A massive association around the obscured open cluster RSGC3
  Four clusters of red supergiants have been discovered in a region of the
Milky Way close to base of the Scutum-Crux Arm and the tip of the Long Bar.
Population synthesis models indicate that they must be very massive to harbour
so many supergiants. The spatial extent of one of these clusters, RSGC3, has
not been investigated. We utilised 2MASS JHK photometry to identify candidate
obscured luminous red stars in the vicinity of RSGC3. We observed a sample of
candidates with the TWIN spectrograph on the 3.5-m telescope at Calar Alto,
obtaining intermediate-resolution spectroscopy in the 8000-9000A range. We
re-evaluated a number of classification criteria proposed in the literature for
this spectral range and found that we could use our spectra to derive spectral
types and luminosity classes. We measured the radial velocity of five members
of RSGC3, finding velocities similar to the average for members of Stephenson2.
Among the candidates observed outside the cluster, our spectra revealed eight
M-type supergiants at distances <18arcmin from the centre of RSGC3, distributed
in two clumps. The southern clump is most likely another cluster of red
supergiants, with reddening and age identical to RSGC3. From 2MASS photometry,
we identified four likely supergiant members of the cluster in addition to the
five spectroscopically observed. The northern clump may be a small cluster with
similar parameters. Photometric analysis of the area around RSGC3 suggests the
presence of a large (>30) population of red supergiants with similar colours,
suggesting that the massive cluster RSGC3 is surrounded by an extended
association, which may be very massive (>10E5M_sun). We also show that
supergiants in the Scutum Complex may be characterised via a combination of
2MASS photometry and intermediate-to-high-resolution spectroscopy in the Z band
(abridged).
