arXiv subjects

Jens Hjorth

Publications and source records attributed to Jens Hjorth.

At least 109 records · Page 6Linked to original sources

arXiv2014

Type Ia Supernova Rate Measurements to Redshift 2.5 from CANDELS : Searching for Prompt Explosions in the Early Universe

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) was a multi-cycle treasury program on the Hubble Space Telescope (HST) that surveyed a total area of ~0.25 deg^2 with ~900 HST orbits spread across 5 fields over 3 years. Within these survey images we discovered 65 supernovae (SN) of all types, out to z~2.5. We classify ~24 of these as Type Ia SN (SN Ia) based on host-galaxy redshifts and SN photometry (supplemented by grism spectroscopy of 6 SN). Here we present a measurement of the volumetric SN Ia rate as a function of redshift, reaching for the first time beyond z=2 and putting new constraints on SN Ia progenitor models. Our highest redshift bin includes detections of SN that exploded when the universe was only ~3 Gyr old and near the peak of the cosmic star-formation history. This gives the CANDELS high-redshift sample unique leverage for evaluating the fraction of SN Ia that explode promptly after formation (<500 Myr). Combining the CANDELS rates with all available SN Ia rate measurements in the literature we find that this prompt SN Ia fraction is fP=0.53 +0.09 -0.10 (stat) +0.10 -0.26 (sys), consistent with a delay time distribution that follows a simple t^{-1} power law for all times t>40 Myr. However, a mild tension is apparent between ground-based low-z surveys and space-based high-z surveys. In both CANDELS and the sister HST program CLASH, we find a low rate of SN Ia at z>1. This could be a hint that prompt progenitors are in fact relatively rare, accounting for only ~20% of all SN Ia explosions -- though further analysis and larger samples will be needed to examine that suggestion.

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arXiv2014

Spectroscopic identification of a redshift 1.55 supernova host galaxy from the Subaru Deep Field Supernova Survey

Context: The Subaru Deep Field (SDF) Supernova Survey discovered 10 Type Ia supernovae (SNe Ia) in the redshift range 1.5 1.5 SDF SN Ia host galaxy candidate Methods: We use the X-shooter (U-to-K-band) spectrograph on the Very Large Telescope to allow the detection of different emission lines in a wide spectral range. Results: We measure a spectroscopic redshift of 1.54563 +/- 0.00027 of hSDF0705.25, consistent with its photometric redshift of 1.552 +/- 0.018. From the strong emission-line spectrum we rule out AGN activity, thereby confirming the optical transient as a SN. The host galaxy follows the fundamental metallicity relation defined in Mannucci et al. (2010, 2011) showing that the properties of this high-redshift SN Ia host galaxy is similar to other field galaxies. Conclusions: Spectroscopic confirmation of additional SDF SN hosts would be required to confirm the cosmic SN rate evolution measured in the SDF.

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arXiv2014

Shaping the dust mass - star-formation rate relation

There is a remarkably tight relation between the observationally inferred dust masses and star-formation rates (SFRs) of SDSS galaxies, Mdust $\propto$ SFR$^{1.11}$ (Da Cunha et al. 2010). Here we extend the Mdust-SFR relation to the high end and show that it bends over at very large SFRs (i.e., dust masses are lower than predicted for a given SFR). We identify several distinct evolutionary processes in the diagram: (1) A star-bursting phase in which dust builds up rapidly at early times. The maximum attainable dust mass in this process is the cause of the bend-over of the relation. A high dust-formation efficiency, a bottom-light initial mass function, and negligible supernova shock dust destruction are required to produce sufficiently high dust masses. (2) A quiescent star-forming phase in which the subsequent parallel decline in dust mass and SFR gives rise to the Mdust-SFR relation, through astration and dust destruction. The dust-to-gas ratio is approximately constant along the relation. We show that the power-law slope of the Mdust-SFR relation is inversely proportional to the global Schmidt-Kennicutt law exponent (i.e., $\sim 0.9$) in simple chemical evolution models. (3) A quenching phase which causes star formation to drop while the dust mass stays roughly constant or drops proportionally. Combined with merging, these processes, as well as the range in total baryonic mass, give rise to a complex population of the diagram which adds significant scatter to the original Mdust-SFR relation. (4) At very high redshifts, a population of galaxies located significantly below the local relation is predicted.

