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Register a new userKMOS view of the Galactic Centre I. Young stars are centrally concentrated
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Anja Feldmeier-Krause
Publication date
2015
fields
Physics
and research's language is
English
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The Galactic centre hosts a crowded, dense nuclear star cluster with a half-light radius of 4 pc. Most of the stars in the Galactic centre are cool late-type stars, but there are also >100 hot early-type stars in the central parsec of the Milky Way. These stars are only 3-8 Myr old. Our knowledge of the number and distribution of early-type stars in the Galactic centre is incomplete. Only a few spectroscopic observations have been made beyond a projected distance of 0.5 pc of the Galactic centre. The distribution and kinematics of early-type stars are essential to understand the formation and growth of the nuclear star cluster. We cover the central >4pc^2 of the Galactic centre using the integral-field spectrograph KMOS. We extracted more than 1,000 spectra from individual stars and identified early-type stars based on their spectra. Our data set contains 114 bright early-type stars: 6 have narrow emission lines, 23 are Wolf-Rayet stars, 9 stars have featureless spectra, and 76 are O/B type stars. Our wide-field spectroscopic data confirm that the distribution of young stars is compact, with 90% of the young stars identified within 0.5 pc of the nucleus. We identify 24 new O/B stars primarily at large radii. We estimate photometric masses of the O/B stars and show that the total mass in the young population is >12,000M_sun. The O/B stars all appear to be bound to the Milky Way nuclear star cluster, while less than 30% belong to the clockwise rotating disk. The central concentration of the early-type stars is a strong argument that they have formed in situ. A large part of the young O/B stars is not on the disk, which either means that the early-type stars did not all form on the same disk or that the disk is dissolving rapidly. [abridged]
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Knowing the metallicity distribution of stars in the Galactic Centre has important implications for the formation history of the Milky Way nuclear star cluster. However, this distribution is not well known, and is currently based on a small sample of fewer than 100 stars. We obtained near-infrared K-band spectra of more than 700 late-type stars in the central 4 pc^2 of the Milky Way nuclear star cluster with the integral-field spectrograph KMOS (VLT). We analyse the medium-resolution spectra using a full-spectral fitting method employing the Gottingen Spectral library of synthetic PHOENIX spectra. The derived stellar metallicities range from metal-rich [M/H]>+0.3 dex to metal-poor [M/H]<-1.0 dex, with a fraction of 5.2(^{+6.0}+{-3.1}) per cent metal-poor ([M/H]<-0.5 dex) stars. The metal-poor stars are distributed over the entire observed field. The origin of metal-poor stars remains unclear. They could originate from infalling globular clusters. For the metal-rich stellar population ([M/H]>0 dex) a globular cluster origin can be ruled out. As there is only a very low fraction of metal-poor stars in the central 4 pc^2 of the Galactic Centre, we believe that our data can discard a scenario in which the Milky Way nuclear star cluster is purely formed from infalling globular clusters.
We perform a joint-analysis of high spatial resolution molecular gas and star-formation rate (SFR) maps in main-sequence star-forming galaxies experiencing galactic-scale outflows of ionised gas. Our aim is to understand the mechanism that determines which galaxies are able to launch these intense winds. We observed CO(1-0) at 1 resolution with ALMA in 16 edge-on galaxies, which also have 2 spatial resolution optical integral field observations from the SAMI Galaxy Survey. Half the galaxies in the sample were previously identified as harbouring intense and large-scale outflows of ionised gas (outflow-types), the rest serve as control galaxies. The dataset is complemented by integrated CO(1-0) observations from the IRAM 30-m telescope to probe the total molecular gas reservoirs. We find that the galaxies powering outflows do not possess significantly different global gas fractions or star-formation efficiencies when compared with a control sample. However, the ALMA maps reveal that the molecular gas in the outflow-type galaxies is distributed more centrally than in the control galaxies. For our outflow-type objects, molecular gas and star-formation is largely confined within their inner effective radius ($rm r_{eff}$), whereas in the control sample the distribution is more diffuse, extending far beyond $rm r_{eff}$. We infer that outflows in normal star-forming galaxies may be caused by dynamical mechanisms that drive molecular gas into their central regions, which can result in locally-enhanced gas surface density and star-formation.
