Sandra Savaglio

Evolved Galaxies at z > 1.5 from the Gemini Deep Deep Survey: The Formation Epoch of Massive Stellar Systems

We present spectroscopic evidence from the Gemini Deep Deep Survey for a significant population of color-selected red galaxies at 1.3 1.5 old galaxies have a sky density greater than 0.1 arcmin-2. Conservative age estimates for 20 galaxies with z > 1.3, z = 1.49, give a median age of 1.2 Gyr and zf = 2.4. One-quarter of the galaxies have inferred zf > 4. Models restricted to [Fe/H] ? 0 give median ages and zf of 2.3 Gyr and 3.3, respectively. These galaxies are among the most massive and contribute ~50% of the stellar mass density at 1 < z < 2. The derived ages and most probable star formation histories suggest a high star formation rate (~300-500 M? yr-1) phase in the progenitor population. We argue that most of the red galaxies are not descendants of the typical z ~ 3 Lyman break galaxies. Galaxies associated with luminous submillimeter sources have the requisite star formation rates to be the progenitor population. Our results point toward early and rapid formation for a significant fraction of present-day massive galaxies.

Heavy-Element Abundances and Dust Depletions in the Host Galaxies of Three Gamma-Ray Bursts

We have derived the column densities of heavy elements in three gamma-ray burst (GRB) optical transients, associated with the circumburst or interstellar medium (ISM) of the host galaxy. In comparison with the same elements observed in damped Lyα (DLA) systems along QSO sight lines, we find evidence for much higher column densities of Zn II. The gap between the QSO-DLA and GRB-DLA distributions is smoothly bridged by observations of the interstellar absorption in the Milky Way and the Magellanic Clouds. Very small [Fe/Zn], [Si/Zn], and [Cr/Zn] values in GRB-DLAs indicate large dust depletions. Once the dust-to-metals ratios are determined, we find an optical extinction AV ≈ 1 mag, to be compared with typical AV 0.1 in most QSO-DLAs. Our inference of high dust content appears to be in contradiction with the typical low reddening previously found in GRBs. One possible way to reconcile is a dust grain size distribution biased toward big grains, which would give a gray extinction. Possibly, the small dust grains have been destroyed by the GRBs themselves. Our findings support the idea that primarily optically selected QSOs probe mainly low-gas/dust regions of high-redshift galaxies, while the more powerful GRBs can be detected through denser regions of their ISM (molecular clouds and star-forming regions). Therefore, GRB-DLAs and QSO-DLAs together provide a more complete picture of the global properties of the interstellar medium in high-redshift galaxies.

Dust depletion and extinction in a gamma-ray burst afterglow

We put stringent constraints for the first time on the dust pro pe ties in the circumburst medium of a gammaray burst (GRB) afterglow. This is based on the optical spect rum of GRB 020813 ( z = 1.255), obtained with Keck-I/LRIS 4.65 hours after the burst (Barth et al. 2003). F rom the absorption lines in the spectrum, we derive very high column densities for 6 heavy elements with differe nt r fractory properties. The relative abundances resemble the dust depletion patterns in the Milky Way, from w hich we infer a visual extinction of AV ≃ 0.4 mag andAV > 0.3 at 95% confidence level. However, the high columns of metals and dust contrast with an observed UV continuum spectrum which is remarkably close to a p wer law of the formFν ∝ ν−0.9, with no sign of curvature, or 2200 Å extinction feature, suggesting low reddening. The Milky Way or Magellanic Cloud reddening is possible only for very low visual extinct ons (AV < 0.08 orAV < 0.2, respectively at 95% confidence), inconsistent with the AV values inferred from the depletion analysis. If we assume a G RB intrinsic spectrumF i λ = FV (5500/λ) α and an extinction law of the forms Aλ = AV (5500/λ), we obtain (forAV = 0.4) the following constraints from continuum spectrum: γ < 0.82 andα < 1.74. Subject headings: cosmology: observations – gamma rays: bursts – galaxies: ab und nces – ISM: dust, extinction

