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Register a new userIdentifying Bright Stars in Crowded Environments Using Velocity Dispersion Measurements, and an Application to the Center of M32
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The identification of individual stars in crowded environments using photometric information alone is confounded by source confusion. However, with the addition of spectroscopic information it is possible to distinguish between blends and areas where the light is dominated by a single star using the widths of absorption features. We describe a procedure for identifying locations in kinematically hot environments where the light is dominated by a single star, and apply this method to spectra with 0.1 arcsec angular resolution covering the 2.1 - 2.3 micron interval in the central regions of M32. Targets for detailed investigation are selected as areas of localized brightness enhancement. Three locations where at least 60% of the K-band light comes from a single bright star, and another with light that is dominated by two stars with very different velocities, are identified. The dominant stars are evolving near the tip of the asymptotic giant branch (AGB), and have M5 III spectral type. The lack of a dispersion in spectral-type suggests that the upper AGB within the central arcsec of M32 has a dispersion in J-K of only a few hundreths of a magnitude, in agreement with what is seen at larger radii. One star has weaker atomic absorption lines than the others, such that [M/H] is 0.2 dex lower. Such a difference in metallicity is consistent with the metallicity dispersion inferred from the width of the AGB in M32. The use of line width to distinguish between blends involving many relatively faint stars, none of which dominate the light output, and areas that are dominated by a single intrinsically bright star could be extended to crowded environments in other nearby galaxies.
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We use velocity dispersion measurements of 21 individual cluster members in the core of Abell 383, obtained with MMT Hectospec, to separate the galaxy and the smooth dark halo (DH) lensing contributions. While lensing usually constrains the overall, projected mass density, the innovative use of velocity dispersion measurements as a proxy for masses of individual cluster members breaks inherent degeneracies and allows us to (a) refine the constraints on single galaxy masses and on the galaxy mass-to-light scaling relation and, as a result, (b) refine the constraints on the DM-only map, a high-end goal of lens modelling. The knowledge of cluster member velocity dispersions improves the fit by 17% in terms of the image reproduction $chi^2$, or 20% in terms of the rms. The constraints on the mass parameters improve by ~10% for the DH, while for the galaxy component, they are refined correspondingly by ~50%, including the galaxy halo truncation radius. For an L$^*$ galaxy with M$^*_B$=-20.96, for example, we obtain best fitting truncation radius r$^*_{tr}=20.5^{+9.6}_{-6.7}$ kpc and velocity dispersion $sigma^*=324pm17 km/s$. Moreover, by performing the surface brightness reconstruction of the southern giant arc, we improve the constraints on r$_{tr}$ of two nearby cluster members, which have measured velocity dispersions, by more than ~30%. We estimate the stripped mass for these two galaxies, getting results that are consistent with numerical simulations. In the future, we plan to apply this analysis to other galaxy clusters for which velocity dispersions of member galaxies are available.
Satellite galaxies are commonly used as tracers to measure the line-of-sight velocity dispersion ($sigma_{rm LOS}$) of the dark matter halo associated with their central galaxy, and thereby to estimate the halos mass. Recent observational dispersion estimates of the Local Group, including the Milky Way and M31, suggest $sigmasim$50 km/s, which is surprisingly low when compared to the theoretical expectation of $sigmasim$100s km/s for systems of their mass. Does this pose a problem for $Lambda$CDM? We explore this tension using the {small{SURFS}} suite of $N$-body simulations, containing over 10000 (sub)haloes with well tracked orbits. We test how well a central galaxys host halo velocity dispersion can be recovered by sampling $sigma_{rm LOS}$ of subhaloes and surrounding haloes. Our results demonstrate that $sigma_{rm LOS}$ is biased mass proxy. We define an optimal window in $v_{rm LOS}$ and projected distance ($D_p$) -- $0.5lesssim D_p/R_{rm vir}lesssim1.0$ and $v_{rm LOS} lesssim0.5V_{rm esc}$, where $R_{rm vir}$ is the virial radius and $V_{rm esc}$ is the escape velocity -- such that the scatter in LOS to halo dispersion is minimised - $sigma_{rm LOS}=(0.5pm0.1)sigma_{v,{rm H}}$. We argue that this window should be used to measure line-of-sight dispersions as a proxy for mass, as it minimises scatter in the $sigma_{rm LOS}-M_{rm vir}$ relation. This bias also naturally explains the results from cite{mcconnachie2012a}, who used similar cuts when estimating $sigma_{rm LOS,LG}$, producing a bias of $sigma_{rm LG}=(0.44pm0.14)sigma_{v,{rm H}}$. We conclude that the Local Groups velocity dispersion does not pose a problem for $Lambda$CDM and has a mass of $log M_{rm LG, vir}/M_odot=12.0^{+0.8}_{-2.0}$.
