arXiv subjects

G. Ostlin

Publications and source records attributed to G. Ostlin.

49 records · Page 3Linked to original sources

arXiv2004

Modelling the red halos of blue compact galaxies

Optical/near-IR broadband photometry of the halos of blue compact galaxies (BCGs) have revealed a very red spectral energy distribution, which cannot easily be reconciled with any normal, metal-poor stellar population. Here, three possible explanations for the red excess are explored: nebular emission, metal-rich stars and a stellar population with a very bottom-heavy initial mass function (IMF). We find, that nebular emission in BCG halos would produce very blue near-IR colours, and hence fails to explain the observed red excess. Although metal-rich stars may in principle explain the colours observed, the required stellar metallicity is very high (solar or higher), which would be a curious halo property given the low gas metallicity (~10% solar) of the central starburst in these objects. A stellar population with a low to intermediate metallicity and a very bottom-heavy IMF does however adequately reproduce the observed BCG halo colours. A bottom-heavy IMF also proves successful in explaining a similar red excess observed in the halos of stacked edge-on disk galaxies from the Sloan Digital Sky Survey. This may indicate that halos dominated by low-mass stars is a phenomenon common to galaxies of very different Hubble types.

astro-ph↗

arXiv2004

The new infra-red view of evolved stars in IZw18

We report results from near-infrared imaging of IZw18, the most metal-poor galaxy in the local universe with NICMOS on board the Hubble Space Telescope (HST). Observations were obtained in the broad F205W filter ($\sim$ K) and in the medium-wide bands F171M and F180M, the latter which cover strong molecular absorption features in cool stars. The new data, together with previously obtained HST/NICMOS images in the F110W ($\sim$ J) and F160W ($\sim$ H) filters, provide a census of the cool stellar population in this chemically unevolved galaxy. We find that stars as old as $\sim 1$ Gyr are required to explain the observed colour-magnitude diagrams. Combining broad and medium-wide band photometry, we have classified the observed stars. The observed stellar populations, to the depth of our data, are dominated by luminous red supergiant stars. However, an intermediate-age component is also present. We have identified carbon star candidates, and show that they dominate the stellar content for intermediate mass and age. We show that low metallicity intermediate-age stars have, for a given colour, a F110W ($\sim$ J-band) excess compared to local carbon stars. The carbon stars identified in IZw18 are the most distant (more than 10 Mpc) resolved examples yet discovered.

astro-ph↗

arXiv2004

The fate of Lyman photons in local Starburst: new ACS/HST images

We review the imaging and spectroscopic properties of the Lyman alpha emission in starburst galaxies. The Lyman alpha photons escape is largely driven by kinematical and orientation effects related to the presence of large scale expanding shells of neutral hydrogen. The various Lyman alpha profiles can be linked to the evolutionary state of the starburst. Our recent Lyman alpha imaging study using the HST-ACS is presented here: it highlights the presence of diffuse extended emission of low intensity leaking out the diffuse HI components. The similarities with the line profiles observed on high redshift galaxies is an important fact that cautions the use of Lyman alpha to derive the cosmic star formation rate and constrains the search for high redshift galaxies.

astro-ph↗

arXiv2004

Stellar dynamics of blue compact galaxies: I. Decoupled star-gas kinematics in ESO 400-G43

In this letter we present the first results from a program aimed at measuring the stellar kinematics of blue compact galaxies by observing the near-infrared Calcium triplet. We show the first results for ESO 400-G43 based on deep VLT/FORS2 spectroscopy. The instabilities found in the central gaseous velocity field are not seen in the solid body stellar rotation curve, indicating that stars and gas are kinematically decoupled in this galaxy. Even if this galaxy has a perturbed gaseous velocity field, the stellar velocity dispersion on average agrees well with that derived from the nebular lines.

astro-ph↗

arXiv2003

The First Deep ACS Lyman alpha Images of Local Starburst Galaxies

We report the first results from a deep Lya 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 Lya morphology, with sometimes strong offsets between the emission of Lya and the location of stellar light, ionized gas traced by Halpha, and the neutral gas. Overall, more Lya photons escape from the more metal- and dust-rich galaxy ESO 350-IG038. The absence of clear SBS 0335-052 Lya 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 Lya photons by dust grains. Rather, the results are in qualitative agreement with models suggesting the kinematic properties of the gas as the dominant Lya escape regulator. If the properties of the two observed galaxies are representative for starburst galaxies in general, Lya will be difficult to interpret as a star-formation indicator, in particular if based on Lya imaging at low spatial resolution.

astro-ph↗

arXiv2003

The temporal and spatial evolution of the starburst in ESO 338-IG04 as probed by its star clusters

In this paper we use ultra-violet (UV) and optical HST photometry in five bands, and an extensive set of spectral evolutionary synthesis scenarios to investigate the age and masses of 124 star clusters in the luminous blue compact galaxy ESO338-IG04 (Tololo 1924-416). The very small internal reddening makes ESO 338-IG04 an excellent laboratory for studying the formation of massive star clusters. We have used the star clusters to trace the temporal and spatial evolution of the starburst, and to put constraints on the star formation activity over a cosmological time-scale. The present starburst has been active for about 40 Myr. A standard Salpeter initial mass function (IMF) extending up to 120 solar masses provides the best fit to the data, although a flatter IMF cannot be excluded. The compact star clusters provide 30-40 percent of the UV luminosity and star formation activity. We find no evidence for dust obscuration even among the youngest (< 1 Myr) clusters. The fraction of stellar mass contained in compact star clusters is found to be several percent, which is an unusually high value. The intermediate age clusters show a flattened space distribution which agrees with the isophotal shape of the galaxy, whereas the oldest clusters seem to have a spherical distribution.(abridged)

