Discovery of tidal debris stars from G1/Mayall-II in M31

The object Mayall II or G1 is the brightest globular cluster belonging to M31. Because of its extreme properties for a globular cluster, it has been speculated that G1 is the remnant nucleus of a dwarf galaxy that has been stripped by the tidal field of M31. Using the Keck DEIMOS spectrograph, we have conducted a survey for tidally stripped stars from G1, obtaining a sample of 351 stellar velocities over ∼ 320 sq. arcminutes of sky centered on G1. Thirteen are within $25~{\, \rm km\, s^{-1}\, }$ of the systemic velocity of G1, and exhibit spatial and velocity correlations consistent with being dynamically associated with G1, and all thirteen are well outside the tidal radius of the cluster. These thirteen stars could be either (i) the remnants of an almost completely evaporated stellar envelope, or (ii) G1 member stars lost through tidal interaction with M31. Estimates of the implied mass loss rate based on our data suggest a short dissolution time-scale for G1, thus favouring the stellar envelope hypothesis for the origin of the tidal tail stars, or, at the very least, an advanced stage of cluster dissolution. In either case, G1, and by extension compact stellar systems in general, have likely played a significant role in building the halo of M31.

Tidal tails of open star clusters as probes of early gas expulsion

Context. Star clusters form in the densest parts of infrared dark clouds. The emergence of massive stars expels the residual gas that has not formed stars yet. Gas expulsion lowers the gravitational potential of the embedded cluster, unbinding many of the cluster stars. These stars then move on their own trajectories in the external gravitational field of the Galaxy, forming a tidal tail. Aims. We investigate, for the first time, the formation and evolution of a tidal tail that forms due to expulsion of primordial gas. We contrast the morphology and kinematics of this tail with that of another tidal tail that forms by gradual dynamical evaporation of the star cluster. We intend to provide predictions that can determine the dynamical origin of possibly observed tidal tails around dynamically evolved (age ≳ 100 Myr) galactic star clusters by the Gaia mission. These observations might estimate the fraction of the initial cluster population that gets released in the gas expulsion event. The severity of the initial gas expulsion is given by the star formation efficiency and the timescale of gas expulsion for the cluster when it was still embedded in its natal gas. A study with a more extended parameter space of the initial conditions is performed in the follow up paper. Methods. We provide a semi-analytical model for the tail evolution. The model is compared against direct numerical simulations using NBODY6. Results. Tidal tails released during gas expulsion have different kinematic properties than the tails gradually forming due to evaporation; the latter kind have been extensively studied. The gas expulsion tidal tail shows non-monotonic expansion with time, where longer epochs of expansion are interspersed with shorter epochs of contraction. The tail thickness and velocity dispersions vary strongly, but not exactly periodically, with time. The times of minima of tail thickness and velocity dispersions are given only by the properties of the galactic potential, and not by the properties of the cluster. The estimates provided by the (semi-)analytical model for the extent of the tail, the minima of tail thickness, and velocity dispersions are in a very good agreement with the NBODY6 simulations. This implies that the semi-analytic model can be used to estimate the properties of the gas expulsion tidal tail for a cluster of a given age and orbital parameters without the necessity of performing numerical simulations.

An extended Pal 5 stream in Gaia DR2

ABSTRACT We present the results of a detailed search for members of the Pal 5 tidal tail system in Gaia Data Release 2 (DR2). Tidal tails provide a sensitive method for measuring the current and past gravitational potential of their host galaxy as well as for testing predictions for the abundance of dark matter subhaloes. The Pal 5 globular cluster and its associated tails are an excellent candidate for such analysis; however, only ∼23° of arc are currently known, with in particular the leading tail much shorter than the trailing. Using Gaia DR2 and its precise astrometry, we extend the known extent of the Pal 5 tail to ∼30°, 7° of which are newly detected along the leading arm. The detected leading and trailing arms are symmetric in length and remain near constant width. This detection constrains proposed models in which the Galactic bar truncates Pal 5’s leading arm. Follow-up spectroscopic observations are necessary to verify the candidate stream stars are consistent with the known tidal tails. If confirmed, this Pal 5 stream extension opens up new possibilities to constrain the Galactic potential.

