Resolving stellar populations with crowded field 3D spectroscopy

(Abridged) We describe a new method to extract spectra of stars from observations of crowded stellar fields with integral field spectroscopy (IFS). Our approach extends the well-established concept of crowded field photometry in images into the domain of 3-dimensional spectroscopic datacubes. The main features of our algorithm are: (1) We assume that a high-fidelity input source catalogue already exists and that it is not needed to perform sophisticated source detection in the IFS data. (2) Source positions and properties of the point spread function (PSF) vary smoothly between spectral layers of the datacube, and these variations can be described by simple fitting functions. (3) The shape of the PSF can be adequately described by an analytical function. Even without isolated PSF calibrator stars we can therefore estimate the PSF by a model fit to the full ensemble of stars visible within the field of view. (4) By using sparse matrices to describe the sources, the problem of extracting the spectra of many stars simultaneously becomes computationally tractable. We present extensive performance and validation tests of our algorithm using realistic simulated datacubes that closely reproduce actual IFS observations of the central regions of Galactic globular clusters. We investigate the quality of the extracted spectra under the effects of crowding. The main effect of blending between two nearby stars is a decrease in the S/N in their spectra. The effect increases with the crowding in the field in a way that the maximum number of stars with useful spectra is always ~0.2 per spatial resolution element. This balance breaks down when exceeding a total source density of ~1 significantly detected star per resolution element. We close with an outlook by applying our method to a simulated globular cluster observation with the upcoming MUSE instrument at the ESO-VLT.Comment: accepted for publication in A&A, 19 pages, 19 figure

On the origin of UV-dim stars: a population of rapidly rotating shell stars?

The importance of stellar rotation in setting the observed properties of young star clusters has become clearer over the past decade, with rotation being identified as the main cause of the observed extended main sequence turn-off (eMSTO) phenomenon and split main-sequences. Additionally, young star clusters are observed to host large fractions of rapidly rotating Be stars, many of which are seen nearly equator-on through decretion disks that cause self-extinction (the so called "shell stars"). Recently, a new phenomenon has been reported in the $\sim1.5$ Gyr star cluster NGC 1783, where a fraction of the main sequence turn-off stars appears abnormally dim in the UV. We investigate the origin of these "UV-dim" stars by comparing the UV colour-magnitude diagrams of NGC 1850 ($\sim100$ Myr), NGC 1783 ($\sim1.5$ Gyr), NGC 1978 ($\sim2$ Gyr) and NGC 2121 ($\sim2.5$ Gyr), massive star clusters in the Large Magellanic Cloud. While the younger clusters show a non-negligible fraction of UV-dim stars, we find a significant drop of such stars in the two older clusters. This is remarkable as clusters older than $\sim$2 Gyr do not have an eMSTO, thus a large populations of rapidly rotating stars, because their main sequence turn-off stars are low enough in mass to slow down due to magnetic braking. We conclude that the UV-dim stars are likely rapidly rotating stars with decretion disks seen nearly equator-on (i.e., are shell stars) and discuss future observations that can confirm or refute our hypothesis.Comment: 9 pages, 6 Figures. Accepted for publication in MNRA

The central dynamics of M3, M13, and M92: Stringent limits on the masses of intermediate-mass black holes

We used the PMAS integral field spectrograph to obtain large sets of radial velocities in the central regions of three northern Galactic globular clusters: M3, M13, and M92. By applying the novel technique of crowded field 3D spectroscopy, we measured radial velocities for about 80 stars within the central ~ 10 arcsec of each cluster. These are by far the largest spectroscopic datasets obtained in the innermost parts of these clusters up to now. To obtain kinematical data across the whole extent of the clusters, we complement our data with measurements available in the literature. We combine our velocity measurements with surface brightness profiles to analyse the internal dynamics of each cluster using spherical Jeans models, and investigate whether our data provide evidence for an intermediate-mass black hole in any of the clusters. The surface brightness profiles reveal that all three clusters are consistent with a core profile, although shallow cusps cannot be excluded. We find that spherical Jeans models with a constant mass-to-light ratio provide a good overall representation of the kinematical data. A massive black hole is required in none of the three clusters to explain the observed kinematics. Our 1sigma (3sigma) upper limits are 5300 M_sun (12000 M_sun) for M3, 8600 M_sun (13000 M_sun) for M13, and 980 M_sun (2700 M_sun) for M92. A puzzling circumstance is the existence of several potential high velocity stars in M3 and M13, as their presence can account for the majority of the discrepancies that we find in our mass limits compared to M92.Comment: accepted for publication in A&A, 20 pages, 15 figures, tables D1 to D6 only available at CD

