arXiv subjects

E. J. Bernard

Publications and source records attributed to E. J. Bernard.

31 records · Page 2Linked to original sources

arXiv2015

The Globular Cluster System of NGC 6822

We present a comprehensive analysis of the globular cluster (GC) system of the Local Group dwarf irregular galaxy NGC 6822. Our study is based on homogeneous optical and near-IR photometry, as well as long-slit spectroscopic observations which are used to determine new radial velocities for 6 GCs, two of which had no previous spectroscopic information. We construct optical-near IR colour-colour diagrams and through comparison to simple stellar population models infer that the GCs have old ages consistent with being 9 Gyr or older, while their metallicities are in the range between -1.6 < [Fe/H] < -0.4. We conduct a kinematic analysis of the GC population and find tentative evidence for weak net rotation of the GC system, in the same sense as that exhibited by the underlying spheroid. The most likely amplitude of rotation is ~10 km/s, approximately half the magnitude of the observed velocity dispersion. Finally, we use the GCs to estimate the dynamical mass of NGC 6822 within 11 kpc and we formally find it to be in the range between (3-4)10^9 Msun. This implies an overall mass-to-light ratio in the range of ~ 30-40 and indicates that NGC 6822 is highly dark matter dominated. The mass and the corresponding mass-to-light ratio estimates are affected by various additional systematic effects due to limitations of the data and the model that are not necessary reflected in the formal uncertainties.

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arXiv2014

A PAndAS view of M31 dwarf elliptical satellites: NGC147 and NGC185

We exploit data from the Pan-Andromeda Archaeological Survey (PAndAS) to study the extended structures of M31's dwarf elliptical companions, NGC147 and NGC185. Our wide-field, homogeneous photometry allows to construct deep colour-magnitude diagrams (CMDs) which reach down to $\sim3$ mag below the red giant branch (RGB) tip. We trace the stellar components of the galaxies to surface brightness of $μ_g \sim 32$ mag arcsec$^{-2}$ and show they have much larger extents ($\sim5$ kpc radii) than previously recognised. While NGC185 retains a regular shape in its peripheral regions, NGC147 exhibits pronounced isophotal twisting due to the emergence of symmetric tidal tails. We fit single Sersic models to composite surface brightness profiles constructed from diffuse light and star counts and find that NGC147 has an effective radius almost 3 times that of NGC185. In both cases, the effective radii that we calculate are larger by a factor of $\sim2$ compared to most literature values. We also calculate revised total magnitudes of $M_g=-15.36\pm0.04$ for NGC185 and $M_g=-16.36\pm0.04$ for NGC147. Using photometric metallicities computed for RGB stars, we find NGC185 to exhibit a metallicity gradient of [Fe/H]$\sim-0.15$ dex/kpc over the radial range 0.125 to 0.5 deg. On the other hand, NGC147 exhibits almost no metallicity gradient, $\sim-0.02$ dex/kpc from 0.2 to 0.6 deg. The differences in the structure and stellar populations in the outskirts of these systems suggest that tidal influences have played an important role in governing the evolution of NGC147.

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arXiv2014

The outer halo globular cluster system of M31 - II. Kinematics

We present a detailed kinematic analysis of the outer halo globular cluster (GC) system of M31. Our basis for this is a set of new spectroscopic observations for 78 clusters lying at projected distances between Rproj ~20-140 kpc from the M31 centre. These are largely drawn from the recent PAndAS globular cluster catalogue; 63 of our targets have no previous velocity data. Via a Bayesian maximum likelihood analysis we find that GCs with Rproj > 30 kpc exhibit coherent rotation around the minor optical axis of M31, in the same direction as more centrally- located GCs, but with a smaller amplitude of 86+/-17 km s-1. There is also evidence that the velocity dispersion of the outer halo GC system decreases as a function of projected distance from the M31 centre, and that this relation can be well described by a power law of index ~ -0.5. The velocity dispersion profile of the outer halo GCs is quite similar to that of the halo stars, at least out to the radius up to which there is available information on the stellar kinematics. We detect and discuss various velocity correlations amongst subgroups of GCs that lie on stellar debris streams in the M31 halo. Many of these subgroups are dynamically cold, exhibiting internal velocity dispersions consistent with zero. Simple Monte Carlo experiments imply that such configurations are unlikely to form by chance, adding weight to the notion that a significant fraction of the outer halo GCs in M31 have been accreted alongside their parent dwarf galaxies. We also estimate the M31 mass within 200 kpc via the Tracer Mass Estimator, finding (1.2 - 1.6) +/- 0.2 10^{12}M_sun. This quantity is subject to additional systematic effects due to various limitations of the data, and assumptions built in into the TME. Finally, we discuss our results in the context of formation scenarios for the M31 halo.