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arXiv2014

Statistical mechanics of collisionless orbits. IV. Distribution of angular momentum

It has been shown in previous work that DARKexp, which is a theoretically derived, maximum entropy, one shape parameter model for isotropic collisionless systems, provides very good fits to simulated and observed dark-matter halos. Specifically, it fits the energy distribution, N(E), and the density profiles, including the central cusp. Here, we extend DARKexp N(E) to include the distribution in angular momentum, L^2, for spherically symmetric systems. First, we argue, based on theoretical, semi-analytical, and simulation results, that while dark-matter halos are relaxed in energy, they are not nearly as relaxed in angular momentum, which precludes using maximum entropy to uniquely derive N(E,L^2). Instead, we require that when integrating N(E,L^2) over squared angular momenta one retrieves the DARKexp N(E). Starting with a general expression for N(E,L^2) we show how the distribution of particles in L^2 is related to the shape of the velocity distribution function, VDF, and velocity anisotropy profile, β(r). We then demonstrate that astrophysically realistic halos, as judged by the VDF shape and β(r), must have linear or convex distributions in L^2, for each separate energy bin. The distribution in energy of the most bound particles must be nearly flat, and become more tilted in favor of radial orbits for less bound particles. These results are consistent with numerical simulations and represent an important step towards deriving the full distribution function for spherically symmetric dark-matter halos.

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arXiv2013

GRB 081007 and GRB 090424: the surrounding medium, outflows and supernovae

We discuss the results of the analysis of multi-wavelength data for the afterglows of GRB 081007 and GRB 090424, two bursts detected by Swift. One of them, GRB 081007, also shows a spectroscopically confirmed supernova, SN 2008hw, which resembles SN 1998bw in its absorption features, while the maximum luminosity is only about half as large as that of SN 1998bw. Bright optical flashes have been detected in both events, which allows us to derive solid constraints on the circumburst-matter density profile. This is particularly interesting in the case of GRB 081007, whose afterglow is found to be propagating into a constant-density medium, yielding yet another example of a GRB clearly associated with a massive star progenitor which did not sculpt the surroundings with its stellar wind. There is no supernova component detected in the afterglow of GRB 090424, likely due to the brightness of the host galaxy, comparable to the Milky Way. We show that the afterglow data are consistent with the presence of both forward- and reverse-shock emission powered by relativistic outflows expanding into the interstellar medium. The absence of optical peaks due to the forward shock strongly suggests that the reverse shock regions should be mildly magnetized. The initial Lorentz factor of outflow of GRB 081007 is estimated to be Γ~ 200, while for GRB 090424 a lower limit of Γ> 170 is derived. We also discuss the prompt emission of GRB 081007, which consists of just a single pulse. We argue that neither the external forward-shock model nor the shock-breakout model can account for the prompt emission data and suggest that the single-pulse-like prompt emission may be due to magnetic energy dissipation of a Poynting-flux dominated outflow or to a dissipative photosphere.

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arXiv2013

SN 2007uy - metamorphosis of an aspheric Type Ib explosion

The supernovae of Type Ibc are rare and the detailed characteristics of these explosions have been studied only for a few events. Unlike Type II SNe, the progenitors of Type Ibc have never been detected in pre-explosion images. So, to understand the nature of their progenitors and the characteristics of the explosions, investigation of proximate events are necessary. Here we present the results of multi-wavelength observations of Type Ib SN 2007uy in the nearby ($\sim$ 29.5 Mpc) galaxy NGC 2770. Analysis of the photometric observations revealed this explosion as an energetic event with peak absolute R band magnitude $-18.5\pm0.16$, which is about one mag brighter than the mean value ($-17.6\pm0.6$) derived for well observed Type Ibc events. The SN is highly extinguished, E(B-V) = 0.63$\pm$0.15 mag, mainly due to foreground material present in the host galaxy. From optical light curve modeling we determine that about 0.3 M$_{\odot}$ radioactive $^{56}$Ni is produced and roughly 4.4 M$_{\odot}$ material is ejected during this explosion with liberated energy $\sim 15\times10^{51}$ erg, indicating the event to be an energetic one. Through optical spectroscopy, we have noticed a clear aspheric evolution of several line forming regions, but no dependency of asymmetry is seen on the distribution of $^{56}$Ni inside the ejecta. The SN shock interaction with the circumburst material is clearly noticeable in radio follow-up, presenting a Synchrotron Self Absorption (SSA) dominated light curve with a contribution of Free Free Absorption (FFA) during the early phases. Assuming a WR star, with wind velocity $\ga 10^3 {\rm km s}^{-1}$, as a progenitor, we derive a lower limit to the mass loss rate inferred from the radio data as $\dot{M} \ga 2.4\times10^{-5}$ M$_{\odot}$, yr$^{-1}$, which is consistent with the results obtained for other Type Ibc SNe bright at radio frequencies.