Towards DIB mapping in galaxies beyond 100 Mpc. A radial profile of the λ\lambda5780.5 diffuse interstellar band in AM 1353-272 B

Diffuse Interstellar Bands (DIBs) are non-stellar weak absorption features of unknown origin found in the spectra of stars viewed through one or several clouds of Interstellar Medium (ISM). Research of DIBs outside the Milky Way is currently very limited. Specifically spatially resolved investigations of DIBs outside of the Local Group is, to our knowledge, inexistent. Here, we explore the capability of the high sensitivity Integral Field Spectrograph, MUSE, as a tool to map diffuse interstellar bands at distances larger than 100 Mpc. We use MUSE commissioning data for AM 1353-272 B, the member with highest extinction of the "The Dentist's Chair", an interacting system of two spiral galaxies. High signal-to-noise spectra were created by co-adding the signal of many spatial elements distributed in a geometry of concentric elliptical half-rings. We derived decreasing radial profiles for the equivalent width of the $\lambda$5780.5 DIB both in the receding and approaching side of the companion galaxy up to distances of $\sim$4.6 kpc from the center of the galaxy. Likewise, interstellar extinction, as derived from the Halpha/Hbeta line ratio displays a similar trend, with decreasing values towards the external parts. This translates into an intrinsic correlation between the strength of the DIB and the extinction within AM 1353-272 B consistent with the current existing global trend between these quantities when using measurements for both Galactic and extragalactic sight lines. Mapping of DIB strength in the Local Universe as up to now only done for the Milky Way seems feasible. This offers a new approach to study the relationship between DIBs and other characteristics and species of the ISM in different conditions as those found in our Galaxy to the use of galaxies in the Local Group and/or single sightlines towards supernovae, quasars and galaxies outside the Local Group.Comment: 4 pages, 4 figures, accepted for publication as a Letter in Astronomy and Astrophysics; Received 10 February 2015 / Accepted 20 February 2015 ; English corrections include

Discovery of an old nova remnant in the Galactic globular cluster M 22

A nova is a cataclysmic event on the surface of a white dwarf in a binary system that increases the overall brightness by several orders of magnitude. Although binary systems with a white dwarf are expected to be overabundant in globular clusters (GCs) compared to the Galaxy, only two novae from Galactic globular clusters have been observed. We present the discovery of an emission nebula in the Galactic globular cluster M 22 (NGC 6656) in observations made with the integral-field spectrograph MUSE. We extract the spectrum of the nebula and use the radial velocity determined from the emission lines to confirm that the nebula is part of NGC 6656. Emission-line ratios are used to determine the electron temperature and density. It is estimated to have a mass of 1 to $17 \times 10^{-5}$ solar masses. This mass and the emission-line ratios indicate that the nebula is a nova remnant. Its position coincides with the reported location of a 'guest star', an ancient Chinese term for transients, observed in May 48 BCE. With this discovery, this nova may be one of the oldest confirmed extrasolar events recorded in human history.Comment: 7 pages, 3 figures; accepted for publication in Astronomy & Astrophysic

Updated radial velocities and new constraints on the nature of the unseen source in NGC1850 BH1