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arXiv2013

Kinematics of Outer Halo Globular Clusters in M31

We present the first kinematic analysis of the far outer halo globular cluster (GC) population in the Local Group galaxy M31. Our sample contains 53 objects with projected radii of ~20-130 kpc, of which 44 have no previous spectroscopic information. GCs with projected radii >30 kpc are found to exhibit net rotation around the minor axis of M31, in the same sense as the inner GCs, albeit with a smaller amplitude of 79 +/-19 km/s. The rotation-corrected velocity dispersion of the full halo GC sample is 106 +/-12 km/s, which we observe to decrease with increasing projected radius. We find compelling evidence for kinematic-coherence amongst GCs which project on top of halo substructure, including a clear signature of infall for GCs lying along the North-West stream. Using the tracer mass estimator, we estimate the dynamical mass of M31 within 200 kpc to be M_M31 = (1.2-1.5) +/- 0.2 x 10^12 M_sun. This value is highly dependent on the chosen model and assumptions within.

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arXiv2013

The Outer Halo of the Nearest Giant Elliptical: A VLT/VIMOS Survey of the Resolved Stellar Populations in Centaurus A to 85 kpc

We present the first deep survey of resolved stellar populations in the remote outer halo of our nearest giant elliptical (gE), Centaurus A (D=3.8 Mpc). Using the VIMOS/VLT optical camera, we obtained deep photometry for four fields along the major and minor axes at projected elliptical radii of ~30-85 kpc (corresponding to ~5-14 R_{eff}). We use resolved star counts to map the spatial and colour distribution of red giant branch (RGB) stars down to ~2 magnitudes below the RGB tip. We detect an extended halo out to the furthermost elliptical radius probed (~85 kpc or ~14 R_{eff}), demonstrating the vast extent of this system. We detect a localised substructure in these parts, visible in both (old) RGB and (intermediate-age) luminous asymptotic giant branch stars, and there is some evidence that the outer halo becomes more elliptical and has a shallower surface brightness profile. We derive photometric metallicity distribution functions for halo RGB stars and find relatively high median metallicity values ([Fe/H]_{med} -0.9 to -1.0 dex) that change very little with radius over the extent of our survey. Radial metallicity gradients are measured to be ~-0.002 to -0.004 dex/kpc and the fraction of metal-poor stars (defined as [Fe/H]<-1.0) is ~40-50% at all radii. We discuss these findings in the context of galaxy formation models for the buildup of gE haloes.

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arXiv2012

A Pan-STARRS1 View of the Bifurcated Sagittarius Stream

We use data from the Pan-STARRS1 survey to present a panoramic view of the Sagittarius tidal stream in the southern Galactic hemisphere. As a result of the extensive sky coverage of Pan-STARRS1, the southern stream is visible along more than 60 degrees of its orbit, nearly double the length seen by the SDSS. The recently discovered southern bifurcation of the stream is also apparent, with the fainter branch of the stream visible over at least 30 degrees. Using a combination of fitting both the main sequence turn-off and the red clump, we measure the distance to both arms of the stream in the south. We find that the distances to the bright arm of the stream agree very well with the N-body models of Law & Majewski (2010). We also find that the faint arm lies ~5 kpc closer to the Sun than the bright arm, similar to the behavior seen in the northern hemisphere.

