HIGH ANGULAR RESOLUTION INTEGRAL-FIELD SPECTROSCOPY OF THE GALAXY'S NUCLEAR CLUSTER: A MISSING STELLAR CUSP?

By Tuan Do, A. Ghez, M. Morris, Jessica R. Lu, K. Matthews, S. Yelda, James Larkin University of California, Los Angeles, Caltech, Pasadena. 2009

140 citations recorded by the source corpus.

We report on the structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants. Using laser guide star adaptive optics in conjunction with the integral field spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate between the older, late-type (∼ 1 Gyr) stars, which are presumed to be dynamically relaxed, and the unrelaxed young (∼ 6 Myr) population. This distinction is crucial for testing models of stellar cusp formation in the vicinity of a black hole, as the models assume that the cusp stars are in dynamical equilibrium in the black hole potential. In the survey region, we classified 60 stars as early-type (22 newly identified) and 74 stars as late-type (61 newly identified). We find that contamination from young stars is significant, with more than twice as many young stars as old stars in our sensitivity range (K′ < 15.5) within the central arcsecond. Based on the late-type stars alone, the surface stellar number density profile, Σ(R) ∝ R−Γ, is flat, with Γ = −0.27 ± 0.19. Monte Carlo simulations of the possible de-projected volume density profile, n(r) ∝r−γ, show that γ is less than 1.0 at the 99.7% confidence level. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than that predicted by most cusp formation theories, and even allows for the presence of a central hole in the stellar distribution. Of the possible dynamical interactions that can lead to the depletion of the red giants observable in this survey—stellar collisions, mass segregation from stellar remnants, or a recent merger event—mass segregation is the only one that can be ruled out as the dominant depletion mechanism. The lack of a stellar cusp around a supermassive black hole would have important implications for black hole growth models and inferences on the presence of a black hole based upon stellar distributions.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/256998de0e35a20fc0374c2264ec589282e1c5d0

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Citations in the corpus

• Stellar Dynamics and Stellar Phenomena Near a Massive Black Hole (2017, background): “16m) (Do et al. 2009; Buchholz, Schödel & Eckart 2009; Bartko et al. 2010), which are currently the only directly detectable tracers of the faint old population.”
• The old Nuclear Star Cluster in the Milky Way (2014, result): “In contrast, observations by Do et al. (2009); Buchholz et al. (2009); Bartko et al. (2010) show that the distribution of old stars near the SMBH appears to have a core.”
• Peering through the veil: near-infrared photometry and extinction for the Galactic nuclear star cluster (2009, methodology): “These extinction values are different from the ones that have typically been used in the analysis of NIR lightcurves from Sgr A*, e.g., AH = 4.3, AK = 2.8, AL′ = 1.8 (Genzel et al. 2003; Eckart et al. 2006), AK = 3.3 (Do et al. 2009b), or AL′ = 1.8 (Ghez et al. 2004).”
• DISSIPATIONLESS FORMATION AND EVOLUTION OF THE MILKY WAY NUCLEAR STAR CLUSTER (2011, background): “This is too long for a BahcallWolf (1976) cusp to have formed over the Galaxy’s lifetime, consistent with the observed lack of a cusp (Buchholz et al. 2009; Do et al. 2009; Bartko et al. 2010).”
• Nuclear star clusters (2009, background): “In addition to the presence of a young stellar component very close to the SMBH, another puzzling phenomenon is the apparent absence of red giant branch (RGB) stars near the center of the Milky Way (e.g., Do et al. 2009).”
• Surface brightness profile of the Milky Way’s nuclear star cluster (2014, methodology): “A different approach, e.g., with a broken power-law should then be chosen (see Do et al. 2009; Schödel et al. 2009).”
• SECULAR EVOLUTION OF COMPACT BINARIES NEAR MASSIVE BLACK HOLES: GRAVITATIONAL WAVE SOURCES AND OTHER EXOTICA (2012, background): “…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 a dominating…”
• 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.”