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TOPIC: Classification of Galaxies

Blobrana

L

Posts: 131433

Date: May 20 10:55 2015

Galaxy morphology	Permalink

Title: Galaxy morphology - an unsupervised machine learning approach Author: Andrew Schutter, Lior Shamir Structural properties posses valuable information about the formation and evolution of galaxies, and are important for understanding the past, present, and future universe. Here we use unsupervised machine learning methodology to analyse a network of similarities between galaxy morphological types, and automatically deduce a morphological sequence of galaxies. Application of the method to the EFIGI catalogue show that the morphological scheme produced by the algorithm is largely in agreement with the De Vaucouleurs system, demonstrating the ability of computer vision and machine learning methods to automatically profile galaxy morphological sequences. The unsupervised analysis method is based on comprehensive computer vision techniques that compute the visual similarities between the different morphological types. Rather than relying on human cognition, the proposed system deduces the similarities between sets of galaxy images in an automatic manner, and is therefore not limited by the number of galaxies being analysed. The source code of the method is publicly available, and the protocol of the experiment is included in the paper so that the experiment can be replicated, and the method can be used to analyse user-defined datasets of galaxy images. Read more (1813kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Jan 27 07:51 2015

RE: Classification of Galaxies	Permalink

Title: Galaxy Zoo: Evidence for Diverse Star Formation Histories through the Green Valley Author: R. J. Smethurst, C. J. Lintott, B. D. Simmons, K. Schawinski, P. J. Marshall, S. Bamford, L. Fortson, S. Kaviraj, K. L. Masters, T. Melvin, R. C. Nichol, R. A. Skibba, K. W. Willett Does galaxy evolution proceed through the green valley via multiple pathways or as a single population? Motivated by recent results highlighting radically different evolutionary pathways between early- and late-type galaxies, we present results from a simple Bayesian approach to this problem wherein we model the star formation history (SFH) of a galaxy with two parameters, [t, \tau] and compare the predicted and observed optical and near-ultraviolet colours. We use a novel method to investigate the morphological differences between the most probable SFHs for both disc-like and smooth-like populations of galaxies, by using a sample of 126,316 galaxies (0.01 < z < 0.25) with probabilistic estimates of morphology from Galaxy Zoo. We find a clear difference between the quenching timescales preferred by smooth- and disc-like galaxies, with three possible routes through the green valley dominated by smooth- (rapid timescales, attributed to major mergers), intermediate- (intermediate timescales, attributed to minor mergers and galaxy interactions) and disc-like (slow timescales, attributed to secular evolution) galaxies. We hypothesise that morphological changes occur in systems which have undergone quenching with an exponential timescale \tau < 1.5 Gyr, in order for the evolution of galaxies in the green valley to match the ratio of smooth to disc galaxies observed in the red sequence. These rapid timescales are instrumental in the formation of the red sequence at earlier times; however we find that galaxies currently passing through the green valley typically do so at intermediate timescales. Read more (9786kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Jun 18 06:39 2013

Galaxy morphology	Permalink

Title: The Intrinsic Shape of Galaxies in SDSS/Galaxy Zoo Authors: Silvio Rodríguez, Nelson D. Padilla By modelling the axis ratio distribution of SDSS DR8 galaxies we find the intrinsic 3D shapes of spirals and ellipticals. We use morphological information from the Galaxy Zoo project and assume a non-parametric distribution intrinsic of shapes, while taking into account dust extinction. We measure the dust extinction of the full sample of spiral galaxies and find a smaller value than previous estimations, with an edge-on extinction of E_0 = 0.284^{+0.015}_{-0.026} in the SDSS r band. We also find that the distribution of minor to major axis ratio has a mean value of 0.267 ± 0.009, slightly larger than previous estimates mainly due to the lower extinction used; the same affects the circularity of galactic discs, which are found to be less round in shape than in previous studies, with a mean ellipticity of 0.215 ± 0.013. For elliptical galaxies, we find that the minor to major axis ratio, with a mean value of 0.584 ± 0.006, is larger than previous estimations due to the removal of spiral interlopers present in samples with morphological information from photometric profiles. These interlopers are removed when selecting ellipticals using Galaxy Zoo data. We find that the intrinsic shapes of galaxies and their dust extinction vary with absolute magnitude, colour and physical size. We find that bright elliptical galaxies are more spherical than faint ones, a trend that is also present with galaxy size, and that there is no dependence of elliptical galaxy shape with colour. For spiral galaxies we find that the reddest ones have higher dust extinction as expected, due to the fact that this reddening is mainly due to dust. We also find that the thickness of discs increases with luminosity and size, and that brighter, smaller and redder galaxies have less round discs. Read more (374kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Jan 12 02:59 2012

Morphology of galaxies	Permalink

Title: Morphology of galaxies Authors: Yogesh Wadadekar The study of the morphology of galaxies is important in order to understand the formation and evolution of galaxies and their sub-components as a function of luminosity, environment, and star-formation and galaxy assembly over cosmic time. Disentangling the many variables that affect galaxy evolution and morphology, requires large galaxy samples and automated ways to measure morphology. The advent of large digital sky surveys, with unprecedented depth and resolution, coupled with sophisticated quantitative methods for morphology measurement are providing new insights in this fast evolving field of astronomical research. Read more (5042kb, PDF) __________________

Blobrana

L

Posts: 131433

Date: Jan 10 22:44 2012

Classification of Galaxies	Permalink

Title: A Revised Parallel-Sequence Morphological Classification of Galaxies: Structure and Formation of S0 and Spheroidal Galaxies Authors: John Kormendy, Ralf Bender We update van den Bergh's parallel sequence galaxy classification in which S0 galaxies form a sequence S0a-S0b-S0c that parallels the sequence Sa-Sb-Sc of spiral galaxies. The ratio B/T of bulge to total light defines the position of a galaxy in each sequence. Our classification makes one improvement. We extend the S0a-S0b-S0c sequence to spheroidal ("Sph'") galaxies that are positioned in parallel to irregular galaxies in a similarly extended Sa-Sb-Sc-Im sequence. This provides a natural "home" for spheroidals, which previously were omitted from galaxy classifications. To motivate our juxtaposition of Sph and irregular galaxies, we present photometry and bulge-disk decompositions of Virgo S0s, including late-type S0s that bridge the gap between S0b and Sph galaxies. NGC 4762 is a SB0bc with B/T = 0.13. NGC 4452 is a SB0c galaxy with an even tinier pseudobulge. VCC 2048 and NGC 4638 have properties of both S0cs and Sphs. We update the structural parameter correlations Sphs, irregulars, bulges, and disks. We show that spheroidals of increasing luminosity form a continuous sequence with the disks (but not bulges) of S0c-S0b-S0a galaxies. Remarkably, the Sph--S0-disk sequence is almost identical to that of irregular galaxies and spiral galaxy disks. We review published observations for galaxy transformation processes, particularly ram-pressure stripping of cold gas. We suggest that Sph galaxies are transformed, "red and dead" Scd--Im galaxies in the same way that many S0 galaxies are transformed, red and dead Sa-Sc spiral galaxies Read more (4335kb, PDF) __________________

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