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Environmental quantification and Halpha characterisation of the most isolated galaxies in the local Universe

Abstract : The role of the environment on galaxy evolution is still not fully understood. In order to quantify and set limits on the role of nurture one must identify and study a sample of isolated galaxies. The AMIGA project "Analysis of the Interstellar Medium of Isolated Galaxies" is doing a multi-wavelength study of a large sample of isolated galaxies in order to examine their interstellar medium and star formation activity.

We processed 950 galaxies from the Catalogue of Isolated Galaxies (Karachentseva, 1973) and evaluated their isolation using an automated star-galaxy classification procedure (down to M_B = 17.5) on large digitised POSS-I fields surrounding each isolated galaxy. We defined, compared and discussed various criteria to quantify the degree of isolation for these galaxies: e.g. Karachentseva's revised criterion, local surface density computations, estimation of the external tidal force affecting each isolated galaxy. We find galaxies violating Karachentseva's original criterion, and we define various subsamples of galaxies according to their degree of isolation. Additionally, we sought for the redshifts of the primary and companion galaxies to access the radial dimension and have an accurate three dimensional picture of the surroundings. Finally, we applied our pipeline to triplets, compact groups and clusters and interpret the isolated galaxy population in light of these control samples.

The star formation is known to be affected by the local environment of the galaxies, but the star formation rate also highly depends on the intrinsic interstellar medium features. Disentangling these two effects is still a challenging subject. To address this issue, we observed and gathered photometric data (Halpha narrow- & r Gunn broad-band filters) for 200 spiral galaxies from the Catalogue of Isolated Galaxies which are, by definition, in low-density regions. We subsequently studied the Halpha morphological aspect of the 45 biggest and less inclined galaxies. Using Fast Fourier Transform techniques, we focus on the modes of the spiral arms, quantify the strength of the bars, and we give the torques between the newly formed stars and the bulk of the optical matter. We interpret the various bar and Halpha morphologies observed in terms of the secular evolution experienced by galaxies in isolation. The observed frequency of particular patterns bring constraints on the lifetime of bars, and their fading time-scales. Through numerical simulations, trying to fit the Halpha distributions yields constraints on the star formation law, which is likely to differ from a simple Schmidt law.
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Contributor : Simon Verley <>
Submitted on : Monday, December 24, 2007 - 11:56:27 AM
Last modification on : Thursday, December 10, 2020 - 12:37:25 PM
Long-term archiving on: : Tuesday, April 13, 2010 - 3:38:34 PM


  • HAL Id : tel-00201125, version 1


Simon Verley. Environmental quantification and Halpha characterisation of the most isolated galaxies in the local Universe. Astrophysics [astro-ph]. Observatoire de Paris, 2005. English. ⟨tel-00201125⟩



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