Showing papers by "Gerry Gilmore published in 2004"

Mass loss from dwarf spheroidal galaxies: the origins of shallow dark matter cores and exponential surface brightness profiles

Abstract: Dwarf spheroidal galaxies have shallow central dark matter density profiles, low angular momentum and approximately exponential surface brightness distributions. Through N-body simulations and analytic calculations we investigate the extent to which these properties can be generated from ``typical'' $\Lambda$CDM galaxies, which differ in all of these properties, by the dynamical consequences of feedback. We find that, for a wide range of initial conditions, one impulsive mass loss event will naturally produce a surface brightness profile in the remaining stellar component of a dwarf spheroidal galaxy (dSph) which is well fit over many scale lengths by an exponential, in good qualitative agreement with observations of Local Group dSphs. Furthermore, two impulsive mass loss phases, punctuated by significant gas re-accretion, are found to be sufficient to transform a central density cusp in the dark matter profile into a near-constant density core. This may then provide the missing link between current cosmological simulations, which predict a central cusp in the dark matter density profile, and current observations, which find much shallower central density profiles. We also look at the angular momentum history of dSphs and demonstrate that if these galaxies have spent most of their lifetime in tidal isolation from massive galaxies then they cannot have formed from high angular momentum gas discs.

Kinematically Cold Populations at Large Radii in the Draco and Ursa Minor Dwarf Spheroidals

Abstract: We present projected velocity dispersion profiles for the Draco and Ursa Minor (UMi) dwarf spheroidal galaxies based on 207 and 162 discrete stellar velocities, respectively. Both profiles show a sharp decline in the velocity dispersion outside ~30 arcmin (Draco) and ~40 arcmin (UMi). New, deep photometry of Draco reveals a break in the light profile at ~25 arcmin. These data imply the existence of a kinematically cold population in the outer parts of both galaxies. Possible explanations of both the photometric and kinematic data in terms of both equilibrium and non-equilibrium models are discussed in detail. We conclude that these data challenge the picture of dSphs as simple, isolated stellar systems.

Characterising stellar micro-variability for planetary transit searches

Abstract: A method for simulating light curves containing stellar micro-variability for a range of spectral types and ages is presented. It is based on parameter-by-parameter scaling of a multi-component fit to the solar irradiance power spectrum (based on VIRGO/PMO6 data), and scaling laws derived from ground based observations of various stellar samples. A correlation is observed in the Sun between the amplitude of the power spectrum on long (weeks) timescales and the BBSO Ca  K-line index of chromospheric activity. On the basis of this evidence, the chromospheric activity level, predicted from rotation period and B − V colour estimates according to the relationship first introduced by Noyes (1983) and Noyes et al. (1984), is used to predict the variability power on weeks time scales. The rotation period is estimated on the basis of a fit to the distribution of rotation period versus B−V observed in the Hyades and the Skumanich (1972) spin-down law. The characteristic timescale of the variability is also scaled according to the rotation period. This model is used to estimate the impact of the target star spectral type and age on the detection capability of space based transit searches such as Eddington and Kepler. K stars are found to be the most promising targets, while the performance drops significantly for stars earlier than G and younger than 2.0 Gyr. Simulations also show that Eddington should detect terrestrial planets orbiting solar-age stars in most of the habitable zone for G2 types and all of it for K0 and K5 types.

A photometrically and kinematically distinct core in the Sextans dwarf spheroidal galaxy

Abstract: We present the line of sight radial velocity dispersion profile of the Sextans dwarf spheroidal galaxy (dSph), based on a sample of 88 stars extending to about 1 degree (about 1.5 kpc). Like the Draco and Ursa Minor dSphs, Sextans shows some evidence of a fall-off in the velocity dispersion at large projected radii, with significance p=0.96. Surprisingly, the dispersion at the very centre of Sextans is close to zero (with significance p=0.96). We present evidence which suggests that this latter change in the stellar kinematics coincides with changes in the stellar populations within the dSph. We discuss possible scenarios which could lead to a kinematically and photometrically distinct population at the centre of Sextans.

Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys – I. The old Large Magellanic Cloud clusters NGC 1928, 1939 and Reticulum

Abstract: We present the results of photometric measurements from images of the Large Magellanic Cloud (LMC) globular clusters NGC 1928, 1939 and Reticulum taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Exposures through the F555W and F814W filters result in high-accuracy colour-magnitude diagrams (CMDs) for these three clusters. This is the first time that CMDs for NGC 1928 and 1939 have been published. All three clusters possess CMDs with features indicating them to be > 10 Gyr old, including main-sequence turn-offs at V ∼ 23 and well-populated horizontal branches (HBs). We use the CMDs to obtain metallicity and reddening estimates for each cluster. NGC 1939 is a metal-poor cluster, with [Fe/H] = -2.10 ± 0.19, while NGC 1928 is significantly more metal rich, with [Fe/H] = -1.27 ± 0.14. The abundance of Reticulum is intermediate between the two, with [Fe/H] = -1.66 ± 0.12 - a measurement which matches well with previous estimates. All three clusters are moderately reddened, with values ranging from E(V - I) = 0.07 ± 0.02 for Reticulum and E(V - I) = 0.08 ± 0.02 for NGC 1928, to E(V - I) = 0.16 ± 0.03 for NGC 1939. After correcting the CMDs for extinction we estimate the HB morphology of each cluster. NGC 1928 and 1939 possess HBs consisting almost exclusively of stars to the blue of the instability strip, with NGC 1928 in addition showing evidence for an extended blue HB. In contrast, Reticulum has an intermediate HB morphology, with stars across the instability strip. Using a variety of dating techniques we show that these three clusters are coeval with each other and the oldest Galactic and LMC globular clusters, to within ∼2 Gyr. The census of known old LMC globular clusters therefore now numbers 15 plus the unique, younger cluster ESO121-SC03. The NGC 1939 field contains another cluster in the line of sight, NGC 1938. A CMD for this object shows it to be less than ∼400 Myr old, and it is therefore unlikely to be physically associated with NGC 1939.

The Short Spectacular Life of a Superstar

Abstract: Supernovae are spectacular events--the death throes of stars that can be seen across the universe--and they can leave behind puzzling remnants in the form of neutron stars or black holes. In his Perspective, Gilmore discusses results reported in the same issue by Bietenholz et al. on the observation of a radio-emitting remnant in supernova 1986J. This unusual compact energetic object is apparently very similar to the supernova remnant in the Crab nebula, and represents the youngest neutron star or black hole associated with a supernova yet observed.

Extragalactic Integral Field Spectroscopy on the Gemini Telescopes

Abstract: We have been undertaking a programme on the Gemini 8-m telescopes to demonstrate the power of integral field spectroscopy, using the optical GMOS spectrograph, and the new CIRPASS instrument in the near-infrared. Here we present some preliminary results from 3D spectroscopy of extra-galactic objects, mapping the emission lines in a 3CR radio galaxy and in a gravitationally lensed arc, exploring dark matter sub-structure through observations of an Einstein Cross gravitational lens, and the star formation time-scales of young massive clusters in the starburst galaxy NGC 1140.

Extragalactic integral field spectroscopy on the Gemini telescopes

Abstract: We have been undertaking a programme on the Gemini 8-m telescopes to demonstrate the power of integral field spectroscopy, using the optical GMOS spectrograph, and the new CIRPASS instrument in the near-infrared. Here we present some preliminary results from 3D spectroscopy of extra-galactic objects, mapping the emission lines in a 3CR radio galaxy and in a gravitationally lensed arc, exploring dark matter sub-structure through observations of an Einstein Cross gravitational lens, and the star formation time-scales of young massive clusters in the starburst galaxy NGC 1140. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Chemical Evolution in the Carina Dwarf Spheroidal

Abstract: We present metallicities for 487 red giants in the Carina dwarf spheroidal (dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT) spectroscopy. We find a mean [Fe/H] of -1.91 dex with an intrinsic dispersion of 0.25 dex, whereas the full spread in metallicities is at least one dex. The analysis of the radial distribution of metallicities reveals that an excess of metal poor stars resides in a region of larger axis distances. These results can constrain evolutionary models and are discussed in the context of chemical evolution in the Carina dSph.