Daniel Kunth - Academia.edu (original) (raw)

Papers by Daniel Kunth

Research paper thumbnail of Strong Na I D absorption in the spectrum of the QSO PKS 1327-206 due to the galaxy MCG 03-34-085

Monthly Notices of the Royal Astronomical Society, 1984

Research paper thumbnail of Continuity of Spectroscopic Properties from Nearby Active Galaxy Nuclei to Intermediate Redshift Quasars

What are quasars? Astronomers have wondered ever since their discovery, more than 15 years ago, a... more What are quasars? Astronomers have wondered ever since their discovery, more than 15 years ago, as a class of highly enigmatic astronomical objects. Now, however, a better understanding of the physical processes in their interiors is slowly emerging, thanks to improved observations, and it appears more or less confirmed that (most of the known) quasars are actually rather similar to the nuclei of nearby, active galaxies. Correlations between strengths of emission lines and absolute magnitude have been found. Drs. Jacqueline Bergeron and Daniel Kunth of the ESO Scientific Group in Geneva are presently studying the important question of radio-strong vs. radio-quiet quasars and report some extremely interesting observations with the 3.6 m telescope.

Research paper thumbnail of Comprehensive study of the QSO-galaxy pair 1327-206 and the extent of low ionization galactic haloes

Astronomy and Astrophysics, May 30, 1987

Research paper thumbnail of High redshift and primeval galaxies; Proceedings of the Third IAP Workshop, Paris, France, June 29July 3, 1987

Various papers on high-redshift and primeval galaxies are presented. The topics considered includ... more Various papers on high-redshift and primeval galaxies are presented. The topics considered include: galaxy surveys at high redshift, spectroscopic surveys of high-z galaxies, angular correlations of faint galaxies, CCD galaxy counts to B = 25 m, selection in H-alpha prism surveys, Durham faint galaxy redshift survey, Lyman-alpha galaxies, IR and optical observations of distant radio galaxies, radio galaxies at high redshift, sub-mjy radio sources, age of nearby clusters, very deep IRAS survey at the north Galactic pole, disk galaxy bulges, evolution of cluster galaxies, cluster evolution, spectrophotometric evolution of distant galaxies, and evolution of optically selected QSOs. Also discussed are: intrinsic properties of H II galaxies, appearance of primeval galaxies, high-redshift cooling flows, statistics on low-redshift absorption systems in quasar spectra, high-redshift absorption systems in QSO spectra and galaxies, clustering of high-redshift metal line absorption systems, st...

Research paper thumbnail of Dispersal and mixing of oxygen in the interstellar medium of gas-rich galaxies

Arxiv preprint astro-ph/9410023, 1994

Stellar and nebular abundance indicators reveal that there exists significant abundance fluctuati... more Stellar and nebular abundance indicators reveal that there exists significant abundance fluctuations in the interstellar medium (ISM) of gas-rich galaxies. It is shown that at the present observed solar level of O/H ∼ 6 × 10 −4 , abundance differences of a factor of two, such as existing between the Sun and the nearby Orion Nebula, are many times larger than expected. We examine a variety of hydrodynamical processes operating at scales ranging from 1 pc to greater than 10 kpc, and show that the ISM should appear better homogenized chemically than it actually is: (i) on large galactic scales (1 ≥ l ≥ 10 kpc), turbulent diffusion of interstellar clouds in the shear flow of galactic differential rotation is able to wipe out azimuthal O/H fluctuations in less than 10 9 yrs; (ii) at the intermediate scale (100 ≥ l ≥ 1000 pc), cloud collisions and expanding supershells driven by evolving associations of massive stars, differential rotation and triggered star formation will redistribute and mix gas efficiently in about 10 8 yrs; (iii) at small scales (1 ≥ l ≥ 100 pc), turbulent diffusion may be the dominant mechanism in cold clouds, while Rayleigh-Taylor and Kelvin-Helmhotz instabilities quickly develop in regions of gas ionized by massive stars, leading to full mixing in ≤ 2 × 10 6 yrs. It is suggested that the relatively large O/H fluctuations observed in large disk galaxies may be due to retention, in sites favored by triggered star formation, of freshly enriched ejecta from SNR and supershells expanding in a differentially rotating disk, plus, possibly, inf all of low metallicity material from individual clouds like high velocity clouds which splash on the disk on timecales shorter than the local mixing time. In low-mass galaxies, stimulated star formation is much less efficient, and the most effective mixing mechanisms are absent; the escape of newly enriched material due to galactic winds powered by the starburst events, the lack of large-scale stirring, and the long dormant phase between successive star forming episodes make possible the survival of large abundance discontinuities.