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arXiv2013

The supernova/gamma-ray burst/jet connection

The observed association between supernovae and gamma-ray bursts represents a cornerstone in our understanding of the nature of gamma-ray bursts. The collapsar model provides a theoretical framework for this connection. A key element is the launch of a bi-polar jet (seen as a gamma-ray burst). The resulting hot cocoon disrupts the star while the 56Ni produced gives rise to radioactive heating of the ejecta, seen as a supernova. In this discussion paper I summarise the observational status of the supernova/gamma-ray burst connection in the context of the 'engine' picture of jet-driven supernovae and highlight SN 2012bz/GRB 120422A -- with its luminous supernova but intermediate high-energy luminosity -- as a possible transition object between low-luminosity and jet gamma-ray bursts. The jet channel for supernova explosions may provide new insight into supernova explosions in general.

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arXiv2013

The Discovery of the Most Distant Known Type Ia Supernova at Redshift 1.914

We present the discovery of a Type Ia supernova (SN) at redshift $z = 1.914$ from the CANDELS multi-cycle treasury program on the \textit{Hubble Space Telescope (HST)}. This SN was discovered in the infrared using the Wide-Field Camera 3, and it is the highest-redshift Type Ia SN yet observed. We classify this object as a SN\,Ia by comparing its light curve and spectrum with those of a large sample of Type Ia and core-collapse supernovae (SNe). Its apparent magnitude is consistent with that expected from the $Λ$CDM concordance cosmology. We discuss the use of spectral evidence for classification of $z > 1.5$ SNe\,Ia using {\it HST} grism simulations, finding that spectral data alone can frequently rule out SNe\,II, but distinguishing between SNe\,Ia and SNe\,Ib/c can require prohibitively long exposures. In such cases, a quantitative analysis of the light curve may be necessary for classification. Our photometric and spectroscopic classification methods can aid the determination of SN rates and cosmological parameters from the full high-redshift CANDELS SN sample.

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arXiv2013

On inferring extinction laws in z~6 quasars as signatures of supernova dust

Unusual extinction curves of high-redshift QSOs have been taken as evidence that dust is primarily produced by supernovae at high redshift. In particular, the 3000 A Todini-Ferrara-Maiolino kink in the extinction curve of the z = 6.20 SDSS J1048+4637 has been attributed to supernova dust. Here we discuss the challenges in inferring robust extinction curves of high-redshift QSOs and critically assess previous claims of detection of supernova dust. In particular, we address the sensitivity to the choice of intrinsic QSO spectrum, the need for a long wavelength baseline, and the drawbacks in fitting theoretical extinction curves. In a sample of 21 QSOs at z ~ 6 we detect significant ultraviolet extinction using existing broad-band optical, near-infrared, and Spitzer photometry. The median extinction curve is consistent with a Small Magellanic Cloud curve with A_1450 ~ 0.7 mag and does not exhibit any conspicuous (restframe) 2175 A or 3000 A features. For two QSOs, SDSS J1044-0125 at z = 5.78 and SDSS J1030+0524 at z = 6.31, we further present X-shooter spectra covering the wavelength range 0.9-2.5 um. The resulting non-parametric extinction curves do not exhibit the 3000 A kink. Finally, in a re-analysis of literature spectra of SDSS J1048+4637, we do not find evidence for a conspicuous kink. We conclude that the existing evidence for a 3000 A feature is weak and that the overall dust properties at high and low redshift show no significant differences. This, however, does not preclude supernovae from dominating the dust budget at high redshift.

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arXiv2012

Helium in natal HII regions: the origin of the X-ray absorption in gamma-ray burst afterglows

Soft X-ray absorption in excess of Galactic is observed in the afterglows of most gamma-ray bursts (GRBs), but the correct solution to its origin has not been arrived at after more than a decade of work, preventing its use as a powerful diagnostic tool. We resolve this long-standing problem and find that He in the GRB's host HII region is responsible for most of the absorption. We show that the X-ray absorbing column density (N_Hx) is correlated with both the neutral gas column density and with the optical afterglow extinction (Av). This correlation explains the connection between dark bursts and bursts with high N_Hx values. From these correlations we exclude an origin of the X-ray absorption which is not related to the host galaxy, i.e. the intergalactic medium or intervening absorbers are not responsible. We find that the correlation with the dust column has a strong redshift evolution, whereas the correlation with the neutral gas does not. From this we conclude that the column density of the X-ray absorption is correlated with the total gas column density in the host galaxy rather than the metal column density, in spite of the fact that X-ray absorption is typically dominated by metals. The strong redshift evolution of N_Hx/Av is thus a reflection of the cosmic metallicity evolution of star-forming galaxies. We conclude that the absorption of X-rays in GRB afterglows is caused by He in the HII region hosting the GRB. While dust is destroyed and metals are stripped of all of their electrons by the GRB to great distances, the abundance of He saturates the He-ionising UV continuum much closer to the GRB, allowing it to remain in the neutral or singly-ionised state. Helium X-ray absorption explains the correlation with total gas, the lack of strong evolution with redshift as well as the absence of dust, metal or hydrogen absorption features in the optical-UV spectra.

