A Virtual Observatory Census to Address Dwarfs Origins (AVOCADO)

I. Science goals, sample selection, and analysis tools

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
e-mail: rsanchez@eso.org
² Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía S/N, 18008 Granada, Spain
³ FRACTAL SLNE, C/Tulipán 2, p13 1-A, 28231 Las Rozas, Madrid, Spain
⁴ Centro de Astrofísica and Faculdade de Ciências, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁵ GEPI, Paris-Meudon Observatory, 5 place Jules Janssen, 92190 Meudon, France
⁶ Centro de Astrobiología (INTA-CSIC), Departamento de Astrofísica, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
⁷ Spanish Virtual Observatory, 2869 Madrid, Spain
⁸ Saint Louis University, Madrid Campus, Division of Science and Engineering, Avenida del Valle 34, 28003 Madrid, Spain
⁹ Departamento de Investigación Básica, CIEMAT, Avda. Complutense 40, 28040 Madrid, Spain
¹⁰ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹¹ Departamento de Astrofísica, Universidad de La Laguna, 38071 La Laguna, Tenerife, Spain

Received: 19 June 2012
Accepted: 18 January 2013

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 h₁₀₀^-1 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 use optical spectra and UV-to-NIR imaging of the dwarf sample to derive star formation rates, stellar masses, ages, and metallicities – which are 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 enables us 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 h₁₀₀^-1 Mpc) of ≳ L^∗ companions, consistent with recent results. While star-forming dwarfs are preferentially found at separations of the order of 1 h₁₀₀^-1 Mpc, there appears to be a tail towards low separations (≲ 100 h₁₀₀^-1 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.