2009 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/332
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Item Understanding the redshift evolution of the luminosity functions of Lyman-alpha emitters(2009-06-01) Samui, Saumyadip; Srianand, R.; Subramanian, KandaswamyWe present a semi-analytical model of star formation which explains simultaneously the observed UV luminosity function of high redshift Lyman break galaxies (LBGs) and luminosity functions of Lyman-α emitters. We consider both models that use the Press-Schechter (PS) and Sheth-Tormen (ST) halo mass functions to calculate the abundances of dark matter halos. The Lyman-α luminosity functions at z . 4 are well reproduced with only . 10% of the LBGs emitting Lyman-α lines with rest equivalent width greater than the limiting equivalent width of the narrow band surveys. However, the observed luminosity function at z > 5 can be reproduced only when we assume that nearly all LBGs are Lyman-α emitters. Thus it appears that 4 < z < 5 marks the epoch when a clear change occurs in the physical properties of the high redshift galaxies. As Lyman-α escape depends on dust and gas kinematics of the inter stellar medium (ISM), this could mean that on an average the ISM at z > 5 could be less dusty, more clumpy and having more complex velocity field. All of these will enable easier escape of the Lyman-α photons. At z > 5 the observed Lyman-α luminosity function are well reproduced with the evolution in the halo mass function along with very minor evolution in the physical properties of high redshift galaxies. In particular, upto z = 6.5, we do not see the effect of evolving inter galactic medium (IGM) opacity on the Lyman-α escape from these galaxiesItem Diffuse molecular gas at high redshift: Detection of CO molecules and the 2175 A* dust feature at z=1.64(2009-06-01) Noterdaeme, P.; Ledoux, C.; Srianand, R.We present the detection of carbon monoxide molecules (CO) at z = 1.6408 towards the quasar SDSS J160457.50+220300.5 using the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. CO absorption is detected in at least two components in the first six A-X bands and one d-X(5-0) inter-band system. This is the second detection of this kind along a quasar line of sight. The CO absorption profiles are well modelled assuming a rotational excitation of CO in the range 6 < Tex < 16 K, which is consistent with or higher than the temperature of the Cosmic Microwave Background Radiation at this redshift. We derive a total CO column density of N(CO) = 4 × 1014 cm−2 . The measured column densities of S i, Mg i, Zn ii, Fe ii and Si ii indicate a dust depletion pattern typical of cold gas in the Galactic disc. The background quasar spectrum is significantly reddened (u−K ∼ 4.5 mag) and presents a pronounced 2175 Å dust absorption feature at the redshift of the CO absorber. Using a control sample of ∼500 quasars we find the chance probability for this feature to be spurious is ∼0.3%. We show that the spectral energy distribution (SED) of the quasar is well fitted with a QSO composite spectrum reddened with a Large Magellanic Cloud supershell extinction law at the redshift of the absorber. It is noticeable that this quasar is absent from the colour-selected SDSS quasar sample. This demonstrates our current view of the Universe may be biased against dusty sightlines. These direct observations of carbonaceous molecules and dust open up the possibility of studying physical conditions and chemistry of diffuse molecular gas in high redshift galaxies.Item Models of high redshift luminosity functions and galactic outflows: The dependence on halo mass function(2009-02-01) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.The form of the halo mass function is a basic ingredient in any semi-analytical galaxy formation model. We study the existing forms of the mass functions in the literature and compare their predictions for semi-analytical galaxy formation models. Two methods are used in the literature to compute the net formation rate of halos, one by simply taking the derivative of the halo mass function and the other using the prescription due to Sasaki (1994). For the historically used Press-Schechter (PS) mass function, we compare various model predictions, using these two methods. However, as the Sasaki formalism cannot be easily generalized for other mass functions, we use the derivative while comparing model predictions of di erent mass functions. We show that the reionization history and UV luminosity function of Lyman break galaxies (LBGs) predicted by the PS mass function di ers from those using any other existing mass function, like Sheth-Tormen (ST) mass func- tion. In particular the reionization e ciency of molecular cooled halos has to be substantially reduced when one uses the ST and other mass functions obtained from the simulation instead of the PS mass function. Using 2 - minimization, we nd that the observed UV luminosity functions of LBGs at 3:0 z 7:4 are better reproduced by models using the ST mass function compared to models that use the PS mass function. On the other hand, the volume lling factor of the metals expelled from the galaxies through super- novae driven out ows di ers very little between models with di erent mass functions. It depends on the way we treat merging out ows. We also show that the porosity weighted average quantities related to the out ow are not very sensitive to the di erences in the halo mass function