the young massive stars embedded in the H ii region. We derived the column densities of H i, N i, O i, Si ii, P ii, Ar i, and Fe ii, fitting the line profiles with either a single component or several components. We used CLOUDY to correct for contamination from the ionized gas. Archival HST/STIS spectra across NGC 604 allowed us to investigate how inhomogeneities affect the final H i column density. Results: .Kinematics show that the neutral gas is physically related to the H ii region. The STIS spectra reveal H i column density fluctuations up to 1 dex across NCG 604. Nevertheless, we find that the H i column density determined from the global STIS spectrum does not differ significantly from the average over the individual sightlines. Our net results using the column densities derived with FUSE, assuming a single component, show that N, O, Si, and Ar are apparently underabundant in the neutral phase by a factor of ˜ 10 or more with respect to the ionized phase, while Fe is the same. However, we discuss the possibility that the absorption lines are made of individual unresolved components, and find that only P ii, Ar i, and Fe ii lines should not be affected by the presence of hidden saturated components, while N i, O i, and Si ii might be much more affected. Conclusions: .If N, O, and Si are actually underabundant in the neutral gas of NGC 604 with respect to the ionized gas, this would confirm earlier results obtained for the blue compact dwarfs, and their interpretations. However, a deeper analysis focused on P, Ar, and Fe mitigates the above conclusion and indicates that the neutral gas and ionized gas could have similar abundances.
We present high dispersion HST GHRS UV spectroscopic observations of 8 H II galaxies covering a w... more We present high dispersion HST GHRS UV spectroscopic observations of 8 H II galaxies covering a wide range of metallicities and physical properties. We have found Lyα emission in 4 galaxies with blueshifted absorption features, leading to P Cygni like profiles in 3 of them. In all these objects the O I and Si II absorption lines are also blueshifted with respect to the ionized gas, indicating that the neutral gas is outflowing in these galaxies with velocities up to 200 km s −1 or more. The rest of the sample shows broad damped Lyα absorption profiles centered at the wavelength corresponding to the redshift of the H II emitting gas. We therefore find that the velocity structure of the neutral gas in these galaxies is the driving factor that determines the detectability of Lyα in emission. Relatively small column densities of neutral gas with even very small dust content would destroy the Lyα emission if this gas is static with respect to the ionized region where Lyα photons originate. The situation changes dramatically when most of the neutral gas is velocity-shifted with respect to the ionized regions because resonant scattering by neutral hydrogen will be most efficient at wavelengths shorter than the Lyα emission, allowing the Lyα photons to escape (at least partially). This mechanism complements the effect of porosity in the neutral interstellar medium discussed by other authors, which allows to explain the escape of Lyα photons in regions surrounded by static neutral gas, but with only partial covering factors. The anisotropy of these gas flows and their dependence on the intrinsic properties of the violent star-forming episodes taking place in these objects (age, strength, gas geometry,...) might explain (in part) the apparent lack of correlation between other properties (like metallicity) and the frequency of occurence and strength of Lyα emission in star-forming galaxies. Attempts to derive the comoving starformation rate at high redshifts from Lyα emission searches are highly questionable.
We present a FUSE spectroscopic study of the nearby gas--rich, metal--deficient blue compact dwar... more We present a FUSE spectroscopic study of the nearby gas--rich, metal--deficient blue compact dwarf (BCD) galaxy IZw 36. Atomic hydrogen and many metal lines are observed in absorption against the stellar continuum of young, massive stars embedded in the ionized region. Profile fitting of absorption lines allowed us to determine abundances and investigate the chemical composition of the neutral gas. This study present strong evidences that nitrogen is 16 (+7 -8) times less abundant in the neutral gas with respect to the ionized gas (all uncertainties are 2 sigma. Similarly, the oxygen abundance estimated using phosphorus as a tracer is lower in the neutral gas by a factor 8 (+17 -5). We also find that argon is underabundant by a factor 32 (+8 -7) and that log (ArI/OI) < -3.0 which is inconsistent with the solar value -2.1 (+0.1 -0.1) (Lodders2003), implying that argon is likely ionized into ArII in the neutral medium.
