A Virtual Observatory Census to Address Dwarfs Origins
AVOCADO - I. Science goals, sample selection and analysis tools

R. Sánchez-Janssen, R. O. Amorín, M. García-Vargas, J. M. Gomes, M. Huertas-Company, F. Jiménez-Esteban, M. Mollá, P. Papaderos, E. Pérez-Montero, C. Rodrigo, J. Sánchez Almeida, E. Solano

Abstract
Context. Even though they are by far the most abundant of all galaxy types, the detailed properties of dwarf galaxies are still only poorly characterised – especially because of the observational challenge that their intrinsic faintness and weak clustering properties represent.
Aims. AVOCADO aims at establishing firm conclusions on the formation and evolution of dwarf galaxies by constructing and analysing a homogeneous, multiwavelength dataset for a statistically significant sample of approximately 6500 nearby dwarfs (M_i - 5 log h₁₀₀ > -18 mag). The sample is selected to lie within the 20 < D < 60 h^-1₁₀₀ Mpc volume covered by the SDSS-DR7 footprint, and is thus volume-limited for M_i - 5 log h₁₀₀ < -16 mag dwarfs – but includes ≈ 1500 fainter systems. We will investigate the roles of mass and environment in determining the current properties of the different dwarf morphological types – including their structure, their star formation activity, their chemical enrichment history and a breakdown of their stellar, dust and gas content.
Methods. We present the sample selection criteria and describe the suite of analysis tools, some of them developed in the framework of the Virtual Observatory. We make use of optical spectra and UV-to-NIR imaging of the dwarf sample to derive star formation rates, stellar masses, ages and metallicities – which are further supplemented with structural parameters that are used to classify them morphologically. This unique dataset, coupled with a detailed characterisation of each dwarf’s environment, allows for a fully comprehensive investigation of their origins and to track the (potential) evolutionary paths between the different dwarf types.
Results. We characterise the local environment of all dwarfs in our sample, paying special attention to trends with current star formation activity. We find that virtually all quiescent dwarfs are located in the vicinity (projected distances ≲1.5 h^-1₁₀₀ Mpc) of ≳ L^* companions, consistent with recent results. While star-forming dwarfs are preferentially found at separations of the order of 1 h^-1₁₀₀ Mpc, there appears to be a tail towards low separations (≲100 h^-1₁₀₀ kpc) in the distribution of projected distances. We speculate that, modulo projection effects, this probably represents a genuine population of late-type dwarfs caught upon first infall about their host and before environmental quenching has fully operated. In this context, these results suggest that internal mechanisms –such as gas exhaustion via star formation or feedback effects– are not sufficient to completely cease the star formation activity in dwarf galaxies, and that becoming the satellite of a massive central galaxy appears to be a necessary condition to create a quiescent dwarf.

Keywords
galaxies: dwarf – galaxies: evolution – galaxies: star formation – galaxies: structure – galaxies: formation – galaxies: fundamental parameters

Astronomy and Astrophysics
Volume 554, Page A20_1
June 2013

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