2008 (IPP)
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Item Towards a robust estimate of the merger rate evolution using near-IR photometry(2008-04-01) Rawat, A.; Kembhavi, A.K.We use a combination of deep, high angular resolution imaging data from the CDFS (HST/ACS GOODS survey) and ground based near-IR Ks images to derive the evolution of the galaxy major merger rate in the redshift range 0.2 ≤ z ≤ 1.2. We select galaxies on the sole basis of their J-band rest-frame, absolute magnitude, which is a good tracer of the stellar mass. We find steep evolution with redshift, with the merger rate ∝ (1 + z)3.43±0.49 for optically selected pairs, and ∝ (1 + z)2.18±0.18 for pairs selected in the near-IR. Our result is unlikely to be affected by luminosity evolution which is relatively modest when using restframe J band selection. The apparently more rapid evolution that we find in the visible is likely caused by biases relating to incompleteness and spatial resolution affecting the ground based near IR photometry, underestimating pair counts at higher redshifts in the near-IR. The major merger rate was ∼5.6 times higher at z ∼ 1.2 than at the current epoch. Overall 41%×(0.5Gyr/τ ) of all galaxies with MJ ≤ −19.5 have undergone a major merger in the last ∼ 8Gyr, where τ is the merger timescale. Interestingly, we find no effect on the derived major merger rate due to the presence of the large scale structure at z = 0.735 in the CDFS.Item Dielectronic recombination and stability of warm gas in active galactic nuclei(2008-09) Chakravorty, Susmita; Kembhavi, A.K.Understanding the thermal equilibrium (stability) curve may offer insights into the nature of the warm absorbers often found in active galactic nuclei. Its shape is determined by factors such as the spectrum of the ionizing continuum and the chemical composition of the gas. We find that the stability curves obtained under the same set of the above-mentioned physical factors, but using recently derived dielectronic recombination rates, give significantly different results, especially in the regions corresponding towarmabsorbers, leading to different physical predictions. Using the current rates we find a larger probability of having a thermally stable warmabsorber at 105 Kthan previous predictions and also a greater possibility for itsmultiphase nature. The results obtained with the current dielectronic recombination rate coefficients are more reliable because the warm absorber models along the stability curve have computed coefficient values, whereas previous calculations relied on guessed averages for these because of a lack of available data.