Research paper thumbnail of Evidences for an expanding shell in the Blue Compact Dwarf Galaxy Haro 2

arXiv (Cornell University), Oct 1, 1997

Long-slit observations of the blue compact galaxy Haro 2 have been performed around Hα and Hβ. Th... more Long-slit observations of the blue compact galaxy Haro 2 have been performed around Hα and Hβ. The main aim of these observations was to detect the Hα emission originating in the partially ionized wind outflowing at 200 km/s, that had been previously detected with the Hubble Space Telescope (HST). A shallow broadening of the Hα line wings has been observed, consistent with the existence of an expanding shell. The rotation curve shows two dips at the same systemic velocity as the nucleus. We interpret this feature as an evidence that the expanding shell is decoupled from the disk rotation. At the positions of the dips the Hα line is clearly broadened with respect to the central core. This broadening is produced by the outer layers of the expanding shell. From the position of these dips we estimate the size of the shell to be around 20" in diameter, with a corresponding kinematical age between 5 and 6 Myr. This shell has most certainly been powered by the massive star formation process that takes place in the central region of this galaxy. A comparison of the Hα and Lyα profiles shows that Lyα is significantly broader than Hα, with an additional emission in the red wing. We interpret this redshifted source of Lyα emission as line photons backscattered by the receding part of the expanding shell. These observations outline the extremely high sensitivity of the Lyα line to the structure and kinematics of the interstellar medium (ISM). Finally the analysis of stellar Balmer lines in the Hβ region indicates that stars less massive than 10 M⊙ have probably been formed.

Research paper thumbnail of A comprehensive study of massive star formation episodes in Blue Compact Galaxies

We have analyzed the properties of the star formation episodes taking place in a sample of Blue C... more We have analyzed the properties of the star formation episodes taking place in a sample of Blue Compact and irregular galaxies by comparing their multiwavelength observational properties with the predictions of evolutionary population synthesis models. This method has allowed us to constrain the age, star formation regime (instantaneous or extended) and Initial Mass Function slope, as well as the shape and strentgh of the interstellar extinction in these regions. We find that star formation episodes are essentially short with a mean age of 3.5 Myrs. Some galaxies may be undergoing their first global episode of star formation while for the rest of the sample older stars contribute to at most half the optical emission. The Wolf-Rayet star population (WR) is well reproduced by the models and provides the strongest argument in favor of a short duration of the star formation episode. Supernova rates are relatively large. The accumulation of supernova explosions within few Myr has contrib...

Research paper thumbnail of The First Deep ACS Ly α Images of Local Starburst Galaxies

We report the first results from a deep Lyα imaging program of local starburst galaxies with the ... more We report the first results from a deep Lyα imaging program of local starburst galaxies with the Advanced Camera for Surveys (ACS) of the Hubble Space Telescope. The two observed galaxies ESO 350-IG038 and SBS 0335-052 have luminosities similar to those of the Magellanic Clouds but differ in their chemical composition. ESO 350-IG038 has an oxygen abundance of 1/8 solar, whereas SBS 0335-052 is known to have one of the lowest abundances among blue galaxies (∼1/30). The ACS imaging reveals a complex Lyα morphology, with sometimes strong offsets between the emission of Lyα and the location of stellar light, ionized gas traced by Hα, and the neutral gas. Overall, more Lyα photons escape from the more metal-and dust-rich galaxy ESO 350-IG038. The absence of clear SBS 0335-052 Lyα emission over all observed knots, whatever their dust content or/and color indices, contradicts model expectations of a lower escape fraction from dust-rich gas due to destruction of Lyα photons by dust grains. Rather, the results are in qualitative agreement with models suggesting the kinematic properties of the gas as the dominant Lyα escape regulator. If the properties of the two observed galaxies are representative for starburst galaxies in general, Lyα will be difficult to interpret as a star-formation indicator, in particular if based on Lyα imaging at low spatial resolution.

Research paper thumbnail of Lya Emission in Starbursts: Implications for Galaxies at High Redshift

Research paper thumbnail of Local Lyman a emitters and their relevance to high-redshift star-forming galaxies

Astrophys Space Sci, 2009

Research paper thumbnail of THE LYMAN ALPHA REFERENCE SAMPLE. V. THE IMPACT OF NEUTRAL ISM KINEMATICS AND GEOMETRY ON LyαESCAPE

The Astrophysical Journal, 2015

We present high-resolution far-UV spectroscopy of the 14 galaxies of the Lyman Alpha Reference Sa... more We present high-resolution far-UV spectroscopy of the 14 galaxies of the Lyman Alpha Reference Sample; a sample of strongly star-forming galaxies at low redshifts (0.028 < z < 0.18). We compare the derived properties to global properties derived from multi band imaging and 21 cm HI interferometry and single dish observations, as well as archival optical SDSS spectra. Besides the Lyman α line, the spectra contain a number of metal absorption features allowing us to probe the kinematics of the neutral ISM and evaluate the optical depth and and covering fraction of the neutral medium as a function of line-of-sight velocity. Furthermore, we show how this, in combination with precise determination of systemic velocity and good Lyα spectra, can be used to distinguish a model in which separate clumps together fully cover the background source, from the "picket fence" model named by Heckman et al. (2011). We find that no one single effect dominates in governing Lyα radiative transfer and escape. Lyα escape in our sample coincides with a maximum velocity-binned covering fraction of 0.9 and bulk outflow velocities of 50 km s −1 , although a number of galaxies show these characteristics and yet little or no Lyα escape. We find that Lyα peak velocities, where available, are not consistent with a strong backscattered component, but rather with a simpler model of an intrinsic emission line overlaid by a blueshifted absorption profile from the outflowing wind. Finally, we find a strong anticorrelation between Hα equivalent width and maximum velocity-binned covering factor, and propose a heuristic explanatory model.

