Evidence for non-axisymmetry in M31 from wide-field kinematics of stars and gas

As the nearest large spiral galaxy, M31 provides a unique opportunity to learn about the structure and evolutionary history of this galaxy type in great detail. Among the many observing programs aimed at M31 are microlensing studies, which require good three-dimensional models of the stellar mass distribution. Possible non-axisymmetric structures like a bar need to be taken into account. Due to M31's high inclination, the bar is difficult to detect in photometry alone. Therefore, detailed kinematic measurements are needed to constrain the possible existence and position of a bar in M31. We obtained $\approx$ 220 separate fields with the optical IFU spectrograph VIRUS-W, covering the whole bulge region of M31 and parts of the disk. We derive stellar line-of-sight velocity distributions from the stellar absorption lines, as well as velocity distributions and line fluxes of the emission lines H$\beta$, [OIII] and [NI]. Our data supersede any previous study in terms of spacial coverage and spectral resolution. We find several features that are indicative of a bar in the kinematics of the stars, we see intermediate plateaus in the velocity and the velocity dispersion, and correlation between the higher moment $h3$ and the velocity. The gas kinematics is highly irregular, but is consistent with non-triaxial streaming motions caused by a bar. The morphology of the gas shows a spiral pattern, with seemingly lower inclination than the stellar disk. We also look at the ionization mechanisms of the gas, which happens mostly through shocks and not through starbursts.Comment: 23 pages, 39 figures; accepted for publication in A&

Unravelling stellar populations in the Andromeda Galaxy

To understand the history and formation mechanisms of galaxies it is crucial to determine their current multidimensional structure. Here we focus on stellar population properties, such as metallicity and [$\alpha$/Fe] enhancement. We devise a new technique to recover the distribution of these parameters using spatially resolved, line-of-sight averaged data. Our chemodynamical method is based on the made-to-measure (M2M) framework and results in an $N$-body model for the abundance distribution. We test our method on a mock data set and find that the radial and azimuthal profiles are well-recovered, however only the overall shape of the vertical profile matches the true profile. We apply our procedure to spatially resolved maps of mean [Z/H] and [$\alpha$/Fe] for the Andromeda Galaxy, using an earlier barred dynamical model of M31. We find that the metallicity is enhanced along the bar, with possible maxima at the ansae. In the edge-on view the [Z/H] distribution has an X shape due to the boxy/peanut bulge; the average vertical metallicity gradient is equal to $-0.133\pm0.006$ dex/kpc. We identify a metallicity-enhanced ring around the bar, which also has relatively lower [$\alpha$/Fe]. The highest [$\alpha$/Fe] is found in the centre, due to the classical bulge. Away from the centre, the $\alpha$-overabundance in the bar region increases with height, which could be an indication of a thick disc. We argue that the galaxy assembly resulted in a sharp peak of metallicity in the central few hundred parsecs and a more gentle negative gradient in the remaining disc, but no [$\alpha$/Fe] gradient. The formation of the bar lead to the re-arrangement of the [Z/H] distribution, causing a flat gradient along the bar. Subsequent star formation close to the bar ends may have produced the metallicity enhancements at the ansae and the [Z/H] enhanced lower-$\alpha$ ring.Comment: 20 pages, 17 figures, 1.9MB, accepted to A&

A 2 epoch proper motion catalogue from the UKIDSS Large Area Survey

This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly citedThe UKIDSS Large Area Survey (LAS) began in 2005, with the start of the UKIDSS program as a 7 year effort to survey roughly 4000 square degrees at high galactic latitudes in Y, J, H and K bands. The survey also included a significant quantity of 2-epoch J band observations, with epoch baselines ranging from 2 to 7 years. We present a proper motion catalogue for the 1500 square degrees of the 2 epoch LAS data, which includes some 800,000 sources with motions detected above the 5 sigma level. We developed a bespoke proper motion pipeline which applies a source-unique second order polynomial transformation to UKIDSS array coordinates of each source to counter potential local non-uniformity in the focal plane. Our catalogue agrees well with the proper motion data supplied in the current WFCAM Science Archive (WSA) DR9 catalogue where there is overlap, and in various optical catalogues, but it benefits from some improvements. One improvement is that we provide absolute proper motions, using LAS galaxies for the relative to absolute correction. Also, by using unique, local, 2nd order polynomial tranformations, as opposed to the linear transformations in the WSA, we correct better for any local distortions in the focal plane, not including the radial distortion that is removed by their pipeline

