arXiv subjects

Johan P. U. Fynbo

Publications and source records attributed to Johan P. U. Fynbo.

At least 91 records · Page 5Linked to original sources

arXiv2011

The extinction curves of star-forming regions from z=0.1 to 6.7 using GRB afterglow spectroscopy

GRB afterglows are well suited to extinction studies due to their brightness, simple power-law spectra and the occurrence of GRBs in distant star forming galaxies. In this paper we present results from the SED analysis of a sample of 41 GRB afterglows, from X-ray to NIR wavelengths. This is the largest sample of extinction curves outside the Local Group and, to date, the only extragalactic sample of absolute extinction curves based on spectroscopy. Visual extinction correlation with HI column density as well as total and gas-phase metal column density are examined. Approximately half the sample require a cooling break between the optical and X-ray regimes. The broken power-law SEDs show an average change in the spectral index of delta_beta=0.51 with a standard deviation of 0.02. This is consistent with the expectation from a simple synchrotron model. Of the sample, 63% are well described by the SMC-type extinction curve and have moderate or low extinction, with AV<0.65. Almost a quarter of our sample is consistent with no significant extinction (typically AV<0.1). The 2175AA extinction bump is detected unequivocally in 7% of our sample (3 GRBs), which all have A_V>1.0. We find an anti-correlation between gas-to-dust ratio and metallicity consistent with the Local Group relation. Our metals-to-dust ratios derived from the soft X-ray absorption are always larger (3-30 times) than the Local Group value, which may mean that GRB hosts may be less efficient at turning their metals into dust. However, we find that gas, dust, and metal column densities are all likely to be influenced by photo-ionization and dust destruction effects from the GRB. [abridged]

astro-ph.CO↗

arXiv2011

A Lyα blob and zabs {\approx} zem damped Lyα absorber in the dark matter halo of the binary quasar Q 0151+048

Q0151+048 is a physical QSO pair at z ~ 1.929 with a separation of 3.3 arcsec on the sky. In the spectrum of Q0151+048A (qA), a DLA is observed at a higher redshift. We have previously detected the host galaxies of both QSOs, as well as a Lya blob. We performed low-resolution spectroscopy with the slit aligned with the extended emission. We also observed the system using the medium-resolution VLT/X-shooter spectrograph and the slit aligned with the two QSOs. We measure systemic redshifts of zem(A)=1.92924{\pm}0.00036 and zem(B)=1.92863{\pm}0.00042 from the Hβ and Hα emission lines, respectively. We estimate the masses of the black holes of the two QSOs to be 10^9.33 M{\odot} and 10^8.38 M{\odot} for qA and qB, respectively. From this we infered the mass of the dark matter halos hosting the two QSOs: 10^13.74 M{\odot} and 10^13.13 M{\odot} for qA and qB, respectively. We observe a velocity gradient along the major axis of the Lya blob consistent with the rotation curve of a large disk galaxy, but it may also be caused by gas inflow or outflow. We detect residual continuum in the DLA trough which we interpret as emission from the host galaxy of qA. The derived H0 column density of the DLA is log NH0 = 20.34 {\pm} 0.02. Metal column densities results in an overall metallicity of 0.01 Z{\odot}. We detect CII* which allows us to make a physical model of the DLA cloud. From the systemic redshifts of the QSOs, we conclude that the Lya blob is associated with qA rather than with the DLA. The DLA must be located in front of both the Lya blob and qA at a distance larger than 30 kpc. The two QSOs accrete at normal eddington ratios. The DM halo of this double quasar will grow to the mass of our local super-cluster at z=0. We point out that those objects therefore form an ideal laboratory to study the physical interactions in a z=2 pre-cursor of our local super-cluster.

astro-ph.CO↗

arXiv2011

Exploring dust extinction at the edge of reionization

The brightness of gamma-ray burst (GRB) afterglows and their occurrence in young, blue galaxies make them excellent probes to study star forming regions in the distant Universe. We here elucidate dust extinction properties in the early Universe through the analysis of the afterglows of all known z > 6 GRBs: GRB 090423, 080913 and 050904, at z = 8.2, 6.69, and 6.295, respectively. We gather all available optical and near-infrared photometry, spectroscopy and X-ray data to construct spectral energy distributions (SEDs) at multiple epochs. We then fit the SEDs at all epochs with a dust-attenuated power-law or broken power-law. We find no evidence for dust extinction in GRB 050904 and GRB 090423, with possible evidence for a low level of extinction in GRB 080913. We compare the high redshift GRBs to a sample of lower redshift GRB extinctions and find a lack of even moderately extinguished events (AV ~ 0.3) above z > 4. In spite of the biased selection and small number statistics, this result hints at a decrease in dust content in star-forming environments at high redshifts.