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arXiv2012

Constraints on the relative sizes of intervening Mg II-absorbing clouds and quasar emitting regions

Context: A significantly higher incidence of strong (rest equivalent width W_r > 1 Å) intervening Mg II absorption is observed along gamma-ray burst (GRB) sight-lines relative to those of quasar sight-lines. A geometrical explanation for this discrepancy has been suggested: the ratio of the beam size of the source to the characteristic size of a Mg II absorption system can influence the observed Mg II equivalent width, if these two sizes are comparable. Aims: We investigate whether the differing beam sizes of the continuum source and broad-line region of Sloan Digital Sky Survey (SDSS) quasars produce a discrepancy between the incidence of strong Mg II absorbers illuminated by the quasar continuum region and those of absorbers illuminated by both continuum and broad-line region light. Methods: We perform a semi-automated search for strong Mg II absorbers in the SDSS Data Release 7 quasar sample. The resulting strong Mg II absorber catalog is available online. We measure the sight-line number density of strong Mg II absorbers superimposed on and off the quasar C IV 1550 Å and C III] 1909 Å emission lines. Results: We see no difference in the sight-line number density of strong Mg II absorbers superimposed on quasar broad emission lines compared to those superimposed on continuum-dominated spectral regions. This suggests that the Mg II-absorbing clouds typically observed as intervening absorbers in quasar spectra are larger than the beam sizes of both the continuum-emitting regions and broad line-emitting regions in the centers of quasars, corresponding to a lower limit of the order of 10^17} cm for the characteristic size of a Mg II absorbing cloud.

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arXiv2012

The rates and time-delay distribution of multiply imaged supernovae behind lensing clusters

Time delays of gravitationally lensed sources can be used to constrain the mass model of a deflector and determine cosmological parameters. We here present an analysis of the time-delay distribution of multiply imaged sources behind 17 strong lensing galaxy clusters with well-calibrated mass models. We find that for time delays less than 1000 days, at z=3.0, their logarithmic probability distribution functions are well represented by P (log Δt)=5.3 x 10^-4 Δt^βM_250^-2β, with β=0.77, where M_250 is the projected cluster mass inside 250 kpc (in 10^14 M_sun), and βis the power-law slope of the distribution. The resultant probability distribution function enables us to estimate the time-delay distribution in a lensing cluster of known mass. For a cluster with M_250=2 x 10^14 M_sun, the fraction of time delays less than 1000 days is approximately 3%. Taking Abell 1689 as an example, its dark halo and brightest galaxies, with central velocity dispersions larger than 500 km/s, mainly produce large time delays, while galaxy-scale mass clumps are responsible for generating smaller time delays. We estimate the probability of observing multiple images of a supernova in the known images of Abell 1689. A two-component model of estimating the supernova rate is applied in this work. For a magnitude threshold of m_AB=26.5, the yearly rate of Type Ia (core-collapse) supernovae with time delays less than 1000 days is 0.004 +- 0.002 (0.029 +- 0.001). If the magnitude threshold is lowered to m_AB ~ 27.0, the rate of core-collapse supernovae suitable for time delay observation is 0.044 +- 0.015 per year.

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arXiv2012

The Dwarf Starburst Host Galaxy of a Type Ia SN at z = 1.55 from CANDELS

We present VLT/X-shooter observations of a high redshift, type Ia supernova host galaxy, discovered with HST/WFC3 as part of the CANDELS Supernova project. The galaxy exhibits strong emission lines of Lyα, [O II], Hβ, [O III], and Hα at z = 1.54992(+0.00008-0.00004). From the emission-line fluxes and SED fitting of broad-band photometry we rule out AGN activity and characterize the host galaxy as a young, low mass, metal poor, starburst galaxy with low intrinsic extinction and high Lyα escape fraction. The host galaxy stands out in terms of the star formation, stellar mass, and metallicity compared to its lower redshift counterparts, mainly because of its high specific star-formation rate. If valid for a larger sample of high-redshift SN Ia host galaxies, such changes in the host galaxy properties with redshift are of interest because of the potential impact on the use of SN Ia as standard candles in cosmology.

