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Register a new userAndromeda chained to the Box -- Dynamical Models for M31: Bulge & Bar
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Authors
Matias Blana
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Andromeda is our nearest neighbouring disk galaxy and a prime target for detailed modelling of the evolutionary processes that shape galaxies. We analyse the nature of M31s triaxial bulge with an extensive set of N-body models, which include Box/Peanut (B/P) bulges as well as initial classical bulges (ICBs). Comparing with IRAC 3.6$mu m$ data, only one model matches simultaneously all the morphological properties of M31s bulge, and requires an ICB and a B/P bulge with 1/3 and 2/3 of the total bulge mass respectively. We find that our pure B/P bulge models do not show concentrations high enough to match the Sersic index ($n$) and the effective radius of M31s bulge. Instead, the best model requires an ICB component with mass $M^{rm ICB}=1.1times10^{10}{rm M_{odot}}$ and three-dimensional half-mass radius $r_{rm half}^{rm ICB}$=0.53 kpc (140 arcsec). The B/P bulge component has a mass of $M^{rm B/P}=2.2times10^{10}{rm M_{odot}}$ and a half-mass radius of $r_{rm half}^{rm B/P}$=1.3 kpc (340 arcsec). The models B/P bulge extends to $r^{rm B/P}$=3.2 kpc (840 arcsec) in the plane of the disk, as does M31s bulge. In this composite bulge model, the ICB component explains the velocity dispersion drop observed in the centre within $R<$190 pc (50 arcsec), while the B/P bulge component reproduces the observed rapid rotation and the kinematic twist of the observed zero velocity line. This models pattern speed is $Omega_p$=38 km/s/kpc, placing corotation at $r_{rm cor}$=5.8 kpc (1500 arcsec). The outer Lindblad resonance (OLR) is then at $r_{rm OLR}$=10.4kpc, near the 10kpc-ring of M31, suggesting that this structure may be related to the bars OLR. By comparison with an earlier snapshot, we estimate that M31s thin bar extends to $r_{rm bar}^{rm thin}sim$4.0 kpc (1000 arcsec) in the disk plane, and in projection extends to $R_{rm bar}^{rm thin}sim$2.3 kpc (600 arcsec).
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We construct dynamical models of the Milky Ways Box/Peanut (B/P) bulge, using the recently measured 3D density of Red Clump Giants (RCGs) as well as kinematic data from the BRAVA survey. We match these data using the NMAGIC Made-to-Measure method, starting with N-body models for barred discs in different dark matter haloes. We determine the total mass in the bulge volume of the RCGs measurement (+-2.2 x +- 1.4 x +- 1.2 kpc) with unprecedented accuracy and robustness to be 1.84 +- 0.07 x10^10 Msun. The stellar mass in this volume varies between 1.25-1.6 x10^10 Msun, depending on the amount of dark matter in the bulge. We evaluate the mass-to-light and mass-to-clump ratios in the bulge and compare them to theoretical predictions from population synthesis models. We find a mass-to-light ratio in the K-band in the range 0.8-1.1. The models are consistent with a Kroupa or Chabrier IMF, but a Salpeter IMF is ruled out for stellar ages of 10 Gyr. To match predictions from the Zoccali IMF derived from the bulge stellar luminosity function requires about 40% or 0.7 x10^10 Msun dark matter in the bulge region. The BRAVA data together with the RCGs 3D density imply a low pattern speed for the Galactic B/P bulge of 25-30 km.s-1.kpc-1. This would place the Galaxy among the slow rotators (R >= 1.5). Finally, we show that the Milky Ways B/P bulge has an off-centred X structure, and that the stellar mass involved in the peanut shape accounts for at least 20% of the stellar mass of the bulge, significantly larger than previously thought.
