The dwarf galaxy population of the Coma cluster to MR=−11: a detailed description

We present the luminosity function and measurements of the scalelengths, colours and radial distribution of dwarf galaxies in the Coma cluster down to R =24. Our survey area is 674 arcmin²; this is the deepest and most detailed survey covering such a large area.
Our measurements agree with those of most previous authors at bright and intermediate magnitudes. The new results are as follows.
(1) Galaxies in the Coma cluster have a luminosity function φ( L )∝ L ^α that is steep (α∼−1.7) for −15< M_R <−11, and is shallower brighter than this. The curvature in the luminosity function at M_R ∼−15 is statistically significant.
(2) The galaxies that contribute most strongly to the luminosity function at −14< M_R <−12 have colours and scalelengths that are consistent with those of local dwarf spheroidal galaxies placed at the distance of Coma.
(3) These galaxies with −14< M_R <−12 have a colour distribution that is very strongly peaked at B − R =1.3. This is suggestive of a substantial degree of homogeneity in their star formation histories and metallicities.
(4) These galaxies with −14< M_R <−12 also appear to be more confined to the cluster core ( r ∼200 kpc) than the brighter galaxies. Alternatively, this observation may be explained in part or whole by the presence of an anomalously high number of background galaxies behind the cluster core. Velocity measurements of these galaxies would distinguish between these two possibilities.     

Comet classification with new methods for gas and dust spectroscopy

We present the results of a program of comet long-slit spectroscopy with the Kast Dual Spectrograph on the 3-m Shane Telescope at Lick Observatory. A total of 26 comets, from a variety of dynamical families, were observed on 39 different nights from 1996 to 2007. A new statistical method extracted the twilight sky from comet frames, because traditional sky subtraction techniques were inadequate. Because previously published Haser model parent and daughter scale lengths did not fit the data well, unbiased ranges of scale lengths were searched for the best-fitting pairs. Coma gas production rates for OH, CN, C₂, C₃, NH, NH₂, and OH confirmed the widely reported carbon-chain depletion for a sub-class of comets, most notably high-perihelion Jupiter-family comets observed at r_h > 1.5 AU, with different behaviors for C₂ and C₃. Our long-slit spectroscopy data was also adapted for the A(θ)fρ dust production parameter. The assumption that A(θ)fρ is constant throughout the nucleus was not upheld. High dust-to-gas ratios for comets with large perihelia were not a selection effect, and suggest that the dust was released earlier in the formation of the coma than the gas. The dust-to-gas ratio did not exhibit any evolutionary traces between different comet dynamical families. The comet survey illuminates the diversity among comets, including the unusually carbon poor Comet 96P/Machholz.     

Monte Carlo modeling of neutral gas and dust in the coma of Comet 1P/Halley

The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. We compare the results from a kinetic model of the neutral cometary environment with measurements from the Neutral Mass Spectrometer and the Dust Impact Detection System onboard the Giotto spacecraft taken during the fly-by at Comet 1P/Halley in 1986. We also show that our model is in good agreement with contemporaneous measurements obtained by the International Ultraviolet Explorer, sounding rocket experiments, and various ground based observations.The model solves the Boltzmann equation with a Direct Simulation Monte Carlo technique (Tenishev, V., Combi, M., Davidsson, B. [2008]. Astrophys. J. 685, 659-677) by tracking trajectories of gas molecules and dust grains under the influence of the comet’s weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus is considered to be the source of dust and the parent species (in our model: H₂O, CO, H₂CO, CO₂, CH₃OH, C₂H₆, C₂H₄, C₂H₂, HCN, NH₃, and CH₄) in the coma. Subsequently our model also tracks the corresponding dissociation products (H, H₂, O, OH, C, CH, CH₂, CH₃, N, NH, NH₂, C₂, C₂H, C₂H₅, CN, and HCO) from the comet’s surface all the way out to 10⁶ km.As a result we are able to further constrain cometary the gas production rates of CO (13%), CO₂ (2.5%), and H₂CO (1.5%) relative to water without invoking unknown extended sources.     