We present a calibration of the massive star formation rate vs. [CII] luminosity relation based o... more We present a calibration of the massive star formation rate vs. [CII] luminosity relation based on a sample of nearby, late-type galaxies observed with ISO-LWS and imaged in the Halpha+[NII] line. The relation holds for far-IR luminosities 10^8 < L_{FIR} < 10^{10.5} L (solar units). The derived star formation rates have an uncertainty of about a factor of 10. Part of this uncertainty is due to the different mix of contributions to the [CII] emission from the different components of the interstellar medium in individual galaxies, as discussed in an appendix.
We present Mid (MIR) and Far (FIR) Infrared observations of 18 spiral/irregular galaxies belongin... more We present Mid (MIR) and Far (FIR) Infrared observations of 18 spiral/irregular galaxies belonging to the Coma and A1367 clusters, carried out with the CAM and PHOT instruments on board the ISO satellite. Complementary photometry from the UV to the Near Infrared (NIR) together with Halpha imaging, HI and 12CO line measurements allow us to study the relationships between the
We present mid-IR images of a 15 ×15 field in the south-west part of the Andromeda galaxy M 31 ob... more We present mid-IR images of a 15 ×15 field in the south-west part of the Andromeda galaxy M 31 obtained with the ISOCAM camera (6 pixels) on board ISO. These broad-band images complement spectro-imaging observations of smaller fields . 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 inter-compared and also compared with H i, CO(1-0) and Hα maps. The mid-IR emission as seen through wide-band filters centered at 7 and 15 µm is extremely well correlated with the distribution of neutral gas as shown by the H i and CO(1-0) maps, while the correlation is poorer with the distribution of the ionized gas seen through its Hα 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 spectroimaging observations of show that the mid-IR spectra of several regions of M 31, two of which are in the presently studied area, are dominated by a strong emission band at 11.3 µm while emission in the other classical Aromatic Infrared Bands (AIBs) at 6.2, 7.7 and 8.6 µm is faint or absent. This result is precised, and 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 general. 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 2 times that near the Sun.
We present ISOCAM imaging data at 6.75 and 15 µm for 145 galaxies in the Virgo cluster and in the... more We present ISOCAM imaging data at 6.75 and 15 µm for 145 galaxies in the Virgo cluster and in the Coma/A1367 supercluster. Of these, 99 form a complete, optically-selected, volume-limited sample including galaxies in the core and in the periphery of the Virgo cluster, suitable for statistical analysis. 34 of the Virgo galaxies were resolved by ISOCAM: for these objects we present mid-IR images, radial light and colour profiles, as well as effective and isophotal radii, surface brightness and light concentration indices. The mid-IR colours of the target galaxies show a weak trend with the H band luminosity, with values of F(6.75 µm)/F(15 µm) ≥ 1 found generally in massive objects (L H ≥ 10 10 L H ), and F(6.75 µm)/F(15 µm) ≤ 1 in low-mass (L H ≤ 10 10 L H ) dwarf galaxies. All early-type galaxies (type ≤ S0a) have F(6.75 µm)/F(15 µm) ≥ 1, as expected when the mid-IR emission is dominated by the photosphere of the cold stellar population. The mid-IR, near-IR and visible light concentration indices of bright galaxies are tightly correlated with one another, indicating that the spatial distribution of the mid-IR emitting sources is, to the first order, similar to that of the stars.
We present maps of dust emission at 7 µm and 15 µm/7 µm intensity ratios of selected regions in 4... more We present maps of dust emission at 7 µm and 15 µm/7 µm 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.
