J/A+A/568/A56 mm continuum and line images of G0.253+0.016 (Johnston+, 2014)
The dynamics and star-forming potential of the massive Galactic centre cloud
G0.253+0.016.
Johnston K.G., Beuther H., Linz H., Schmiedeke A., Ragan S.E., Henning T.
<Astron. Astrophys. 568, A56 (2014)>
=2014A&A...568A..56J 2014A&A...568A..56J
ADC_Keywords: Molecular clouds ; Carbon monoxide ; Combined data ;
Radio continuum ; Radio lines
Keywords: stars: formation - ISM: clouds - (ISM:) dust, extinction -
ISM: kinematics and dynamics - ISM: structure - Galaxy: center
Abstract:
The massive infrared dark cloud G0.253+0.016 projected ∼45pc from the
Galactic centre contains ∼105M☉ of dense gas whilst being
mostly devoid of observed star-formation tracers.
Our goals are therefore to scrutinise the physical properties,
dynamics and structure of this cloud with reference to its
star-forming potential.
We have carried out a concerted SMA and IRAM 30m study of this
enigmatic cloud in dust continuum, CO isotopologues, several shock
tracing molecules, as well as H2CO to trace the gas temperature. In
addition, we include ancillary far-IR and sub-mm Herschel and SCUBA
data in our analysis.
We detect and characterise a total of 36 dust cores within
G0.253+0.016 at 1.3mm and 1.37mm, with masses between 25 and
approximately 250M☉, and find that the kinetic temperature of
the gas traced by H2CO ratios is >320K on size-scales of ∼0.15pc.
Analysis of the position-velocity diagrams of our observed lines show
broad linewidths and strong shock emission in the south of the cloud,
indicating that G0.253+0.016 is colliding with another cloud at
vLSR∼70km/s. We confirm via an analysis of the observed dynamics in
the Central Molecular Zone that it is an elongated structure,
orientated with Sgr B2 closer to the Sun than Sgr A*, however our
results suggest that the actual geometry may be more complex than an
elliptical ring. We find that the column density Probability
Distribution Function (PDF) of G0.253+0.016 derived from SMA and SCUBA
dust continuum emission is log-normal with no discernible power-law
tail, consistent with little star formation, and that its width can be
explained in the framework of theory predicting the density structure
of clouds created by supersonic, magnetised turbulence. We also
present the Delta-variance spectrum of this region, a proxy for the
density power spectrum of the cloud, and show it is consistent with
that expected for clouds with no current star formation. Finally, we
show that even after determining a scaled column density threshold for
star formation by incorporating the effects of the increased
turbulence in the cloud, we would still expect ten stars with masses
>15M☉ to form in G0.253+0.016. If these cannot be accounted for
by new radio continuum observations, then further physical aspects may
be important, such as the background column density level, which would
turn an absolute column density threshold for star formation into a
critical over-density.
We conclude that G0.253+0.016 contains high-temperatures and
wide-spread shocks, displaying evidence of interaction with a nearby
cloud which we identify at vLSR∼70km/s. Our analysis of the
structure of the cloud can be well-explained by theory of magnetised
turbulence, and is consistent with little or no current star
formation. Using G0.253+0.016 as a test-bed of the conditions required
for star formation in a different physical environment to that of
nearby clouds, we also conclude that there is not one column density
threshold for star formation, but instead this value is dependant on
the local physical conditions.
Description:
Observations of G0.253+0.016 were conducted on 9 June 2012 with the
SMA in its compact array configuration. The two 4GHz sidebands were
placed at 218.9 and 230.9GHz (1.37 and 1.30mm). Observations were also
carried out on 16 and 22 October 2012 using the Eight MIxer Receiver
(EMIR) installed on the IRAM 30m telescope on Pico Veleta, Spain. We
also made use of archival data from the Herschel satellite.
FITS images corresponding to Figs. 2, 4, 10 and 11.
Figure 2 presents 1.37 and 1.3mm continuum images taken with the
Submillimeter Array (SMA). Fig. 4 contains an SMA 1.3mm continuum
image combined with a model of the larger-scale 1.3mm continuum
emission derived from an 450micron continuum image taken by the
Submillimetre Common-User Bolometer Array (SCUBA). Figure 10 displays
integrated maps derived from: a combined SMA and IRAM 30m 13CO image
cube, and SMA C18O, CH3OH 4(2,2) - 3(1,2)-E, SiO, HNCO and H2CO
3(0,3) - 2(0,2) image cubes. Figure 11 displays the channel maps of
the combined SMA and IRAM 30m 13CO image cube mentioned above.
Further details of the observations which produced these images are
given in Section 2 of the paper.
Objects:
------------------------------------------------------------------
RA (2000) DE Designation(s)
------------------------------------------------------------------
17 46 09.7 -28 43 31 GAL 000.253+00.016 = GCM +0.25 +0.01
------------------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
list.dat 132 9 List of FITS files
fits/* . 9 Individual FITS files
--------------------------------------------------------------------------------
Byte-by-byte Description of file: list.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 22 F3.1 arcsec/pix scale [0.5] Scale of the image
24- 26 I3 --- Nx [272] Number of pixels along X-axis
28- 30 I3 --- Ny [447] Number of pixels along Y-axis
32- 33 I2 --- Nz [30/31]? Number of slices for cubes
35- 41 F7.3 GHz Freq Freqency (of continuum or of line)
43- 45 I3 km/s Vmin ? Minimal velocity (for cubes)
47- 50 I4 km/s Vmax ? Maximal velocity (for cubes)
52- 54 F3.1 km/s dV ? Velocity resolution (for cubes)
68- 72 I5 Kibyte size [484/14732] Size of FITS file
74- 85 A12 --- FileName Name of FITS file in subdirectory fits
87-132 A46 --- Title Title of the file
--------------------------------------------------------------------------------
Acknowledgements:
Katharine Johnston, johnston(at)mpia.de
(End) Patricia Vannier [CDS] 18-Jun-2014