astro-ph.CO↗

arXiv2011

The nature of z ~ 2.3 Lyman-alpha emitters

We study the multi-wavelength properties of a set of 171 Ly-alpha emitting candidates at redshift z = 2.25 found in the COSMOS field, with the aim of understanding the underlying stellar populations in the galaxies. We especially seek to understand what the dust contents, ages and stellar masses of the galaxies are, and how they relate to similar properties of Ly-alpha emitters at other redshifts. The candidates here are shown to have different properties from those of Ly-alpha emitters found at higher redshift, by fitting the spectral energy distributions (SEDs) using a Monte-Carlo Markov-Chain technique and including nebular emission in the spectra. The stellar masses, and possibly the dust contents, are higher, with stellar masses in the range log M_* = 8.5 - 11.0 M_sun and A_V = 0.0 - 2.5 mag. Young population ages are well constrained, but the ages of older populations are typically unconstrained. In 15% of the galaxies only a single, young population of stars is observed. We show that the Ly-alpha fluxes of the best fit galaxies are correlated with their dust properties, with higher dust extinction in Ly-alpha faint galaxies. Testing for whether results derived from a light-weighted stack of objects correlate to those found when fitting individual objects we see that stellar masses are robust to stacking, but ages and especially dust extinctions are derived incorrectly from stacks. We conclude that the stellar properties of Ly-alpha emitters at z = 2.25 are different from those at higher redshift and that they are diverse. Ly-alpha selection appears to be tracing systematically different galaxies at different redshifts.

astro-ph.CO↗

arXiv2010

GRB 090417B and its Host Galaxy: A Step Towards an Understanding of Optically-Dark Gamma-Ray Bursts

GRB 090417B was an unusually long burst with a T_90 duration of at least 2130 s and a multi-peaked light curve at energies of 15-150 keV. It was optically dark and has been associated with a bright star-forming galaxy at a redshift of 0.345 that is broadly similar to the Milky Way. This is one of the few cases where a host galaxy has been clearly identified for a dark gamma-ray burst and thus an ideal candidate for studying the origin of dark bursts. We find that the dark nature of GRB 090417B cannot be explained by high redshift, incomplete observations, or unusual physics in the production of the afterglow. Assuming the standard relativistic fireball model for the afterglow we find that the optical flux is at least 2.5 mag fainter than predicted by the X-ray flux. The Swift/XRT X -ray data are consistent with the afterglow being obscured by a dense, localized sheet of dust approximately 30-80 pc from the burst along the line of sight. Our results suggest that this dust sheet imparts an extinction of A_V >~ 12 mag, which is sufficient to explain the missing optical flux. GRB 090417B is an example of a gamma-ray burst that is dark due to the localized dust structure in its host galaxy.

astro-ph.CO↗

arXiv2010

No evidence for dust extinction in GRB 050904 at z ~ 6.3

Context: GRB afterglows are excellent probes of gas and dust in star-forming galaxies at all epochs. It has been posited that dust in the early Universe must be different from dust at lower z. To date two reports directly support this contention, one of which is based on the spectral shape of GRB 050904 at z = 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. We address the claimed evidence for unusual (SN-origin) dust in its host galaxy by simultaneously examining the X-ray and optical/NIR spectrophotometric data. Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4 days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunn photometric data, the spectroscopic and z'-band photometric data at ~3 days from the Subaru telescope, as well as the critical UKIRT Z-band photometry at 0.47 days, upon which the claim of dust detection largely relies. Results: We find no evidence of dust extinction in the SED. We compute flux densities at lambda_rest = 1250 AA directly from the observed counts at all epochs. In the earliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3 +- 0.2 mag) than previously reported and statistically insignificant (<1.5 sigma). Furthermore we find that the photometry of this band is unstable and difficult to calibrate. Conclusions: From the afterglow SED we demonstrate that there is no evidence for dust extinction -- the SED at all times can be reproduced without dust, and at 1.25 days in particular, significant extinction can be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-type extinction curve. We conclude that there is no evidence of any extinction in the afterglow of GRB 050904 and that the presence of SN-origin dust in the host of GRB 050904 must be viewed skeptically. [abridged]

astro-ph.CO↗

arXiv2009

The nature of HI absorbers in GRB afterglows: clues from hydrodynamic simulations

