Chemical evolution of classical and ultra-faint dwarf spheroidal galaxies

Abstract

We present updated chemical evolution models of two dwarf spheroidal galaxies (Sculptor and Carina) and the first detailed chemical evolution models of two ultra-faint dwarfs (Hercules and Boötes I). Our results suggest that the dwarf spheroidals evolve with a low efficiency of star formation, confirming previous results, and the ultra-faint dwarfs with an even lower one. Under these assumptions, we can reproduce the stellar metallicity distribution function, the [α/Fe] versus [Fe/H] abundance patterns and the total stellar and gas masses observed at the present time in these objects. In particular, for the ultra-faint dwarfs we assume a strong initial burst of star formation, with the mass of the system being already in place at early times. On the other hand, for the classical dwarf spheroidals the agreement with the data is found by assuming the star formation histories suggested by the colour-magnitude diagrams and a longer timescale of formation via gas infall. We find that all these galaxies should experience galactic winds, starting in all cases before 1 Gyr from the beginning of their evolution. From comparison with Galaxy data, we conclude that it is unlikely that the ultra-faint dwarfs have been the building blocks of the whole Galactic halo, although more data are necessary before drawing firm conclusions.

The Impact of Modeling Assumptions in Galactic Chemical Evolution Models

“…In the nearby galaxy sample of Leroy et al (2008), dwarf irregular galaxies have a current specific star formation efficiency of roughly between 10 −11 and 10 −9 yr −1 . Although our models are broadly consistent with those observations (see also Lanfranchi & Matteucci 2004;Vincenzo et al 2014), we cannot use those values as direct constraints, since they represent the current state of gas-rich systems as opposed to the past history of one specific spheroidal galaxy.…”

“…On the other hand, outflows remove metals from galaxies and tend to slow down the growth of their metal content at later time. These ingredients are usually included in the simulation of dwarf galaxies, but with different levels of complexity and different numerical methods, including one-zone models (e.g., Carigi et al 2002;Lanfranchi & Matteucci 2003, 2004Fenner et al 2006;Gibson 2007;Lanfranchi & Matteucci 2010;Lanfranchi et al 2006;Vincenzo et al 2014;Homma et al 2015;Kobayashi et al 2015;Ural et al 2015a), semi-analytical models (e.g., Romano & Starkenburg 2013;Romano et al 2015), and hydrodynamical simulations (e.g., Kawata et al 2006;Marcolini et al 2008;Revaz et al 2009;Pilkington et al 2012;Revaz & Jablonka 2012;Hirai et al 2015;Revaz et al 2016).…”

Are ancient dwarf satellites the building blocks of the Galactic halo?

“…To model the chemical evolution of dSph and UfD galaxies, we refer to the work of Vincenzo et al (2014). In Tables 2 and 3, the main parameters of generic models for 'classical' dSph and UfD galaxies are reported, respectively.…”

“…Weisz, Johnson & Conroy 2014). We point out that, in the modelling of the dSphs and UfDs, we did not consider any threshold in the gas density for SF, as in Vincenzo et al (2014).…”

“…Major issues are the still relatively small number of discovered Galaxy satellites (the so-called missing satellite problem; e.g. Klypin et al 1999; Moore et al 1999;Bullock 2010) and the different chemical abundance patterns in halo and dSph stars (Shetrone, Côté & Sargent 2001;Venn et al 2004;Vincenzo et al 2014).…”