astro-ph↗

arXiv2003

Stellar Masses of Luminous Compact Blue Galaxies at Redshifts z=0.4-1.2

We present stellar mass measurements for a sample of 36 Luminous Compact Blue Galaxies (LCBGs) at redshifts z = 0.4-1.2 in the Flanking Fields around the Hubble Deep Field North. The technique is based on fitting a two-component galaxy population model to multi-broadband photometry. Best-fit models are found to be largely independent on the assumed values for the IMF and the metallicity of the stellar populations, but are sensitive to the amount of extinction and the extinction law adopted. On average, the best-fit model corresponds to a LMC extinction law with E(B-V)=0.5. Stellar mass estimates, however, are remarkably independent on the final model choice. Using a Salpeter IMF, the derived median stellar mass for this sample is 5 x 10^9 Mo, i.e., ~2 times smaller than previous virial mass estimates. Despite uncertainties of a factor 2-3, our results strengthen prior claims that L* CBGs at intermediate redshifts are, on average, about 10 times less massive than a typical L* galaxy today.

astro-ph↗

arXiv2002

Massive (?) starburst hosts of blue compact galaxies (BCGs)

We present optical spectroscopy and deep optical/near-IR photometry of 4 luminous metal-poor blue compact galaxies (BCGs) and two of their companions. With the aid of spectral evolutionary models (SEMs) and structural parameters derived from the surface photometry we discuss the properties of the central starbursts and the halo populations of the galaxies. Special attention is paid to the effects of dust, chemical inhomogeneities and contamination of nebular emission to the halo light. The optical/near-IR colour index profiles show a sharp distinction between the starburst and the host. The hosts have luminosity profiles characteristic of massive ellipticals and remarkably red colours, typical of a relatively {\it metal-rich} stellar population of {\it old age}. These properties are in conflict with the relatively low luminosities. The situation can best be explained if the hosts have an unusually large amount of dark matter that can hinder the outflow of metals from the system. The indicated difference in metallicity between the halo and the young starburst disproves the recurrent burst scenario and supports different origins of the two populations. We conclude that these BCGs are undergoing mergers between early type galaxies/thick disks and gas-rich galaxies or intergalactic HI clouds, in many respects reminiscent of a retarded formation of massive ellipticals.

astro-ph↗

arXiv2001

Dynamics of blue compact galaxies, as revealed by their H$α$ velocity fields II. Mass models and the starburst triggering mechanism

The H$α$ velocity fields of a sample of six luminous blue compact galaxies (BCGs) and two companions have been obtained by observations with a scanning Fabry-Perot interferometer. The Fabry-Perot images, velocity fields and rotations curves have been presented in a previous paper (Paper I). In general, the velocity fields are irregular and often contain secondary dynamical components, but display overall rotation. The two companions have more regular velocity fields and rotation curves. In this article we analyse the velocity fields and dynamics together with the morphology of the studied BCGs, and present detailed mass models. In addition, we model the stellar mass content by means of multicolour surface photometry and spectral evolutionary synthesis analysis. By comparison of the masses of stars and those derived from the rotation curve, we show that about half of the galaxies cannot be supported by rotation alone. The morphology and dynamics of the BCGs suggest that the starburst activity in these galaxies are most likely triggered by mergers involving gas-rich dwarf galaxies and/or massive gas clouds.

astro-ph↗

arXiv2001

Are there any Young Galaxies in our local universe?

We find no evidence for the presence of genuinely young galaxies in our Local universe. We argue that the metal deficiencies and the behavior of the N/O and C/O ratios meet several plausible explanations involving the primary and secondary nature of these elements and their dispersion and mixing by massive stars and intermediate mass stars. Colors are to be analysed using surface photometry and comparing several model scenario since an instant burst model unavoidably provides a lower limit to the age. Ultimately colour-magnitude diagram techniques will be the only direct test to be performed at least on a small number local starburst galaxies. Our present analysis provides important warning for assessing the true age of distant galaxies.

astro-ph↗

arXiv2000

HST/NICMOS Observations of IZw18 - revealing a population of old AGB stars

I present the first results from a HST/NICMOS imaging study of the most metal-poor blue compact dwarf galaxy IZw18. The near infrared color-magnitude diagram (CMD) is dominated by two populations, one 10-20 Myr population of red super giants and one 0.5-5 Gyr population of asymptotic giant branch stars. Stars older than 1 Gyr are required to explain the observed CMD at the adopted distance of 12.6 Mpc, showing that IZw18 is not a young galaxy. The results hold also if the distance to IZw18 i s significantly larger. This rules out the possibility that IZw18 is a truly young galaxy formed recently in the local universe.

astro-ph↗

arXiv2000

Massive star clusters in dwarf starburst galaxies

I will discuss the presence of massive star clusters in starburst galaxies with an emphasis on low mass galaxies outside the local group. I will show that such galaxies, with respect to their mass and luminosity, may be very rich in young luminous clusters.

astro-ph↗

arXiv2000

The most metal-poor galaxies

Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial role in the cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields, outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metal-poor galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and the physical processes that are at work in these objects. The question on how they are related and may evolve from one class of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe, we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in the universe. We discuss what QSO absorption lines and known distant galaxies tell us already? We illustrate the importance of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator and discuss the possibility to detect nearly metal-free galaxies at high redshift from Ly$α$ emission.

astro-ph↗