Using the DIANOGA hydrodynamical zoom-in simulation set of galaxy clusters, we analyze the dynamics traced by stars belonging to the Brightest Cluster Galaxies (BCGs) and their surrounding diffuse component, forming the intracluster light (ICL), and compare it to the dynamics traced by dark matter and galaxies identified in the simulations. We compute scaling relations between the BCG and cluster velocity dispersions and their corresponding masses (i.e. $M_mathrm{BCG}^{star}$- $sigma_mathrm{BCG}^{star}$, $M_{200}$- $sigma_{200}$, $M_mathrm{BCG}^{star}$- $M_{200}$, $sigma_mathrm{BCG}^{star}$- $sigma_{200}$), we find in general a good agreement with observational results. Our simulations also predict $sigma_mathrm{BCG}^{star}$- $sigma_{200}$ relation to not change significantly up to redshift $z=1$, in line with a relatively slow accretion of the BCG stellar mass at late times. We analyze the main features of the velocity dispersion profiles, as traced by stars, dark matter, and galaxies. As a result, we discuss that observed stellar velocity dispersion profiles in the inner cluster regions are in excellent agreement with simulations. We also report that the slopes of the BCG velocity dispersion profile from simulations agree with what is measured in observations, confirming the existence of a robust correlation between the stellar velocity dispersion slope and the cluster velocity dispersion (thus, cluster mass) when the former is computed within $0.1 R_{500}$. Our results demonstrate that simulations can correctly describe the dynamics of BCGs and their surrounding stellar envelope, as determined by the past star-formation and assembly histories of the most massive galaxies of the Universe.
In various fields of physics and astronomy, access to experimental facilities or to telescopes is becoming more and more competitive and limited. It becomes therefore important to optimize the type of measurements and their scheduling to reach a given scientific objective and to increase the chances of success of a scientific project. In this communication, extending the work of Ford (2008) and of Loredo et al. (2012), we present an efficient adaptive scheduling tool aimed at prioritzing measurements in order to reach a scientific goal. The algorithm, based on the Fisher matrix, can be applied to a wide class of measurements. We present this algorithm in detail and discuss some practicalities such as systematic errors or measurements losses due to contigencies (such as weather, experimental failure, ...). As an illustration, we consider measurements of the short-period star S0-2 in our Galactic Center. We show that the radial velocity measurements at the two turning points of the radial velocity curve are more powerful for detecting the gravitational redshift than measurements at the maximal relativistic signal. We also explicitly present the methodology that was used to plan measurements in order to detect the relativistic redshift considering systematics and possible measurements losses. For the future, we identify the astrometric turning points to be highly sensitive to the relativistic advance of the periastron. Finally, we also identify measurements particularly sensitive to the distance to our Galactic Center: the radial velocities around periastron and the astrometric measurements just before closest approach and at the maximal right ascension astrometric turning point.
The unusual morphologies of the Andromeda spiral galaxy (M31) and its dwarf companion M32 have been characterized observationally in great detail. The two galaxies apparent proximity suggests that Andromedas prominent star-forming ring as well as M32s compact elliptical structure may result from a recent collision. Here we present the first self-consistent model of the M31-M32 interaction that simultaneously reproduces observed positions, velocities, and morphologies for both galaxies. Andromedas spiral structure is resolved in unprecedented detail, showing that a rare head-on orbit is not necessary to match Andromedas ring-like morphology. The passage of M32 through Andromedas disk perturbs the disk velocity structure. We find tidal stripping of M32s stars to be inefficient during the interaction, suggesting that some cEs are intrinsically compact. Additionally, the orbital solution implies that M32 is currently closer to the Milky Way than models have typically assumed, a prediction that may be testable with upcoming observations.
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