Chemodynamical properties of the Anticentre Stream: a surviving disc fossil from a past satellite interaction

ABSTRACT Using Gaia second data release (DR2), we trace the Anticentre Stream (ACS) in various stellar populations across the sky and find that it is kinematically and spatially decoupled from the Monoceros Ring. Using stars from lamost and segue, we show that the ACS is systematically more metal-poor than Monoceros by 0.1 dex with indications of a narrower metallicity spread. Furthermore, the ACS is predominantly populated of old stars ($\sim 10\, \rm {Gyr}$), whereas Monoceros has a pronounced tail of younger stars ($6-10\, \rm {Gyr}$) as revealed by their cumulative age distributions. Put together, all of this evidence support predictions from simulations of the interaction of the Sagittarius dwarf with the Milky Way, which argue that the ACS is the remains of a tidal tail of the Galaxy excited during Sgr’s first pericentric passage after it crossed the virial radius, whereas Monoceros consists of the composite stellar populations excited during the more extended phases of the interaction. Importantly, the ACS can be viewed as a stand-alone fossil of the chemical enrichment history of the Galactic disc.

Detecting tidal tail of the globular cluster Whiting 1

Whiting 1 is a faint and young globular cluster embedded in the Sag dSph. It has similar distance, metalicity and radial velocity with the trailing stream of the Sag. Due to these special properties, Whiting 1 was suggested to be associated with the trailing stream of Sag. However, its origin is still unclear and further investigation of its relation with Sgr dSph is needed. In this work, we use DECaLS data to search the tidal tail of Whiting 1, with the aim of looking for spatial connection between Whiting 1 and Sgr dSph. With Matched Filter method, we detected a tidal tail around the main body of Whiting 1. This tail extends to at least 0.5 degree and aligns with the mean orbit direction of Sgr dSph. This tail is newly detected and it provides additional evidence of the association between Whiting 1 and Sgr trailing stream.

The tidal tails of open star clusters produced by early gas expulsion

We investigate, for the first time, the formation and evolution of the tidal tail released from a young Pleiades-like star cluster due to expulsion of primordial gas in a realistic gravitational field of the Galaxy. The tidal tails (as well as clusters) are integrated by nbody6 from their embedded phase for more than 300 Myr. We vary the star formation efficiency (SFE) from 33% to 100% and the timescales of gas expulsion as free parameters, and provide predictions for the morphology and kinematics of the evolved tail for each of the models. The resulting tail properties are intended for comparison with Gaia measurements, where an inverse analysis of our findings might constrain some of the poorly understood conditions and processes in embedded star clusters during the gas phase and gas expulsion.

Discovery of CO absorption at z = 0.05 in G0248+430

Absorption lines in front of distant quasars are quite rare in the millimeter domain. They can, however, bring very useful and complementary information to emission lines. We report here the detection with NOEMA of CO(1–0) and CN(1–0) lines in absorption, and the confirmation of CO emission in the quasar/galaxy pair Q0248+430/G0248+430. The system G0248+430 corresponds to two merging galaxies (a Seyfert and a LINER) at z = 0.0519 with a tidal tail just on the line of sight to the background quasar Q0248+430 at z = 1.313. Optical (CaII, NaI), H I 21 cm, and OH-1667 MHz absorption lines associated with the tidal tail of the foreground system have previously been detected toward the quasar, while four CO lines at different rotation J levels have been detected in emission from the foreground galaxies. New H I 21 cm line observations with the upgraded GMRT array are also presented. We discuss the molecular content of the merging galaxies, and the physical conditions in the absorbing interstellar medium of the tidal tail.