Research paper thumbnail of Large Number of Wolf-Rayet Stars in Emission-Line Galaxies

Wolf-Rayet Stars and Interrelations with other Massive Stars in Galaxies, 1991

We present synthetic populations of massive stars that constrain the properties of the star forma... more We present synthetic populations of massive stars that constrain the properties of the star formation episodes, predict the occurence, subtype distribution and evolution of WR star populations at solar and SMC metallicities. Instantaneous burst and continuous star formation rates have been considered. Our predictions have been tested on a sample of 17 galaxies. The evolution of the WR stars is qualitatively well reproduced. The IMF slope is flatter on average than in the solar vicinity and is not correlated with the metallicity. The extinction law is in general similar to that of the SMC, independently of metallicity.

Research paper thumbnail of Stellar population history of Seyfert 2 nuclei

Proceedings of the International Astronomical Union, 2004

We present the results of a new stellar population synthesis code which mixes both evolutionary a... more We present the results of a new stellar population synthesis code which mixes both evolutionary and empirical population synthesis techniques. The code decomposes a given galaxy spectrum into a base of high resolution spectra from the recent evolutionary synthesis models of Bruzual & Charlot (2003). The main novelty is that now we synthesize the whole F λ spectrum, rather than a handful of absorption lines and colors.

Research paper thumbnail of THE Lyα REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259

The Astrophysical Journal, 2014

The Lyman-alpha reference sample (LARS) is a substantial program with the Hubble Space Telescope ... more The Lyman-alpha reference sample (LARS) is a substantial program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the detailed astrophysics of the visibility and strength of the Lyman-alpha (Lyα) line of neutral hydrogen. Lyα is the dominant spectral line in use for characterizing high redshift (z) galaxies. This article presents an overview of the survey, its selection function and HST imaging observations. The sample was selected from the combined GALEX+SDSS catalogue at z = 0.028−0.19, in order to allow Lyα to be captured with combinations of long pass filters in the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) on board HST. In addition, LARS utilises Hα and Hβ narrow, and u, b, i broad-band imaging with ACS and the Wide Field Camera 3 (WFC3). In order to study galaxies in which large numbers of Lyα photons are produced (whether or not they escape) we demanded an Hα equivalent width W(Hα)≥ 100Å. The final sample of 14 galaxies covers far UV (FUV, λ ∼ 1500Å) luminosities that overlaps with those of high-z Lyα emitters and Lyman Break Galaxies (LBGs), making LARS a valid comparison sample. We present the reduction steps used to obtain the Lyα images, including our LARS eXtraction software (LaXs) which utilises pixel-by-pixel spectral synthesis fitting of the energy distribution to determine and subtract the continuum at Lyα. We demonstrate that the use of SBC long pass filter combinations increase the signal to noise with an order of magnitude compared to the nominal Lyα filter available in SBC. To exemplify the science potential of LARS, we also present some first results for a single galaxy, Mrk 259 (LARS #1). This irregular galaxy shows bright extended (indicative of resonance scattering), but strongly asymmetric Lyα emission. Spectroscopy from HST/COS centered on the brightest UV knot show a moderate outflow in the neutral interstellar medium (probed by low ionization stage absorption features), and Lyα emission with an asymmetric profile. Radiative transfer modeling is able to reproduce the essential features of the Lyα line profile, and confirms the presence of an outflow. From the integrated photometry we measure a Lyα luminosity of L Lyα = 1.3 × 10 42 erg/s an equivalent width W(Lyα)=45Å and a far UV absolute magnitude M FUV = −19.2 (AB). Mrk 259 would hence be detectable in high-z Lyα and LBG surveys. The total Lyα escape fraction is 12 %. This number is higher than the low-z average, but similar to that at z > 4 demonstrating that LARS provides a valid comparison sample for high-z galaxy studies.

Research paper thumbnail of The Lyman Alpha Reference Sample. III. Properties of the Neutral Ism from GBT and Vla Observations

The Astrophysical Journal, 2014

We present new H I imaging and spectroscopy of the 14 UV-selected starforming galaxies in the Lym... more We present new H I imaging and spectroscopy of the 14 UV-selected starforming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a detailed study of the processes governing the production, propagation, and escape of Lyα photons. New H I spectroscopy, obtained with the 100m Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the 14 observed LARS galaxies (although the profiles of two of the galaxies are likely confused by other sources within the GBT beam); the three highest redshift galaxies are not detected at our current sensitivity limits. The GBT profiles are used to derive fundamental H I line properties of the LARS galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging localizes the H I gas and provides a measurement of the total H I mass in each galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal an enormous tidal structure that extends over 160 kpc from the main interacting systems and that contains >10 9 M of H I. We compare various H I properties with global Lyα quantities derived from HST measurements. The measurements of the Lyα escape fraction are coupled with the new direct measurements of H I mass and significantly disturbed H I velocities. Our robustly detected sample reveals that both total H I mass and linewidth are tentatively correlated with key Lyα tracers. Further, on global scales, these data support a complex coupling between Lyα propagation and the H I properties of the surrounding medium.

