Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy

ABSTRACT We report evidence from APOGEE for the presence of a new metal-poor stellar structure located within ∼4 kpc of the Galactic Centre. Characterized by a chemical composition resembling those of low-mass satellites of the Milky Way, this new inner Galaxy structure (IGS) seems to be chemically and dynamically detached from more metal-rich populations in the inner Galaxy. We conjecture that this structure is associated with an accretion event that likely occurred in the early life of the Milky Way. Comparing the mean elemental abundances of this structure with predictions from cosmological numerical simulations, we estimate that the progenitor system had a stellar mass of ∼5 × 108 M⊙, or approximately twice the mass of the recently discovered Gaia-Enceladus/Sausage system. We find that the accreted:in situ ratio within our metal-poor ([Fe/H] < –0.8) bulge sample is somewhere between 1:3 and 1:2, confirming predictions of cosmological numerical simulations by various groups.

Candidate fossil groups in the CFHTLS: a probabilistic approach

Context. Fossil groups (FGs) have been discovered 25 years ago, and are now defined as galaxy groups with an X-ray luminosity higher than $ 10^{42}\,h_{50}^{-2} $ erg s−1 and a brightest group galaxy brighter than the other group members by at least two magnitudes. However, the scenario of their formation remains controversial. Aims. We propose here a probabilistic analysis of FGs, extracted from the large catalog of candidate groups and clusters previously detected in the CFHTLS survey based on photometric redshifts to investigate their position in the cosmic web and probe their environment. Methods. Based on spectroscopic and photometric redshifts, we estimated the probability of galaxies to belong to a galaxy structure, and by imposing the condition that the brightest group galaxy is at least brighter than the others by two magnitudes, we computed the probability for a given galaxy structure to be a FG. We analyzed the mass distribution of these candidate FGs, and estimated their distance to the filaments and nodes of the cosmic web in which they are embedded. Results. We find that structures with masses lower than 2.4 × 1014 M⊙ have the highest probabilities of being fossil groups (PFG). Overall, structures with PFG ≥ 50% are located close to the cosmic web filaments (87% are located closer than 1 Mpc to their nearest filament). They are preferentially four times more distant from their nearest node than from their nearest filament. Conclusions. We confirm that FGs have low masses and are rare. They seem to reside closely to cosmic filaments and do not survive in nodes. Being in a poor environment might therefore be the driver of FG formation because the number of nearby galaxies is not sufficient to compensate for the cannibalism of the central group galaxy.

Connecting galaxy structure and star formation: the role of environment in formation of S0 galaxies

In this work, we investigate the reason behind the increased occurrence of S0 galaxies in high-density environments. Our sample comprises of ∼2500 spiral and ∼2000 S0 galaxies spanning a wide range of environments. Dividing the galaxies into categories of classical and pseudo-bulge hosting spiral and S0 galaxies, we have studied their properties as a function of the environment. We find that the fraction of pseudo-bulge hosting disc galaxies decreases with increase in density. The classical bulge hosting spirals and S0 galaxies follow a similar trend in less dense environments but towards higher densities, we observe an increase in the fraction of classical bulge host S0 galaxies at the expense of spirals. Comparing the structural and the star formation properties of galaxies on the size–mass and NUV − r colour–mass planes, respectively, we infer that classical bulge hosting spirals are likely to get transformed into S0 morphology. We notice a trend of galaxy structure with environment such that the fraction of classical bulge hosting spiral galaxies is found to increase with environment density. We also find that among classical bulge hosting spirals, the fraction of quenched galaxies increases in denser environments. We surmise that the existence of more classical bulge hosting spirals galaxies and more efficient quenching leads to the observed increased occurrence of S0 galaxies in high-density environments. The relation between galaxy structure and environment also exists for the disc galaxies irrespective of their visual morphology, which is driven mainly by halo mass.

Curvature of galaxy brightness profiles

Galaxy morphologies reflect the shapes of galaxies and their structural components, such as bulges, discs, bars, spiral arms, etc. The detailed knowledge of the morphology of a galaxy provides understanding of the physics behind its evolution, since the time of its formation, including interaction processes and influence of the environment. Thus, the more precisely we can describe a galaxy structure, the more we may understand about its formation and evolution. We present a method that measures curvature, using images, to describe galaxy structure and to infer the morphology of each component of a galaxy. We also include some preliminary results of curvature measurements for galaxies of the Southern Photometric Local Universe Survey (S-PLUS) DR1 data release and for jellyfish galaxies of the Omega Survey. We find that the median of the curvature parameter and the integrated area under the curvature give us clues on the morphology of a galaxy.