THE PAndAS VIEW OF THE ANDROMEDA SATELLITE SYSTEM. II. DETAILED PROPERTIES OF 23 M31 DWARF SPHEROIDAL GALAXIES

Abstract

We present a comprehensive analysis of the structural properties and luminosities of the 23 dwarf spheroidal galaxies that fall within the footprint of the Pan-Andromeda Archaeological Survey (PAndAS). These dwarf galaxies represent the large majority of Andromeda's known satellite dwarf galaxies and cover a wide range in luminosity (−11.6 ∼ < M V ∼ < −5.8 or 10 4.2 ∼ < L ∼ < 10 6.5 L ⊙ ) and surface brightness (25.1 ∼ < µ 0 ∼ < 29.3 mag/arcsec 2 ). We confirm most previous measurements, but find And XIX to be significantly larger than before (r h = 3065 +1065 −935 pc, M V = −10.1 +0.8 −0.4 ) and cannot derive parameters for And XXVII as it is likely not a bound stellar system. We also significantly revise downward the luminosities of And XV and And XVI, which are now M V ∼ −7.5 or L ∼ 10 5 L ⊙ . Finally, we provide the first detailed analysis of Cas II/And XXX, a fairly faint system (M V = −8.03 ) of typical size (r h = 270 ± 50 pc), located in close proximity to the two bright elliptical dwarf galaxies NGC 147 & 185. Combined with the set of homogeneous distances published in an earlier contribution, our analysis dutifully tracks all relevant sources of uncertainty in the determination of the properties of the dwarf galaxies from the PAndAS photometric catalogue. We further publish the posterior probability distribution functions of all the parameters we fit for in the form of MCMC chains available online; these inputs should be used in any analysis that aims to remain truthful to the data and properly account for covariance between parameters.

A Rogues’ Gallery of Andromeda's Dwarf Galaxies. I. A Predominance of Red Horizontal Branches

“…This is even more evident in the CMDs of Figure 3 that are zoomed-in on the HB region. Even a very faint system like And XIII (M V =− 6.5 +0.7 −0.5 ; Martin et al 2016) has a clear population of RHB stars, which is unlike MW satellites of the same luminosity (e.g., Hercules, M V ∼−6.6) that have exclusively blue HBs (BHBs; Brown et al 2014). Only And XVII and XI show predominantly blue HB and And XIV hosts a more balanced HB.…”

“…The six ISLAndS systems were quenched more than 5 Gyr ago (i.e., they contain few or no stars younger than 5 Gyr), at odds with MW systems like Carina, Fornax, or Leo I despite being of similar luminosities and host distances (Skillman et al 2017). Moreover, whereas MW satellites below ∼10 5.5 L e (M V ∼ −9.0) tend to harbor only old stars (e.g., Draco, Hercules; Weisz et al 2014a), the faintest system in the ISLAndS program, And XVI (10 4.8±0.1 L e ; Martin et al 2016) has an SFH that extends for ∼8 Gyr, which challenges the notion that low-mass galaxies are ubiquitously quenched by reionization. Shallower CMDs of other faint M31 companions are also suggestive of similarly extended SFHs (e.g., Weisz et al 2014a), reinforcing the conclusions of first generation photometric studies of the M31 satellites that noted the presence of unusually red horizontal branch (HB) populations (Da Costa et al 1996, 2000, 2002, indicative of extended episodes of star formation in the early ages of the galaxies' evolution.…”

“…In order to minimize contamination from foreground stars, we restrict each CMD to the region within 2r h of each dwarf galaxy, with r h values taken from Martin et al (2013c), Martin et al (2013b), Slater et al (2015), Martin et al (2016), and Rhode et al (2017). The CMDs are ordered by decreasing galaxy luminosity.…”

“…In parallel, we observed a single pointing with the Wide Field Camera 3 (WFC3) in F606W and F814W. In half of the cases, the ∼6′ angular separation between ACS and WFC3 meant the WFC3 field only sampled the very outer region of the target galaxy, where the expected stellar densities from ground-based imaging were known to be vanishingly low (Martin et al 2016). …”

“…While dedicated imaging surveys such as the Pan-Andromeda Archaeological Survey (PAndAS; McConnachie et al 2009) have proven invaluable for discovery and structural characterization of M31 satellites (e.g., Martin et al 2016), the resulting colormagnitude diagrams (CMDs) are generally limited to the luminous, evolved phases of evolution (e.g., upper red giant branch (RGB) stars), which, because of the age-metallicity degeneracy, provide only coarse information about star formation histories (SFHs) over cosmologically relevant timescales (e.g., Aparicio & Hidalgo 2009;Weisz et al 2011;Dolphin 2012).…”