EXCITATION of COUPLED STELLAR MOTIONS in the GALACTIC DISK by ORBITING SATELLITES

We use a set of high-resolution N-body simulations of the Galactic disk to study its interactions with the population of cosmologically predicted satellites. One simulation illustrates that multiple passages of massive satellites with different velocities through the disk generate a wobble, which has the appearance of rings in face-on projections of the stellar disk. They also produce flares in the outer disk parts and gradually heat the disk through bending waves. A different numerical experiment shows that an individual satellite as massive as the Sagittarius dwarf galaxy passing through the disk will drive coupled horizontal and vertical oscillations of stars in underdense regions with small associated heating. This experiment shows that vertical excursions of stars in these low-density regions can exceed 1 kpc in the Solar neighborhood, resembling the recently locally detected coherent vertical oscillations. They can also induce non-zero vertical streaming motions as large as 10-20 km s-1, which is consistent with recent observations in the Galactic disk. This phenomenon appears as a local ring with modest associated disk heating. © 2016. The American Astronomical Society. All rights reserved

THE IMPRINT of RADIAL MIGRATION on the VERTICAL STRUCTURE of GALAXY DISKS

We use numerical simulations to examine the effects of radial migration on the vertical structure of galaxy disks. The simulations follow three exponential disks of different mass but similar circular velocity, radial scalelength, and (constant) scale height. The disks develop different non-axisymmetric patterns, ranging from feeble, long-lived multiple arms to strong, rapidly evolving few-armed spirals. These fluctuations induce radial migration through secular changes in the angular momentum of disk particles, mixing the disk radially and blurring pre-existing gradients. Migration primarily affects stars with small vertical excursions, regardless of spiral pattern. This "provenance bias" largely determines the vertical structure of migrating stars: inward migrators thin down as they move in, whereas outward migrators do not thicken up but rather preserve the disk scale height at their destination. Migrators of equal birth radius thus develop a strong scale-height gradient, not by flaring out as commonly assumed, but by thinning down as they spread inward. Similar gradients have been observed for low-[α/Fe] mono-abundance populations (MAPs) in the Galaxy, but our results argue against interpreting them as a consequence of radial migration. This is because outward migration does not lead to thickening, implying that the maximum scale height of any population should reflect its value at birth. In contrast, Galactic MAPs have scale heights that increase monotonically outward, reaching values that greatly exceed those at their presumed birth radii. Given the strong vertical bias affecting migration, a proper assessment of the importance of radial migration in the Galaxy should take carefully into account the strong radial dependence of the scale heights of the various stellar populations. © 2016. The American Astronomical Society. All rights reserved

ON the CONSERVATION of the VERTICAL ACTION in GALACTIC DISKS

We employ high-resolution N-body simulations of isolated spiral galaxy models, from low-amplitude, multi-armed galaxies to Milky Way-like disks, to estimate the vertical action of ensembles of stars in an axisymmetrical potential. In the multi-armed galaxy the low-amplitude arms represent tiny perturbations of the potential, hence the vertical action for a set of stars is conserved, although after several orbital periods of revolution the conservation degrades significantly. For a Milky Way-like galaxy with vigorous spiral activity and the formation of a bar, our results show that the potential is far from steady, implying that the action is not a constant of motion. Furthermore, because of the presence of high-amplitude arms and the bar, considerable in-plane and vertical heating occurs that forces stars to deviate from near-circular orbits, reducing the degree at which the actions are conserved for individual stars, in agreement with previous results, but also for ensembles of stars. If confirmed, this result has several implications, including the assertion that the thick disk of our Galaxy forms by radial migration of stars, under the assumption of the conservation of the action describing the vertical motion of stars. © 2016. The American Astronomical Society. All rights reserved

