The Starburst Properties of NGC660

Last Changed: JeffMangum - 23 June 2011

Table of Contents:

Student's To Do List

1. Read background information on molecular excitation, star formation, formaldehyde, ammonia, etc.
2. Review and "follow leads" in literature on NGC660. This investigation should result in a detailed overall understanding of the current understanding of the properties of NGC660.
3. Review data:
   1. H2CO and NH3:
      1. Review work already competed on H2CO and NH3 transition ratios to derive spatial density and kinetic temperature. Specifically:
         1. Review Mangum et. al. (2008) paper on starburst galaxy densitometry to understand how H2CO is used as a density probe.
         2. Review "Molecular Column Density Calculation" tutorial by Mangum and Shirley to understand how NH3 is used as a kinetic temperature probe.
      2. Analyze dependence of line ratios on density, temperature, and abundance to understand limitations of line ratio measurements for deriving physical conditions (specifically, density) in star formation regions. Use radiative transfer model (LVG) in this investigation.
         1. Peruse the following LVG model fits cubes to understand density, temperature, and column density dependence of H2CO transitions:
            1. TROH2CO01100x100x57.fits
            2. TROH2CO04100x100x57.fits
            3. RatCube-ExgalKDoubMeasurements-H2COJ1-H2COJ2.fits
      3. Analyze GBT H2CO and NH3 measurements of NGC660:
         1. GBTIDL data file is "NGC660-H2CO-NH3.fits".
         2. Spectral smooth (as needed), baseline, and gaussian fit all spectra.
         3. Use "NGC660-H2CO-NH3-Analysis-Results.txt" as a guide.
         4. If limitations of GBTIDL become an issue (as they likely will), use CLASS to analyze spectra. CLASS format data file is NGC660-H2CO-NH3.gbt in /export/data_1/jmangum.
      4. Apply GBT H2CO and NH3 measurement results to LVG models to derive spatial density and temperature(s) in NGC660.
      5. Summarize results of H2CO and NH3 line ratio analyses and their application to the problem of deriving spatial density and kinetic temperature in NGC660.
   2. CO (from Eva Schinnerer)
      1. Study intensity and velocity structure in these data and correlate with information gleaned from literature search. Use "casaview" as a tool. This will require running "casa" and importing the FITS cubes into casa using "importfits".
   3. HI (from Eva Schinnerer)
      1. Study intensity and velocity structure in these data and correlate with information gleaned from literature search. Use "casaview" as a tool. This will require running "casa" and importing the FITS cubes into casa using "importfits".

Background

The Physical Conditions Within Starburst Galaxies

Studies of the distribution of Carbon Monoxide (CO) emission in external galaxies (cf. Young & Scoville (1991)) have pointed to the presence of large quantities of molecular material in these systems. These studies have yielded a detailed picture of the molecular mass in many external galaxies. But, because emission from the abundant CO molecule is generally dominated by radiative transfer effects, such as high optical depth, it is not a reliable monitor of the physical conditions, such as spatial density and kinetic temperature, quantities necessary to assess the possibility of star formation. Emission from less-abundant, higher-dipole moment molecules are better-suited to the task of deriving the spatial density and kinetic temperature of the dense gas in our and external galaxies. For this reason, emission line studies from a variety of molecules have been made toward mainly nearby galaxies (see Mauersberger & Henkel (1989) (CS), Gao & Solomon (2004a) (HCN), Nguyen-Q-Rieu et al. (1992) (HCO+), Mauersberger et al. (1990) and Meier & Turner (2005) (HC3N), Mauersberger et al. (2003) (NH3), or Henkel, Baan, & Mauersberger (1991) for a review). The most extensive sets of measurements of molecular line emission in external galaxies has been done using the J=1-0 transitions of CO (Helfer et al. 2003) and HCN (Gao & Solomon 2004a). Since the J=1-0 transitions of CO and HCN are good tracers of the more generally distributed and the denser gas, respectively, but do not provide comprehensive information about the individual physical conditions of the dense, potentially star-forming gas, another molecule must be observed for this purpose.

Density Measurement Using Formaldehyde

Formaldehyde (H2CO) has proven to be a reliable density and kinetic temperature probe in Galactic molecular clouds. Existing measurements of the H2CO 1(10)-1(11) and 2(11)-2(12) emission in a wide variety of galaxies by Baan et al. (1986), Baan et al. (1990), Baan et al. (1993), and Araya et al. (2004) have mainly concentrated on measurements of the 1(10)-1(11) transition. One of our goals with the present study was to obtain a uniform set of measurements of both K-doublet transitions with which the physical conditions, specifically the spatial density, in the extragalactic context could be derived. Using the unique density selectivity of the K-doublet transitions of H2CO we have measured the spatial density in a sample of galaxies exhibiting starburst phenomena and/or high infrared luminosity.

Results from the first phase of this work, which was a "pilot survey" of a sample of mainly nearby galaxies measured using the GBT (Mangum et al . (2008) has shown that H2CO is a reliable and accurate density probe for extragalactic environments where the kinetic temperature is known. See our recent poster describing this work presented at the Infrared Emission, ISM and Star Formation workshop held at MPIA in Heidelberg, Germany, February 22-24.

Kinetic Temperature Measurement Using Ammonia

The derivation of n(H2) in our sample of starburst galaxies currently relies upon assumed kinetic temperatures. The inversion transitions of NH3 and the rotational transitions of H2CO possess very similar excitation conditions, thus likely trace similar dense gas environments. Using the unique sensitivities to kinetic temperature afforded by the excitation characteristics of several inversion transitions of NH3 , we can continue our characterization of the dense gas in galaxies exhibiting starbursts by measuring the kinetic temperature in a sample of galaxies selected for their high infrared luminosity. This extension of our successful galaxy survey will allow us to further study the range of mean physical conditions which give rise to star formation in some of the most starburst-active galaxies known.

