Composition of the galactic center star cluster - Population analysis from adaptive optics narrow band spectral energy distributions

By R. Buchholz, R. Schodel, A. Eckart. 2009

154 citations recorded by the source corpus.

Context. The GC is the closest galactic nucleus, offering the unique possibility of studying the population of a dense stellar cluster surrounding an SMBH. Aims. The goals of this work are to develop a new method of separating early and late type stellar components of a dense stellar cluster based on narrow band filters, applying it to the central parsec of the GC, and conducting a population analysis of this area. Methods. We use AO assisted observations obtained at the ESO VLT in the NIR H-band and 7 intermediate bands covering the NIR K -band. A comparison of the resulting SEDs with a blackbody of variable extinction then allows us to determine the presence and strength of a CO absorption feature to distinguish between early and late type stars. Results. This new method is suitable for classifying K giants (and later), as well as B2 main sequence (and earlier) stars that are brighter than 15.5 mag in the K band in the central parsec. Compared to previous spectroscopic investigations that are limited to 13–14 mag, this represents a major improvement in the depth of the observations and reduces the needed observation time. Extremely red objects and foreground sources can also be reliably removed from the sample. Comparison to sources of known classification indicates that the method has an accuracy of better than ~87%. We classify 312 stars as early type candidates out of a sample of 5914 sources. Several results, such as the shape of the KLF and the spatial distribution of both early and late type stars, confirm and extend previous works. The distribution of the early type stars can be fitted with a steep power law ($\beta_{1''}$ = -1.49  ±  0.12), alternatively with a broken power law, $\beta_{1-10''}$ = -1.08 ± 0.12, $\beta_{10-20''}$ = -3.46 ± 0.58, since we find a drop in the early type density at ~10″. We also detect early type candidates outside of 0.5 pc in significant numbers for the first time. The late type density function shows an inversion in the inner 6″, with a power-law slope of $\beta_{R ± 0.09. The late type KLF has a power-law slope of 0.30 ± 0.01, closely resembling the KLF obtained for the bulge of the Milky Way. The early type KLF has a much flatter slope of (0.14 ± 0.02). Our results agree best with an in-situ star formation scenario.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/c494b5bfefd40803430a789057da63126f5e74b5

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In Astronomy and Astrophysics

1. F. Antonini
2. T. Alexander
3. O. Gerhard
4. R. Genzel
5. Joss Bland-Hawthorn
6. F. Eisenhauer
7. S. Gillessen
8. T. Ott

In Instrumentation

1. F. Antonini
2. T. Alexander
3. R. Genzel
4. O. Gerhard
5. N. Neumayer
6. Joss Bland-Hawthorn
7. R. Schödel
8. C. Wegg

Citations in the corpus

• SECULAR EVOLUTION OF COMPACT BINARIES NEAR MASSIVE BLACK HOLES: GRAVITATIONAL WAVE SOURCES AND OTHER EXOTICA (2012, background): “…of the inner density profile we assumed two different values: γ = 0.5, representative of the observed distribution of stars at the Galactic center (Buchholz et al. 2009; Do et al. 2009; Bartko et al. 2010), and γ = 1.8 corresponding approximately to a nearly-relaxed configuration of stars near…”
• The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, background): “Another unsolved problem is the apparent core in the NSC density profile (Buchholz et al. 2009); this might indicate that the NSC is not fully relaxed, consistent with the relaxation time estimated as ≈10 Gyr throughout the NSC (Merritt 2013).”
• DISSIPATIONLESS FORMATION AND EVOLUTION OF THE MILKY WAY NUCLEAR STAR CLUSTER (2011, background): “In addition, the central region (r < rb) of our model exhibits a shallow density profile (or core) near the SMBH, also in agreement with observations (Buchholz et al. 2009).”
• Nuclear star clusters (2009, background): “The presence or absence of this Bahcall-Wolf cusp in NSCs, especially the Milky Way NSC, is debated in both the theoretical (e.g., Merritt 2010; Vasiliev 2017) and observational literature (e.g., Buchholz et al. 2009; Schödel et al. 2014a; Gallego-Cano et al. 2018).”
• HIGH ANGULAR RESOLUTION INTEGRAL-FIELD SPECTROSCOPY OF THE GALAXY'S NUCLEAR CLUSTER: A MISSING STELLAR CUSP? (2009, methodology): “More recently, Buchholz et al. (2009) took deeper narrowband images (K < 15.5) in CO and found an even more shallow slope in the inner region of the cluster.”
• Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster (2009, methodology): “How this can be done in spite of the difficulties described above has been demonstrated recently by Buchholz et al. (2009).”
• AN EXTREMELY TOP-HEAVY INITIAL MASS FUNCTION IN THE GALACTIC CENTER STELLAR DISKS (2009, background): “The large central core (and perhaps even central hole, see Buchholz et al. 2009; Do et al. 2009) in the density distribution of late type stars of all luminosities is puzzling and currently not understood.”
• The old Nuclear Star Cluster in the Milky Way (2014, background): “…constraints: The slope of the inner component should be γ1 > 0.5 because isotropic models with a black hole and γ1 < 0.5 are unphysical (Tremaine et al. 1994), but it should be close to the limiting value of 0.5 to better approximate the observed core near the center (Buchholz et al. 2009).”
• Surface brightness profile of the Milky Way’s nuclear star cluster (2014, methodology): “…dynamics (e.g., Alexander 2006; Merritt 2006; Preto & Amaro-Seoane 2010), the distribution of the old – and therefore dynamically relaxed – stars within 0.5 pc of Sgr A* is significantly flatter than predicted by theory (Buchholz et al. 2009; Do et al. 2009; Bartko et al. 2010; Do et al. 2013).”