We present a suite of semi-analytic disk-bulge-halo models for the Andromeda galaxy (M31) which satisfy three fundamental conditions: (1) internal self-consistency; (2) consistency with observational data; and (3) stability of the disk against the formation of a central bar. The models are chosen from a set first constructed by Kuijken and Dubinski. We develop an algorithm to search the parameter space for this set in order to best match observations of the M31 rotation curve, inner velocity dispersion profile, and surface brightness profile. Models are obtained for a large range of bulge and disk masses; we find that the disk mass must be of order 8 * 10^10 M_sun and that the preferred value for the bulge mass is 2.5 * 10^10 M_sun. N-body simulations are carried out to test the stability of our models against the formation of a bar within the disk. We also calculate the baryon fraction and halo concentration parameter for a subset of our models and show that the results are consistent with the predictions from cosmological theories of structure formation. In addition, we describe how gravitational microlensing surveys and dynamical studies of globular clusters and satellites can further constrain the models.
As part of the Panchromatic Hubble Andromeda Treasury (PHAT) multi-cycle program, we observed a 12 times 6.5 area of the bulge of M31 with the WFC3/UVIS filters F275W and F336W. From these data we have assembled a sample of sim4000 UV-bright, old stars, vastly larger than previously available. We use updated Padova stellar evolutionary tracks to classify these hot stars into three classes: Post-AGB stars (P-AGB), Post-Early AGB (PE-AGB) stars and AGB-manque stars. P-AGB stars are the end result of the asymptotic giant branch (AGB) phase and are expected in a wide range of stellar populations, whereas PE-AGB and AGB-manque (together referred to as the hot post-horizontal branch; HP-HB) stars are the result of insufficient envelope masses to allow a full AGB phase, and are expected to be particularly prominent at high helium or {alpha} abundances when the mass loss on the RGB is high. Our data support previous claims that most UV-bright sources in the bulge are likely hot (extreme) horizontal branch stars (EHB) and their progeny. We construct the first radial profiles of these stellar populations, and show that they are highly centrally concentrated, even more so than the integrated UV or optical light. However, we find that this UV-bright population does not dominate the total UV luminosity at any radius, as we are detecting only the progeny of the EHB stars that are the likely source of the UVX. We calculate that only a few percent of MS stars in the central bulge can have gone through the HP-HB phase and that this percentage decreases strongly with distance from the center. We also find that the surface density of hot UV-bright stars has the same radial variation as that of low-mass X-ray binaries. We discuss age, metallicity, and abundance variations as possible explanations for the observed radial variation in the UV-bright population.
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 construct a large set of dynamical models of the galactic bulge, bar and inner disk using the Made-to-Measure method. Our models are constrained to match the red clump giant density from a combination of the VVV, UKIDSS and 2MASS infrared surveys together with stellar kinematics in the bulge from the BRAVA and OGLE surveys, and in the entire bar region from the ARGOS survey. We are able to recover the bar pattern speed and the stellar and dark matter mass distributions in the bar region, thus recovering the entire galactic effective potential. We find a bar pattern speed of $39.0 pm 3.5 ,rm{km,s^{-1},kpc^{-1}}$, placing the bar corotation radius at $6.1 pm 0.5 rm{kpc}$ and making the Milky Way bar a typical fast rotator. We evaluate the stellar mass of the long bar and bulge structure to be $M_{rm{bar/bulge}} = 1.88 pm 0.12 times 10^{10} , rm{M}_{odot}$, larger than the mass of disk in the bar region, $M_{rm{inner disk}} = 1.29pm0.12 times 10^{10} , rm{M}_{odot}$. The total dynamical mass in the bulge volume is $1.85pm0.05times 10^{10} , rm{M}_{odot}$. Thanks to more extended kinematic data sets and recent measurement of the bulge IMF our models have a low dark matter fraction in the bulge of $17%pm2%$. We find a dark matter density profile which flattens to a shallow cusp or core in the bulge region. Finally, we find dynamical evidence for an extra central mass of $sim0.2times10^{10} ,rm{M}_{odot}$, probably in a nuclear disk or disky pseudobulge.
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