Morphological study of 0.1 ≤ z ≤ 1.3 galaxies : comparison to artificially redshifted local galaxies
Lesonen, Juho-Petteri (2017-11-02)
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In this thesis, the goal is to disentangle the real morphological evolution of galaxies from the observational effects, caused by redshift. This was done by taking a sample of ~1300 high-redshift (0.1 ≤ z ≤ 1.3) galaxies, observed with HST (Hubble Space telescope), from the GOODS-south field and then dividing them into six redshift bins of ∆z ≈ 0.2 and further into absolute magnitude bins of ∆M = 0.5. The sample was then matched to low-redshift (z ≤ 0.025) galaxies from the SDSS with the same comoving number density per ∆M-bin to create a sample of galaxies of the possible descendants of the high-redshift galaxies. Spectroscopic redshifts of the high-redshift sample were fetched from data provided by the 3DHST project. The low-redshift sample was then spatially scaled, dimmed and k-corrected to simulate redshift effects caused by the accelerating expansion of space. To appear as observed with the HST, the images were convolved with a new point-spread function (PSF), created by deconvolving the HST PSF with the SDSS PSF. This results in a change of the PSF 2D-distribution and a resolution change from 0.396”/pix (SDSS) to 0.03”/pix (HST). To achieve this the FERENGI and the KCORRECT codes were used. All three (low-redshift, high-redshift, redshifted) samples were then classified both visually and with CAS (concentration, asymmetry, smoothness) parameters. The results were compared between these samples. The redshifting process did not produce clumpy structures in the low-redshift galaxies as seen in real high-redshift galaxies. Visual classification becomes increasingly difficult for z ≥ 0.55. The concentration index hardly changes with redshift. The asymmetry of the high-redshift galaxies seem to be genuinely higher than that of the low-redshift galaxies. The smoothness parameter values increase with redshift, implying an increasing clumpiness, but some of the change could be due to the redshift effects. For the high-redshift sample, all the CAS parameters between redshift bins show very little evolution for a given morphological type and could be used to differentiate between them. We also note that we found a significant amount of “clumpy” galaxy types, already in the nearest redshift bin of 0.1 ≤ z ≤ 0.3, as opposed to previous studies.
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