APEX SV projects

CO Emission in Nearby Dwarf Galaxies

Coordinator: J. Cannon and F. Walter

Abstract:

Data:
Program is available and data products can be downloaded

Scientific justification:

The transition from neutral to molecular gas, and hence the behavior of star formation, at low metallicities is a fundamental yet poorly understood process. Theoretical studies suggest that, as the metallicity falls, the CO-emitting cores of molecular clouds shrink in response to the higher UV radi- ation fields and lower dust contents of low-metallicity galaxies. This scenario has been borne out in numerous observations of nearby metal-poor galaxies. However, since CO represents the only easily- observed tracer of cold (i.e., < 100 K) molecular material in galaxies, it is imperative that sensitive observations of nearby star-forming dwarf galaxies of low metallicity be undertaken. The unprece- dented sensitivity of the new APEX telescope and receivers will allow detailed observations ( 20 ) of the CO (32) transition at 345.8 GHZ. We propose to observe this transition in a carefully-selected sample of nearby star-forming dwarf galaxies for which we have collected a a multitude of supporting multi-wavelength data, including JCMT and OVRO CO observations. Most notably, we have access to Spitzer IRAC and MIPS imaging as well as IRS spectroscopy for all targets. The resulting datasets will allow detailed studies of the nature of star formation in the dwarf galaxy ISM. Note that ideally, we would also like to observe these galaxies in the CI lines, using the FLASH instrument - however given the short time between the announcement and the deadline for the submission of the APEX verification proposals, we were unable to establish contact with the PI, Dr. R. Guesten.

Proposed Targets/Pointings
Based on the available Spitzer, OVRO CO and HI observations, we have selected 8 pointings in three nearby dwarf galaxies. All dwarf galaxies have metallicities of 1/4 solar (i.e. CO is detectable). The observations will not only shed light on the physical properties of the ISM in the systems under study but will also serve as important tests for designing future APEX (and ALMA) observations of lower metallicity systems. Assuming a system temperature of 400 K, typical line strengths of 0.4 Jy (narrow lines: 20 km/s) we estimate the need for 1 hour of observing time for the individual positions (i.e., 8 hours total). The individual targets are described in the following:

NGC 6822
Important parameters: D = 470 ą 40 kpc, Z 20% Z , Heliocentric radial velocity range = -100 - + 12 km s-1
Supporting observations: Archival SEST observations of the CO (10) transition at 115.3 GHz; detections of CO in 5 positions. Spitzer imaging (whole galaxy) and spectral maps (selected pointings).
In addition, we have highresolution HI imaging available for the entire galaxy.
Proposed pointings:

RA,DEC (J2000)	19:44:31.64	-14:42:01.18	CO(10) = 0.39 K km/sec,	V LSR = -68
RA,DEC (J2000)	19:44:48.93	-14:52:38.05	CO(10) = 0.80 K km/sec,	V LSR = -50
RA,DEC (J2000)	19:44:52.85	-14:43:10.68	CO(10) = 0.77 K km/sec,	V LSR = -41
RA,DEC (J2000)	19:44:57.74	-14:47:51.47	CO(10) = 0.48 K km/sec,	V LSR = -53
RA,DEC (J2000)	19:44:58.93	-14:47:33.39	CO(10) = 0.90 K km/sec,	V LSR = -50

NGC 625
Important parameters: D = 3.9 ą 0.2 Mpc, Z 30% Z , Heliocentric radial velocitys range = +360-+ 460 km s-1
Supporting observations: Archival JCMT observations of the CO (21) transition at 230 GHz; detections of CO in central dust cloud.
Proposed pointing:

RA,DEC (J2000)

01:35:04.7

-41:26:16

CO(21) = 1.13 K km/sec

V LSR = 396 km/sec.s

NGC 1705
Important parameters: D = 5.1 ą 0.6 Mpc, Z 35% Z , Heliocentric radial velocity range = +550 - + 710 km s-1
Supporting observations: None in CO lines. Spitzer imaging (whole galaxy) and spectral maps (selected pointings).
Proposed pointings:

RA,DEC (J2000)	04:54:14.66	-53:21:38.23	VLSR = +550 - + 710 km/sec.
RA,DEC (J2000)	04:54:12.57	-53:21:44.70	VLSR = +550 - + 710 km/sec.