We study the properties of the galaxies hosting the first 19 tidal disruption events (TDEs) detected with the Zwicky Transient Facility (ZTF) within the context of a carefully constructed, representative host galaxy sample. We find that the ZTF sample of TDE hosts is dominated by compact green valley galaxies. After we restrict the comparison sample to galaxies with a similar concentration, as measured by Sersic index, we find this green valley over representation is even larger. That is, concentrated red sequence galaxies are not producing TDEs at elevated levels. We present host galaxy spectra which show that E+A galaxies are overrepresented in the ZTF sample by a factor of $approx$22, which is lower than previous TDE host galaxy studies have found. We find that this overrepresentation can be fully accounted for when taking into account the masses, colors, and Sersic indices of the ZTF TDE hosts. The combination of both green colors and high Sersic index of the typical TDE host galaxy could be explained if the TDE rate is temporarily enhanced following a merger that leads to a higher central concentration of stars.
Two groups of astronomers used large telescopes Keck and VLT for decades to observe trajectories of bright stars near the Galactic Centre. Based on results of their observations astronomers concluded that trajectories of the stars are roughly elliptical and foci of the orbits are approximately coincide with the Galactic Centre position. It gives an opportunity to claim that the Newtonian potential of point like mass around $4.3times 10^6 M_odot$ is a good initial approximation for the gravitational potential near the Galactic Centre. In the last years, the astronomers found that gravitational redshift of S2 star near pericenter passage in May 2018 is in accordance with general relativity predictions. In 2020 the GRAVITY team found that the observed relativistic precession of S2 star orbit is also consistent with theoretical estimates calculated for a weak gravitational field approximation in a Schwarzschild black hole. In last years a a self-gravitating dark matter core--halo distribution suggested by Ruffini, Arguelles and Rueda (MNRAS, 2015) (RAR model) was proposed and recently Becerra-Vergara et al. (MNRAS, 2021) claimed that this model provides a better fit of trajectories of bright stars in comparison with the conventional model with the supermassive black hole. We confirm that in the case of this dark matter distribution model for a dense core trajectories of test bodies are elliptical but in this case centers (not foci) of these ellipses should coincide with the Galactic Centre and orbital periods do not depend on semi-major axis and it contradicts observational data and therefore, we concluded supermassive black hole is a preferable model in comparison with the a dense core--diluted halo density profile for the Galactic Centre.
The Central Molecular Zone (CMZ), a $sim$200 pc sized region around the Galactic Centre, is peculiar in that it shows a star formation rate (SFR) that is suppressed with respect to the available dense gas. To study the SFR in the CMZ, young stellar objects (YSOs) can be investigated. Here we present radio observations of 334 2.2 $mu$m infrared sources that have been identified as YSO candidates. Our goal is to investigate the presence of centimetre wavelength radio continuum counterparts to this sample of YSO candidates which we use to constrain the current SFR in the CMZ. As part of the GLOSTAR survey, D-configuration VLA data was obtained for the Galactic Centre, covering -2$^{circ}<l<$2$^{circ}$ and -1$^{circ}<b<$1$^{circ}$, with a frequency coverage of 4-8 GHz. We matched YSOs with radio continuum sources based on selection criteria and classified these radio sources as potential HII regions and determined their physical properties. Of the 334 YSO candidates, we found 35 with radio continuum counterparts. We find that 94 YSOs are associated with dense dust condensations identified in the 870 $mu$m ATLASGAL survey, of which 14 have a GLOSTAR counterpart. Of the 35 YSOs with radio counterparts, 11 are confirmed as HII regions, based on their spectral indices and the literature. We estimated their Lyman continuum photon flux in order to estimate the mass of the ionising star. Combining these with known sources, the present-day SFR in the CMZ is calculated to be $sim$0.068 M$_{odot}$ yr$^{-1}$, which is $sim$6.8$%$ of the Galactic SFR. Candidate YSOs that lack radio counterparts may not have yet evolved to the stage of exhibiting an HII region or, conversely, are older and have dispersed their natal clouds. Since many lack dust emission, the latter is more likely. Our SFR estimate in the CMZ is in agreement with previous estimates in the literature.
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