Stellar chemo-kinematics of isolated Local Group dwarf galaxies.

Abstract

The study of dwarf galaxies is of great importance to understand galaxy formation and evolution on the smallest mass scales. According to the current cosmological paradigm, galaxies grows hierarchically from the accretion of smaller systems. The dwarf galaxies that we observe today are therefore a living record of the building blocks that drove the formation of larger galactic systems. Being the first galaxies to be formed, they may provide information about the nature of dark matter, the formation of first stars and the era of reionization.

In the Local Group, our galactic neighbourhood, dwarf galaxies can be studied in great details. Apparently simple, these systems have actually proven to cover a variety of properties, be them kinematic, chemical, and structural. They also span a wide range in stellar masses, inhabit different environments, and show complex star formation histories.

There are currently almost a hundred dwarf galaxies known to be part of the Local Group, more than half of which have been discovered in the last two decades only. The majority of these systems are gas-poor spheroidals usually found as satellites of the largest spirals, namely the Milky Way and M31, while the rest are gas-richer systems, typically found in the field. Due to their small masses, the formation and evolution of dwarf galaxies could be strongly influenced by internal and environmental effects. In particular, the evolution of satellite dwarfs have been probably influenced by their host galaxy. The isolated systems then, having spent much of their time in a more benign environment, offer a valuable opportunity to better understand what are the intrinsic properties of dwarf galaxies.

In this thesis we have studied a sample of Local Group dwarf galaxies found in isolation. From the analysis of sizeable samples of spectroscopic data for individual stars in each of these systems, we have obtained accurate line-of-sight velocity and metallicity measurements from their individual stars distributed over a wide spatial area. We were able to determine the galaxies chemo-kinematic properties, along with their internal mass content. The obtained results enabled us to perform a comparison with the other dwarf galaxies that populate the Local Group, drawing general conclusions on the evolutionary role played by the environment and their internal mechanisms.

In the first part of this work, we present results from the analysis of Cetus and Tucana, two of the three dwarf spheroidals of the Local Group found in isolation. These systems are of great interest because they are directly comparable to their satellite counterparts. We analysed spectroscopic data of individual target stars mainly collected with the FORS2 instrument on the Very Large Telescope. Results from the kinematic analysis showed that their internal motion is dominated by velocity dispersion, with no significant signs of rotation. Making use of extensive mock-tests, we also showed that an eventual rotation signal inside the half-light radius would be weak and not capable of producing the observed ellipticity of these systems.

In the case of Tucana, previous spectroscopic works reported results substantially different from ours. We re-reduced and homogeneously analysed these archival data, reporting consistent results among the datasets. In particular, the velocity dispersion value resulted much smaller than what was previously found, implying a lower dark matter content and a less centrally-concentrated dark-matter-halo, now in line with similarly luminous systems.

The internal kinematics of Cetus and Tucana therefore resembles that of the Milky Way satellites, challenging those models wherein the evolution of dwarf spheroidals has been strongly influenced by the repeated interaction with a larger galaxy. The chemical properties resulted normal for these systems. Cetus showed a radial metallicity gradient, like other dwarf galaxies of similar luminosity but inhabiting very different environments.

The second part of the text is devoted to the analysis of the gas-rich isolated systems IC~1613 and Aquarius, finally completed by a comparison between the observed kinematic, chemical and mass properties of the dwarf galaxies of the Local Group.

The study of IC~1613 focused on the analysis of a large spectroscopic dataset obtained with the integral-field-unit MUSE instrument, also on the Very Large Telescope. We performed the spectral classification and determination of radial velocities for more than 800 stars. At the same time we obtained metallicities for a selected subsample of red giant branch stars. The kinematic analysis confirmed for the first time the presence of a strong stellar rotation signal with a high statistical significance. Stars appeared to rotate in the same direction, and with a similar intensity, as the neutral gas. From the chemical analysis, we find no signs of a radial metallicity gradient, as observed in other systems with a similar luminosity.

On the other hand, the study of Aquarius also showed the presence of a significant internal rotation, which resulted however peculiarly misaligned with respect to that of the neutral gas, being the first time that such effect is observed in the Local Group for a dwarf galaxy of this luminosity.

In the comparative analysis, we showed that the dynamical masses obtained for our sample add to those of the other Local Group systems that reside in dark-matter-halos less massive than predicted by dark-matter-only simulations. This issue, also known as the too-big-to-fail problem, therefore persists regardless of the environment in which dwarf galaxies reside. We also performed a homogeneous analysis of the dwarf systems in the Local Group aimed at finding spatial variations in their metallicity measurements. Several dwarfs showed indeed mild metallicity gradients, with a large scatter of values among the fainter systems. The brightest dwarfs instead had values often consistent with zero. In general, we found that the environment seems to play a minor role in the formation of such gradients, while past mergers are the probable cause of the steepest slopes.