Connecting the first galaxies with ultra faint dwarfs in the Local Group: chemical signatures of Population~III stars

We investigate the star formation history and chemical evolution of isolated analogues of Local Group (LG) ultra faint dwarf galaxies (UFDs; stellar mass range of 10^2 solar mass < M_star <10^5 solar mass) and gas rich, low mass dwarfs (Leo P analogs; stellar mass range of 10^5 solar mass < M_star <10^6 solar mass). We perform a suite of cosmological hydrodynamic zoom-in simulations to follow their evolution from the era of the first generation of stars down to z=0. We confirm that reionization, combined with supernova (SN) feedback, is primarily responsible for the truncated star formation in UFDs. Specifically, haloes with a virial mass of M_vir 90\% of stars prior to reionization. Our work further demonstrates the importance of Population~III (Pop~III) stars, with their intrinsically high $\rm [C/Fe]$ yields, and the associated external metal-enrichment, in producing low-metallicity stars ($\rm [Fe/H]\lesssim-4$) and carbon-enhanced metal-poor (CEMP) stars. We find that UFDs are composite systems, assembled from multiple progenitor haloes, some of which hosted only Population~II (Pop~II) stars formed in environments externally enriched by SNe in neighboring haloes, naturally producing, extremely low-metallicity Pop~II stars. We illustrate how the simulated chemical enrichment may be used to constrain the star formation histories (SFHs) of true observed UFDs. We find that Leo P analogs can form in haloes with M_vir ~ 4 x 10^9 solar mass (z=0). Such systems are less affected by reionization and continue to form stars until z=0, causing higher metallicity tails. Finally, we predict the existence of extremely low-metallicity stars in LG UFD galaxies that preserve the pure chemical signatures of Pop~III nucleosynthesis.Comment: 22 pages, 13 figures, Accepted for publication in Ap

Dynamics of the Magellanic Clouds in a LCDM Universe

We examine Milky Way-Magellanic Cloud systems selected from the Millennium-II Simulation in order to place the orbits of the Magellanic Clouds in a cosmological context. Our analysis shows that satellites massive enough to be LMC analogs are typically accreted at late times. Moreover, those that are accreted at early times and survive to the present have orbital properties that are discrepant with those observed for the LMC. The high velocity of the LMC, coupled with the dearth of unbound orbits seen in the simulation, argues that the mass of the MW's halo is unlikely to be less than 2 x 10^12 Msun. This conclusion is further supported by statistics of halos hosting satellites with masses, velocities, and separations comparable to those of the LMC. We further show that: (1) LMC and SMC-mass objects are not particularly uncommon in MW-mass halos; (2) the apparently high angular momentum of the LMC is not cosmologically unusual; and (3) it is rare for a MW halo to host a LMC-SMC binary system at z=0, but high speed binary pairs accreted at late times are possible. Based on these results, we conclude that the LMC was accreted within the past four Gyr and is currently making its first pericentric passage about the MW.Comment: 14 pages, 13 figures; MNRAS, in press. Minor revisions, conclusions unchange

Probing the Assembly of Dwarf Galaxies through Cosmic Time with Damped Lyα Absorption Spectroscopy

We investigate the absorption features associated with a gas-rich dwarf galaxy, using cosmological hydrodynamics simulations. Our goal is to explore whether the progenitors of the lowest-mass dwarf galaxies known to harbor neutral hydrogen today (M-* approximate to 10(6) M-circle dot, M-halo = 4 x 10(9) M-circle dot) could possibly be detected as Damped Ly alpha Absorbers (DLAs) over cosmic time. We trace the evolution of a single dwarf galaxy, preselected to contain DLAs, from the era of the first metal-free, so-called Population III (Pop III) stars, down to z = 0, thus allowing us to study the metal enrichment history of DLAs associated with the simulated galaxy. We find that the progenitors of the simulated dwarf are expected to be seen for most of their evolution as DLAs that are contaminated by normal, Population II stars. The time period during which DLAs are only metal-enriched by Pop III stars, on the other hand, is likely very brief, confined to high redshifts, z greater than or similar to 6. The susceptibility of the dwarfs to the external UV radiation background allows them to preserve neutral gas only at the center (a few similar to 100 pc). This results in a small probability that the simulated dwarf would be observed as a DLA. This study suggests that DLAs are unlikely to be hosted in the lowest-mass dwarfs that can harbor neutral gas (M-halo greater than or similar to 4 x 10(9 )M(circle dot)), below which neutral gas is unlikely to exist. However, this study does illustrate that, when detected, absorption lines provide a powerful method for probing ISM conditions inside the smallest dwarf galaxies at intermediate to high redshifts.HST Grant [15030]; National Science Foundation [1228509]; National Research Foundation of Korea (NRF) [NRF-2018R1C1B6004304]; Korean government (MSIT); NSF [AST-1413501]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Formation of Narrow Dust Rings in Circumstellar Debris Disks