Research paper thumbnail of The Lyman Alpha Reference Sample. II.HUBBLE Space Telescopeimaging Results, Integrated Properties, and Trends

The Astrophysical Journal, 2014

We report new results regarding the Lyα output of galaxies, derived from the Lyman Alpha Referenc... more We report new results regarding the Lyα output of galaxies, derived from the Lyman Alpha Reference Sample, and focused on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Lyα, Hα, and UV, and maps of Hα/Hβ, Lyα equivalent width (EW), and Lyα/Hα. We present Lyα and UV radial light profiles and show they are well-fitted by Sérsic profiles, but Lyα profiles show indices systematically lower than those of the UV (n ≈ 1-2 instead of 4). This reveals a general lack of the central concentration in Lyα that is ubiquitous in the UV. Photometric growth curves increase more slowly for Lyα than the far ultraviolet, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii ≈10 kpc, suggesting that if placed at high-z only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total Lyα luminosities to be independent of all other quantities. Normalized Lyα throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and nebular quantities that suggest harder ionizing continuum and lower metallicity. Six galaxies would be selected as high-z Lyα emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Lyα and UV samples. A few galaxies have EWs above 50 Å, and one shows f Lyα esc of 80%; such objects have not previously been reported at low-z.

Research paper thumbnail of The Lyman Alpha Reference Sample: Extended Lyman Alpha Halos Produced at Low Dust Content

The Astrophysical Journal, 2013

We report on new imaging observations of the Lyman alpha emission line (Lyα), performed with the ... more We report on new imaging observations of the Lyman alpha emission line (Lyα), performed with the Hubble Space Telescope, that comprise the backbone of the Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies at redshifts 0.028 < z < 0.18 in continuum-subtracted Lyα, Hα, and the far ultraviolet continuum. We show that Lyα is emitted on scales that systematically exceed those of the massive stellar population and recombination nebulae: as measured by the Petrosian 20 percent radius, R P20 , Lyα radii are larger than those of Hα by factors ranging from 1 to 3.6, with an average of 2.4. The average ratio of Lyα-to-FUV radii is 2.9. This suggests that much of the Lyα light is pushed to large radii by resonance scattering. Defining the Relative Petrosian Extension of Lyα compared to Hα, ξ Lyα = R Lyα P20 /R Hα P20 , we find ξ Lyα to be uncorrelated with total Lyα luminosity. However ξ Lyα is strongly correlated with quantities that scale with dust content, in the sense that a low dust abundance is a necessary requirement (although not the only one) in order to spread Lyα photons throughout the interstellar medium and drive a large extended Lyα halo.

Research paper thumbnail of Red supergiant stars in blue compact dwarf galaxies: The case of Tol 0610-387

Astronomy & Astrophysics, 2001

We have investigated the stellar content of the blue compact dwarf galaxies (BCDG) Tol 0610-387 b... more We have investigated the stellar content of the blue compact dwarf galaxies (BCDG) Tol 0610-387 by combining near infrared and optical images, complemented by spectroscopic data. The color maps, combined with the Hα image allowed us to identify clusters possibly dominated by red supergiant stars (RSG) associated with the current ongoing starburst episode. The analysis shows that the local colors and Hα emission, when compared with the predictions from population synthesis models, are consistent with an aging stellar population of around 10-15 Myrs dominated by red supergiant stars. This galaxy could therefore become a preferred target for the spectroscopic confirmation and analysis of its RSG population. Since the presence of such stars in BCDGs is seldom observed, we discuss the implications of RSG detection for the study of the starburst dynamics and evolution.

Research paper thumbnail of POX 186: the ultracompact Blue Compact Dwarf Galaxy reveals its nature

Astronomy and Astrophysics

. High resolution, ground based R and I band observations of the ultra compact dwarf galaxy POX 1... more . High resolution, ground based R and I band observations of the ultra compact dwarf galaxy POX 186 are presented. The data, obtained with the ESO New Technology Telescope (NTT), are analyzed using a new deconvolution algorithm which allows one to resolve the innermost regions of this stellar-like object into three SuperStar Clusters (SSC). Upper limits to both masses (M 10 5 M fi ) and the physical sizes (60pc) of the SSCs are set. In addition, and maybe most importantly, extended light emission underlying the compact star-forming region is clearly detected in both bands. The R Gamma I color rules out nebular Hff contamination and is consistent with an old stellar population. This casts doubt on the hypothesis that Blue Compact Dwarf Galaxies (BCDG) are young galaxies. Key words: Galaxies: compact, dwarf, evolution, formation, photometry, starburst 1. Introduction Among the Blue Compact Dwarf Galaxy (BCDG) population, there exists a small class of very compact starforming objec...

Research paper thumbnail of Extended Tidal Structure In Two Lyman Alpha Emitting Starburst Galaxies

Bulletin of the American Astronomical Society, Dec 1, 2003

We present new VLA C-array HI imaging of the Lyman alpha emitting starburst galaxies Tol 1924-416... more We present new VLA C-array HI imaging of the Lyman alpha emitting starburst galaxies Tol 1924-416 and IRAS 08339+6517. The effective resolution probes neutral gas structures larger than 4.7 kpc in Tol 1924-416, and larger than 7.7 kpc in IRAS 08339+6517. Both systems are revealed to be tidally interacting: Tol 1924-416 with ESO 338-IG04B (6.6 arcminutes = 72 kpc minimum separation), and IRAS 08339+6517 with 2MASX J08380769+6508579 (3.4 arcminutes = 75 kpc minimum separation). The HI emission is extended in these systems, with tidal tails and debris between the target galaxies and their companions. These observations argue for the importance of interactions in triggering massive star formation events. Furthermore, as Lyman alpha emission has been detected from both of these primary systems, it confirms the hypothesis that the geometry of the ISM is a controlling factor in the escape of Lyman alpha emission from starburst environments. This work has been supported by NASA GSRP Fellowship NGT 5-50346, the National Radio Astronomy Observatory, and NASA through grant GO-9470 from STScI.