In recent work, we have shown that it is possible to link quantitatively many aspects of damped Lyman alpha (DLA) absorbers in the spectra of quasars to high resolution simulations of galaxy formation. Using runs from the same series of hydrodynamic numerical studies, we consider the expected properties of Lyman alpha absorbers seen in the spectra of high redshift (z>2) gamma ray burst afterglows (GRB-DLAs). If GRBs are associated with the death of massive stars, their afterglows provide insights into otherwise unprobed regions of protogalactic objects, but detailed physical interpretations are currently embryonic. We find that median impact parameters (measured from the potential minimum) are approximately 1 kpc for GRBs compared with 4 kpc for QSO-DLAs. However, an equally important difference is that GRB-DLAs are predominantly associated with halos of mass 10^10 10^19 cm^-2, but they somewhat underpredict the incidence of low column densities N_HI<10^19 cm^-2. Line-of-sight neutral gas metallicities predicted by our simulations (10^-2 < Z/Z_sol < 1) are consistent with the modest observational constraints. Because of large internal dispersions in gas metallicities, this agreement is not significantly compromised by imposing a cut-off on the metallicity of stars able to launch GRBs (Z_star<Z_sol/3), confounding claims that the observed metallicity of GRB-DLAs poses a challenge to current GRB models. (Abridged.)

astro-ph.CO↗

arXiv2009

On the dependence between UV luminosity and Lyman-alpha equivalent width in high redshift galaxies

We show that with the simple assumption of no correlation between the Ly-alpha equivalent width and the UV luminosity of a galaxy, the observed distribution of high redshift galaxies in an equivalent width - absolute UV magnitude plane can be reproduced. We further show that there is no dependence between Ly-alpha equivalent width and Ly-alpha luminosity in a sample of Ly-alpha emitters. The test was expanded to Lyman-break galaxies and again no dependence was found. Simultaneously, we show that a recently proposed lack of large equivalent width, UV bright galaxies (Ando et al. 2006) can be explained by a simple observational effect, based on too small survey volumes.

astro-ph.CO↗

arXiv2009

Evolution in the properties of Lyman-alpha emitters from redshifts z ~ 3 to z ~ 2

Context: Narrow-band surveys for Ly-alpha emitters (LAEs) is a powerful tool in detecting high, and very high, redshift galaxies. Even though samples are growing at redshifts z = 3 - 6, the nature of these galaxies is still poorly known. Aims: To study the properties of z = 2.25 LAEs and compare those with the properties of z > 3 LAEs. Methods: We present narrow-band imaging made with the MPG/ESO 2.2m telescope with the WFI detector. We have made a selection for emission-line objects and find 170 candidate typical LAEs and 17 candidates which we regard as high UV-transmission LAEs. We have derived the magnitudes of these objects in 8 bands from u* to Ks, and studied if they have X-ray and/or radio counterparts. Results: We show that there has been significant evolution in the properties of LAEs 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 Ly-alpha emission compared to that derived from the UV emission are lower by on average a factor of ~ 1.8, indicating a large absorption of dust. Conclusion: LAEs at redshift z = 2.25 may be more evolved than LAEs at higher redshift. The red SEDs imply more massive, older and/or more dusty 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 LAE population. [Abridged]

astro-ph↗

arXiv2009

The properties of the host galaxy and the immediate environment of GRB 980425 / SN 1998bw from the multi-wavelength spectral energy distribution

We present an analysis of the spectral energy distribution (SED) of the galaxy ESO 184-G82, the host of the closest known long gamma-ray burst (GRB) 980425 and its associated supernova (SN) 1998bw. We use our observations obtained at the Australia Telescope Compact Array (the third >3 sigma radio detection of a GRB host) as well as archival infrared and ultraviolet (UV) observations to estimate its star formation state. We find that ESO 184-G82 has a UV star formation rate (SFR) and stellar mass consistent with the population of cosmological GRB hosts and of local dwarf galaxies. However, it has a higher specific SFR (per unit stellar mass) than luminous spiral galaxies. The mass of ESO 184-G82 is dominated by an older stellar population in contrast to the majority of GRB hosts. The Wolf-Rayet region ~800 pc from the SN site experienced a starburst episode during which the majority of its stellar population was built up. Unlike that of the entire galaxy, its SED is similar to those of cosmological submillimeter/radio-bright GRB hosts with hot dust content. These findings add to the picture that in general, the environments of GRBs on 1-3 kpc scales are associated with high specific SFR and hot dust.

astro-ph↗

arXiv2008

NGC 2770 - a supernova Ib factory?

