Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems

Recent photometric observations by the {\it Hubble Space Telescope (HST)} have revealed the physical properties of stellar galactic nuclei in nucleated dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the formation processes of nucleated dwarfs, we numerically investigate gas dynamics, star formation, and chemical evolution within the central 1 kpc of gas disks embedded within the galactic stellar components of non-nucleated dwarfs. We find that high density, compact stellar systems can be formed in the central regions of dwarfs as a result of dissipative, repeated merging of massive stellar and gaseous clumps developed from nuclear gaseous spiral arms as a result of local gravitational instability. The central stellar components are found to have stellar masses which are typically $~$5% of their host dwarfs and show very flattened shapes, rotational kinematics, and central velocity dispersions significantly smaller than those of their host dwarfs. We also find that more massive dwarfs can develop more massive, more metal-rich, and higher density stellar systems in their central regions, because star formation and chemical enrichment proceed more efficiently owing to the less dramatic suppression of star formation by supernovae feedback effects in more massive dwarfs. Based on these results, we suggest that gas-rich, non-nucleated dwarfs can be transformed into nucleated ones as a result of dissipative gas dynamics in their central regions. We discuss the origin of the observed correlations between physical properties of stellar galactic nuclei and those of their host galaxies.Comment: 13 pages, 4 figures (1 color), ApJL in pres

Episodic disk accretion in the halo of the 'old' Pre-Main Sequence cluster Eta Chamaeleontis

We present multi-epoch medium-resolution observations of two M4.5 candidate members in the halo of the ~8 Myr Eta Chamaeleontis open cluster. Over six months of observations both stars exhibited variations in their H-alpha line profiles on timescales of days to months, with at least one episode of substantial activity attributable to accretion from a circumstellar disk. We derive an accretion rate ~10^-8.7 Msun/yr for this event, with a rate of ~10^-10.6 Msun/yr in quiescence. Episodic accretion like that observed here means existing surveys of accreting Weak-lined T-Tauri Stars in young clusters are likely incomplete and that gas dissipation timescales calculated from the fraction of accreting objects are underestimates.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in MNRAS Letter

Abell 1201: a Minor merger at second core passage

We present an analysis of the structures and dynamics of the merging cluster Abell~1201, which has two sloshing cold fronts around a cooling core, and an offset gas core approximately 500kpc northwest of the center. New Chandra and XMM-Newton data reveal a region of enhanced brightness east of the offset core, with breaks in surface brightness along its boundary to the north and east. This is interpreted as a tail of gas stripped from the offset core. Gas in the offset core and the tail is distinguished from other gas at the same distance from the cluster center chiefly by having higher density, hence lower entropy. In addition, the offset core shows marginally lower temperature and metallicity than the surrounding area. The metallicity in the cool core is high and there is an abrupt drop in metallicity across the southern cold front. We interpret the observed properties of the system, including the placement of the cold fronts, the offset core and its tail in terms of a simple merger scenario. The offset core is the remnant of a merging subcluster, which first passed pericenter southeast of the center of the primary cluster and is now close to its second pericenter passage, moving at ~1000 km/s. Sloshing excited by the merger gave rise to the two cold fronts and the disposition of the cold fronts reveals that we view the merger from close to the plane of the orbit of the offset core.Comment: accepted by Ap

A synthetic sample of short-cadence solar-like oscillators for TESS

NASA's Transiting Exoplanet Survey Satellite (TESS) has begun a two-year survey of most of the sky, which will include lightcurves for thousands of solar-like oscillators sampled at a cadence of two minutes. To prepare for this steady stream of data, we present a mock catalogue of lightcurves, designed to realistically mimic the properties of the TESS sample. In the process, we also present the first public release of the asteroFLAG Artificial Dataset Generator, which simulates lightcurves of solar-like oscillators based on input mode properties. The targets are drawn from a simulation of the Milky Way's populations and are selected in the same way as TESS's true Asteroseismic Target List. The lightcurves are produced by combining stellar models, pulsation calculations and semi-empirical models of solar-like oscillators. We describe the details of the catalogue and provide several examples. We provide pristine lightcurves to which noise can be added easily. This mock catalogue will be valuable in testing asteroseismology pipelines for TESS and our methods can be applied in preparation and planning for other observatories and observing campaigns.Comment: 14 pages, 6 figures, accepted for publication in ApJS. Archives containing the mock catalogue are available at https://doi.org/10.5281/zenodo.1470155 and the pipeline to produce it at https://github.com/warrickball/s4tess . The first public release of the asteroFLAG Artificial Dataset Generator v3 (AADG3) is described at https://warrickball.github.io/AADG3

A Dynamical Study of Optically Selected Distant Clusters

We present a programme of spectroscopic observations of galaxies in a sample of optically-selected clusters taken from the catalogue of Couch et al (1991). Previous ROSAT observations of these clusters have shown them to have lower X-ray luminosities, given their optical richness, than might be expected on the basis of local samples. In the present paper we extend this work by determining velocity dispersions of a subsample of the clusters. We confirm the dynamical reality of all but one of the original sample, and find velocity dispersions comparable with present-day clusters of equivalent comoving space density. Thus, in the context of the $L_X-\sigma$ relation for present-day clusters, there is evidence for a higher velocity dispersion at fixed X-ray luminosity. A key question is whether the high velocity dispersions are indicative of the gravitational potential. If they are, the X-ray luminosities measured in Bower et al., 1994 (Paper I), would then imply an implausibly low efficiency of X-ray generation. Alternatively, the discrepancy could be explained if the clusters were systems of lower virial temperature, in which the apparent velocity dispersion is inflated by an infalling, unrelaxed halo. This might result either from an increase with redshift in the infall rate for clusters, or from the preferential selection of clusters embedded in filaments oriented along the line of sight. Since clusters with similar properties can be found in local optically selected catalogues, we suggest that the latter explanation is more likely.Comment: Accepted for publication in MNRAS. 13 pages plain TeX (not Latex). Uses macro files psfig.tex and mn.tex. Figures and tables included; finding charts available from http://star-www.dur.ac.uk/~rgb