Three Lyα emitters at z ≈ 6: Early GMOS/gemini data from the GLARE project

We report spectroscopic detection of three z ∼ 6 Lyα-emitting galaxies, in the vicinity of the Hubble Ultra Deep Field, from the early data of the Gemini Lyman Alpha at Reionisation Era (GLARE) project. Two objects, GLARE 3001 (z = 5.79) and GLARE 3011 (z = 5.94), are new detections and are fainter in z′ (z = 26.37 and 27.15) than any Lyman break galaxy previously detected in Lyα. A third object, GLARE 1042 (z = 5.83), has previously been detected in line emission from the ground; we report here a new spectroscopic continuum detection. Gemini/GMOS-South spectra of these objects, obtained using nod and shuffle, are presented together with a discussion of their photometric properties. All three objects were selected for spectroscopy via the i-drop Lyman break technique, the two new detections from the GOODS version 1.0 imaging data. The red i′-z′ colors and high equivalent widths of these objects suggest a high-confidence z > 5 Lyα identification of the emission lines. This brings the total number of known z > 5 galaxies within 9a of the Hubble Ultra Deep Field to four, of which three are at the same redshift (z = 5.8 within 2000 km s-1), suggesting the existence of a large-scale structure at this redshift.

Abundances in the Neutral Interstellar Medium of I Zw 18 from Far Ultraviolet Spectroscopic Explorer Observations

We report on new FUSE far-UV spectroscopy of the most metal-poor blue compact dwarf galaxy, I Zw 18. The new data represent an improvement over previous FUSE spectra by a factor of 1.7 in the signal-to-noise ratio. Together with a larger spectral coverage (λλ = 917-1188 A), this allows us to characterize absorption lines in the interstellar medium with unprecedented accuracy. The kinematics averaged over the large sampled region shows no clear evidence of gas inflows or outflows. The H I absorption is interstellar with a column density of 2.2 × 1021 cm-2. A conservative 3 σ upper limit of 5.25 × 1014 cm-2 is derived for the column density of diffuse H2. From a simultaneous fitting of metal absorption lines in the interstellar medium, we infer the following abundances: [Fe/H] = -1.76 ± 0.12, [O/H] = -2.06 ± 0.28, [Si/H] = -2.09 ± 0.12, [Ar/H] = -2.27 ± 0.13, and [N/H] = -2.88 ± 0.11. This is in general several times lower than in the H II regions. The only exception is iron, whose abundance is the same. The abundance pattern of the interstellar medium suggests ancient star formation activity with an age of at least 1 Gyr that enriched the H I phase. Around 470 SNe Ia are required to produce the iron content. A more recent episode that started 10 to several hundred megayears ago is responsible for the additional enrichment of α-elements and nitrogen in the H II regions.We report on new FUSE far-UV spectroscopy of the most metal-poor blue compact dwarf galaxy I Zw 18. The new data represent an improvement over previous FUSE spectra by a factor of 1.7 in the signal-to-noise. Together with a larger spectral coverage (λλ = 917−1188 Å), this allows us to characterize absorption lines in the interstellar medium with unprecedented accuracy. The kinematics averaged over the large sampled region shows no clear evidence of gas inflows or outflows. The H i absorption is interstellar with a column density of 2.2 − 0.5 × 10 cm. A conservative 3σ upper limit of 5.25× 10 cm is derived for the column density of diffuse H2. From a simultaneous fitting of metal absorption lines in the interstellar medium, we infer the following abundances: [Fe/H] = − 1.76 ± 0.12, [O/H] = − 2.06 ± 0.28, [Si/H] = − 2.09 ± 0.12, [Ar/H] = − 2.27 ± 0.13, and [N/H] = − 2.88 ± 0.11. This is in general several times lower than in the H ii regions. The only exception is iron, whose abundance is the same. The abundance pattern of the interstellar medium suggests ancient star-formation activity with an age of at least a Gyr that enriched the H i phase. Around 470 SNe Ia are required to produce the iron content. A more recent episode that started 10 to several 100 Myr ago is responsible for the additional enrichment of α-elements and nitrogen in the H ii regions. Subject headings: galaxies: dwarf — galaxies: starburst — galaxies: individual (I Zw 18) — galaxies: ISM — ISM: abundances Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218 On leave of absence from INAF-Osservatorio Astronomico di Roma, Italy Space Telescope Science Institute, Baltimore, MD 21218