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arXiv2012

On the central helium-burning variable stars of the LeoI dwarf spheroidal galaxy

We present a study of short period, central helium-burning variable stars in the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and 51 Cepheids. So far, this is the largest sample of Cepheids and the largest Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with other Local Group dwarf spheroidals, Carina and Fornax, shows that the period distribution of RR Lyrae stars is quite similar, suggesting similar properties of the parent populations, whereas the Cepheid period distribution in LeoI peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a broader range, from 0.5 to 1.78d. Evolutionary and pulsation predictions indicate, assuming a mean metallicity peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids traces a unique mix of Anomalous Cepheids (blue extent of the red--clump, partially electron degenerate central helium-burning stars) and short-period classical Cepheids (blue-loop, quiescent central helium-burning stars). Current evolutionary prescriptions also indicate that the transition mass between the two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different implications of the current findings on the star formation history of LeoI.

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arXiv2012

Variable Stars in the Cetus dSph Galaxy: Population Gradients and Connections with the Star Formation History

We investigate the variable star content of the isolated, Local Group, dwarf spheroidal galaxy (dSph) Cetus. Multi-epoch, wide-field images collected with the VLT/VIMOS camera allowed us to detect 638 variable stars (630 RR Lyrae stars and 8 Anomalous Cepheids), 475 of which are new detections. We present a full catalogue of periods, amplitudes, and mean magnitudes. Motivated by the recent discovery that the pulsational properties of the RR Lyrae stars in the Tucana dSph revealed the presence of a metallicity gradient within the oldest (>10 Gyr old) stellar populations, we investigated the possibility of an analogous effect in Cetus. We found that, despite the obvious radial gradient in the Horizontal Branch (HB) and Red Giant Branch (RGB) morphologies, both becoming bluer on average for increasing distance from the center of Cetus, the properties of the RR Lyrae stars are homogeneous within the investigated area (out to r~15'), with no significant evidence of a radial gradient. We discuss this in connection with the star formation history (SFH) previously derived for the two galaxies. The observed differences between these two systems show that even systems this small show a variety of early evolutionary histories. These differences could be due to different merger or accretion histories.

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arXiv2011

The ACS LCID project VII: the blue stragglers population in the isolated dSph galaxies Cetus and Tucana

We present the first investigation of the Blue Straggler star (BSS) population in two isolated dwarf spheroidal galaxies of the Local Group, Cetus and Tucana. Deep HST/ACS photometry allowed us to identify samples of 940 and 1214 candidates, respectively. The analysis of the star formation histories of the two galaxies suggests that both host a population of BSSs. Specifically, if the BSS candidates are interpreted as young main sequence stars, they do not conform to their galaxy's age-metallicity relationship. The analysis of the luminosity function and the radial distributions support this conclusion, and suggest a non-collisional mechanism for the BSS formation, from the evolution of primordial binaries. This scenario is also supported by the results of new dynamical simulations presented here. Both galaxies coincide with the relationship between the BSS frequency and the absolute visual magnitude Mv found by Momany et al (2007). If this relationship is confirmed by larger sample, then it could be a valuable tool to discriminate between the presence of BSSs and galaxies hosting truly young populations.

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arXiv2010

The ACS LCID project. VI. The SFH of the Tucana dSph and the relative ages of the isolated dSph galaxies

