A 3mm Emission Line Survey of the 30 Dor Molecular Ridge

Question. How does the strength of the interstellar radiation field influence the properties of molecular gas?

Background. In regions with strong radiation fields and low dust-to-gas ratios, far-UV photons are able to penetrate deeper into molecular clouds, enhancing the abundance of ionized and atomic carbon relative to carbon monoxide, since CO is efficiently photodissociated by FUV photons. Star formation may therefore proceed differently in molecular clouds situated in environments with strong radiation fields and low dust abundance, since the structure and thermal balance of the molecular gas is different to molecular gas in the solar neighbourhood. Environments with strong radiation fields and low dust content are of particular interest since these conditions are thought to have prevailed in the early Universe.

30 Doradus in the Large Magellanic Cloud (LMC) is the most active star-forming region in the Local Group, containing more than 60 O stars, which are the dominant source of far-UV radiation in galaxies. There is a long ridge of molecular gas that extends southwards from 30 Doradus star-forming region for almost 2 kpc. Along this molecular ridge, we can therefore study the properties of molecular gas as a function of the radiation field.

Experiment. In this experiment, you will measure the spectral line emission from different molecular species at several locations along the molecular ridge. The aim of your observations is to obtain empirical input data for models that calculate the physical conditions (temperature, density) within molecular gas using intensity ratios from different molecular line transitions.

1. Use SIMBAD to find the location of 30 Doradus.

2. Inspect the files ridge.peakT.fits and dust.colorT.fits in ds9. Select three or four locations along the molecular ridge that are bright in CO emission. Note the peak CO brightness at these positions, and verify that the ratio of the 60um to 100um emission (which we are using as a proxy for the interstellar radiation field) is different at each of the locations that you have chosen.

3. Use the COORD tool at:

http://www.parkes.atnf.csiro.au/cgi-bin/utilities/coord.cgi

to calculate the LST range when these positions are observable from Mopra. Note that the Mopra dish can be steered to a limiting elevation of 12.5°, but that the signal-to-noise of millimetre observations deteriorates rapidly below elevations of about 35° due to moisture in the Earth’s atmosphere.

4. Look at the frequencies of some commonly observed spectral lines on this page:

http://www.narrabri.atnf.csiro.au/observing/spectral.html

You need to observe the ¹²CO(J=1-0) emission line at 115.271 GHz, as this is the reference molecule for nearly all models of molecular cloud chemistry. What is the total bandwidth of the Mopra spectrometer (MOPS)? What are some other spectral lines that you could observe simultaneously with ¹²CO? At what frequency would you place the centre of the MOPS band (this is called the central observing frequency)? A longer list of spectral lines can be found using the Lovas catalogue:

http://physics.nist.gov/cgi-bin/micro/table5/start.pl

4b. You would also like to observe the HNC/HCN ratio. Are there emission lines from these molecules that are accessible using the Mopra 3mm receiver? What are some other spectral lines that you could observe simultaneously with HCN and HNC? What central observing frequency would you use in this case?

5. Refer to Table 3 of Chin et al (1997). What is the ratio of the 12CO(J=1-0) peak brightness temperature (Tmb) to the 13CO(J=1-0) peak brightness for their observations of N113 (a molecular cloud in another part of the LMC)? What is the ratio of the 12CO peak brightness to the HCO+ peak brightness? Assuming similar line ratios throughout the molecular ridge, what brightness temperatures do you expect for 13CO and HCO+ at the positions that you have chosen in the molecular ridge?

Use Table 3 of Chin et al (1997), Table 5 of Heikkila et al (1999) or the Lovas catalogue to estimate the peak brightness temperature of other spectral lines that you have selected for your different positions within the molecular ridge. (For this estimate, you will need to assume that the line ratios are the same for different sources in the Milky Way and the LMC.)

6. Use the Mopra position-switching sensitivity calculator at:

http://www.narrabri.atnf.csiro.au/mopra/sensitivity_mopra_pswitch.html

to estimate how long you will need to integrate on each of your spectral lines at each position in order to achieve a 5-sigma detection across a 1 km/s channel. What about a 4-sigma detection across a 2 km/s channel?

You want to limit the total time request for your observing proposal to be less than 100 hours, including time for overheads (i.e. calibrations plus bad weather). Given this time constraint, which spectral lines would be realistic to include in your proposal?

Within your group, discuss the implications for the total time request of your proposal if you decided to:

i) include observations of more positions within the molecular ridge

ii) include spectral lines that require observations using additional central observing frequencies

iii) increased the sensitivity of your observations e.g. by a factor of three.

7. Use the ATNF velocity-to-frequency calculator at:

http://www.narrabri.atnf.csiro.au/observing/obstools/velo.html

to estimate the sky frequencies of the spectral lines that you are planning to observe. For this estimate, you can use the average radial velocity of the N159 clouds (see Heikkila et al. 1999). Do you need to revise your i) central frequencies ii) spectral line list or iii) integration time estimates because of Doppler shift of spectral lines in the LMC? What if you wanted to use Mopra to observe a galaxy at z=0.1 that had similar line strengths as the LMC*? What about a galaxy at z=1.0?

Note that the velocity-to-frequency calculator is just a tool to solve the relativistic Doppler shift equation, i.e.

i) l_o/l_s= f_s/f_o = sqrt((1+b)/(1-b))

ii) b=v/c

iii) z = sqrt((1+b)/(1-b)) - 1

where

f_s - frequency of radiation at source

f_o- frequency of radiation at observer

l_s - wavelength of radiation at source

l_o - wavelength of radiation at observer

v - relative velocity of source and observer

c - velocity of light

z - redshift of source

*Also note that it is not realistic to assume that distant galaxies will have similar line strengths as the LMC!

8. You should now have sufficient information to complete the cover sheet and the observations table of your proposal. Go to http://opal.atnf.csiro.au

9. Using the articles in the recommended reading list as a starting point, write a couple of paragraphs that explain the scientific motivation for your observations and why you have adopted your observing strategy.

10. Go to the ATNF online data archive:

http://atoa.atnf.csiro.au

and check if Mopra data already exists that you could have used for this project. (Note that this should have been the first step in preparing your observing proposal, not the last!)

Recommended Reading

You will need to refer to papers marked with an asterisk to complete this exercise. Other papers are for interest only and can be found using ADS (http://adsabs.harvard.edu/abstract_service.html) or the astro-ph preprint server (http://xxx.lanl.gov/).

*Heikkila et al (1999) “Molecular Abundance Variations In The Magellanic Clouds”

*Chin et al (1997) “Molecular Abundances In The Magellanic Clouds I. A Multiline Study Of Five Cloud Cores”

Hollenbach & Tielens (1999) “Photodissociation Regions In The Interstellar Medium Of Galaxies”

Pineda et al (2008) “Submillimeter Line Emission From LMC N159W: A Dense, Clumpy PDR In A Low Metallicity Environment”

Pak et al (1998) “Molecular Cloud Structure In The Magellanic Clouds: Effect Of Metallicity”