Kinematic and metallicity properties of the Aquarius dwarf galaxy from FORS2 MXU spectroscopy

Abstract

Context. Dwarf galaxies found in isolation in the Local Group (LG) are unlikely to have interacted with the large LG spirals, and therefore environmental effects such as tidal and ram-pressure stripping should not be the main drivers of their evolution. Aims. We aim to provide insight into the internal mechanisms shaping LG dwarf galaxies by increasing our knowledge of the internal properties of isolated systems. Here we focus on the evolved stellar component of the Aquarius dwarf galaxy, whose kinematic and metallicity properties have only recently started to be explored. Methods. Spectroscopic data in the region of the near-infrared Ca II triplet lines has been obtained with FORS2 at the Very Large Telescope for 53 red giant branch (RGB) stars. These data are used to derive line-of-sight velocities and [Fe/H] of the individual RGB stars. Results. We have derived a systemic velocity of −142.2 +1.8 −1.8 km s −1 , in agreement with previous determinations from both the HI gas and stars. The internal kinematics of Aquarius appears to be best modelled by a combination of random motions (l.o.s. velocity dispersion of 10.3 +1.6 −1.3 km s −1 ) and linear rotation (with a gradient −5.0 +1.6 −1.9 km s −1 arcmin −1 ) along a P.A. = 139 +17 −27 deg, broadly consistent with the optical projected major axis. This rotation signal is significantly misaligned or even counter-rotating to that derived from the HI gas. We also find the tentative presence of a mild negative metallicity gradient and indications that the metal-rich stars have a colder velocity dispersion than the metalpoor ones. Conclusions. This work represents a significant improvement with respect to previous measurements of the RGB stars of Aquarius, as it doubles the number of member stars already studied in the literature. We speculate that the misaligned rotation between the HI gas and evolved stellar component might have been the result of recent accretion of HI gas, or re-accretion after gas-loss due to internal stellar feedback.

Chemical and stellar properties of star-forming dwarf galaxies

“…Studies of the chemical properties in SFDs are mostly based on optical and near-infrared spectroscopy of bright HII regions [48][49][50][51][52] since, even with the most advanced instrumentation on 8-10 m telescopes, metallicity measurements in individual (giant and super-giant) stars are only feasible at distances 1 Mpc [e.g., [53][54][55][56][57]. Comparison of population synthesis models with multiband photometry and stellar absorption lines measured in the integrated-light spectra of galaxies and star clusters has sometimes been pursued to infer the stellar chemical content of SFDs [58,59], but the uncertainties provided by this approach are typically large (∼0.3-0.5 dex).…”

The baryonic specific angular momentum of disc galaxies

“…We exploit the relation between V c and σ * ,z 0 to estimate the latter. We compile both parameters for a set of galaxies in the literature, ranging from massive spirals to small dwarf irregulars (Bottema 1993;Swaters 1999;van der Marel et al 2002;Hunter et al 2005;Leaman et al 2012;Martinsson et al 2013;Johnson et al 2015;Hermosa Muñoz et al 2020), as shown in Figure 2. A second-order polynomial provides a good fit to the points through the relation:…”

Solo dwarfs IV: comparing and contrasting satellite and isolated dwarf galaxies in the Local Group

“…The HI and stellar components show some offset and differences in spatial structure . Hermosa Muñoz et al (2020) looked at the metallicity and kinematics of stars, finding significant misalignment of the HI and stellar components. They suggested that this may be due to recent HI accretion or reaccretion of gas after feedback from star formation.…”

“…demonstrate the complex structure of this galaxy in terms of the differences in the radial gradients of its different stellar populations, and the apparent disconnect between the spatial properties of the stars and HI distribution. Hermosa Muñoz et al (2020) obtained spectroscopic measurements for 53 RGB stars and determine the axis of rotation for the stellar component to be 139 ). In our analysis, we find that this galaxy is possibly better described with a two component profile, but again it is hard to determine if this represents two "old" components, or whether it reflects the age segregation in the stellar populations noted by Mc-Connachie et al (2006).…”