Göran Östlin

The most metal-poor galaxies

Summary. 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 rôle 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 do 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

On the Redshift Evolution of the Lyα Escape Fraction and the Dust Content of Galaxies

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The Spectral Evolution of the First Galaxies. I. James Webb Space Telescope Detection Limits and Color Criteria for Population III Galaxies

emission.

Escape of about five per cent of Lyman-alpha photons from high-redshift star-forming galaxies.

The Lyα emission line has been proven to be a powerful tool for studying evolving galaxies at the highest redshift. However, in order to use Lyα as a physical probe of galaxies, it becomes vital to know the Lyα escape fraction (f^(Lyα)_(esc)). Unfortunately, due to the resonant nature of Lyα, f^(Lyα)_(esc) may vary unpredictably and requires empirical measurement. Here, we compile Lyα luminosity functions (LFs) between redshifts z = 0 and 8 and, combined with Hα and ultraviolet data, assess how f^(Lyα)_(esc) evolves with redshift. We find a strong upward evolution in f^(Lyα)_(esc) over the range z = 0.3-6, which is well fit by the power law f^(Lyα)_(esc)α(1 + z)^ξwith ξ = (2.57^(+0.19)_(–0.12)). This predicts that f^(Lyα)_(esc) should reach unity at z = 11.1. By comparing f^(Lyα)_(esc) and E_(B–V) in individual galaxies we derive an empirical relationship between f^(Lyα)_(esc) and E_(B–V) , which includes resonance scattering and can explain the redshift evolution of f^(Lyα)_(esc) between z = 0 and 6 purely as a function of the evolution in the dust content of galaxies. Beyond z ≈ 6.5, f^(Lyα)_(esc) drops more substantially, an effect attributed to either ionizing photon leakage, or an increase in the neutral gas fraction of the intergalactic medium. While distinguishing between these two scenarios may be extremely challenging, by framing the problem this way we remove the uncertainty of the halo mass from Lyα-based tests of reionization. We finally derive a new method by which to estimate the dust content of galaxies, based purely upon the observed Lyα and UV LFs. These data are characterized by an exponential with an e-folding scale of z_(EBV) ≈ 3.4.

Evolution in the properties of Lyman-

The James Webb Space Telescope (JWST) is expected to revolutionize our understanding of the high-redshift universe, and may be able to test the prediction that the first, chemically pristine (Popul ...

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The Lyman-α (Lyα) emission line is the primary observational signature of star-forming galaxies at the highest redshifts, and has enabled the compilation of large samples of galaxies with which to study cosmic evolution. The resonant nature of the line, however, means that Lyα photons scatter in the neutral interstellar medium of their host galaxies, and their sensitivity to absorption by interstellar dust may therefore be greatly enhanced. This implies that the Lyα luminosity may be significantly reduced, or even completely suppressed. Hitherto, no unbiased empirical test of the escaping fraction (fesc) of Lyα photons has been performed at high redshifts. Here we report that the average fesc from star-forming galaxies at redshift z = 2.2 is just 5 per cent by performing a blind narrowband survey in Lyα and Hα. This implies that numerous conclusions based on Lyα-selected samples will require upwards revision by an order of magnitude and we provide a benchmark for this revision. We demonstrate that almost 90 per cent of star-forming galaxies emit insufficient Lyα to be detected by standard selection criteria. Both samples show an anti-correlation of fesc with dust content, and we show that Lyα- and Hα-selection recovers populations that differ substantially in dust content and fesc.

emitters from redshifts z ~ 3 to z ~ 2

Context: Narrow-band surveys to detect Lyα emitters are powerful tools for identifying high, and very high, redshift galaxies. Although samples are increasing at redshifts z = 3 {-} 6, the nature of these galaxies is still poorly known. The number of galaxies detected at redshifts below z ˜ 3 are also small. Aims: We study the properties of z = 2.25 Lyα emitters and compare them with those of z > 3 Lyα emitters. Methods: We present narrow-band imaging made with the MPG/ESO 2.2m telescope and the WFI (Wide Field Imager) detector. Using this data, we have searched for emission-line objects. We find 170 candidate typical Lyα emitters and 17 candidates that we regard as high UV-transmission Lyα emitters. We have derived the magnitudes of these objects in 8 photometric bands from u* to K_s, and studied whether they have X-ray and/or radio counterparts. Results: We demonstrate that there has been significant evolution in the properties of Lyα emitters between redshift z ˜ 3 and z = 2.25. The spread in spectral energy distributions (SEDs) at the lower redshift is larger and we detect a significant AGN contribution in the sample. The distribution of the equivalent widths is narrower than at z ˜ 3, with only a few candidates with rest-frame equivalent width above the predicted limit of 240 A. The star formation rates derived from the Lyα emission compared to those derived from the UV emission are lower by on average a factor of ˜ 1.8, indicative of a significant absorption by dust. Conclusions: Lyα emitters at redshift z = 2.25 may be more evolved than Lyα emitters at higher redshift. The red SEDs imply more massive, older and/or dustier galaxies at lower redshift than observed at higher redshifts. The decrease in equivalent widths and star formation rates indicate more quiescent galaxies, with in general less star formation than in higher redshift galaxies. At z = 2.25, AGN appear to be more abundant and also to contribute more to the Lyα emitting population. Full Table 2 is only available in electronic form at http://www.aanda.org

