PhD thesis defense by Tayyaba Zafar – University of Copenhagen

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PhD thesis defense by Tayyaba Zafar

Tayyaba Zafar will defend her PhD thesis Thursday 14 April 2011 at 15:15 in Auditorium A at the Niels Bohr Institute. 


Spectroscopy of high redshift sightlines
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This thesis deals with the absorption studies of two cosmological objects: Gamma-ray bursts (GRBs) and quasars (QSO), using spectroscopy and spectral energy distribution (SED) analysis.

GRBs are the most powerful explosions in the Universe. After the discovery of these cosmological events in 1967, a lot of progress has been made in investigating their properties which divided them into two subcategories of long and short bursts. Both GRB classes have different origins and properties. Long duration GRBs are signposts of star formation due to their association with the deaths of short-lived massive stars. The launch of the Swift satellite in 2004, mainly devoted to GRB observations, has marked a dramatic improvement in our understanding of GRB physics. The initial burst of gamma-rays should be followed by slowly fading emission at low frequencies, which is termed the “afterglow”. GRB afterglows are excellent and sensitive probes of gas and dust in star-forming galaxies at all epochs. The X-ray to optical/near-infrared SED analysis of GRB afterglows can reveal intrinsic host galaxy properties. The brightness of these transient sources and their occurrence in young, blue galaxies make them excellent tools to study star forming regions in the distant Universe.

The first chapter presents an introduction to the history of GRB research, different progenitor models and afterglow phases. It also summarizes the different dust models used for afterglow SED modeling. The chapter also provides an introduction to the Damped Ly-alpha absorbers (DLAs) usually seen in the spectra of background QSOs.

The second and third chapter provides the SED analysis of all known z > 6 GRB afterglows: GRB090423, 080913 and 050904, at z = 8.2, 6.69, and 6.295, respectively. A fresh reduction of the available photometric, spectroscopic and the X-ray data is made and multi-epoch SEDs are generated. The X-ray-NIR SEDs are jointly fitted with single and broken power-laws including dust models. This provides information about the kind of dust and how much dust content is present at these cosmic epochs. For GRB050904, the claimed evidence for unusual (supernova (SN)-origin) dust in its host galaxy is addressed by the revised SED analysis. I find that the SED at all times can be reproduced without dust, and at 1.25 days in particular, significant extinction can be excluded. The data of GRB090423 were well fit with no dust using a broken power-law. There is possible evidence for a low level of extinction in GRB080913. In this analysis I find no GRB at high redshift with AV > 0.1. Comparison with my lower redshift sample where the average extinguishable burst has AV = 0.3, suggests a decrease in dust content in star-forming environments at z > 4.  The third chapter focuses on the SED analysis of a sample of 41 GRBs using spectroscopy. This is the largest ever sample studied to derive absolute extinction curves outside the Local Group. The chapter discusses the spectral shapes of GRBs and their consistency with the synchrotron emission model. In our spectroscopic GRB sample, a spectral break between the X-ray and optical wavelengths is found for 22 cases. More than 95% of this subsample is well described by a spectral change of Δβ =β_2 - β_1 = 0.5 as predicted by the fireball model. In this chapter, I also examine GRB circumburst environments by comparing their gas-to-dust and metals-to-dust ratios, and metallicities to that of Local Group environments. Gas-to-dust ratios of GRBs display a large scatter suggesting that a large fraction of gas is ionized by the GRB. While the metals-to-dust ratios of GRB environments are found always to be at least 10 times higher than the Local Group, suggesting that this may be an intrinsic pre-burst lower limit on the metals-to-dust ratio in these galaxies from the Local Group. GRBs span a range of metallicities from approximately solar to » 1/100 solar, hinting that this remarkable variation in metals-to-dust ratio are not due to any metallicity effects. The chapter also gives an overview of the extinction properties of GRBs, suggesting that SMC-type extinction often fits the observed data. Three of approximately twelve objects where a 2175 Å bump could have been detected had a bump. This fraction of 25%, may represent an approximate lower limit: the spectra with a bump are all more extinguished than any other burst in our sample (AV > 1), suggesting that this type of extinction is in fact common and we may indeed be missing many more highly extinguished events with bumps because of the selection criteria.

Chapter four discusses the interesting case, other than GRBs, of a QSO pair Q0151+048. The complex system brings together several types of objects at z » 2, which are usually seem separately on the sky: a QSO pair, a Ly-α blob and a DLA. In the spectrum of the brighter member of this pair, Q0151+048A, a DLA is observed at a higher redshift than the QSO. We seek to constrain the geometry of the system and understand the possible relations between the DLA, the Ly-α blob and the two QSOs. We measure systemic redshifts of z_em(A) = 1.92924±0.00036 and z_em(B) = 1.92863±0.00042 from the Hβ and Hα emission lines, respectively. In other words, the two QSOs have identical redshifts within 2σ. We employ line-profile fitting techniques, to measure the metallicity of the DLA, and photo-ionization modeling, to characterize the DLA further. We detect residual continuum in the DLA trough which we interpret as emission from the host galaxy of Q0151+048A. No absorption is detected in the spectrum of Q0151+048B at or near the redshift of the DLA which implies that the extent of the DLA must be smaller than about 28 kpc.