The recent discovery of a large fraction of DCN in the galactic center is interpreted in the fram... more The recent discovery of a large fraction of DCN in the galactic center is interpreted in the framework of current models of chemical evolution. It implies that deuterium must be continuously replenished in the highly processed galactic center Several hypotheses are considered: the most likely are either mixing of external gas with cosmological deuterium or the presence in the galactic center of important fluxes of galactic cosmic rays producing deuterium by spallation reactions; less probable possibilities are a recent large production of deuterium by supernova outbursts or by a hypothetical central supermassive object. (AIP)
Faint emission in the CO(1-0) and CO(2-1) spectral lines at mm wavelengths has been discovered fr... more Faint emission in the CO(1-0) and CO(2-1) spectral lines at mm wavelengths has been discovered from dark dust clouds in the inner disk of M31. These dust clouds are located in regions of the galaxy where there is little evidence of star-forming activity. The CO emission from these clouds is resolved, and comes from roughly the same area covered by
We discuss recent observations of CO, CS, and NH3 emission lines from two molecular clouds in the... more We discuss recent observations of CO, CS, and NH3 emission lines from two molecular clouds in the inner regions of M31. The intensities of the (12)CO(1-0), (12)CO(2-1), (13)CO(1-0), and (13)CO(2-1) lines have been measured with the IRAM 30 m telescope at the same angular resolution of 23 sec. Upper limits have been obtained in the CS(2-1), CS(3-2) and in the
We have identified a population of B stars in the field whose distribution is also well correlate... more We have identified a population of B stars in the field whose distribution is also well correlated with the extended HI distribution. Evidently, star formation is both ongoing and wide spread in the outer disk of M 31. According to the current view of the star formation process, molecular gas is therefore also expected to be present.
The paper presents spectrophotometric observations of H II regions in the irregular and blue comp... more The paper presents spectrophotometric observations of H II regions in the irregular and blue compact galaxies. These observations are used to derive abundances of He, N, O, Ne, S, and Ar relative to H, and the results are consistent with the solution that nitrogen is in part a product of primary nucleosynthesis, and that the ratio of primary production of nitrogen to oxygen is N/O approximately equals 0.02. The relationship between the heavy element abundance and the total mass of galaxies is determined and it is found that the heavy element yield is constant. A model of galactic chemical evolution based on a new initial mass function and on stellar evolutionary models with mass loss agrees with the observations; it is concluded that irregular and blue compact galaxies seem to differ only in the present rate of star formation, but it was not determined whether they are old systems with a present burst, or systems where star formation started only recently.
Aims.We present FUSE spectra of the giant H ii region NGC 604 in the spiral galaxy M 33. Chemical... more Aims.We present FUSE spectra of the giant H ii region NGC 604 in the spiral galaxy M 33. Chemical abundances are tentatively derived from far-UV absorption lines and compared to those derived from optical emission lines. Methods: .Absorption lines from neutral hydrogen and heavy elements were observed against the continuum provided by the young massive stars embedded in the H ii region. We derived the column densities of H i, N i, O i, Si ii, P ii, Ar i, and Fe ii, fitting the line profiles with either a single component or several components. We used CLOUDY to correct for contamination from the ionized gas. Archival HST/STIS spectra across NGC 604 allowed us to investigate how inhomogeneities affect the final H i column density. Results: .Kinematics show that the neutral gas is physically related to the H ii region. The STIS spectra reveal H i column density fluctuations up to 1 dex across NCG 604. Nevertheless, we find that the H i column density determined from the global STIS spectrum does not differ significantly from the average over the individual sightlines. Our net results using the column densities derived with FUSE, assuming a single component, show that N, O, Si, and Ar are apparently underabundant in the neutral phase by a factor of ˜ 10 or more with respect to the ionized phase, while Fe is the same. However, we discuss the possibility that the absorption lines are made of individual unresolved components, and find that only P ii, Ar i, and Fe ii lines should not be affected by the presence of hidden saturated components, while N i, O i, and Si ii might be much more affected. Conclusions: .If N, O, and Si are actually underabundant in the neutral gas of NGC 604 with respect to the ionized gas, this would confirm earlier results obtained for the blue compact dwarfs, and their interpretations. However, a deeper analysis focused on P, Ar, and Fe mitigates the above conclusion and indicates that the neutral gas and ionized gas could have similar abundances.