Research paper thumbnail of Strong Na I D absorption in the spectrum of the QSO PKS 1327-206 due to the galaxy MCG 03-34-085

Monthly Notices of the Royal Astronomical Society, 1984

Research paper thumbnail of Continuity of Spectroscopic Properties from Nearby Active Galaxy Nuclei to Intermediate Redshift Quasars

What are quasars? Astronomers have wondered ever since their discovery, more than 15 years ago, a... more What are quasars? Astronomers have wondered ever since their discovery, more than 15 years ago, as a class of highly enigmatic astronomical objects. Now, however, a better understanding of the physical processes in their interiors is slowly emerging, thanks to improved observations, and it appears more or less confirmed that (most of the known) quasars are actually rather similar to the nuclei of nearby, active galaxies. Correlations between strengths of emission lines and absolute magnitude have been found. Drs. Jacqueline Bergeron and Daniel Kunth of the ESO Scientific Group in Geneva are presently studying the important question of radio-strong vs. radio-quiet quasars and report some extremely interesting observations with the 3.6 m telescope.

Research paper thumbnail of Comprehensive study of the QSO-galaxy pair 1327-206 and the extent of low ionization galactic haloes

Astronomy and Astrophysics, May 30, 1987

Research paper thumbnail of High redshift and primeval galaxies; Proceedings of the Third IAP Workshop, Paris, France, June 29July 3, 1987

Various papers on high-redshift and primeval galaxies are presented. The topics considered includ... more Various papers on high-redshift and primeval galaxies are presented. The topics considered include: galaxy surveys at high redshift, spectroscopic surveys of high-z galaxies, angular correlations of faint galaxies, CCD galaxy counts to B = 25 m, selection in H-alpha prism surveys, Durham faint galaxy redshift survey, Lyman-alpha galaxies, IR and optical observations of distant radio galaxies, radio galaxies at high redshift, sub-mjy radio sources, age of nearby clusters, very deep IRAS survey at the north Galactic pole, disk galaxy bulges, evolution of cluster galaxies, cluster evolution, spectrophotometric evolution of distant galaxies, and evolution of optically selected QSOs. Also discussed are: intrinsic properties of H II galaxies, appearance of primeval galaxies, high-redshift cooling flows, statistics on low-redshift absorption systems in quasar spectra, high-redshift absorption systems in QSO spectra and galaxies, clustering of high-redshift metal line absorption systems, st...

Research paper thumbnail of Dispersal and mixing of oxygen in the interstellar medium of gas-rich galaxies

Arxiv preprint astro-ph/9410023, 1994

Stellar and nebular abundance indicators reveal that there exists significant abundance fluctuati... more Stellar and nebular abundance indicators reveal that there exists significant abundance fluctuations in the interstellar medium (ISM) of gas-rich galaxies. It is shown that at the present observed solar level of O/H ∼ 6 × 10 −4 , abundance differences of a factor of two, such as existing between the Sun and the nearby Orion Nebula, are many times larger than expected. We examine a variety of hydrodynamical processes operating at scales ranging from 1 pc to greater than 10 kpc, and show that the ISM should appear better homogenized chemically than it actually is: (i) on large galactic scales (1 ≥ l ≥ 10 kpc), turbulent diffusion of interstellar clouds in the shear flow of galactic differential rotation is able to wipe out azimuthal O/H fluctuations in less than 10 9 yrs; (ii) at the intermediate scale (100 ≥ l ≥ 1000 pc), cloud collisions and expanding supershells driven by evolving associations of massive stars, differential rotation and triggered star formation will redistribute and mix gas efficiently in about 10 8 yrs; (iii) at small scales (1 ≥ l ≥ 100 pc), turbulent diffusion may be the dominant mechanism in cold clouds, while Rayleigh-Taylor and Kelvin-Helmhotz instabilities quickly develop in regions of gas ionized by massive stars, leading to full mixing in ≤ 2 × 10 6 yrs. It is suggested that the relatively large O/H fluctuations observed in large disk galaxies may be due to retention, in sites favored by triggered star formation, of freshly enriched ejecta from SNR and supershells expanding in a differentially rotating disk, plus, possibly, inf all of low metallicity material from individual clouds like high velocity clouds which splash on the disk on timecales shorter than the local mixing time. In low-mass galaxies, stimulated star formation is much less efficient, and the most effective mixing mechanisms are absent; the escape of newly enriched material due to galactic winds powered by the starburst events, the lack of large-scale stirring, and the long dormant phase between successive star forming episodes make possible the survival of large abundance discontinuities.

Research paper thumbnail of Evidences for an expanding shell in the Blue Compact Dwarf Galaxy Haro 2

arXiv (Cornell University), Oct 1, 1997

Long-slit observations of the blue compact galaxy Haro 2 have been performed around Hα and Hβ. Th... more Long-slit observations of the blue compact galaxy Haro 2 have been performed around Hα and Hβ. The main aim of these observations was to detect the Hα emission originating in the partially ionized wind outflowing at 200 km/s, that had been previously detected with the Hubble Space Telescope (HST). A shallow broadening of the Hα line wings has been observed, consistent with the existence of an expanding shell. The rotation curve shows two dips at the same systemic velocity as the nucleus. We interpret this feature as an evidence that the expanding shell is decoupled from the disk rotation. At the positions of the dips the Hα line is clearly broadened with respect to the central core. This broadening is produced by the outer layers of the expanding shell. From the position of these dips we estimate the size of the shell to be around 20" in diameter, with a corresponding kinematical age between 5 and 6 Myr. This shell has most certainly been powered by the massive star formation process that takes place in the central region of this galaxy. A comparison of the Hα and Lyα profiles shows that Lyα is significantly broader than Hα, with an additional emission in the red wing. We interpret this redshifted source of Lyα emission as line photons backscattered by the receding part of the expanding shell. These observations outline the extremely high sensitivity of the Lyα line to the structure and kinematics of the interstellar medium (ISM). Finally the analysis of stellar Balmer lines in the Hβ region indicates that stars less massive than 10 M⊙ have probably been formed.

