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Register a new user3-Dimensional Kinematics in low foreground extinction windows of the Galactic Bulge: Radial Velocities for 6 bulge fields
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M. Soto
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The detailed structure of the Galactic bulge still remain uncertain. The strong difficulties of obtaining observations of stars in the Galactic bulge have hindered the acquisition of a kinematic representation for the inner kpc of the Milky Way. The observation of the 3-d kinematics in several low foreground extinction windows can solve this problem. We have developed a new technique, which combines precise stellar HST positions and proper motions with integral field spectroscopy, in order to obtain reliable 3-d stellar kinematics in crowded fields of the Galactic center. In addition, we present results using the new techniques for six fields in our project. A significant vertex deviation has been found in some of the fields in agreement with previous determinations. This result confirms the presence of a stellar bar in the Galactic bulge.
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Several recent studies have demonstrated that the Galactic bulge hosts two components with different mean metallicities, and possibly different spatial distribution and kinematics. As a consequence, both the metallicity distribution and the radial velocity of bulge stars vary across different line of sights. We present here the metallicity distribution function of red clump stars in 26 fields spread across a wide area of the bulge, with special emphasis on fields close to Galactic plane, at latitudes b=-2 and b=-1, that were not explored before. This paper includes new metallicities from a sample of about 5000 K giant stars, observed at spectral resolution R=6500, in the Calcium II Triplet region. They are the main dataset of the GIRAFFE Inner Bulge Survey. As part of the same survey we have previously published results for a sample of about 600 K giant stars, at latitude b=-4 , derived from higher resolution spectra (R=22,500). Results. The combined sample allows us to trace and characterize the metal poor and metal rich bulge populations down to the inner bulge. We present a density map for each of the two components. Contrary to the expectations from previous works, we found the metal poor population to be more centrally concentrated than the metal rich one, and with a more axisymmetric spatial distribution. The metal rich population, on the other hand, is arranged in a boxy distribution, consistent with an edge-on bar. By coupling metallicities and radial velocities we show that the metal poor population has a velocity dispersion that varies rather mildly with latitude. On the contrary, the metal rich population has a low velocity dispersion far from the plane (b=-8.5), but it has a steeper gradient with latitude, becoming higher than the metal poor one in the innermost field (b=-1). [abridged]
We present the reddening (E(V-I)) and Extinction maps in V-band (A_V) and I-band (A_I) for 48 Optical Gravitational Lensing Experiment II (OGLE-II) Galactic bulge (GB) fields, covering a range of $-11^circ <l< 11^circ$, with the total area close to 11 square degrees. These measurements are based on two-band photometry of Red Clump Giant (RCG) stars in OGLE-II VI maps of GB. We confirm the anomalous value of the ratio of total to selective extinction $R_{VI} = A_V / E(V-I) = 1.9 sim 2.1$, depending on the line of sight, as measured by Udalski (2003). By using the average value of $R_{VI}=1.964$ with the standard deviation sdev=0.085, we measured E(V-I), A_V and A_I, and we obtained extinction and reddening maps with a high spatial resolution of $ 26.7sim 106.8$, depending on the stellar density of each field. We assumed that average, reddening corrected colours of red clump giants are the same in every field. The maps cover the range 0.42<E(V-I)<3.5, 0.83<A_V<6.9 and 0.41<A_I<3.4 mag respectively. The zero points of these maps are calibrated by using V-K colours of 20 RR Lyrae ab variables (RRab) in Baades window. The apparent reddening corrected I-band magnitudes of the RCGs change by +0.4 mag while the Galactic coordinate l varies from $+5^{circ}$ to $-5^{circ}$, indicating that these stars are in the Galactic Bar. The reddening corrected colour of RRab and RCGs in GB are consistent with colours of local stars, while in the past these colours were claimed to be different.
We present new results from BRAVA, a large scale radial velocity survey of the Galactic bulge, using M giant stars selected from the Two Micron All Sky Survey catalogue as targets for the Cerro Tololo Inter-American Observatory 4m Hydra multi-object spectrograph. The purpose of this survey is to construct a new generation of self-consistent bar models that conform to these observations. We report the dynamics for fields at the edge of the Galactic bulge at latitudes b=-8 deg. and compare to the dynamics at b=-4 deg. We find that the rotation curve V(r) is the same at b=-8 deg. as at b=-4 deg. That is, the Galactic boxy bulge rotates cylindrically, as do boxy bulges of other galaxies. The summed line of sight velocity distribution at b=-8 deg. is Gaussian, and the binned longitude-velocity plot shows no evidence for either a (disk) population with cold dynamics or for a (classical bulge) population with hot dynamics. The observed kinematics are well modeled by an edge-on N-body bar, in agreement with published structural evidence. Our kinematic observations indicate that the Galactic bulge is a prototypical product of secular evolution in galaxy disks, in contrast with stellar population results that are most easily understood if major mergers were the dominant formation process.
We have used the AAOMEGA spectrograph to obtain R $sim 1500$ spectra of 714 stars that are members of two red clumps in the Plaut Window Galactic bulge field $(l,b)=0^{circ},-8^{circ}$. We discern no difference between the clump populations based on radial velocities or abundances measured from the Mg$b$ index. The velocity dispersion has a strong trend with Mg$b$-index metallicity, in the sense of a declining velocity dispersion at higher metallicity. We also find a strong trend in mean radial velocity with abundance. Our red clump sample shows distinctly different kinematics for stars with [Fe/H] $<-1$, which may plausibly be attributable to a minority classical bulge or inner halo population. The transition between the two groups is smooth. The chemo-dynamical properties of our sample are reminiscent of those of the Milky Way globular cluster system. If correct, this argues for no bulge/halo dichotomy and a relatively rapid star formation history. Large surveys of the composition and kinematics of the bulge clump and red giant branch are needed to define further these trends.
The HST/WFC3 multiband photometry spanning from the UV to the near-IR of four fields in the Galactic bulge, together with that for six template globular and open clusters, are used to photometrically tag the metallicity [Fe/H] of stars in these fields after proper-motion rejecting most foreground disk contaminants. Color-magnitude diagrams and luminosity functions are then constructed, in particular for the most metal rich and most metal poor stars in each field. We do not find any significant difference between the $I$-band and $H$-band luminosity functions, hence turnoff luminosity and age, of the metal rich and metal poor components which therefore appear essentially coeval. In particular, we find that no more than $sim 3%$ of the metal-rich component can be $sim 5$ Gyr old, or younger. Conversely, theoretical luminosity functions give a good match to the observed ones for an age of ~10 Gyr. Assuming this age is representative for the bulk of bulge stars, we then recall the observed properties of star-forming galaxies at 10 Gyr lookback time, i.e., at z~2, and speculate about bulge formation in that context. We argue that bar formation and buckling instabilities leading to the observed boxy/peanut, X-shaped bulge may have arisen late in the history of the Milky Way galaxy, once its gas fraction had decreased compared to the high values typical of high-redshift galaxies. This paper follows the public release of the photometric and astrometric catalogs for the measured stars in the four fields.
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