A black hole candidate orbiting a luminous star in the Large Magellanic Cloud young cluster NGC 1850 ($\sim100$Myr) has recently been reported based on radial velocity and light curve modelling. Subsequently, an alternative explanation has been suggested for the system: a bloated post-mass transfer secondary star (M$_{\rm initial} \sim 4-5M_{\odot}$, M$_{\rm current} \sim 1-2M_{\odot}$) with a more massive, yet luminous companion (the primary). Upon reanalysis of the MUSE spectra, we found that the radial velocity variations originally reported were underestimated ($K_{\rm 2,revised} = 176\pm3$km/s vs $K_{\rm 2,original} = 140\pm3$km/s) because of the weighting scheme adopted in the full-spectrum fitting analysis. The increased radial velocity semi-amplitude translates into a system mass function larger than previously deduced ($f_{\rm revised}$=2.83$M_{\odot}$ vs $f_{\rm original}$=1.42$M_{\odot}$). By exploiting the spectral disentangling technique, we place an upper limit of 10\% of a luminous primary source to the observed optical light in NGC1850 BH1, assuming that the primary and secondary are the only components contributing to the system. Furthermore, by analysing archival near-infrared data, we find clues to the presence of an accretion disk in the system. These constraints support a low-mass post-mass transfer star but do not provide a definitive answer whether the unseen component in NGC1850 BH1 is indeed a black hole. These results predict a scenario where, if a primary luminous source of mass M $\ge 4.7M_{\odot}$, is present in the system (given the inclination and secondary mass constraints), it must be hidden in a optically thick disk to be undetected in the MUSE spectra.Comment: 10 pages, 8 Figures and 2 Tables. Accepted for publication by MNRA

Kinematic differences between multiple populations in Galactic globular clusters

The formation process of multiple populations in globular clusters is still up for debate. Kinematic differences between the populations are particularly interesting in this respect, because they allow us to distinguish between single-epoch formation scenarios and multi-epoch formation scenarios. We analyze the kinematics of 25 globular clusters and aim to find kinematic differences between multiple populations to constrain their formation process. We split red-giant branch (RGB) stars in each cluster into three populations (P1, P2, P3) for the type-II clusters and two populations (P1 and P2) otherwise using Hubble photometry. We derive the rotation and dispersion profiles for each cluster and its populations by using all stars with radial velocity measurements obtained from MUSE spectroscopy. Based on these profiles, we calculate the rotation strength in terms of ordered-over-random motion $\left(v/\sigma\right)_\mathrm{HL}$ evaluated at the half-light radius of the cluster. We detect rotation in all but four clusters. For NGC~104, NGC~1851, NGC~2808, NGC~5286, NGC~5904, NGC~6093, NGC~6388, NGC~6541, NGC~7078 and NGC~7089 we also detect rotation for P1 and/or P2 stars. For NGC~2808, NGC~6093 and NGC~7078 we find differences in $\left(v/\sigma\right)_\mathrm{HL}$ between P1 and P2 that are larger than $1\sigma$. Whereas we find that P2 rotates faster than P1 for NGC~6093 and NGC~7078, the opposite is true for NGC~2808. However, even for these three clusters, the differences are still of low significance. We find that the strength of rotation of a cluster generally scales with its median relaxation time. For P1 and P2, the corresponding relation is very weak at best. We observe no correlation between the difference in rotation strength between P1 and P2 and cluster relaxation time. The MUSE stellar radial velocities that this analysis is based on are made publicly available

A MUSE map of the central Orion Nebula (M 42)

We present a new integral-field spectroscopic dataset of the central part of the Orion Nebula (M 42), observed with the MUSE instrument at the ESO VLT. We reduced the data with the public MUSE pipeline. The output products are two FITS cubes with a spatial size of ~5.9'x4.9' (corresponding to ~0.76 pc x 0.63 pc) and a contiguous wavelength coverage of 4595...9366 Angstrom, spatially sampled at 0.2". We provide two versions with a sampling of 1.25 Angstrom and 0.85 Angstrom in dispersion direction. Together with variance cubes these files have a size of 75 and 110 GiB on disk. They represent one of the largest integral field mosaics to date in terms of information content. We make them available for use in the community. To validate this dataset, we compare world coordinates, reconstructed magnitudes, velocities, and absolute and relative emission line fluxes to the literature and find excellent agreement. We derive a two-dimensional map of extinction and present de-reddened flux maps of several individual emission lines and of diagnostic line ratios. We estimate physical properties of the Orion Nebula, using the emission line ratios [N II] and [S III] (for the electron temperature $T_e$) and [S II] and [Cl III] (for the electron density $N_e$), and show two-dimensional images of the velocity measured from several bright emission lines.Comment: Resubmitted to A&A after incorporating referee comments; access to full dataset via http://muse-vlt.eu/science/data-release