Research paper thumbnail of A comprehensive study of massive star formation episodes in Blue Compact Galaxies

We have analyzed the properties of the star formation episodes taking place in a sample of Blue C... more We have analyzed the properties of the star formation episodes taking place in a sample of Blue Compact and irregular galaxies by comparing their multiwavelength observational properties with the predictions of evolutionary population synthesis models. This method has allowed us to constrain the age, star formation regime (instantaneous or extended) and Initial Mass Function slope, as well as the shape and strentgh of the interstellar extinction in these regions. We find that star formation episodes are essentially short with a mean age of 3.5 Myrs. Some galaxies may be undergoing their first global episode of star formation while for the rest of the sample older stars contribute to at most half the optical emission. The Wolf-Rayet star population (WR) is well reproduced by the models and provides the strongest argument in favor of a short duration of the star formation episode. Supernova rates are relatively large. The accumulation of supernova explosions within few Myr has contrib...

Research paper thumbnail of The First Deep ACS Ly α Images of Local Starburst Galaxies

We report the first results from a deep Lyα imaging program of local starburst galaxies with the ... more We report the first results from a deep Lyα imaging program of local starburst galaxies with the Advanced Camera for Surveys (ACS) of the Hubble Space Telescope. The two observed galaxies ESO 350-IG038 and SBS 0335-052 have luminosities similar to those of the Magellanic Clouds but differ in their chemical composition. ESO 350-IG038 has an oxygen abundance of 1/8 solar, whereas SBS 0335-052 is known to have one of the lowest abundances among blue galaxies (∼1/30). The ACS imaging reveals a complex Lyα morphology, with sometimes strong offsets between the emission of Lyα and the location of stellar light, ionized gas traced by Hα, and the neutral gas. Overall, more Lyα photons escape from the more metal-and dust-rich galaxy ESO 350-IG038. The absence of clear SBS 0335-052 Lyα emission over all observed knots, whatever their dust content or/and color indices, contradicts model expectations of a lower escape fraction from dust-rich gas due to destruction of Lyα photons by dust grains. Rather, the results are in qualitative agreement with models suggesting the kinematic properties of the gas as the dominant Lyα escape regulator. If the properties of the two observed galaxies are representative for starburst galaxies in general, Lyα will be difficult to interpret as a star-formation indicator, in particular if based on Lyα imaging at low spatial resolution.

Research paper thumbnail of Lya Emission in Starbursts: Implications for Galaxies at High Redshift

Research paper thumbnail of Local Lyman a emitters and their relevance to high-redshift star-forming galaxies

Astrophys Space Sci, 2009

Research paper thumbnail of THE LYMAN ALPHA REFERENCE SAMPLE. V. THE IMPACT OF NEUTRAL ISM KINEMATICS AND GEOMETRY ON LyαESCAPE

The Astrophysical Journal, 2015

We present high-resolution far-UV spectroscopy of the 14 galaxies of the Lyman Alpha Reference Sa... more We present high-resolution far-UV spectroscopy of the 14 galaxies of the Lyman Alpha Reference Sample; a sample of strongly star-forming galaxies at low redshifts (0.028 < z < 0.18). We compare the derived properties to global properties derived from multi band imaging and 21 cm HI interferometry and single dish observations, as well as archival optical SDSS spectra. Besides the Lyman α line, the spectra contain a number of metal absorption features allowing us to probe the kinematics of the neutral ISM and evaluate the optical depth and and covering fraction of the neutral medium as a function of line-of-sight velocity. Furthermore, we show how this, in combination with precise determination of systemic velocity and good Lyα spectra, can be used to distinguish a model in which separate clumps together fully cover the background source, from the "picket fence" model named by Heckman et al. (2011). We find that no one single effect dominates in governing Lyα radiative transfer and escape. Lyα escape in our sample coincides with a maximum velocity-binned covering fraction of 0.9 and bulk outflow velocities of 50 km s −1 , although a number of galaxies show these characteristics and yet little or no Lyα escape. We find that Lyα peak velocities, where available, are not consistent with a strong backscattered component, but rather with a simpler model of an intrinsic emission line overlaid by a blueshifted absorption profile from the outflowing wind. Finally, we find a strong anticorrelation between Hα equivalent width and maximum velocity-binned covering factor, and propose a heuristic explanatory model.