Formaldehyde as a Spatial Density Probe

Formaldehyde is a proven tracer of the high density environs of molecular clouds. It is ubiquitous: H2CO is associated with 80% of the HII regions surveyed by Downes et al. (1980), and possesses a large number of observationally accessible transitions from centimeter to far-infrared wavelengths. Because H2 CO is a slightly asymmetric rotor molecule, most rotational energy levels are split by this asymmetry into two energy levels. Therefore, the energy levels must be designated by a total angular momentum quantum number, J, the projection of J along the symmetry axis for a limiting prolate symmetric top, K-1 , and the projection of J along the symmetry axis for a limiting oblate symmetric top, K+1 . This splitting leads to two basic types of transitions: the high-frequency ∆J = 1, ∆K−1 = 0, K+1 = −1 “P-branch” transitions and the lower-frequency ∆J = 0, ∆K−1 = 0, ∆K+1 = ±1 “Q-branch” transitions, popularly known as the "K-doublet” transitions (see discussion in Mangum & Wootten (1993)). The P-branch transitions are only seen in emission in regions where n(H2 ) >= 10^4 cm−3 . The excitation of the K-doublet transitions, though, is not so simple. For n(H2 ) < 10^5.5 cm−3 , the lower energy states of the 1(10)−1(11) through 5(14)−5(15) K-doublet transitions become overpopulated due to a collisional selection effect (Evans et al. (1975); Garrison et al. (1975)). This overpopulation cools the J ≤ 5 K-doublets to excitation temperatures lower than that of the cosmic microwave background, causing them to appear in absorption. For n(H2 ) > 10^5.5 cm−3 , this collisional pump is quenched and the J ≤ 5 K-doublets are then seen in emission over a wide range of kinetic temperatures and abundances (see Figure 1 in Mangum etal (2008)).

Ammonia as a Kinetic Temperature Probe

Ammonia is a proven and unbiased tracer of the high density regions within molecular clouds in a variety of galactic and extragalactic environments (cf. Walmsley & Ungerechts (1983), Mauersberger et al . (2003)). Because NH3 is a symmetric top molecule (energy levels given by quantum numbers (J,K)), exchange of population between the K-ladders within a given symmetry state (ortho or para) occurs only via collisional processes. The relative intensity of these rotational energy levels then represents the urgently needed direct measure of the kinetic temperature (cf. Figure 1), breaking the Tk – n(H2 ) degeneracy. The inversion transitions of NH3 at 23–27 GHz have been used to monitor the kinetic temperature in both cool (TK ≃ 20 K) and warm (TK ≃ 300 K) galactic and extragalactic star formation environments. For example, a study of the NH3 (1,1) through (9,9) transitions toward NGC 253, IC 342, and Maffei 2 by Mauersberger et al . (2003) revealed warm (TK = 100–140 K) gas toward NGC 253, IC 342, and Maffei 2 and cooler gas (TK = 60 K) toward M82. In this study, the NH3 (2,2)/(1,1) line ratio monitors lower kinetic temperatures (<= 40 K), while the NH3 (4,4)/(2,2) ratio monitors the higher kinetic temperatures (<= 150 K) in our starburst galaxy sample.

The Big Picture

Ultimately, we want to know how galaxies evolve. The density and temperature measurements conducted with this work is a major step in this direction, as it will ultimately allow us to determine the density and temperature evolution of galaxies from the Milky Way to the edges of the universe.

Reference Material

• "Formaldehyde Densitometry of Starburst Galaxies" by Mangum, Darling, Menten, & Henkel (2008), ApJ, 673, 832. Results from our "pilot survey" of H2CO in starburst galaxies.
• "Ammonia as a Molecular Cloud Thermometer" by Walmsley & Ungerecths (1983), A&A, 122, 164. How to use NH3 as a kinetic temperature probe.
• "Formaldehyde as a Probe of Physical Conditions in Dense Molecular Clouds" by Mangum & Wootten (1993), ApJS, 89, 123. Detailed discussion of H2CO as a probe of physical conditions in star formation regions. Specifically, describes details on how one uses H2CO as a temperature probe (good to compare to NH3).
• "HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies" by Gao, Y. & Solomon, P. M. (2004), ApJS, 152, 63. The seminal work on the characterization of dense gas in starburst galaxies. Very good introduction to the correlation between dense gas content and other starburst properties (infrared luminosity, mass, etc.).

Observations To Be Studied

Green Bank Telescope (GBT)

Formaldehyde Measurements

The Green Bank Telescope (GBT) has been used to make single-pointing measurements of the 1(10)-1(11) (4.8 GHz) and 2(11)-2(12) GHz) K-doublet transitions of H2CO toward a sample of 52 galaxies exhibiting various aspects of starburst activity. The first phase of this project was published in Mangum etal. (2008). A follow-up paper, which adds to the data presented in our 2008 paper, is currently being analyzed. Some of the measurements of the H2CO emission and absorption in the 1(10)-1(11) and 2(11)-2(12) transitions toward our starburst galaxy sample are shown in the following figures.

NGC660 H2CO

Ammonia Measurements

The Green Bank Telescope (GBT) has been used to make single-pointing measurements of the (1,1), (2,2), and (4,4) transitions transitions of NH3 near 24 GHz toward starburst galaxies where we have detected H2CO. The major portion of this project is currently scheduled on the GBT and is in-progress. A sample of NH3 (1,1), (2,2), and (4,4) measurements toward three starburst galaxies is shown in the following figures.

NGC660 NH3 (2,1), (1,1), (2,2), and (4,4)

NGC660 NH3 (3,3), (5,5), (6,6), and (7,7)

-- JeffMangum - 2011-06-14