The Supermassive Black Hole and Double Nucleus of the Core Elliptical NGC 5419

We obtained adaptive-optics assisted SINFONI observations of the central regions of the giant elliptical galaxy NGC5419 with a spatial resolution of 0.2 arcsec (≈55 pc). NGC5419 has a large depleted stellar core with a radius of 1.58 arcsec (430 pc). HST and SINFONI images show a point source located at the galaxy\u27s photocentre, which is likely associated with the low-luminosity AGN previously detected in NGC5419. Both the HST and SINFONI images also show a second nucleus, off-centred by 0.25 arcsec (≈70 pc). Outside of the central double nucleus, we measure an almost constant velocity dispersion of σ∼350 km/s. In the region where the double nucleus is located, the dispersion rises steeply to a peak value of ∼420 km/s. In addition to the SINFONI data, we also obtained stellar kinematics at larger radii from the South African Large Telescope. While NGC5419 shows low rotation (v\u3c50 km/s), the central regions (inside ∼4rb) clearly rotate in the opposite direction to the galaxy\u27s outer parts. We use orbit-based dynamical models to measure the black hole mass of NGC5419 from the kinematical data outside of the double nuclear structure. The models imply MBH=7.2+2.7−1.9×109 M⊙. The enhanced velocity dispersion in the region of the double nucleus suggests that NGC5419 possibly hosts two supermassive black holes at its centre, separated by only ≈70 pc. Yet our measured MBH is consistent with the black hole mass expected from the size of the galaxy\u27s depleted stellar core. This suggests, that systematic uncertainties in MBH related to the secondary nucleus are small

Re-growth of stellar disks in mature galaxies: The two component nature of NGC 7217 revisited with VIRUS-W

Previous studies have reported the existence of two counter-rotating stellar disks in the early-type spiral galaxy NGC7217. We have obtained high-resolution optical spectroscopic data (R ~ 9000) with the new fiber-based Integral Field Unit instrument VIRUS-W at the 2.7m telescope of the McDonald Observatory in Texas. Our analysis confirms the existence of two components. However, we find them to be co-rotating. The first component is the more luminous (~ 77% of the total light), has the higher velocity dispersion (~ 170 km/s) and rotates relatively slowly (projected $v_{max}$ = 50 km/s). The lower luminosity second component, (~ 23% of the total light), has a low velocity dispersion (~ 20 km/s) and rotates quickly (projected $v_{max}$ = 150 km/s). The difference in the kinematics of the two stellar components allows us to perform a kinematic decomposition and to measure the strengths of their Mg and Fe Lick indices separately. The rotational velocities and dispersions of the less luminous and faster component are very similar to those of the interstellar gas as measured from the [OIII] emission. Morphological evidence of active star formation in this component further suggests that NGC7217 may be in the process of (re)growing a disk inside a more massive and higher dispersion stellar halo. The kinematically cold and regular structure of the gas disk in combination with the central almost dust-free morphology allows us to compare the dynamical mass inside of the central 500pc with predictions from a stellar population analysis. We find agreement between the two if a Kroupa stellar initial mass function is assumed.Comment: accepted for publication by MNRA

Central rotations of Milky Way Globular Clusters

Most Milky Way globular clusters (GCs) exhibit measurable flattening, even if on a very low level. Both cluster rotation and tidal fields are thought to cause this flattening. Nevertheless, rotation has only been confirmed in a handful of GCs, based mostly on individual radial velocities at large radii. We are conducting a survey of the central kinematics of Galactic GCs using the new Integral Field Unit instrument VIRUS-W. We detect rotation in all 11 GCs that we have observed so far, rendering it likely that a large majority of the Milky Way GCs rotate. We use published catalogs of the ACS survey of GCs to derive central ellipticities and position angles. We show that in all cases where the central ellipticity permits an accurate measurement of the position angle, those angles are in excellent agreement with the kinematic position angles that we derive from the VIRUS-W velocity fields. We find an unexpected tight correlation between central rotation and outer ellipticity, indicating that rotation drives flattening for the objects in our sample. We also find a tight correlation between central rotation and published values for the central velocity dispersion, most likely due to rotation impacting the old dispersion measurements.Comment: 6 pages, 3 figures; accepted for publication in ApJ Letter