J/AJ/109/960 Young star clusters in The Antennae (Whitmore+ 1995)
Hubble Space Telescope observations of young star clusters in
NGC 4038/4039, "The Antennae" galaxies
Whitmore B.C., Schweizer F.
<Astron. J. 109, 960 (1995)>
=1995AJ....109..960W 1995AJ....109..960W
ADC_Keywords: Associations, stellar ; Photometry
Mission_Name: HST
Abstract:
New high-resolution images of the disks of NGC 4038/4039 obtained with
the Wide Field Camera of the Hubble Space Telescope (HST) are
presented. NGC 4038/4039, nicknamed "The Antennae," is a prototypical
example of a pair of colliding galaxies believed to be at an early
stage of a merger. Down to the limiting magnitude of V∼23mag, the HST
images reveal a population of over 700 blue pointlike objects within
the disks. The mean absolute magnitude of these objects is
MV=-11mag, with the brightest objects reaching MV~-15. Their mean
apparent color indices are U-V=-0.7mag and V-I=0.8mag on the Johnson
UVI passband system, while their mean indices corrected for internal
reddening are (U-V)0=-1.0mag and (V-I)0=0.5. Their mean effective
radius, determined from slightly resolved images, is 18pc
(for H0=50km/s/Mpc). Based on their luminosities and resolution, most
of these objects cannot be individual stars, but are likely young
compact star clusters. The brighter ones are similar to the objects
found in NGC 1275 and NGC 7252, which appear to be young globular
clusters formed during recent galaxy mergers. Based on their U-V and
V-I colors, the brightest, bluest clusters of NGC 4038/4039 appear to
be less than 10Myr old. Most of these bright clusters are relatively
tightly clustered themselves, with typically a dozen individual
clusters belonging to a complex identified as a giant H II region from
ground-based observations. The cluster luminosity function (LF) is
approximately a power law, Φ(L)dL is proportional to
L(-1.78±0.05)dL, with no hint of a turnover at fainter magnitudes.
This power-law shape agrees with the LF of Magellanic Cloud clusters
and Galactic open clusters, but differs from the LF of old globular
cluster systems that is typically Gaussian with a FWHM of ∼3mag.
Possible explanations for this apparent difference include: (1) We
have not observed faint enough to see the turnover, (2) the initial LF
of star clusters is a power law but the fainter objects dissolve with
time, (3) conditions at the present epoch favor the formation of a
wide range of cluster masses while conditions at earlier epochs
favored the formation of massive clusters, and (4) the NGC 4038/4039
clusters may not evolve into normal globular clusters. Besides the
blue clusters, we also find about a dozen extremely red objects with
V-I>3.0. The highest number density of these red objects is found in
the SE quadrant, where star formation appears to be most recent. We
propose that these objects may be very young star clusters still
embedded in their placental dust cocoons.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 64 728 Positions, magnitudes, and sizes of the
pointlike objects in NGC 4038/4039
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Obj Object number
4- 5 A2 --- Note *[a-d ] Table note reference letter
7- 13 F7.2 arcsec oRA *RA offset from object #442
14- 20 F7.2 arcsec oDE *Dec offset from object #442
22- 27 F6.2 mag Vmag ? Johnson V magnitude
29- 33 F5.2 mag e_Vmag ? Uncertainty in Vmag
35- 39 F5.2 mag V-I *? Johnson V-I color
41- 45 F5.2 mag e_V-I *? Uncertainty in V-I
47- 52 F6.2 mag U-V *? Johnson U-V color
54- 58 F5.2 mag e_U-V *? Uncertainty in U-V
60- 64 F5.1 pc Reff ? Effective radius of the object
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Note on Note:
a) Very red object. In this case, the V-I and e_V-I fields are
instead Johnson I magnitudes and errors respectively. Also
the U-V and e_U-V fields are actually V-I colors and error,
respectively.
b) Nucleus of NGC 4039
c) Evaluated as if a star cluster. However, this is almost certainly
a normal galactic nucleus, hence the magnitude and radius are
misleading since the object is very extended.
d) Probably a star, since Reff is ←10pc (i.e., below the cutoff),
and the I image shows possible tendrils which are characteristic
of a point source.
Note on oRA, oDE:
Offset in RA and DEC from object # 442 at a position RA=12h01m52.891s,
DEC=-18deg52'09.49" (J2000 coordinates in the coordinate frame of the
Guide Star Catalog)
Note on V-I, e_V-I, U-V, e_U-V:
If Note field contains letter 'a', these fields are actually the
Johnson I magnitude, error in I magnitude, V-I color and error in
V-I color, respectively.
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Origin: AAS CD-ROM series, Volume 4, 1995 Lee Brotzman [ADS] 03-May-1995
(End) [CDS] 11-Jul-1995