J/MNRAS/488/5340 No evidence for IMBHs in ω Cen/NGC6624 (Baumgardt+, 2019)
No evidence for intermediate-mass black holes in the globular clusters
ω Cen and NGC 6624.
Baumgardt H., He C., Sweet S.M., Drinkwater M., Sollima A., Hurley J.,
Usher C., Kamann S., Dalgleish H., Dreizler S., Husser T.-O.
<Mon. Not. R. Astron. Soc., 488, 5340-5351 (2019)>
=2019MNRAS.488.5340B 2019MNRAS.488.5340B (SIMBAD/NED BibCode)
ADC_Keywords: Radial velocities ; Spectra, infrared ; Photometry, infrared ;
Clusters, globular
Keywords: stars: luminosity function, mass function - globular clusters: general
Abstract:
We compare the results of a large grid of N-body simulations with the
surface brightness and velocity dispersion profiles of the globular
clusters ω Cen and NGC 6624. Our models include clusters with
varying stellar-mass black hole retention fractions and varying masses
of a central intermediate-mass black hole (IMBH). We find that an
∼45000M☉ IMBH, whose presence has been suggested based on the
measured velocity dispersion profile of ω Cen, predicts the
existence of about 20 fast-moving, m>0.5M☉, main-sequence stars
with a (1D) velocity v>60km/s in the central 20arcsec of ω Cen.
However, no such star is present in the HST/ACS proper motion
catalogue of Bellini et al. (2017ApJ...842....6B 2017ApJ...842....6B, Cat. J/ApJ/842/6),
strongly ruling out the presence of a massive IMBH in the core of
ω Cen. Instead, we find that all available data can be fitted by
a model that contains 4.6 per cent of the mass of ω Cen in a
centrally concentrated cluster of stellar-mass black holes. We show
that this mass fraction in stellar-mass BHs is compatible with the
predictions of stellar evolution models of massive stars. We also
compare our grid of N-body simulations with NGC 6624, a cluster
recently claimed to harbour a 20000M☉ black hole based on timing
observations of millisecond pulsars. However, we find that models with
MIMBH>1000M☉ IMBHs are incompatible with the observed velocity
dispersion and surface brightness profile of NGC 6624, ruling out the
presence of a massive IMBH in this cluster. Models without an IMBH
provide again an excellent fit to NGC 6624.
Description:
In order to increase the number of stars with measured radial
velocities in the outer cluster parts, we observed NGC 6624 for one
half-night using the DEIMOS spectrograph (Faber et al.
2003SPIE.4841.1657F 2003SPIE.4841.1657F) on the Keck II telescope (Programm ID: Z252, PI:
M. Drinkwater). Observations were performed on 2017 July 19 with
0.6arcsec seeing and some thin cirrus, using the 1200G grating with a
central wavelength of 8000Å and the OG550 order-blocking filter.
Four slitmasks were observed at position angles (PAs) of 0, 90, 270,
and 315deg, in order to maximize the number of stars near the centre
of the cluster, where the proportion of member stars is expected to be
higher, and to maximize the spatial coverage of the outer regions.
Slits were placed on a total of 685 unique targets, comprised of 545
stars with 2MASS coordinates and 140 with positions derived from HST
imaging. Each mask had seven stars in common with the VLT/FLAMES data
set to assist in radial velocity calibration. Exposure times were
3x800s for the mask with PA=0, 3x850s for PA=270, and 3x900s for the
masks with PA=90 and 315.
The DEIMOS spectra were reduced with the help of the DEEP2 data
reduction pipeline developed by the DEEP2 survey team (Cooper et al.
2012ascl.soft03003C; Newman et al. 2013ApJS..208....5N 2013ApJS..208....5N). In total, we
were able to determine the radial velocities of 264 stars from the
DEIMOS spectra. Table A1 gives the individual radial velocities that
we have derived from the DEIMOS spectra.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 93 259 DEIMOS stellar radial velocities for stars in
the field of NGC 6624
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 16 A16 --- Name Star name from 2MASS (HHMMSSss+DDMMSSs)
18- 27 F10.6 deg RAdeg Right ascension (J2000)
29- 38 F10.6 deg DEdeg Declination (J2000)
40- 45 F6.2 km/s RV Heliocentric radial velocity
47- 50 F4.2 km/s e_RV Error on RV
52- 57 F6.2 arcmin d Distance from the cluster center
59- 63 F5.2 mag Jmag ? 2MASS J-band magnitude
65- 68 F4.2 mag e_Jmag ? Error on Jmag
70- 74 F5.2 mag Ksmag ? 2MASS K-band magnitude
76- 79 F4.2 mag e_Ksmag ? Error on Ksmag
81- 85 F5.3 --- Pmemb Membership probability based on the radial
velocity (1)
87 I1 --- o_RV Number of radial velocity measurements
89- 93 F5.3 --- PRVcst For stars with multiple radial velocity
measurements, the probability that the star
has a constant radial velocity
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Note (1): The membership probabilities in Table A1 are calculated as described
in Baumgardt & Hilker (2018MNRAS.478.1520B 2018MNRAS.478.1520B, Cat. J/MNRAS/478/1520)
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 29-Dec-2022