Supernova 2006aj and the associated X-Ray Flash 060218

Aims. We have studied the afterglow of the gamma-ray burst (GRB) of February 18, 2006. This is a nearby long GRB, with a very low peak energy, and is therefore classified as an X-ray Flash (XRF). XRF 060218 is clearly associated with a supernova – dubbed SN 2006aj. Methods. We present early spectra for SN 2006aj as well as optical lightcurves reaching out to 50 days past explosion. Results. Our optical lightcurves define the rise times, the lightcurve shapes and the absolute magnitudes in the U, V and R bands, and we compare these data with data for other relevant supernovae. SN 2006aj evolved quite fast, somewhat similarly to SN 2002ap, but not as fast as SN 1994I. Our spectra show the evolution of the supernova over the peak, when the U-band portion of the spectrum rapidly fades due to extensive line blanketing. We compare to similar spectra of very energetic type Ic supernovae. Our first spectra are earlier than spectra for any other GRB-SN. The spectrum taken 12 days after burst in the rest frame is similar to somewhat later spectra of both SN 1998bw and SN 2003dh, implying a rapid early evolution. This is consistent with the fast lightcurve. From the narrow emission lines from the host galaxy we derive a redshift of z = 0.0331 ± 0.0007. This makes XRF 060218 the second closest gamma-ray burst detected. The flux of these emission lines indicate a high-excitation state, and a modest metallicity and star formation rate of the host galaxy.

Spatially Resolved Properties of the GRB 060505 Host: Implications for the Nature of the Progenitor*

GRB 060505 was the first well-observed nearby possible long-duration gamma-ray burst ( GRB) that had no associated supernova. Here we present spatially resolved spectra of the host galaxy of GRB 060505, an Sbc spiral, at redshift z = 0.0889. The GRB occurred inside a star-forming region in the northern spiral arm at 6.5 kpc from the center. From the position of the emission lines, we determine a maximum rotational velocity for the galaxy of v similar to 212 km s(-1), corresponding to a mass of 1.14 x 10(11) M (circle dot) within 11 kpc from the center. By fitting single-age spectral synthesis models to the stellar continuum, we derive a very young age for the GRB site, confirmed by photometric and H alpha line measurements, of around similar to 6 Myr, which corresponds to the lifetime of a 32M(circle dot) star. The metallicity derived from several emission-line measurements varies throughout the galaxy and is lowest at the GRB site. Using the Two Degree Field Galaxy Redshift Survey we can locate the host galaxy in its large-scale (similar to Mpc) environment. The galaxy lies in the foreground of a filamentary overdensity, extending southwest from the galaxy cluster Abell 3837 at z = 0.0896. The properties of the GRB site are similar to those found for other long-duration GRB host galaxies with high specific star formation rate and low metallicity, which is an indication that GRB 060505 originated from a young, massive star that died without making a supernova.

On the nature of nearby GRB/SN host galaxies ⋆

We present and discuss optical diagnostics of the low redshift (z< 0.2) galaxies that are known to have hosted supernovae associated withγ-ray bursts (GRBs). The three galaxies are all actively starforming sub-luminous (L< L ⋆ ) galaxies with relatively low metallicities (Z ∼ Z⊙). We find no evidence for substantial internal extinction wi thin any of the galaxies. We derive star formation rates (SFR) based on Hα luminosities, as well as specific star formation rates (SSFR, star formation rate per uni t luminosity). For GRB 980425 (SN 1998bw) we use photometry of the supernova environment to estimate the mass of the progentitor to ∼ 30 M⊙. These three host galaxies have global properties (luminosities, SFR, SSFR, metallicity, colour, reddening) that rese mble those of more distant GRB host galaxies. We also compare the host galaxies with a sample of Blue Compact Galaxies (BCGs) in the local universe, and show that these samples have similar properties.