Narrow dust rings observed around some young stars (e.g., HR 4796A) need to be confined. We present a possible explanation for the formation and confinement of such rings in optically thin circumstellar disks, without invoking shepherding planets. If an enhancement of dust grains (e.g., due to a catastrophic collision) occurs somewhere in the disk, photoelectric emission from the grains can heat the gas to temperatures well above that of the dust. The gas orbits with super(sub)-Keplerian speeds inward (outward) of the associated pressure maximum. This tends to concentrate the grains into a narrow region. The rise in dust density leads to further heating and a stronger concentration of grains. A narrow dust ring forms as a result of this instability. We show that this mechanism not only operates around early-type stars that have high UV fluxes, but also around stars with spectral types as late as K. This implies that this process is generic and may have occurred during the lifetime of each circumstellar disk. We examine the stringent upper-limit on the H2 column density in the HR 4796A disk and find it to be compatible with the presence of a significant amount of hydrogen gas in the disk. We also compute the OI and CII infrared line fluxes expected from various debris disks and show that these will be easily detectable by the upcoming Herschel mission. Herschel will be instrumental in detecting and characterizing gas in these disks.Comment: Accepted for publication in ApJ; 14 pages, 7 figure

The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun

We study the future orbital evolution and merging of the MW-M31-M33 system, using a combination of collisionless N-body simulations and semi-analytic orbit integrations. Monte-Carlo simulations are used to explore the consequences of varying the initial phase-space and mass parameters within their observational uncertainties. The observed M31 transverse velocity implies that the MW and M31 will merge t = 5.86 (+1.61-0.72) Gyr from now, after a first pericenter at t = 3.87 (+0.42-0.32) Gyr. M31 may (probability p=41%) make a direct hit with the MW (defined here as a first-pericenter distance less than 25 kpc). Most likely, the MW and M31 will merge first, with M33 settling onto an orbit around them. Alternatively, M33 may make a direct hit with the MW first (p=9%), or M33 may get ejected from the Local Group (p=7%). The MW-M31 merger remnant will resemble an elliptical galaxy. The Sun will most likely (p=85%) end up at larger radius from the center of the MW-M31 merger remnant than its current distance from the MW center, possibly further than 50 kpc (p=10%). The Sun may (p=20%) at some time in the next 10 Gyr find itself moving through M33 (within 10 kpc), but while dynamically still bound to the MW-M31 merger remnant. The arrival and possible collision of M31 (and possibly M33) with the MW is the next major cosmic event affecting the environment of our Sun and solar system that can be predicted with some certainty. (Abridged)Comment: 58 pages, 16 figures, to be published in ApJ. Version with high resolution figures and N-body movies available at http://www.stsci.edu/~marel/M31 . Press materials, graphics, and visualizations available at http://hubblesite.org/newscenter/archive/releases/2012/2

And yet it moves: The dangers of artificially fixing the Milky Way center of mass in the presence of a massive Large Magellanic Cloud

Motivated by recent studies suggesting that the Large Magellanic Cloud (LMC) could be significantly more massive than previously thought, we explore whether the approximation of an inertial Galactocentric reference frame is still valid in the presence of such a massive LMC. We find that previous estimates of the LMC's orbital period and apocentric distance derived assuming a fixed Milky Way are significantly shortened for models where the Milky Way is allowed to move freely in response to the gravitational pull of the LMC. Holding other parameters fixed, the fraction of models favoring first infall is reduced. Due to this interaction, the Milky Way center of mass within the inner 50 kpc can be significantly displaced in phase-space in a very short period of time that ranges from 0.3 to 0.5 Gyr by as much as 30 kpc and 75 km/s. Furthermore, we show that the gravitational pull of the LMC and response of the Milky Way are likely to significantly affect the orbit and phase space distribution of tidal debris from the Sagittarius dwarf galaxy (Sgr). Such effects are larger than previous estimates based on the torque of the LMC alone. As a result, Sgr deposits debris in regions of the sky that are not aligned with the present-day Sgr orbital plane. In addition, we find that properly accounting for the movement of the Milky Way around its common center of mass with the LMC significantly modifies the angular distance between apocenters and tilts its orbital pole, alleviating tensions between previous models and observations. While these models are preliminary in nature, they highlight the central importance of accounting for the mutual gravitational interaction between the MW and LMC when modeling the kinematics of objects in the Milky Way and Local Group.Comment: Accepted for publication in ApJ; 16 pages, 11 figure