25 research outputs found
Densitometry and Thermometry of Starburst Galaxies
With a goal toward deriving the physical conditions in external galaxies, we
present a survey of formaldehyde (H2CO) and ammonia (NH3) emission and
absorption in a sample of starburst galaxies using the Green Bank Telescope. By
extending well-established techniques used to derive the spatial density in
star formation regions in our own Galaxy, we show how the relative intensity of
the 1(10)-1(11) and 2(11)-2(12) K-doublet transitions of H2CO can provide an
accurate densitometer for the active star formation environments found in
starburst galaxies (c.f. Mangum et al. 2008). Similarly, we employ the
well-established technique of using the relative intensities of the (1,1),
(2,2), and (4,4) transitions of NH3 to derive the kinetic temperature in
starburst galaxies. Our measurements of the kinetic temperature constrained
spatial density in our starburst galaxy sample represent the first mean density
measurements made toward starburst galaxies. We note a disparity between
kinetic temperature measurements derived assuming direct coupling to dust and
those derived from our NH3 measurements which points to the absolute need for
direct gas kinetic temperature measurements using an appropriate molecular
probe. Finally, our spatial density measurements point to a rough constancy to
the spatial density (10^{4.5} to 10^{5.5} cm^{-3}) in our starburst galaxy
sample. This implies that the Schmidt-Kennicutt relation between L_{IR} and
M_{dense}: (1) Is a measure of the dense gas mass reservoir available to form
stars, and (2) Is not directly dependent upon a higher average density driving
the star formation process in the most luminous starburst galaxies.Comment: 4 pages, to appear in proceedings of The 5th Zermatt ISM Symposiu
Exploring morphological correlations among H2CO, 12CO, MSX and continuum mappings
There are relatively few H2CO mappings of large-area giant molecular cloud
(GMCs). H2CO absorption lines are good tracers for low-temperature molecular
clouds towards star formation regions. Thus, the aim of the study was to
identify H2CO distributions in ambient molecular clouds. We investigated
morphologic relations among 6-cm continuum brightness temperature (CBT) data
and H2CO (111-110; Nanshan 25-m radio telescope), 12CO (1--0; 1.2-m CfA
telescope) and midcourse space experiment (MSX) data, and considered the impact
of background components on foreground clouds. We report simultaneous 6-cm H2CO
absorption lines and H110\alpha radio recombination line observations and give
several large-area mappings at 4.8 GHz toward W49 (50'\times50'), W3
(70'\times90'), DR21/W75 (60'\times90') and NGC2024/NGC2023 (50'\times100')
GMCs. By superimposing H2CO and 12CO contours onto the MSX color map, we can
compare correlations. The resolution for H2CO, 12CO and MSX data was about 10',
8' and 18.3", respectively. Comparison of H2CO and 12CO contours, 8.28-\mu m
MSX colorscale and CBT data revealed great morphological correlation in the
large area, although there are some discrepancies between 12CO and H2CO peaks
in small areas. The NGC2024/NGC2023 GMC is a large area of HII regions with a
high CBT, but a H2CO cloud to the north is possible against the cosmic
microwave background. A statistical diagram shows that 85.21% of H2CO
absorption lines are distributed in the intensity range from -1.0 to 0 Jy and
the \Delta V range from 1.206 to 5 km/s.Comment: 18 pages, 22 figures, 5 tables. Accepted to be published in
Astrophysics and Space Scienc
Tracing Interstellar heating: an ALCHEMI measurement of the HCN Isomers in NGC 253
Horizon 2020(H2020)833460.viiGalaxie