THE GEMINI DEEP DEEP SURVEY. II. METALS IN STAR-FORMING GALAXIES AT REDSHIFT 1.3 < z < 2

The goal of the Gemini Deep Deep Survey (GDDS) is to study an unbiased sample of K 1.3. The selected objects have colors typical of irregular and Sbc galaxies. Strong [O II] emission indicates high star formation activity in the H II regions (star formation rate ~13-106 M☉ yr-1). The high signal-to-noise ratio composite spectrum shows strong ISM Mg II and Fe II absorption, together with weak Mn II and Mg I lines. The Fe II column density, derived using the curve of growth analysis, is log N = 15.54. This is considerably larger than typical values found in damped Lyα systems (DLAs) along QSO sight lines, where only 10 of 87 (~11%) have log N ≥ 15.2. High Fe II column densities are observed in the z = 2.72 Lyman break galaxy cB58 (log N 15.25) and in gamma-ray burst host galaxies (log N ~ 14.8-15.9). Given our measured Fe II column density and assuming a moderate iron dust depletion (δFe ~ 1 dex), we derive an optical dust extinction of AV ~ 0.6. If the H I column density is log N 0.2. The high completeness of the GDDS sample implies that these results are typical of star-forming galaxies in the 1 < z < 2 redshift range, an epoch that has heretofore been particularly challenging for observational programs.

The Lyα Forest of a Lyman Break Galaxy: Very Large Telescope Spectra of MS 1512–cB58 at z = 2.724

The high-redshift galaxy MS 1512-cB58 (z = 2.724, mV = 20.64) has been observed with the very efficient high-resolution UV-Visual Eschelle Spectograph on the Very Large Telescope. Although this is a very challenging observational program for a Southern Hemisphere telescope (the galaxy is located at +36° declination), high-resolution spectra (FWHM 26 km s-1) have revealed, with unprecedented detail along a galaxy sight line, the Lyα forest due to intervening clouds in the intergalactic medium (IGM). The mean depression DA due to IGM absorption blueward of the galaxy Lyα wavelength and the number density dn/dz of Lyα clouds have been compared with equivalent results obtained for QSO sight lines at similar redshifts. Our results indicate a possible excess of absorption close to the galaxy. The mean depression at ~150 h Mpc comoving (Ωm = 0.3, ΩΛ = 0.7) from the galaxy is DA = 0.36 ± 0.03, to be compared with 0.22 ± 0.04 expected from a best fit to QSO sight lines. In the same region (z 2.610), the number density of lines with H I column density in excess of 1014 atoms cm-2 is also ~3 σ larger than expected. This high-density region is at least 60 h Mpc comoving wide, but the large Lyα absorption of the galaxy itself prevents us from detecting a possible structure extending down to the galaxy. This excess of Lyα clouds is suggestive of two possible scenarios. One is the presence of a supercluster of Lyα clouds not associated with cB58. The other is a high density of gas associated with the environment of cB58. Indeed, a hint of the complexity of cB58 and possibly its environment is given by the huge velocity range (almost 1000 km s-1) between the optical emission of star-forming regions and UV absorption of its interstellar medium.

GRB host studies (GHostS)

The GRB Host Studies (GHostS) is a public archive collecting observed quantities of GRB host galaxies. At this time (January 2006) it contains information on 32 GRB hosts, i.e. about half of the total number of GRBs with known redshift. Here we present some preliminary statistical analysis of the sample, e.g. the total stellar mass, metallicity and star formation rate for the hosts. We found that these are generally low-mass objects, with 79% having M_*<10^10 M_sun. The total stellar mass and the metallicity for a subsample of 7 hosts at 0.4<z<1 are consistent with the mass-metallicity relation recently found for normal star-forming galaxies in the same redshift interval. At least 56% of the total sample are bursty galaxies: their growth time-scale (the time required to form the observed stellar mass assuming that the observed SFR is constant over the entire life of the galaxy) is shorter than 400 Myr.

The Dust Depletion and Extinction of the GRB 020813 Afterglow

The Keck optical spectrum of the GRB 020813 afterglow is the best ever obtained for GRBs. Its large spectral range and very high S/N ratio allowed for the first time the detection of a vast variety of absorption lines, associated with the circumburst medium or interstellar medium of the host. The remarkable similarity of the relative abundances of 8 elements with the dust depletion pattern seen in the Galactic ISM suggests the presence of dust. The derived visual dust extinction A_V=0.40+/-0.06 contradicts the featureless UV spectrum of the afterglow, very well described by a unreddened power law. The forthcoming Swift era will open exciting opportunities to explain similar phenomena in other GRB afterglows.

The mass‐metallicity relation at z ∼ 0.7

The ISM metallicity and the stellar mass are examined in a sample of 66 galaxies at 0.4 < z < 1, selected from the Gemini Deep Deep Survey (GDDS) and the Canada‐France Redshift Survey (CFRS). We observe a mass‐metallicity relation similar to that seen in z ∼ 0.1 SDSS galaxies, but displaced towards higher masses and/or lower metallicities. Using this sample, and a small sample of z ∼ 2.3 LBGs, a redshift dependent mass‐metallicity relation is proposed which describes the observed results.