We present high dispersion HST GHRS UV spectroscopic observations of 8 H II galaxies covering a w... more We present high dispersion HST GHRS UV spectroscopic observations of 8 H II galaxies covering a wide range of metallicities and physical properties. We have found Lyα emission in 4 galaxies with blueshifted absorption features, leading to P Cygni like profiles in 3 of them. In all these objects the O I and Si II absorption lines are also blueshifted with respect to the ionized gas, indicating that the neutral gas is outflowing in these galaxies with velocities up to 200 km s −1 or more. The rest of the sample shows broad damped Lyα absorption profiles centered at the wavelength corresponding to the redshift of the H II emitting gas. We therefore find that the velocity structure of the neutral gas in these galaxies is the driving factor that determines the detectability of Lyα in emission. Relatively small column densities of neutral gas with even very small dust content would destroy the Lyα emission if this gas is static with respect to the ionized region where Lyα photons originate. The situation changes dramatically when most of the neutral gas is velocity-shifted with respect to the ionized regions because resonant scattering by neutral hydrogen will be most efficient at wavelengths shorter than the Lyα emission, allowing the Lyα photons to escape (at least partially). This mechanism complements the effect of porosity in the neutral interstellar medium discussed by other authors, which allows to explain the escape of Lyα photons in regions surrounded by static neutral gas, but with only partial covering factors. The anisotropy of these gas flows and their dependence on the intrinsic properties of the violent star-forming episodes taking place in these objects (age, strength, gas geometry,...) might explain (in part) the apparent lack of correlation between other properties (like metallicity) and the frequency of occurence and strength of Lyα emission in star-forming galaxies. Attempts to derive the comoving starformation rate at high redshifts from Lyα emission searches are highly questionable.
We present a FUSE spectroscopic study of the nearby gas--rich, metal--deficient blue compact dwar... more We present a FUSE spectroscopic study of the nearby gas--rich, metal--deficient blue compact dwarf (BCD) galaxy IZw 36. Atomic hydrogen and many metal lines are observed in absorption against the stellar continuum of young, massive stars embedded in the ionized region. Profile fitting of absorption lines allowed us to determine abundances and investigate the chemical composition of the neutral gas. This study present strong evidences that nitrogen is 16 (+7 -8) times less abundant in the neutral gas with respect to the ionized gas (all uncertainties are 2 sigma. Similarly, the oxygen abundance estimated using phosphorus as a tracer is lower in the neutral gas by a factor 8 (+17 -5). We also find that argon is underabundant by a factor 32 (+8 -7) and that log (ArI/OI) < -3.0 which is inconsistent with the solar value -2.1 (+0.1 -0.1) (Lodders2003), implying that argon is likely ionized into ArII in the neutral medium.
We present a calibration of the massive star formation rate vs. [CII] luminosity relation based o... more We present a calibration of the massive star formation rate vs. [CII] luminosity relation based on a sample of nearby, late-type galaxies observed with ISO-LWS and imaged in the Halpha+[NII] line. The relation holds for far-IR luminosities 10^8 < L_{FIR} < 10^{10.5} L (solar units). The derived star formation rates have an uncertainty of about a factor of 10. Part of this uncertainty is due to the different mix of contributions to the [CII] emission from the different components of the interstellar medium in individual galaxies, as discussed in an appendix.
We present Mid (MIR) and Far (FIR) Infrared observations of 18 spiral/irregular galaxies belongin... more We present Mid (MIR) and Far (FIR) Infrared observations of 18 spiral/irregular galaxies belonging to the Coma and A1367 clusters, carried out with the CAM and PHOT instruments on board the ISO satellite. Complementary photometry from the UV to the Near Infrared (NIR) together with Halpha imaging, HI and 12CO line measurements allow us to study the relationships between the
We present mid-IR images of a 15 ×15 field in the south-west part of the Andromeda galaxy M 31 ob... more We present mid-IR images of a 15 ×15 field in the south-west part of the Andromeda galaxy M 31 obtained with the ISOCAM camera (6 pixels) on board ISO. These broad-band images complement spectro-imaging observations of smaller fields . 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 inter-compared and also compared with H i, CO(1-0) and Hα maps. The mid-IR emission as seen through wide-band filters centered at 7 and 15 µm is extremely well correlated with the distribution of neutral gas as shown by the H i and CO(1-0) maps, while the correlation is poorer with the distribution of the ionized gas seen through its Hα 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 spectroimaging observations of show that the mid-IR spectra of several regions of M 31, two of which are in the presently studied area, are dominated by a strong emission band at 11.3 µm while emission in the other classical Aromatic Infrared Bands (AIBs) at 6.2, 7.7 and 8.6 µm is faint or absent. This result is precised, and 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 general. 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 2 times that near the Sun.