Gas kinetics and dust dynamics in low-density comet comae

Extensive regions of low-density cometary comae are characterized by important deviations from the Maxwell-Boltzmann velocity distribution, i.e. breakdown of thermodynamic equilibrium. The consequences of this on the shapes of emission and absorption lines, and for the acceleration of solid bodies due to gas drag, have rarely been investigated.These problems are studied here to aid in the development of future coma models, and in preparation for observations of Comet 67P/Churyumov-Gerasimenko from the ESA Rosetta spacecraft. Two topics in particular, related to Rosetta, are preparation for in situ observations of water, carbon monoxide, ammonia, and methanol emission lines by the mm/sub-mm spectrometer MIRO, as well as gas drag forces on dust grains and on the Rosetta spacecraft itself.Direct Simulation Monte Carlo (DSMC) modeling of H₂O/CO mixtures in spherically symmetric geometries at various heliocentric distances are used to study the evolution of the (generally non-Maxwellian) velocity distribution function throughout the coma. Such distribution functions are then used to calculate Doppler broadening profiles and drag forces.It is found that deviation from thermodynamic equilibrium indeed is commonplace, and already at 2.5 AU from the Sun the entire comet coma displays manifestations of such breakdown, e.g., non-equal partitioning of energy between kinetic and rotational modes, causing substantial differences between translational and rotational temperatures. We exemplify how deviations from thermodynamic equilibrium affect the properties of Doppler broadened line profiles. Upper limits on the size of liftable dust grains as well as terminal grain velocities are presented. Furthermore, it is demonstrated that the drag-to-gravity force ratio is likely to decrease with decreasing cometocentric distance, which may be of relevance both for Rosetta and for the lander probe Philae.     

Kinematic properties and stellar populations of faint early-type galaxies – I. Velocity dispersion measurements of central Coma galaxies

We present velocity dispersion measurements for 69 faint early-type galaxies in the core of the Coma cluster, spanning  −22.0 ≲ M_R ≲−17.5 mag  . We examine the   L –σ  relation for our sample and compare it to that of bright elliptical galaxies (Es) from the literature. The distribution of the the faint early-type galaxies in the   L –σ  plane follows the relation   L ∝σ^2.01±0.36  , which is significantly shallower from   L ∝σ⁴  as defined for the bright Es. While increased rotational support for fainter early-type galaxies could account for some of the difference in slope, we show that it cannot explain it. We also investigate the colour–σ relation for our Coma galaxies. Using the scatter in this relation, we constrain the range of galaxy ages as a function of their formation epoch for different formation scenarios. Assuming a strong coordination in the formation epoch of faint early-type systems in Coma, we find that most had to be formed at least 6 Gyr ago and over a short 1-Gyr period.     

Imaging Borrelly

The nucleus, coma, and dust jets of short-period Comet 19P/Borrelly were imaged from the Deep Space 1 spacecraft during its close flyby in September 2001. A prominent jet dominated the near-nucleus coma and emanated roughly normal to the long axis of nucleus from a broad central cavity. We show it to have remained fixed in position for more than 34 hr, much longer than the 26-hr rotation period. This confirms earlier suggestions that it is co-aligned with the rotation axis. From a combination of fitting the nucleus light curve from approach images and the nucleus' orientation from stereo images at encounter, we conclude that the sense of rotation is right-handed around the main jet vector. The inferred rotation pole is approximately perpendicular to the long axis of the nucleus, consistent with a simple rotational state. Lacking an existing IAU comet-specific convention but applying a convention provisionally adopted for asteroids, we label this the north pole. This places the sub-solar latitude at ∼60° N at the time of the perihelion with the north pole in constant sunlight and thus receiving maximum average insolation.     

New stellar members of the Coma Berenices open star cluster

We present the results of a survey of the Coma Berenices open star cluster (Melotte 111), undertaken using proper motions from the USNO-B1.0 (United States Naval Observatory) and photometry from the Two-Micron All-Sky Survey (2MASS) Point Source catalogues. We have identified 60 new candidate members with masses in the range  1.007 < M < 0.269 M_⊙  . For each we have estimated a membership probability by extracting control clusters from the proper motion vector diagram. All 60 are found to have greater than 60 per cent probability of being members, more than doubling the number of known cluster members. The new luminosity function for the cluster peaks at bright magnitudes, but is rising at   K ≈ 12  , indicating that it is likely that lower mass members may exist. The mass function also supports this hypothesis.     

Spectroscopic binaries near the North Galactic Pole paper 15: HD 106947

Photoelectric radial-velocity measurements show that HD 106947 is a double-lined spectroscopic binary. The components have spectral types of about F6 V and G5 V and are in a 59-day orbit of moderate eccentricity. The system is a member of the Coma Cluster.