Tidally induced warps of spiral galaxies in IllustrisTNG

Abstract Warps are common features in both stellar and gaseous discs of nearby spiral galaxies with the latter usually easier to detect. Several theories have been proposed in the literature to explain their formation and prevalence, including tidal interactions with external galaxies. Observational correlations also suggest the importance of tides for warp formation. Here, we use the TNG100 run from the magnetohydrodynamical cosmological simulation suite IllustrisTNG to investigate the connection between interactions and the formation of gas warps. We find that in the sample of well-resolved gas-rich spiral galaxies (1010 ≲ M*/M⊙ ≲ 1011 at z = 0) from the simulation TNG100-1, about 16% possess the characteristic S-shaped warp. Around one-third of these objects have their warps induced by interactions with other galaxies. Half of these interactions end with the perturber absorbed by the host by z = 0. We find that warps induced by interactions survive on average for <1 Gyr, similarly to the remaining S-shaped warps. The angle between the orbital angular momentum of the perturber and the angular momentum of the host’s disc that most likely leads to warp formation is around 45 degrees. While our main goal is to investigate tidally induced warps, we find that during interactions in addition to tides, new gas that is accreted from infalling satellites also can contribute to warp formation

Two fossil groups of galaxies at z~0.4 in the COSMOS: accelerated stellar-mass build-up, different progenitors

We report on 2 fossil groups of galaxies at z=0.425 and 0.372 discovered in the Cosmic Evolution Survey (COSMOS) area. Selected as X-ray extended sources, they have total masses (M_200) of 1.9(+/-0.41)E13 and 9.5(+/-0.42)E13 M_sun, respectively, as obtained from a recent X-ray luminosity-mass scaling relation. The lower mass system appears isolated, whereas the other sits in a well-known large-scale structure (LSS) populated by 27 other X-ray emitting groups. The identification as fossil is based on the i-band photometry of all the galaxies with a photo-z consistent with that of the group at the 2-sigma confidence level and within a projected group-centric distance equal to 0.5R_200, and i_AB<=22.5-mag limited spectroscopy. Both fossil groups exhibit high stellar-to-total mass ratios compared to all the X-ray selected groups of similar mass at 0.3<=z<=0.5 in the COSMOS. At variance with the composite galaxy stellar mass functions (GSMFs) of similarly massive systems, both fossil group GSMFs are dominated by passively evolving galaxies down to M^stars~1E10 M_sun (according to the galaxy broad-band spectral energy distributions). The relative lack of star-forming galaxies with 1E10<=M^stars<=1E11 M_sun is confirmed by the galaxy distribution in the b-r vs i color-magnitude diagram. Hence, the 2 fossil groups appear as more mature than the coeval, similarly massive groups. Their overall star formation activity ended rapidly after an accelerated build up of the total stellar mass; no significant infall of galaxies with M^stars>=1E10 M_sun took place in the last 3 to 6 Gyr. This similarity holds although the 2 fossil groups are embedded in two very different density environments of the LSS, which suggests that their galaxy populations were shaped by processes that do not depend on the LSS. However, their progenitors may do so. ...Comment: 12 pages, 5 color figures, 1 table; to be published in the MNRA

The Stromlo Missing Satellites Survey

The Stromlo Missing Satellites (SMS) program is a critical endeavor to investigate whether cold dark matter cosmology is flawed in its ability to describe the matter distribution on galaxy scales or proves itself once again as a powerful theory to make observational predictions. The project will deliver unprecedented results on Milky Way satellite numbers, their distribution and physical properties. It is the deepest, most extended survey for optically elusive dwarf satellite galaxies to date, covering the entire 20,000 sq deg of the Southern hemisphere. 150TB of CCD images will be analysed in six photometric bands, 0.5-1.0 mag fainter than SDSS produced by the ANU SkyMapper telescope over the next five years. (For more details see: http://msowww.anu.edu.au/~jerjen/SMS_Survey.html)Comment: 4 pages, 1 figure, in "Galaxies in the Local Volume" (Sydney, 8-13 July 2007), eds B. Koribalski and H. Jerjen, Springer Astrophysics and Space Science Proceedings, p. 18

Fossil Groups Origins: I. RX J105453.3+552102 a very massive and relaxed system at z~0.5