We present ISOCAM imaging data at 6.75 and 15 µm for 145 galaxies in the Virgo cluster and in the... more We present ISOCAM imaging data at 6.75 and 15 µm for 145 galaxies in the Virgo cluster and in the Coma/A1367 supercluster. Of these, 99 form a complete, optically-selected, volume-limited sample including galaxies in the core and in the periphery of the Virgo cluster, suitable for statistical analysis. 34 of the Virgo galaxies were resolved by ISOCAM: for these objects we present mid-IR images, radial light and colour profiles, as well as effective and isophotal radii, surface brightness and light concentration indices. The mid-IR colours of the target galaxies show a weak trend with the H band luminosity, with values of F(6.75 µm)/F(15 µm) ≥ 1 found generally in massive objects (L H ≥ 10 10 L H ), and F(6.75 µm)/F(15 µm) ≤ 1 in low-mass (L H ≤ 10 10 L H ) dwarf galaxies. All early-type galaxies (type ≤ S0a) have F(6.75 µm)/F(15 µm) ≥ 1, as expected when the mid-IR emission is dominated by the photosphere of the cold stellar population. The mid-IR, near-IR and visible light concentration indices of bright galaxies are tightly correlated with one another, indicating that the spatial distribution of the mid-IR emitting sources is, to the first order, similar to that of the stars.
We present maps of dust emission at 7 µm and 15 µm/7 µm intensity ratios of selected regions in 4... more We present maps of dust emission at 7 µm and 15 µm/7 µm 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.
The recent discovery of a large fraction of DCN in the galactic center is interpreted in the fram... more The recent discovery of a large fraction of DCN in the galactic center is interpreted in the framework of current models of chemical evolution. It implies that deuterium must be continuously replenished in the highly processed galactic center Several hypotheses are considered: the most likely are either mixing of external gas with cosmological deuterium or the presence in the galactic center of important fluxes of galactic cosmic rays producing deuterium by spallation reactions; less probable possibilities are a recent large production of deuterium by supernova outbursts or by a hypothetical central supermassive object. (AIP)
Faint emission in the CO(1-0) and CO(2-1) spectral lines at mm wavelengths has been discovered fr... more Faint emission in the CO(1-0) and CO(2-1) spectral lines at mm wavelengths has been discovered from dark dust clouds in the inner disk of M31. These dust clouds are located in regions of the galaxy where there is little evidence of star-forming activity. The CO emission from these clouds is resolved, and comes from roughly the same area covered by
We discuss recent observations of CO, CS, and NH3 emission lines from two molecular clouds in the... more We discuss recent observations of CO, CS, and NH3 emission lines from two molecular clouds in the inner regions of M31. The intensities of the (12)CO(1-0), (12)CO(2-1), (13)CO(1-0), and (13)CO(2-1) lines have been measured with the IRAM 30 m telescope at the same angular resolution of 23 sec. Upper limits have been obtained in the CS(2-1), CS(3-2) and in the
We have identified a population of B stars in the field whose distribution is also well correlate... more We have identified a population of B stars in the field whose distribution is also well correlated with the extended HI distribution. Evidently, star formation is both ongoing and wide spread in the outer disk of M 31. According to the current view of the star formation process, molecular gas is therefore also expected to be present.
The paper presents spectrophotometric observations of H II regions in the irregular and blue comp... more The paper presents spectrophotometric observations of H II regions in the irregular and blue compact galaxies. These observations are used to derive abundances of He, N, O, Ne, S, and Ar relative to H, and the results are consistent with the solution that nitrogen is in part a product of primary nucleosynthesis, and that the ratio of primary production of nitrogen to oxygen is N/O approximately equals 0.02. The relationship between the heavy element abundance and the total mass of galaxies is determined and it is found that the heavy element yield is constant. A model of galactic chemical evolution based on a new initial mass function and on stellar evolutionary models with mass loss agrees with the observations; it is concluded that irregular and blue compact galaxies seem to differ only in the present rate of star formation, but it was not determined whether they are old systems with a present burst, or systems where star formation started only recently.
Uploads
Papers by James Lequeux