Browsing by Author "Rawat, A."

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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.
Unravelling the morphologies of Luminous Compact Galaxies using the HST/ACS GOODS survey
(2007-04-17) Rawat, A.; Kembhavi, A.K.; Barway, Sudhanshu; et al.
Context. Luminous Compact Galaxies (LCGs) (MB ≤ −20, R1/2 ≤ 4.5 kpc and EW0(OII) ≥15Å) constitute one of the most rapidly evolving galaxy populations over the last ∼8Gyr history of the universe. Due to their inherently compact sizes, any detailed quantitative analysis of their morphologies has proved to be diﬃcult in the past. Hence, the morphologies and thereby the local counterparts of these enigmatic sources have been hotly debated. Aims. Our aim is to use the high angular resolution, deep, multiband HST/ACS imaging data, from the HST/ACS GOODS survey, to study the quantitative morphology of a complete sample of LCGs in the redshift range 0.5 ≤ z ≤ 1.2. Methods. We have derived structural parameters for a representative sample of 39 LCGs selected from the GOODS-S HST/ACS ﬁeld, using full 2-dimensional surface brightness proﬁle ﬁtting of the galaxy images in each of the four ﬁlters available. B435W − z850LP color maps are constructed for the sample to aid in the morphological classiﬁcation. We then use the rest frame B band bulge ﬂux fraction (B/T) to determine the morphological class of galaxies which are well ﬁt by a ulge+disk two dimensional structure. Mergers were essentially identiﬁed visually by the presence of multiple maxima of comparable intensity in the rest frame B band images, aided by the color maps to distinguish them from HII regions. We also make use of the Spitzer 24 m source catalog of sources in the CDFS to derive the dust enshrouded star formation rates (SFR) for some of the sample LCGs Results. We derive the following morphological mix for our sample of intermediate redshift LCGs: Mergers: ∼36%, Disk dominated: ∼22%, S0: ∼20%, Early types: ∼7%, Irr/tadpole: ∼15%.We establish that our sample LCGs are intermediate mass objects with stellar mass ranging from 9.44 ≤ Log10(M/M⊙) ≤ 10.96, with a median mass of Log10(M/M⊙) = 10.32. We also derive SFR values ranging from a few to ∼ 65 M⊙/year as expected for this class of objects. We ﬁnd that LCGs account for ∼26% of the MB ≤ −20 galaxy population in the redshift range 0.5 ≤ z ≤ 1.2. We estimate a factor ∼11 fall in the comoving number density of blue LCGs from redshifts 0.5 ≤ z ≤ 1.2 to the current epoch, even though this number is subject to large uncertainities given the small sample size at zero redshift available from the literature. Conclusions. The strong redshift evolution exhibited by LCGs, and the fact that a signiﬁcant fraction of LCGs are in merging systems, seem to indicate that LCGs might be an important phase in the hierarchical evolution of galaxies. We envisage that some of the LCGs that are classiﬁed as merging systems, might go on to rebuild their disks and evolve into disk galaxies in the local universe.