NGC 2770 has been the host of three supernovae of Type Ib during the last 10 years, SN 1999eh, SN 2007uy and SN 2008D. SN 2008D attracted special attention due to the serendipitous discovery of an associated X-ray transient. In this paper, we study the properties of NGC 2770 and specifically the three SN sites to investigate whether this galaxy is in any way peculiar to cause a high frequency of SNe Ib. We model the global SED of the galaxy from broadband data and derive a star-formation and SN rate comparable to the values of the Milky Way. We further study the galaxy using longslit spectroscopy covering the major axis and the three SN sites. From the spectroscopic study we find subsolar metallicities for the SN sites, a high extinction and a moderate star-formation rate. In a high resolution spectrum, we also detect diffuse interstellar bands in the line-of-sight towards SN 2008. A comparison of NGC 2770 to the global properties of a galaxy sample with high SN occurance (at least 3 SN in the last 100 years) suggests that NGC 2770 is not particularly destined to produce such an enhancement of observed SNe observed. Its properties are also very different from gamma-ray burst host galaxies. Statistical considerations on SN Ib detection rates give a probability of ~1.5% to find a galaxy with three Ib SNe detected in 10 years. The high number of rare Ib SNe in this galaxy is therefore likely to be a coincidence rather than special properties of the galaxy itself. NGC 2770 has a small irregular companion, NGC 2770B, which is highly starforming, has a very low mass and one of the lowest metallicities detected in the nearby universe as derived from longslit spectroscopy. In the most metal poor part, we even detect Wolf-Rayet features, against the current models of WR stars which require high metallicities.

astro-ph↗

arXiv2008

The host of GRB 060206: kinematics of a distant galaxy

We present early (starting 1.6h after the burst) WHT/ISIS optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 detecting a range of metal absorption lines and their fine-structure transitions. Additional information is provided by the afterglow lightcurve. The resolution and wavelength range of the spectra and the bright afterglow facilitate a detailed study and fitting of the absorption line systems in order to derive column densities. We also use deep imaging to detect the host galaxy and probe the nature of an intervening system at z = 1.48 seen in absorption in the afterglow spectra. We detect four discrete velocity systems in the resonant metal absorption lines, best explained by shells within and/or around the host created by starburst winds. The fine-structure lines have no less than three components with strengths decreasing from the redmost components. We therefore suggest that the fine-structure lines are best explained to be produced by UV pumping from which follows that the redmost component is the one closest to the burst where also NV was detected. The host is detected in deep HST imaging with F814W_AB = 27.48 +- 0.19 mag and a 3 sigma upper limit of H = 20.6 mag (Vega) is achieved. A candidate counterpart for the intervening absorption system is detected as well, which is quite exceptional for an absorber in the sightline towards a GRB afterglow. The intervening system shows no temporal evolution as claimed by Hao et al. (2007) which we prove from our WHT spectra taken before and Subaru spectra taken during the observations from Hao et al.

astro-ph↗

arXiv2008

Reconciling the Metallicity Distributions of Gamma-ray Burst, Damped Lyman-alpha, and Lyman-break Galaxies at z=3

We test the hypothesis that the host galaxies of long-duration gamma-ray bursts (GRBs) as well as quasar-selected damped Lyman-alpha (DLA) systems are drawn from the population of UV-selected star-forming, high-z galaxies (generally referred to as Lyman-break galaxies). Specifically, we compare the metallicity distributions of the GRB and DLA populations to simple models where these galaxies are drawn randomly from the distribution of star-forming galaxies according to their star-formation rate and HI cross-section respectively. We find that it is possible to match both observational distributions assuming very simple and constrained relations between luminosity, metallicity and HI sizes. The simple model can be tested by observing the luminosity distribution of GRB host galaxies and by measuring the luminosity and impact parameters of DLA selected galaxies as a function of metallicity. Our results support the expectation that GRB and DLA samples, in contrast to magnitude limited surveys, provide an almost complete census of z=3 star-forming galaxies that are not heavily dust-obscured.

astro-ph↗

arXiv2008

Unraveling the dynamics and kinematics of GRB hosts with high resolution spectroscopy