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arXiv2012

Gravitationally Lensed Galaxies at 2<z<3.5: Direct Abundance Measurements of Lya Emitters

Strong gravitational lensing magnifies the flux from distant galaxies, allowing us to detect emission lines that would otherwise fall below the detection threshold for medium-resolution spectroscopy. Here we present the detection of temperature-sensitive oxygen emission lines from three galaxies at 2 2 for which this has been possible. The three galaxies have ~10% solar oxygen abundances in agreement with strong emission line diagnostics. Carbon and nitrogen ratios relative to oxygen are sub-solar as expected for young metal-poor galaxies. Two of the galaxies are Lya emitters with rest-frame equivalent widths of 20 A and 40 A, respectively, and their high magnification factors allow us for the first time to gain insight into the physical characteristics of high-redshift Lya emitters. Using constraints from the physical properties of the galaxies, we accurately reproduce their line profiles with radiative transfer models. The models show a relatively small outflow in agreement with the observed small velocity offsets between nebular emission and interstellar absorption lines.

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arXiv2012

The optically unbiased GRB host (TOUGH) survey. V. VLT/X-shooter emission-line redshifts for Swift GRBs at z~2

We present simultaneous optical and near-infrared (NIR) spectroscopy of 19 Swift GRB host galaxies with VLT/X-shooter with the aim of measuring their redshifts. Galaxies were selected from The Optically Unbiased GRB Host (TOUGH) survey (15 of the 19 galaxies) or because they hosted GRBs without a bright optical afterglow. Here, we provide emission-line redshifts for 13 of the observed galaxies with brightnesses between F606W > 27 mag and R=22.9 mag (median R=24.6 mag). The median redshift is z=2.1 for all, and z=2.3 for the TOUGH hosts. Our new data significantly improve the redshift completeness of the TOUGH survey, which now stands at 77% (53 out of 69 GRBs). They furthermore provide accurate redshifts for nine prototype-dark GRBs (e.g., GRBs 071021 at z=2.452 and 080207 at z=2.086), which are exemplary of GRBs where redshifts are challenging to obtain via afterglow spectroscopy. This establishes X-shooter spectroscopy as an efficient tool for redshift determination of faint, star-forming, high-redshift galaxies such as GRB hosts. It is hence a further step towards removing the bias in GRB samples that is caused by optically-dark events, and provides the basis for a better understanding of the conditions in which GRBs form. The distribution of column densities as measured from X-ray data (N_{H,X}), for example, is closely related to the darkness of the afterglow and skewed towards low N_{H, X} values in samples that are dominated by bursts with bright optical afterglows.

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arXiv2012

The Low Mass End of the Fundamental Relation for Gravitationally Lensed Star Forming Galaxies at 1<z<6

We present VLT/X-shooter spectra of 13 galaxies in the redshift range 1< z < 6, which are strongly lensed by massive galaxy clusters. Spectroscopic redshifts are measured for nine galaxies, while three sources have redshifts determined from continuum breaks in their spectra. The stellar masses of the galaxies span four orders of magnitude between 10^7 and 10^11 M_sun and have luminosities at 1500 A rest-frame between 0.004 and 9 L^* after correcting for the magnification. This allows us to probe a variety of galaxy types from young, low-mass starburst galaxies to massive evolved galaxies. The lensed galaxies with stellar masses less than 10^10 M_sun have a large scatter compared to the fundamental relation between stellar mass, star formation rates and oxygen abundances. We provide a modified fit to the fundamental relation for low-mass, low-metallicity galaxies with a weaker dependence of the metallicity on either the star formation rate or stellar mass compared to low-redshift, high-mass and high-metallicity SDSS galaxies.

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arXiv2012

TOUGH: Observational aspects of gamma-ray burst host galaxies

GRB-selected galaxies are broadly known to be faint, blue, young, star-forming dwarf galaxies. This insight, however, is based in part on heterogeneous samples of optically selected, lower-redshift galaxies. To study the statistical properties of GRB-selected galaxies we here introduce The Optically Unbiased GRB Host (TOUGH) complete sample of 69 X-ray selected Swift GRB host galaxies spanning the redshift range 0.03-6.30 and summarise the first results of a large observational survey of these galaxies.

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arXiv2012

The optically unbiased GRB 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. 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. Seven hosts have detections of the Ly-alpha emission line and we can exclude an early indication that Ly-alpha emission is ubiquitous among GRB hosts, but confirm that Ly-alpha is stronger in GRB-selected galaxies than in flux-limited samples of Lyman break galaxies.

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