P. Møller

A very energetic supernova associated with the |[gamma]|-ray burst of 29 March 2003

Over the past five years evidence has mounted that long-duration (>2 s) γ-ray bursts (GRBs)—the most luminous of all astronomical explosions—signal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova, but now includes the association of GRBs with regions of massive star formation in distant galaxies, the appearance of supernova-like ‘bumps’ in the optical afterglow light curves of several bursts and lines of freshly synthesized elements in the spectra of a few X-ray afterglows. These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that a very energetic supernova (a hypernova) was temporally and spatially coincident with a GRB at redshift z = 0.1685. The timing of the supernova indicates that it exploded within a few days of the GRB, strongly suggesting that core-collapse events can give rise to GRBs, thereby favouring the ‘collapsar’ model.

An optical supernova associated with the X-ray flash XRF 060218

Long-duration γ-ray bursts (GRBs) are associated with type Ic supernovae that are more luminous than average and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB–supernovae were thought to be rare events. Whether X-ray flashes—analogues of GRBs, but with lower luminosities and fewer γ-rays—can also be associated with supernovae, and whether they are intrinsically ‘weak’ events or typical GRBs viewed off the axis of the burst, is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB–supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output and the supernova radio flux. Our data, combined with radio and X-ray observations, suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB–supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB–supernovae.

UltraVISTA: a new ultra-deep near-infrared survey in COSMOS

In this paper we describe the first data release of the UltraVISTA near-infrared imaging survey of the COSMOS field. We summarise the key goals and design of the survey and provide a detailed description of our data reduction techniques. We provide stacked, sky-subtracted images in YJHK_s and narrow-band filters constructed from data collected during the first year of UltraVISTA observations. Our stacked images reach 5σAB depths in an aperture of 2″ diameter of ~25 in Y and ~24 in JHK_s bands and all have sub-arcsecond seeing. To this 5σ limit, our K_s catalogue contains 216 268 sources. We carry out a series of quality assessment tests on our images and catalogues, comparing our stacks with existing catalogues. The 1σ astrometric rms in both directions for stars selected with 17.0 < K_s(AB) < 19.5 is ~0.08″ in comparison to the publicly-available COSMOS ACS catalogues. Our images are resampled to the same pixel scale and tangent point as the publicly available COSMOS data and so may be easily used to generate multi-colour catalogues using this data. All images and catalogues presented in this paper are publicly available through ESO’s “phase 3” archiving and distribution system and from the UltraVISTA web site.

The host of GRB 030323 at z=3.372: A very high column density DLA system with a low metallicity

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lyα (DLA) absorption and low- and high-ionization lines at a redshift z = 3.3718 ± 0.0005. The inferred neutral hy- drogen column density, log N(Hi) = 21.90 ± 0.07, is larger than any (GRB- or QSO-) DLA H  column density inferred directly from Lyα in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: AV < 0.5 mag. The iron abundance is (Fe/H) = −1.47 ± 0.11, while the metallicity of the gas as measured from sulphur is (S/H) = −1.26 ± 0.20. We derive an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2) + N(Hi)) < 10 −6 .I n the Lyα trough, a Lyα emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Myr −1 . All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, Si *, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the H  volume density to be nHi = 10 2 −10 4 cm −3 .H ST/ACS imaging 4 months after the burst shows an extended AB(F606W) = 28.0 ± 0.3 mag object at a distance of 0.

The host of GRB 030323 at z = 3.372: A very high column density DLA system with a low metallicity

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lyα (DLA) absorption and low- and high-ionization lines at a redshift z = 3.3718 ± 0.0005. The inferred neutral hy- drogen column density, log N(Hi) = 21.90 ± 0.07, is larger than any (GRB- or QSO-) DLA H  column density inferred directly from Lyα in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: AV < 0.5 mag. The iron abundance is (Fe/H) = −1.47 ± 0.11, while the metallicity of the gas as measured from sulphur is (S/H) = −1.26 ± 0.20. We derive an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2) + N(Hi)) < 10 −6 .I n the Lyα trough, a Lyα emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Myr −1 . All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, Si *, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the H  volume density to be nHi = 10 2 −10 4 cm −3 .H ST/ACS imaging 4 months after the burst shows an extended AB(F606W) = 28.0 ± 0.3 mag object at a distance of 0.

Extended Lyα emission from a damped Ly α absorber at z=1.93, and the relation between damped Ly α absorbers and Lyman-break galaxies

The number of damped Lyα absorbers (DLAs) currently known is about 100, but our knowledge of their sizes and morphologies is still very sparse, as very few have been detected in emission. Here we present narrow-band and broad-band observations of a DLA in the field of the quasar pair Q0151+048A (qA) and Q0151+048B (qB). These two quasars have very similar redshifts, zem=1.922 and 1.937 respectively, and an angular separation of 3.27 arcsec. The spectrum of qA contains a DLA at zabs = 1.9342 (close to the emission redshift) which shows an emission line in the trough, detected at 4σ. Our narrow-band image confirms this detection, and we find Lyα emission from an extended area covering 6 × 3 arcsec2, corresponding to 25 × 12 h-2 kpc2 (q0=0.5, H0 = 100 h km s-1). The total Lyα luminosity from the DLA is 1.2 × 1043h-2 erg s-1, which is several times higher than the Lyα luminosity found from other DLAs. The narrow-band image also indicates that qB is not covered by the DLA. This fact, together with the large equivalent width of the emission line from the Lyα cloud, the large luminosity, and the ∼ 300 km s-1 blueshift relative to the DLA, can plausibly be explained if qB is the source of ionizing photons, lying to the near side of the DLA at a distance from the DLA of <20 h-1 kpc. In this case the size of the emission-line region corresponds to the area over which the cloud is optically thick, i.e., is indicative of the size of a Lyman-limit system. We also consider the relation between DLAs and Lyman-break galaxies (LBGs). If DLAs are gaseous discs surrounding LBGs, and if the apparent brightnesses and impact parameters of the few identified DLAs are representative of the brighter members of the population, then the luminosity distribution of DLAs is nearly flat, and we would expect that some 70 per cent of the galaxy counterparts to DLAs at z≈ 3 are fainter than mR=28.