In the last years the research on GRB host galaxies has proceeded to more detailed studies both using high resolution afterglow spectroscopy and spatially resolved spectra of nearby hosts. High resolution spectra give a detailed picture of the kinematic properties of matter along the line-of-sight in the host. Two afterglow spectra show clear indications for outflows from their host, namely GRB 030329 and GRB 060206, derived from the position of absorption and host emission lines, nonvariability of Mg I and the radiation field calculated from fine-structure line detections. In nearby GRB hosts it is possible to resolve the actual GRB site. GRB 060505, a SN-less GRB, originated in a relatively metal poor, star forming region with similar properties as other long-duration hosts. A similar conclusion is reached for the site of GRB 980425/SN 1998bw.

astro-ph↗

arXiv2007

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 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~212km/s corresponding to a mass of 1.14x10^11 M_solar 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 Halpha line measurements, of around 6 Myr which corresponds to the lifetime of a 32 M_solar star. The metallicity derived from several emission line measurements is lowest at the GRB site with 1/5 Z_solar but roughly solar in the rest of the galaxy. Using the 2dF galaxy redshift survey we can locate the host galaxy in its large scale (Mpc) environment. The galaxy lies in the foreground of a filamentary overdensity extending south west 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.

astro-ph↗

arXiv2007

Where are the Cosmic Metals at z~3?

The temperature distribution of the cosmic gas-phase oxygen at z~3 is determined by combining high resolution cosmological simulations with the observed, (rest-frame) V-band galaxy LF. The simulations invoke three different stellar IMFs, a Kroupa (K98), a Salpeter (S) and an Arimoto-Yoshii (AY), spanning a range of a factor of five in chemical yield and specific SNII energy feedback. Oxygen is found in all T phases (cold: logT<~4, warm: logT~4.5, hot: logT>~5), in particular for the (top-heavy) AY IMF. For the K98 and S IMFs, the most important phase is the cold one. Moreover, the cold phase alone contains 1-3 times the mass of oxygen in galactic stars for the three IMFs. The implications of this in relation to DLA studies are discussed. In relation to ``missing metals'' it is found that a significant fraction of the oxygen is located in a warm/hot phase that may be very difficult to detect. Moreover, it is found that less than about 20-25% of the cosmic oxygen is associated with galaxies brighter than M_V~-22, the faintest LBGs probed by current metallicity determinations. From the LBG based, lambda~1500 AA luminosity density history at z>3, the mean oxygen density at z=3 is inferred. Comparing this to the models, it is found that the (solar neighbourhood type) K98 IMF is strongly excluded (too small yield), the S is marginally excluded, and the AY matches the constraint well. The optimal IMF has a yield between the S and AY. A similar conclusion is reached on the basis of the C IV abundance of the moderate density IGM.

astro-ph↗

arXiv2007

The spatially resolved host of GRB 060505 and implications for the nature of the progenitor

We present a study of the host galaxy of the Gamma-Ray Burst (GRB) of May 5 2006 (GRB 060505). The host is spatially resolved in both imaging data and in a long slit spectrum including the GRB site. We find the galaxy to be a Sbc spiral, which is unusual for a long GRB host galaxy. The site of the GRB is considerably different from the rest of the galaxy with intense star formation, low metallicity and a young age. This suggest a massive stellar progenitor rather than a merger of compact objects which has been suggested based on the the relatively short duration of T_90=4s for the prompt emission.

astro-ph↗

arXiv2007

No supernovae associated with two long-duration gamma ray bursts

It is now accepted that long duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The standard "collapsar" model predicts that a broad-lined and luminous Type Ic core-collapse supernova (SN) accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we present observations of two nearby long-duration GRBs that challenge this simple view. In the GRBs 060505 and 060614 we demonstrate that no SN emission accompanied these long-duration bursts, down to limits hundreds of times fainter than the archetypal SN 1998bw that accompanied GRB 980425, and fainter than any Type Ic SN ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration and show that the bursts originated in star-forming regions. The absence of a SN to such deep limits is qualitatively different from all previous nearby long GRBs and suggests a new phenomenological type of massive stellar death. From the supplementary material: Now we have observed SN-less GRBs in star-forming regions, suggesting that a non-detection of a SN does not preclude a massive progenitor. The position of the GRB, i.e. in a star-forming region or in an older component, may be the only way to discriminate between merging compact objects and massive stars as progenitors. In fact, several host galaxies for short GRBs have been found to be as actively star-forming as some host galaxies of long-duration GRBs. The GRB labels "long" and "short" have become synonymous with "massive stars" and "other progenitors". These distinctions may need to be relaxed.

astro-ph↗