Research paper thumbnail of Large Number of Wolf-Rayet Stars in Emission-Line Galaxies

Wolf-Rayet Stars and Interrelations with other Massive Stars in Galaxies, 1991

We present synthetic populations of massive stars that constrain the properties of the star forma... more We present synthetic populations of massive stars that constrain the properties of the star formation episodes, predict the occurence, subtype distribution and evolution of WR star populations at solar and SMC metallicities. Instantaneous burst and continuous star formation rates have been considered. Our predictions have been tested on a sample of 17 galaxies. The evolution of the WR stars is qualitatively well reproduced. The IMF slope is flatter on average than in the solar vicinity and is not correlated with the metallicity. The extinction law is in general similar to that of the SMC, independently of metallicity.

Research paper thumbnail of Stellar population history of Seyfert 2 nuclei

Proceedings of the International Astronomical Union, 2004

We present the results of a new stellar population synthesis code which mixes both evolutionary a... more We present the results of a new stellar population synthesis code which mixes both evolutionary and empirical population synthesis techniques. The code decomposes a given galaxy spectrum into a base of high resolution spectra from the recent evolutionary synthesis models of Bruzual & Charlot (2003). The main novelty is that now we synthesize the whole F λ spectrum, rather than a handful of absorption lines and colors.

Research paper thumbnail of THE Lyα REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259

The Astrophysical Journal, 2014

The Lyman-alpha reference sample (LARS) is a substantial program with the Hubble Space Telescope ... more The Lyman-alpha reference sample (LARS) is a substantial program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the detailed astrophysics of the visibility and strength of the Lyman-alpha (Lyα) line of neutral hydrogen. Lyα is the dominant spectral line in use for characterizing high redshift (z) galaxies. This article presents an overview of the survey, its selection function and HST imaging observations. The sample was selected from the combined GALEX+SDSS catalogue at z = 0.028−0.19, in order to allow Lyα to be captured with combinations of long pass filters in the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) on board HST. In addition, LARS utilises Hα and Hβ narrow, and u, b, i broad-band imaging with ACS and the Wide Field Camera 3 (WFC3). In order to study galaxies in which large numbers of Lyα photons are produced (whether or not they escape) we demanded an Hα equivalent width W(Hα)≥ 100Å. The final sample of 14 galaxies covers far UV (FUV, λ ∼ 1500Å) luminosities that overlaps with those of high-z Lyα emitters and Lyman Break Galaxies (LBGs), making LARS a valid comparison sample. We present the reduction steps used to obtain the Lyα images, including our LARS eXtraction software (LaXs) which utilises pixel-by-pixel spectral synthesis fitting of the energy distribution to determine and subtract the continuum at Lyα. We demonstrate that the use of SBC long pass filter combinations increase the signal to noise with an order of magnitude compared to the nominal Lyα filter available in SBC. To exemplify the science potential of LARS, we also present some first results for a single galaxy, Mrk 259 (LARS #1). This irregular galaxy shows bright extended (indicative of resonance scattering), but strongly asymmetric Lyα emission. Spectroscopy from HST/COS centered on the brightest UV knot show a moderate outflow in the neutral interstellar medium (probed by low ionization stage absorption features), and Lyα emission with an asymmetric profile. Radiative transfer modeling is able to reproduce the essential features of the Lyα line profile, and confirms the presence of an outflow. From the integrated photometry we measure a Lyα luminosity of L Lyα = 1.3 × 10 42 erg/s an equivalent width W(Lyα)=45Å and a far UV absolute magnitude M FUV = −19.2 (AB). Mrk 259 would hence be detectable in high-z Lyα and LBG surveys. The total Lyα escape fraction is 12 %. This number is higher than the low-z average, but similar to that at z > 4 demonstrating that LARS provides a valid comparison sample for high-z galaxy studies.

Research paper thumbnail of The Lyman Alpha Reference Sample. III. Properties of the Neutral Ism from GBT and Vla Observations

The Astrophysical Journal, 2014

We present new H I imaging and spectroscopy of the 14 UV-selected starforming galaxies in the Lym... more We present new H I imaging and spectroscopy of the 14 UV-selected starforming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a detailed study of the processes governing the production, propagation, and escape of Lyα photons. New H I spectroscopy, obtained with the 100m Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the 14 observed LARS galaxies (although the profiles of two of the galaxies are likely confused by other sources within the GBT beam); the three highest redshift galaxies are not detected at our current sensitivity limits. The GBT profiles are used to derive fundamental H I line properties of the LARS galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging localizes the H I gas and provides a measurement of the total H I mass in each galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal an enormous tidal structure that extends over 160 kpc from the main interacting systems and that contains >10 9 M of H I. We compare various H I properties with global Lyα quantities derived from HST measurements. The measurements of the Lyα escape fraction are coupled with the new direct measurements of H I mass and significantly disturbed H I velocities. Our robustly detected sample reveals that both total H I mass and linewidth are tentatively correlated with key Lyα tracers. Further, on global scales, these data support a complex coupling between Lyα propagation and the H I properties of the surrounding medium.

Research paper thumbnail of The Lyman Alpha Reference Sample. II.HUBBLE Space Telescopeimaging Results, Integrated Properties, and Trends

The Astrophysical Journal, 2014

We report new results regarding the Lyα output of galaxies, derived from the Lyman Alpha Referenc... more We report new results regarding the Lyα output of galaxies, derived from the Lyman Alpha Reference Sample, and focused on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Lyα, Hα, and UV, and maps of Hα/Hβ, Lyα equivalent width (EW), and Lyα/Hα. We present Lyα and UV radial light profiles and show they are well-fitted by Sérsic profiles, but Lyα profiles show indices systematically lower than those of the UV (n ≈ 1-2 instead of 4). This reveals a general lack of the central concentration in Lyα that is ubiquitous in the UV. Photometric growth curves increase more slowly for Lyα than the far ultraviolet, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii ≈10 kpc, suggesting that if placed at high-z only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total Lyα luminosities to be independent of all other quantities. Normalized Lyα throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and nebular quantities that suggest harder ionizing continuum and lower metallicity. Six galaxies would be selected as high-z Lyα emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Lyα and UV samples. A few galaxies have EWs above 50 Å, and one shows f Lyα esc of 80%; such objects have not previously been reported at low-z.