We present a detailed study of the star formation history (SFH) of the Tucana dwarf spheroidal galaxy. High quality, deep HST/ACS data, allowed us to obtain the deepest color-magnitude diagram to date, reaching the old main sequence turnoff (F814 ~ 29) with good photometric accuracy. Our analysis, based on three different SFH codes, shows that Tucana is an old and metal-poor stellar system, which experienced a strong initial burst of star formation at a very early epoch (~ 13 Gyr ago) which lasted a maximum of 1 Gyr (sigma value). We are not able to unambiguously answer the question of whether most star formation in Tucana occurred before or after the end of the reionization era, and we analyze alternative scenarios that may explain the transformation of Tucana from a gas-rich galaxy into a dSph. Current measurements of its radial velocity do not preclude that Tucana may have crossed the inner regions of the Local Group once, and so gas stripping by ram pressure and tides due to a close interaction cannot be ruled out. On the other hand, the high star formation rate measured at early times may have injected enough energy into the interstellar medium to blow out a significant fraction of the initial gas content. Gas that is heated but not blown out would also be more easily stripped via ram pressure. We compare the SFH inferred for Tucana with that of Cetus, the other isolated LG dSph galaxy in the LCID sample. We show that the formation time of the bulk of star formation in Cetus is clearly delayed with respect to that of Tucana. This reinforces the conclusion of Monelli et al. (2010) that Cetus formed the vast majority of its stars after the end of the reionization era implying, therefore, that small dwarf galaxies are not necessarily strongly affected by reionization, in agreement with many state-of-the-art cosmological models. [abridged]

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arXiv2010

The ACS LCID project IV: detection of the RGB bump in isolated galaxies of the Local Group

We report the detection and analysis of the red giant branch luminosity function bump in a sample of isolated dwarf galaxies in the Local Group. We have designed a new analysis approach comparing the observed color-magnitude diagrams with theoretical best-fit color-magnitude diagrams derived from precise estimates of the star formation histories of each galaxy. This analysis is based on studying the difference between the V-magnitude of the RGB bump and the horizontal branch at the level of the RR Lyrae instability strip (Delta_vhbb) and we discuss here a technique for reliably measuring this quantity in complex stellar systems. By using this approach, we find that the difference between the observed and predicted values of Delta_vhbb is +0.13 +/- 0.14 mag. This is smaller, by about a factor of two, than the well-known discrepancy between theory and observation at low metallicity commonly derived for Galactic globular clusters. This result is confirmed by a comparison between the adopted theoretical framework and empirical estimates of the Delta_vhbb parameter for both a large database of Galactic globular clusters and for four other dSph galaxies for which this estimate is available in the literature. We also investigate the strength of the red giant branch bump feature (R_bump), and find very good agreement between the observed and theoretically predicted R_bump values. This agreement supports the reliability of the evolutionary lifetimes predicted by theoretical models of the evolution of low-mass stars.

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arXiv2007

The Stellar Structures around Disk Galaxies

We present a brief summary of our current results on the stellar distribution and population gradients of the resolved stars in the surroundings of ~50 nearby disk galaxies, observed with space- (Hubble & Spitzer) and ground-based telescopes (Subaru, VLT, BTA, Palomar, CFHT & INT). We examine the radial (in-plane) and vertical (extraplanar) distributions of resolved stars as a function of stellar age and metallicity by tracking changes in the color-magnitude diagram of face-on and edge-on galaxies. Our data show, that the scale length and height of a stellar population increases with age, with the oldest detected stellar populations identified at a large galactocentric radius or extraplanar height, out to typically a few kpc. In the most massive of the studied galaxies there is evidence for a break in number density and color gradients of evolved stars, which plausibly correspond to the thick disk and halo components of the galaxies. The ratio of intermediate-age to old stars in the outermost fields correlate with the gas fraction, while relative sizes of the thick-to-thin disks anticorrelate with galactic circular velocity.

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arXiv2007

Short-Period Variables in the Local Group Dwarf Galaxies Tucana and LGS3

We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the fraction of the observed field we studied in each galaxy, a total of 133 and 30 variables were found, respectively. For Tucana, we identified 76 of them as RR Lyrae (RRL) stars pulsating in the fundamental mode (RRab) and 32 in the first-overtone mode (RRc), as well as 2 anomalous Cepheids (AC). The mean period of the RRab and RRc is 0.59 and 0.35 days, respectively. In the case of LGS3, we found 24 RRab and 4 RRc, with mean periods 0.61and 0.39 days, respectively, plus two candidate ACs. These values place both galaxies in the Oosterhoff gap.

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