Atlas Image mosaic,
covering 7´ × 7´ on the sky of NGC 6440.
This globular star cluster is near the Galactic center, at a distance of 8.5
kpc (27710 light years) and reddened by about three visual magnitudes
(Ortolani et al. 1994, A&AS 108, 653). NGC 6440 is also interesting because
it has long shown to contain bright X-ray sources; a bright transient source
was detected by the BeppoSAX satellite in 1998 August. This source
has been studied by Verbunt et al. (2000, A&A, 359, 960). The X-ray point
sources are similar in property to
low-mass X-ray binaries seen in the Galactic plane. These interacting binary
systems likely contain ultracompact primary stars, possibly black holes
(Sidoli et al. 2001, A&A, 368, 451). Image mosaic by S. Van Dyk (IPAC).
Atlas Image mosaic,
covering 5´ × 5´ on the sky of NGC 346. This is the
brightest open stellar cluster and HII (or ionized hydrogen) region
in the Small Magellanic Cloud
(SMC), one of the dwarf satellite galaxies to our own Milky Way Galaxy. The
SMC is at a distance from us of ~63 kpc (205,400 light years). In the optical
this cluster is highlighted by many hot, young, massive stars (Massey, Parker,
& Garmany 1989, AJ, 98, 1305). In the near-infrared the most prominent stars
are a much older, underlying population of red giant stars. Also revealed are
several dust-obscured stars. These populations can be seen in the 2MASS
color-color and
color-magnitude diagrams. On the latter diagram in green are shown
isochrones (for the appropriate low metallicity, or heavy element abundance,
of the SMC) for 4 Myr, 15 Myr, and 10 Gyr. The bluer, hot,
young population is faintly detected by 2MASS, but a large ensemble of red
giants with ages of about 10 billion years (not a rigorous fit) is prevalent.
(The three brightest stars seen in the diagrams are likely Galactic foreground
stars.)
The bright red star to the southeast in the 2MASS image has a
Ks magnitude of 12.15 and color J-Ks=2.95, and is likely
a dust-obscured asymptotic giant branch star. Reflected starlight and
likely molecular hydrogen emission from the nebula can also be seen in the
2MASS image. Image mosaic by S. Van Dyk (IPAC).
Atlas Image, covering 8´ × 8´ on the sky of SS 433 and
its environs.
The 433rd entry in the Stephenson-Sanduleak (1977, ApJS, 33, 459) catalog of
H emission stars in the Milky Way, aka V1343 Aquilae, is
one of the most peculiar and possibly unique objects in the Galaxy (it is seen
as the relatively bright star at the center of the 2MASS image). The
strong emission lines seen in the spectrum of SS 433 appear as two sets which
alternately redshift and blueshift in a smooth and regular fashion. The line
sets, when redshifted, reach an impressive maximum of 50,000 km s-1,
while the sets, when blueshifted, reach a maximum of 30,000 km s-1.
These velocities both are a significant fraction of the speed of light! The
basic model hypothesizes that the emission arises from two opposing collimated
jets of matter, which rotate once every 164 days. The jets emanate from an
accretion disk around a neutron star in an interacting binary system, possibly
with a B-type companion star. That the compact object is likely a neutron star
is consistent with the likely association of SS 433 with the supernova remnant
W50; the jets interact with the nebula (Dubner et al. 1998, AJ, 116, 1842).
SS 433, on the date that the routine 2MASS southern operations observed
it (1999 Aug 11 UT), had magnitudes J=9.40, H=8.75, Ks=8.18.
Kodaira, Nakada, & Backman (1985, ApJ, 296, 232) found the near-infrared
emission to be, not unexpectedly, variable and likely mostly arising from the
disk. The 2MASS color-color and
color-magnitude diagrams (SS 433 is the red star
on the latter diagram) indicate that the
recent radio distance (Dubner et al.) of ~3.0 kpc (9780 light years) and a
visual extinction of 8 to 9 magnitudes are consistent with the brightnesses
and colors of many of the stars in SS 433's environment.
Atlas Image, covering 5.5´ × 5.5´ on the sky of
NGC 5253. This is a dwarf
galaxy of Hubble type S0/I0 and is a member of the Centaurus group of galaxies,
along with Messier 83 and
Centaurus A (NGC 5128). Although the
extended light of the galaxy is essentially characterized by a dwarf spheroidal
(or S0), NGC 5253 is highlighted by many recently-formed star clusters within
the central 100 pc (Calzetti et al. 1997, AJ, 114, 1834). These clusters
contain a large number of massive stars, including Wolf-Rayet stars; the
integrated light of these very massive, evolved stars leads to the designation
for NGC 5253 as a "Wolf-Rayet galaxy" (Conti 1991, ApJ, 377, 115).
In the 2MASS
near-infrared image the galaxy looks significantly different from optical
images. The infrared light, from near- to far-, is dominated by the emission
from a "supernebula" (seen in the 2MASS image as the brightest knot just northeast
of the galaxy center). This object is also a bright thermal radio source
(Turner, Beck, & Ho 2000, ApJ, 532, L109). Gorjian, Turner, & Beck (2001, ApJ,
554, L29) recently studied the supernebula in the mid-infrared with the Keck
10-m telescope,
and determined that it is excited by 105-106 stars, with
the size and mass of a typical globular cluster, such as
47 Tucanae in the Milky Way. However, this cluster
is very dust-obscured and likely very young, possibly the youngest known
globular cluster in any galaxy.
Atlas Image mosaic,
covering 11´ × 11´ on the sky of W43, which is one of
the giant HII, or ionized hydrogen, regions in the Milky Way Galaxy. The gas
is ionized by a cluster of dozens of hot, young, massive stars, similar to the
conditions in NGC 3603. However, unlike NGC 3603,
W43 is completely obscured visually. 2MASS reveals the nebula and stars in
this image. 2MASS color-color and
color-magnitude diagrams, generated for an
area with radius 2.5´ centered on W43, show considerable variable
reddening in the field.
The distance to W43 derived from the radio is ~7 kpc
(22820 light years). Foreground stars appear appreciably less reddened, with
the cluster population having J-Ks~2.5 and redward. This
corresponds to visual extinction to the cluster stars of at least 15
magnitudes. Blum,
Damineli, & Conti (1999, AJ, 117, 1392) studied the central region of W43 in
the near-infrared and find that the central dense clustering contains several
very massive stars behind ~34 visual magnitudes of extinction. These are the
red stars between Ks~12 and 9 mag. In the 2MASS
image a number of objects can be seen at primarily Ks, with weak
H and J detections, which may be young stars embedded in their natal cloud on
the outskirts of W43. A scenario where the energy input from the central
cluster leads to the propagation of star formation elsewhere in the molecular
cloud is possible.
The bright star toward the southwest is the late-type pulsating variable
giant star AB Aql (it lies right at the edges of two 2MASS tiles and is not
associated with W43). 2MASS is
ideal for revealing recent star formation throughout the Galaxy.
Image mosaic by S. Van Dyk (IPAC).
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