Are High-Redshift Damped Lyα Galaxies Lyman Break Galaxies?*

We use deep HST STIS and NICMOS images of three spectroscopically confirmed galaxy counterparts of high-redshift damped Ly? (DLA) absorbers (one of which is a new discovery) to test the hypothesis that high-redshift DLA galaxies are Lyman break galaxies. If this hypothesis is correct, the emission properties of DLA galaxies must lie within the range of emission properties measured for Lyman break galaxies of similar absolute magnitude. This will be true regardless of selection biases in the sample of detected DLA galaxies. We test this prediction using several emission properties: half-light radius, radial profile (S?rsic n-parameter), optical to near-infrared color, morphology, Ly? emission equivalent width, and Ly? emission velocity structure. In all cases the measured values for the DLA galaxies lie within the range measured for the population of Lyman break galaxies. None of the measurements is in conflict with the prediction. We conclude that the measured emission properties of the three DLA galaxies studied here are consistent with the conjecture that high-redshift DLA galaxies are Lyman break galaxies. We show that this result does not conflict with the observation that the few high-redshift DLA galaxies discovered are mostly fainter than spectroscopically confirmed L* Lyman break galaxies.

THE OPTICALLY UNBIASED GAMMA-RAY BURST HOST (TOUGH) SURVEY. I. SURVEY DESIGN AND CATALOGS*

Long-duration gamma-ray bursts (GRBs) are powerful tracers of star-forming galaxies. We have defined a homogeneous subsample of 69 Swift GRB-selected galaxies spanning a very wide redshift range. Special attention has been devoted to making the sample optically unbiased through simple and well-defined selection criteria based on the high-energy properties of the bursts and their positions on the sky. Thanks to our extensive follow-up observations, this sample has now achieved a comparatively high degree of redshift completeness, and thus provides a legacy sample, useful for statistical studies of GRBs and their host galaxies. In this paper, we present the survey design and summarize the results of our observing program conducted at the ESO Very Large Telescope (VLT) aimed at obtaining the most basic properties of galaxies in this sample, including a catalog of R and Ks magnitudes and redshifts. We detect the host galaxies for 80% of the GRBs in the sample, although only 42% have Ks -band detections, which confirms that GRB-selected host galaxies are generally blue. The sample is not uniformly blue, however, with two extremely red objects detected. Moreover, galaxies hosting GRBs with no optical/NIR afterglows, whose identification therefore relies on X-ray localizations, are significantly brighter and redder than those with an optical/NIR afterglow. This supports a scenario where GRBs occurring in more massive and dusty galaxies frequently suffer high optical obscuration. Our spectroscopic campaign has resulted in 77% now having redshift measurements, with a median redshift of 2.14 ± 0.18. TOUGH alone includes 17 detected z > 2 Swift GRB host galaxies suitable for individual and statistical studies—a substantial increase over previous samples. Seven hosts have detections of the Lyα emission line and we can exclude an early indication that Lyα emission is ubiquitous among GRB hosts, but confirm that Lyα is stronger in GRB-selected galaxies than in flux-limited samples of Lyman break galaxies.

Probing cosmic chemical evolution with gamma-ray bursts: GRB 060206 at z = 4.048

Aims.We present early optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 with the aim of determining the metallicity of the GRB absorber and the physical conditions in the circumburst medium. We also discuss how GRBs may be important complementary probes of cosmic chemical evolution. Methods.Absorption line study of the GRB afterglow spectrum. Results.We determine the redshift of the GRB to be z=4.04795±0.00020. Based on the measurement of the neutral hydrogen column density from the damped Lyman-alpha line and the metal content from weak, unsaturated S II lines we derive a metallicity of [S/H]=-0.84±0.10. This is one of the highest metallicities measured from absorption lines at z~4. From the very high column densities for the forbidden Si II*, O I*, and O I** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H2 molecules with log N(H_2)~17.0, translating into a molecular fraction of log{f}≈ -3.5 with f=2N(H2)/(2N(H2) + N(H I)). Even if GRBs are only formed by single massive stars with metallicities below ~0.3 Zo, they could still be fairly unbiased tracers of the bulk of the star formation at z>2. Hence, metallicities as derived for GRB 060206 here for a complete sample of GRB afterglows will directly show the distribution of metallicities for representative star-forming galaxies at these redshifts.

The Evolution of Field Early-Type Galaxies to z ∼ 0.7*

We have measured the fundamental plane (FP) parameters for a sample of 30 field early-type galaxies (E/S0s) in the redshift range 0.1 < z < 0.66. We find that (i) the FP is defined and tight out to the highest redshift bin; (ii) the intercept γ evolves as dγ/dz = 0.58 (for Ω = 0.3, ΩΛ = 0.7) or, in terms of average effective mass-to-light ratio, as d log(M/LB)/dz = -0.72, i.e., faster than is observed for cluster E/S0s (-0.49 ± 0.05). In addition, we detect [O II] emission greater than 5 A in 22% of an enlarged sample of 42 massive E/S0s in the range 0.1 < z < 0.73, in contrast with the quiescent population observed in clusters at similar z. We interpret these findings as evidence that a significant fraction of massive field E/S0s experiences secondary episodes of star formation at z < 1.