The extended structure of the dwarf irregular galaxies Sextans A and Sextans B

Abstract

We present a detailed study of the stellar and H  structure of the dwarf irregular galaxies Sextans A and Sextans B, members of the NGC 3109 association. We use newly obtained deep (r 26.5) and wide-field g and r photometry to extend the surface brightness (SB) profiles of the two galaxies down to µ V 31.0 mag/arcsec 2 . We find that both galaxies are significantly more extended than previously traced with surface photometry, out to ∼ 4 kpc from their centres along their major axes. Older stars are found to have more extended distribution than younger populations. We obtain the first estimate of the mean metallicity for the old stars in Sex B, from the colour distribution of the red giant branch, [Fe/H] = −1.6. The SB profiles show significant changes of slope and cannot be fitted with a single Sérsic model. Both galaxies have HI discs as massive as their respective stellar components. In both cases the H  discs display solid-body rotation with maximum amplitude of ∼50 km s −1 (albeit with significant uncertainty due to the poorly constrained inclination), implying a dynamical mass ∼10 9 M , a mass-to-light ratio∼ 25, and a dark-to-baryonic mass ratio of ∼10. The distribution of the stellar components is more extended than the gaseous disc in both galaxies. We find that the main, approximately round, stellar body of Sex A is surrounded by an elongated low-SB stellar halo that can be interpreted as a tidal tail, similar to that found in another member of the same association (Antlia). We discuss these, as well as other evidence of tidal disturbance, in the framework of a past passage of the NGC 3109 association close to the Milky Way, which has been hypothesised by several authors and is also supported by the recently discovered filamentary configuration of the association itself.

Solo dwarfs I: survey introduction and first results for the Sagittarius dwarf irregular galaxy

“…Rather, there is a secondary, much more extended, lower surface brightness and lower luminosity component to the galaxy that surrounds the highly elliptical main body i.e., a stellar halo. Indeed, such a scenario is not particularly unusual; bright galaxies are known to possess multiple stellar components and more recent work on nearby dwarf galaxies suggests that multiple components are ubiquitous even at low stellar mass (e.g., Martínez-Delgado et al 1999;Lee & Kim 2000;Kleyna et al 2003;Tolstoy et al 2004;Kleyna et al 2004;Vansevičius et al 2004;Battaglia et al 2006Battaglia et al , 2008Battaglia et al , 2011McConnachie et al 2007a;Hidalgo et al 2008;Bellazzini et al 2014). We note that the recent analysis of Sag DIG by Beccari et al (2014) trace the profile to approximately 4 arcmin.…”

The StEllar Counterparts of COmpact high velocity clouds (SECCO) survey

“…The average and the standard error of the mean of the independent measures obtained from the different images were adopted as the final values of the instrumental magnitude and of the uncertainty on the relative photometry. It has been demonstrated (Bellazzini et al 2011(Bellazzini et al , 2014) that this procedure works very well on similar datasets aimed at analysising of dwarf galaxies in the same range of distances as we are interested in here.…”

“…As mentioned above, the single 17.3 × 7.7 field covered by the central chip is wide enough to adequately sample the stellar body of typical dwarf galaxies residing in the Local Volume (see Fig. 1, and McConnachie 2012; Bellazzini et al 2011Bellazzini et al , 2014. In this context, it is important to recall that most of the stellar light in gas-rich dwarf galaxies is generally enclosed in a significantly smaller area than is spanned by the neutral gas distribution.…”

“…From our observational experience (Bellazzini et al 2011(Bellazzini et al , 2014, we knew that very deep imaging and photometry for dwarf galaxies in the relevant range of distances (0.25 Mpc ≤ D ≤ 2.0 Mpc) could be obtained, under good seeing conditions, with a minimal observational effort using the LBC cameras (Giallongo et al 2008) mounted on the Large Binocular Telescope (LBT, Mount Graham, AZ) in binocular mode (see below). These observations provide the strongest constraints achievable with ground-based optical observations on the presence or lack of a stellar counterpart in a reasonable amount of observing time.…”

“…To clean the catalogues from spurious and non-stellar sources, we also adopted cuts in the image-quality parameters CHI and SHARP, provided by DAOPHOTII (as done, e.g., in Bellazzini et al 2011Bellazzini et al , 2014. The SHARP (sharpness) parameter provides a classification of detected sources according to the shape of their light distribution with respect to a point source, which should have SHARP = 0.0.…”