A Virtual Observatory Census to Address Dwarfs Origins (AVOCADO)

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 h 100 > −18 mag). The sample is selected to lie within the 20 < D < 60 h −1 100 Mpc volume covered by the SDSS-DR7 footprint, and is thus volume-limited for M i − 5 log h 100 < −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 h −1 100 Mpc) of L * companions, consistent with recent results. While star-forming dwarfs are preferentially found at separations of the order of 1 h −1 100 Mpc, there appears to be a tail towards low separations ( 100 h −1 100 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.

Molecular gas in low-metallicity starburst galaxies:

“…Their results on the CO emission of eight lowmetallicity dwarf galaxies are certainly complementary to the work presented here, and their conclusions on the molecular depletion timescales of these galaxies appear in good agreement with ours. Further insights on the above conclusions would benefit from future studies that use larger samples of star-forming dwarfs, for which large and homogeneous multiwavelength datasets are currently available (e.g., the AVOCADO project, Sánchez-Janssen et al 2013).…”

Colors of Dwarf Ellipticals from GALEX to WISE

“…The mechanisms for halting star formation typical falls into two categories; quenching (Bundy et al 2006) and exhaustion (Sánchez-Janssen et al 2013). For normal ellipticals, who appear to have a majority of their stars in the form of an old, metal-rich population, their α/Fe ratios argues that an exhaustion process dominates.…”

Galaxy pairs in the Sloan Digital Sky Survey – XI. A new method for measuring the influence of the closest companion out to wide separations

“…Various approaches to characterizing environment have been used in the literature, including membership and/or location within structures such as clusters, groups, sheets, filaments and voids (Yang et al 2007;Darvish et al 2014;Eardley et al 2015), central/satellite classification (Kravtsov et al 2004), local number density (Cooper et al 2005;Baldry et al 2006), clustering statistics (Robaina et al 2009;Skibba et al 2013), proximity to the nearest massive galaxy (Geha et al 2012;Sánchez-Janssen et al 2013;Ruiz, Trujillo, & Mármol-Queraltó 2014;Pearson et al 2016), etc. Given the particular goals of this study, and our desire to consistently classify a large number of galaxies within a flux-limited galaxy redshift survey, we elect to use two distinct metrics to describe each galaxy's environment.…”