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Register a new userImproving the triaxial bulge model of M31
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A detailed hydrodynamical model of the gas flow in the triaxial gravitational potential of the bulge of the Andromeda galaxy (M31) has recently been proposed by Berman (2001) astro-ph/0103209, and shown to provide excellent agreement with the CO emission line velocities observed along its major axis. In the present paper, we confirm the validity of that model by showing that it can also reproduce the CO velocities observed off the major axis - a much more robust test. The CO observations, however, tend to span a wider range of velocities than a direct application of the original model of Berman would suggest. This situation can be improved significantly if the molecular disk is made thicker, a requirement already encountered in dynamical simulations of other spiral galaxies, and typically attributed to a broadening of the molecular layer in galactic fountain-like processes. In the central regions of M31, however, it is unclear whether there actually is a thick molecular disk, or whether broadening the molecular layer is merely an artificial theoretical means of accounting for some disk warping. Other effects not included in the model, such as hydraulic jumps, might also contribute to a widening of the velocities.
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We present the study of stellar populations in the central 5.5 (~1.2 kpc) of the M31 bulge by using the optical color magnitude diagram derived from HST ACS WFC/HRC observations. In order to enhance image quality and then obtain deeper photometry, we construct Nyquist-sampled images and use a deconvolution method to detect sources and measure their photometry. We demonstrate that our method performs better than DOLPHOT in the extremely crowded region. The resolved stars in the M31 bulge have been divided into nine annuli and the color magnitude diagram fitting is performed for each of them. We confirm that the majority of stars (> 70%) in the M31 bulge are indeed very old (>5 Gyr) and metal-rich ([Fe/H] > 0.3). At later times, the star formation rate decreased and then experienced a significant rise around 1 Gyr ago, which pervaded the entire M31 bulge. After that, stars formed at less than 500 Myr ago in the central 130. Through simulation, we find that these intermediate-age stars cannot be the artifacts introduced by the blending effect. Our results suggest that although the majority of the M31 bulge are very old, the secular evolutionary process still continuously builds up the M31 bulge slowly. We compare our star formation history with an older analysis derived from the spectral energy distribution fitting, which suggests that the latter one is still a reasonable tool for the study of stellar populations in remote galaxies.
We present the AGAPE astrometric and photometric catalogue of 1579 variable stars in a 14x10 field centred on M31. This work is the first survey devoted to variable stars in the bulge of M31. The R magnitudes of the objects and the B-R colours suggest that our sample is dominated by red long-period variable stars (LPV), with a possible overlap with Cepheid-like type II stars. Twelve nova candidates are identified. Correlations with other catalogues suggest that 2 novae could be recurrent novae and provide possible optical counterparts for 2 supersoft X-ray sources candidates observed with Chandra.
We introduce crowded field integral field (3D) spectrophotometry as a useful technique for the study of resolved stellar populations in nearby galaxies. As a methodological test, we present a pilot study with selected extragalactic planetary nebulae (XPN) in the bulge of M31, demonstrating how 3D spectroscopy is able to improve the limited accuracy of background subtraction which one would normally obtain with classical slit spectroscopy. It is shown that due to the absence of slit effects, 3D is a most suitable technique for spectrophometry. We present spectra and line intensities for 5 XPN in M31, obtained with the MPFS instrument at the Russian 6m BTA, INTEGRAL at the WHT, and with PMAS at the Calar Alto 3.5m Telescope. Using 3D spectra of bright standard stars, we demonstrate that the PSF is sampled with high accuracy, providing a centroiding precision at the milli-arcsec level. Crowded field 3D spectrophotometry and the use of PSF fitting techniques is suggested as the method of choice for a number of similar observational problems, including luminous stars in nearby galaxies, supernovae, QSO host galaxies, gravitationally lensed QSOs, and others.
We study the origin of unresolved X-ray emission from the bulge of M31 based on archival Chandra and XMM-Newton observations. We demonstrate that three different components are present: (i) Broad-band emission from a large number of faint sources -- mainly accreting white dwarfs and active binaries, associated with the old stellar population, similar to the Galactic Ridge X-ray emission of the Milky Way. The X-ray to K-band luminosity ratios are compatible with those for the Milky Way and for M32, in the 2 - 10 keV band it is (3.6 +/- 0.2) x 10^27 erg/s/L_sun. (ii) Soft emission from ionized gas with temperature of about ~ 300 eV and mass of ~ 2 x 10^6 M_sun. The gas distribution is significantly extended along the minor axis of the galaxy suggesting that it may be outflowing in the direction perpendicular to the galactic disk. The mass and energy supply from evolved stars and type Ia supernovae is sufficient to sustain the outflow. We also detect a shadow cast on the gas emission by spiral arms and the 10-kpc star-forming ring, confirming significant extent of the gas in the ``vertical direction. (iii) Hard extended emission from spiral arms, most likely associated with young stellar objects and young stars located in the star-forming regions. The L_X/SFR ratio equals ~ 9 x 10^38 (erg/s)/(M_sun/yr) which is about ~ 1/3 of the HMXBs contribution, determined earlier from Chandra observations of other nearby galaxies.
We present radial velocities for a sample of 723 planetary nebulae (PNe) in the disk and bulge of M31, measured using the WYFFOS fibre spectrograph on the William Herschel telescope. Velocities are determined using the [OIII] 5007 Angstrom emission line. Rotation and velocity dispersion are measured to a radius of 50 arcminutes (11.5 kpc), the first stellar rotation curve and velocity dispersion profile for M31 to such a radius. Our kinematics are consistent with rotational support at radii well beyond the bulge effective radius of 1.4kpc, although our data beyond a radius of 5kpc are limited. We present tentative evidence for kinematic substructure in the bulge of M31 to be studied fully in a later work. This paper is part of an ongoing project to constrain the total mass, mass distribution and velocity anisotropy of the disk, bulge and halo of M31.
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