Research paper thumbnail of The Lyman Alpha Reference Sample: Extended Lyman Alpha Halos Produced at Low Dust Content

The Astrophysical Journal, 2013

We report on new imaging observations of the Lyman alpha emission line (Lyα), performed with the ... more We report on new imaging observations of the Lyman alpha emission line (Lyα), performed with the Hubble Space Telescope, that comprise the backbone of the Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies at redshifts 0.028 < z < 0.18 in continuum-subtracted Lyα, Hα, and the far ultraviolet continuum. We show that Lyα is emitted on scales that systematically exceed those of the massive stellar population and recombination nebulae: as measured by the Petrosian 20 percent radius, R P20 , Lyα radii are larger than those of Hα by factors ranging from 1 to 3.6, with an average of 2.4. The average ratio of Lyα-to-FUV radii is 2.9. This suggests that much of the Lyα light is pushed to large radii by resonance scattering. Defining the Relative Petrosian Extension of Lyα compared to Hα, ξ Lyα = R Lyα P20 /R Hα P20 , we find ξ Lyα to be uncorrelated with total Lyα luminosity. However ξ Lyα is strongly correlated with quantities that scale with dust content, in the sense that a low dust abundance is a necessary requirement (although not the only one) in order to spread Lyα photons throughout the interstellar medium and drive a large extended Lyα halo.

Research paper thumbnail of Red supergiant stars in blue compact dwarf galaxies: The case of Tol 0610-387

Astronomy & Astrophysics, 2001

We have investigated the stellar content of the blue compact dwarf galaxies (BCDG) Tol 0610-387 b... more We have investigated the stellar content of the blue compact dwarf galaxies (BCDG) Tol 0610-387 by combining near infrared and optical images, complemented by spectroscopic data. The color maps, combined with the Hα image allowed us to identify clusters possibly dominated by red supergiant stars (RSG) associated with the current ongoing starburst episode. The analysis shows that the local colors and Hα emission, when compared with the predictions from population synthesis models, are consistent with an aging stellar population of around 10-15 Myrs dominated by red supergiant stars. This galaxy could therefore become a preferred target for the spectroscopic confirmation and analysis of its RSG population. Since the presence of such stars in BCDGs is seldom observed, we discuss the implications of RSG detection for the study of the starburst dynamics and evolution.

Research paper thumbnail of POX 186: the ultracompact Blue Compact Dwarf Galaxy reveals its nature

Astronomy and Astrophysics

. High resolution, ground based R and I band observations of the ultra compact dwarf galaxy POX 1... more . High resolution, ground based R and I band observations of the ultra compact dwarf galaxy POX 186 are presented. The data, obtained with the ESO New Technology Telescope (NTT), are analyzed using a new deconvolution algorithm which allows one to resolve the innermost regions of this stellar-like object into three SuperStar Clusters (SSC). Upper limits to both masses (M 10 5 M fi ) and the physical sizes (60pc) of the SSCs are set. In addition, and maybe most importantly, extended light emission underlying the compact star-forming region is clearly detected in both bands. The R Gamma I color rules out nebular Hff contamination and is consistent with an old stellar population. This casts doubt on the hypothesis that Blue Compact Dwarf Galaxies (BCDG) are young galaxies. Key words: Galaxies: compact, dwarf, evolution, formation, photometry, starburst 1. Introduction Among the Blue Compact Dwarf Galaxy (BCDG) population, there exists a small class of very compact starforming objec...

Research paper thumbnail of Extended Tidal Structure In Two Lyman Alpha Emitting Starburst Galaxies

Bulletin of the American Astronomical Society, Dec 1, 2003

We present new VLA C-array HI imaging of the Lyman alpha emitting starburst galaxies Tol 1924-416... more We present new VLA C-array HI imaging of the Lyman alpha emitting starburst galaxies Tol 1924-416 and IRAS 08339+6517. The effective resolution probes neutral gas structures larger than 4.7 kpc in Tol 1924-416, and larger than 7.7 kpc in IRAS 08339+6517. Both systems are revealed to be tidally interacting: Tol 1924-416 with ESO 338-IG04B (6.6 arcminutes = 72 kpc minimum separation), and IRAS 08339+6517 with 2MASX J08380769+6508579 (3.4 arcminutes = 75 kpc minimum separation). The HI emission is extended in these systems, with tidal tails and debris between the target galaxies and their companions. These observations argue for the importance of interactions in triggering massive star formation events. Furthermore, as Lyman alpha emission has been detected from both of these primary systems, it confirms the hypothesis that the geometry of the ISM is a controlling factor in the escape of Lyman alpha emission from starburst environments. This work has been supported by NASA GSRP Fellowship NGT 5-50346, the National Radio Astronomy Observatory, and NASA through grant GO-9470 from STScI.