The most accepted scenario for the origin of fossil groups (FGs) is that they are galaxy associations in which the merging rate was fast and efficient. These systems have assembled half of their mass at early epoch of the Universe, subsequently growing by minor mergers. They could contain a fossil record of the galaxy structure formation. We have started a project in order to characterize a large sample of FGs. In this paper we present the analysis of the fossil system RX J105453.3+552102. Optical deep images were used for studying the properties of the brightest group galaxy and for computing the photometric luminosity function of the group. We have also performed a detail dynamical analysis of the system based on redshift data for 116 galaxies. This galaxy system is located at z=0.47, and shows a quite large line-of-sight velocity dispersion \sigma_{v}~1000 km/s. Assuming the dynamical equilibrium, we estimated a virial mass of M ~ 10^{15} h_{70} M_{\odot}. No evidence of substructure was found within 1.4 Mpc radius. We found a statistically significant departure from Gaussianity of the group members velocities in the most external regions of the group. This could indicate the presence of galaxies in radial orbits in the external region of the group. We also found that the photometrical luminosity function is bimodal, showing a lack of M_{r} ~ -19.5 galaxies. The brightest group galaxy shows low Sersic parameter (n~2) and a small peculiar velocity. Indeed, our accurate photometry shows that the difference between the brightest and the second brightest galaxies is 1.9 mag in the r-band, while the classical definition of FGs is based on a magnitude gap of 2. We conclude that this fossil system does not follow the empirical definition of FGs. Nevertheless, it is a massive, old and undisturbed galaxy system with little infall of L^{*} galaxies since its initial collapse.Comment: 17 pages, 14 figures, accepted for publication at A&

AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY

We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (μ = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe

Outer-Disk Populations in NGC 7793: Evidence for Stellar Radial Migration

We analyzed the radial surface brightness profile of the spiral galaxy NGC 7793 using HST/ACS images from the GHOSTS survey and a new HST/WFC3 image across the disk break. We used the photometry of resolved stars to select distinct populations covering a wide range of stellar ages. We found breaks in the radial profiles of all stellar populations at 280" (~5.1 kpc). Beyond this disk break, the profiles become steeper for younger populations. This same trend is seen in numerical simulations where the outer disk is formed almost entirely by radial migration. We also found that the older stars of NGC 7793 extend significantly farther than the underlying HI disk. They are thus unlikely to have formed entirely at their current radii, unless the gas disk was substantially larger in the past. These observations thus provide evidence for substantial stellar radial migration in late-type disks.Comment: 8 pages, 6 figure. Accepted for publication in Ap

Formation and structure of halos in a warm dark matter cosmology

(Abridged) Using high-resolution cosmological N-body simulations, we study how the density profiles of dark matter halos are affected by the filtering of the density power spectrum below a given scale length and by the introduction of a thermal velocity dispersion. In the warm dark matter (WDM) scenario, both the free-streaming scale, R_f, and the velocity dispersion, v_w, are determined by the mass m_w of the WDM particle. We find that v_w is too small to affect the density profiles of WDM halos. Down to the resolution attained in our simulations, there is not any significant difference in the density profiles and concentrations of halos obtained in simulations with and without the inclusion of v_w. The density profiles of halos with masses down to ~0.01 the filtering mass M_f can be described by the NFW shape; significant soft cores are not formed. Nevertheless, the concentrations of these halos are lower than those of the CDM counterparts and are approximately independent of mass. The lower concentrations of WDM halos with respect to their CDM counterparts can be accounted for their late formation epoch. Overall, our results point to a series of advantages of a WDM model over the CDM one. In addition to solving the substructure problem, a WDM model with R_f~0.16 Mpc (m_w~0.75 kev; flat cosmology with Omega_L=h=0.7) also predicts concentrations, a Tully-Fisher relation, and formation epochs for small halos which seems to be in better agreement with observations, relative to CDM predictions.Comment: Accepted for publication in ApJ. 34 pages, figs. 1a,1b, and 1c given in separate files (high resolution versions available at ftp://ftp.astroscu.unam.mx/pub/temporal/). Major modifications after referees Report (more simulations and new figures are presented), but main conclusions remain the sam