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تسجيل مستخدم جديدNGC 147, NGC 185 and CassII: a genetic approach to orbital properties, star formation and tidal debris
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NGC147, NGC185 and CassiopeiaII (CassII) have similar positions in the sky, distances and measured line of sight velocities. This proximity in phase space suggests that these three satellites of M31 form a subgroup within the Local Group. Nevertheless, the differences in their star formation history and interstellar medium, and the recent discovery of a stellar stream in NGC~147, combined with the lack of tidal features in the other two satellites, are all indications of complex and diverse interactions between M31 and these three satellites. We use a genetic algorithm to explore the different orbits that these satellites can have and select six sets of orbits that could best explain the observational features of the NGC147, NGC185 and CassII satellites. The parameters of these orbits are then used as a starting point for N-body simulations. We present models for which NGC147, NGC185 and CassII are a bound group for a total time of at least one Gyr but still undergo different interactions with M31 and as a result NGC147 has a clear stellar stream whereas the other two satellites have no significant tidal features. This result shows that it is possible to find solutions that reproduce the contrasting properties of the satellites and for which NGC147-NGC185-CassII have been gravitationally bound.
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We present new mid- to far-infrared images of the two dwarf compact elliptical galaxies that are satellites of M31, NGC 185 and NGC 147, obtained with the Spitzer Space Telescope. Spitzers high sensitivity and spatial resolution enable us for the fir
st time to look directly into the detailed spatial structure and properties of the dust in these systems. The images of NGC 185 at 8 and 24 micron display a mixed morphology characterized by a shell-like diffuse emission region surrounding a central concentration of more intense infrared emission. The lower resolution images at longer wavelengths show the same spatial distribution within the central 50 but beyond this radius, the 160 micron emission is more extended than that at 24 and 70micron. On the other hand, the dwarf galaxy NGC 147 located only a small distance away from NGC 185 shows no significant infrared emission beyond 24 micron and therefore its diffuse infrared emission is mainly stellar in origin. For NGC 185, the derived dust mass based on the best fit to the spectral energy distribution is 1.9e3 Msol, implying a gas mass of ~3.0e5 Msol. These values are in agreement with those previously estimated from infrared as well as from CO and HI observations and are consistent with the predicted mass return from dying stars based on the last burst of star formation ~1.0e9 yr ago. Based on the 70 to 160micron flux density ratio, we estimate a temperature for the dust of ~17K. For NGC 147, we obtain an upper limit for the dust mass of 4.5e2 Msol at 160 micron (assuming a temperature of ~20K), a value consistent with the previous upper limit derived using ISO observations of this galaxy. In the case of NGC 185, we also present full 5-38 micron low-resolution (R~100) spectra of the main emission regions.
NGC 147 and NGC 185 are two of the most massive satellites of the Andromeda galaxy (M 31). Close together in the sky, of similar mass and morphological type dE, they possess different amounts of interstellar gas and tidal distortion. The question the
refore is, how do their histories compare? Here we present the first reconstruction of the star formation histories of NGC 147 and NGC 185 using long-period variable stars. These represent the final phase of evolution of low- and intermediate-mass stars at the asymptotic giant branch, when their luminosity is related to their birth mass. Combining near-infrared photometry with stellar evolution models, we construct the mass function and hence the star formation history. For NGC 185 we found that the main epoch of star formation occurred 8.3 Gyr ago, followed by a much lower, but relatively constant star formation rate. In the case of NGC 147, the star formation rate peaked only 7 Gyr ago, staying intense until ~ 3 Gyr ago, but no star formation has occurred for at least 300 Myr. Despite their similar masses, NGC 147 has evolved more slowly than NGC 185 initially, but more dramatically in more recent times. This is corroborated by the strong tidal distortions of NGC 147 and the presence of gas in the centre of NGC 185.
We present the first proper motion (PM) measurements for the dwarf elliptical galaxies NGC 147 and NGC 185, two satellite galaxies of M31, using multi-epoch HST imaging data with time baselines of $sim 8$ years. For each galaxy, we take an error-weig
hted average of measurements from HST ACS/WFC and WFC3/UVIS to determine the PMs. Our final results for the PMs are $(mu_mathrm{W}, mu_mathrm{N})_mathrm{N147} = (-0.0232, 0.0378) pm (0.0143, 0.0146),mathrm{mas},mathrm{yr}^{-1}$ for NGC 147, and $(mu_mathrm{W}, mu_mathrm{N})_mathrm{N185} = (-0.0242, 0.0058) pm (0.0141, 0.0147),mathrm{mas},mathrm{yr}^{-1}$ for NGC 185. The 2-dimensional direction of motion for NGC 147 about M31 is found to be aligned with its tidal tails. The 3-d positions and velocities of both galaxies are transformed into a common M31-centric coordinate system to study the detailed orbital histories of the combined M31+NGC 147+NGC 185 system via numerical orbit integration. We find that NGC 147 (NGC 185) had its closest passage to M31 0.3-0.5~Gyr ($gtrsim 1.6$~Gyr) within the past 6 Gyr at distances of $sim 70$ kpc (70-260 kpc). The pericentric times of NGC 147/NGC 185 correlate qualitatively well with the presence/absence of tidal tails seen around the galaxies. Our PMs show that the orbital poles of NGC 147, and also NGC 185 albeit to a lesser degree, agree within the uncertainties with the normal of the Great Plane of Andromeda (GPoA). These are the first measurements of the 3-d angular momentum vector of any satellite identified as original GPoA members. Our results strengthen the hypothesis that the GPoA may be a dynamically coherent entity. We revisit previous claims that NGC 147 and NGC 185 are binary galaxies and conclude that it is very unlikely the two galaxies were ever gravitationally bound to each other.
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