Imaging the Spatial Density within Starburst Galaxies

TIP Last Changed: JeffMangum - 23 February 2013

Table of Contents:

The Big Picture

NGC 0253 I FUV g2006.jpg
GALEX image of NGC253 in UV emission.
ngc253mapNH3-33.jpg
Image of NGC253 in NH3 (3,3) emission acquired with the EVLA. Note that the NH3 emission originates from the nucleus of this starburst galaxy.
Ultimately, we want to know how galaxies evolve. This means that we need to understand many aspects of the physical conditions and evolutionary state of galaxies. Starburst galaxies represent one possible state or aspect (it is not clear if all galaxies go through a starburst phase) of galaxy evolution. The physical composition of a galaxy provides the clues to understanding its evolution. The images of NGC253 shown to the right are examples of two aspects of this physical composition, showing emission from stars (UV) and dense gas (NH3), which is the stuff from which stars form.

The density and temperature measurements conducted with this work are a major step toward understanding the dense gas phase of the star formation process in galaxies. Studies of the dense gas from which stars form requires measurements of their molecular emission tracers. Most of the dense molecular gas in galaxies is composed of molecular hydrogen (H2). Since H2 has no permanent dipole moment it does not possess rotational excitation transitions that can be measured. For this reason we resort to a secondary measurement of molecules which are collisionally excited by H2 and which possess rotational transitions that we can measure. Over the past 50 years studies of the emission from molecules including CO, NH3, and H2CO have provided astrophysicists with very powerful tools to study the gas from which stars form in galaxies. Some of these molecules, due to their rotational excitation properties, are valuable probes of the spatial density and kinetic temperature in dense gas. These molecules then allow us to derive the spatial density and temperature within these regions.

In addition to the actual density and temperature of the star formation regions within galaxies, a knowledge of their proximity to previous generations of stars, supernovae, and other phases of the stellar evolutionary content of a galaxy allows us to assemble the life history of a galaxy. Once sufficient information on the physical and geographical dense gas properties within a large enough sample of galaxies is accumulated, we can determine the density and temperature evolution of galaxies from the Milky Way to the edges of the universe.

Student's Task List

  1. Read background information on molecular excitation, star formation, formaldehyde, ammonia, etc.
  2. Review and "follow leads" in literature on galaxies under study. This investigation should result in a detailed overall understanding of the properties of the following galaxies:
    1. NGC253
    2. IC342
    3. Maffei2
    4. M83
    5. NGC3079
    6. Arp220
    7. M82
  3. Review data:
    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;2013) papers 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.
  4. VLA Data Reduction:
    1. Reduce VLA H2CO measurements:
      1. Learn how to use the interferometric data analysis package CASA. Start with online CASA guides. In particular, the VLA tutorial on high-frequency observations of IRC+10216 is the most relevant tutorial to this student project. Work through this tutorial.
      2. Once you are proficient in VLA data analysis with CASA, start reduction of one of the galaxies in your sample (NGC253 is a good one to start with).
      3. Enter data reduction notes (anything you think would be interesting and/or useful to know about the analysis...more is better!) on the project wiki page under the "Data Reduction Summary" topic.
  5. H2CO Data Analysis:
    1. Once the reduction of the first galaxy in your list is complete, analyze results:
      1. Make integrated intensity images of each H2CO line.
      2. Use spectral line image cubes, integrated intensity images, and continuum images for comparisons described below.
      3. Compare your H2CO images to the single-dish measurements made with the GBT. For example, compare flux recovered in VLA interferometer measurements with those derived from GBT single-dish measurements.
      4. Compare your H2CO images with any other molecular spectral line images that you uncovered in your literature search on this galaxy.
      5. Understand our current thinking as to what the kinetic temperature is in this galaxy. This will involve reviewing our existing NH3 measurements of these galaxies.
      6. Summarize results of H2CO analysis before moving-on to data reduction and analysis of next galaxy in sample.
    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 (see me when you are ready to start this comparison):
        1. TROH2CO01100x100x57.fits
        2. TROH2CO04100x100x57.fits
        3. RatCube-ExgalKDoubMeasurements-H2COJ1-H2COJ2.fits
    3. Put it all together into a unified interpretation of the H2CO and NH3 measurements of the galaxies you have studied to state what the dense gas spatial density and kinetic temperature is these objects.
    4. Enter analysis notes (anything you think would be interesting and/or useful to know about the analysis...more is better!) on the project wiki page under the "Data Analysis Summary" topic.
    5. Now that you know how to reduce VLA data, reduce all other galaxies in both C- and Ku-band. Do them in the following order:
      1. M83
      2. NGC3079 (I have already reduced the Ku-band data, but I would like you to reduce it also as a comparison)
      3. Arp220
      4. All the rest in any order you like
    6. Analyze results as you did with NGC253
    7. Enter analysis notes (anything you think would be interesting and/or useful to know about the analysis...more is better!) on the project wiki page under the "Data Analysis Summary" topic.

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, 2010. We have also now published a more extensive survey of H2CO emission in starburst galaxies (Mangum etal. 2013), basically a follow-up to our 2008 survey, where we have derived the spatial density in 13 starburst galaxies, now using more accurate kinetic temperature measurements.

FormaldehydeDensitometryStarburstPosterIR10.jpg
Formaldehyde Densitometry of Starburst Galaxies poster presentation given at the 'IR10' conference February 22-24, 2010.

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, along with ratios of higher-excitation transitions, monitor the higher kinetic temperatures (>= 100 K) in our starburst galaxy sample.

Reading List and Reference Material

Observations To Be Studied

Very Large Array (VLA)

The Expanded Very Large Array (VLA) has been used to image the 1(10)-1(11) (4.8 GHz) and 2(11)-2(12) GHz) K-doublet transitions of H2CO toward a several galaxies in our original 2008 sample of 52 galaxies exhibiting various aspects of starburst activity. The first set of observations for this part of the project were obtained 2011/11 through 2012/01. These measurements will continue to be scheduled on the EVLA through the spring of 2012.

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 in preparation. 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.

NGC253FormSpec.jpg
NGC253 H2CO
M83FormSpec.jpg
M83 H2CO

M82FormSpec.jpg
M82 H2CO
Arp220FormSpec.jpg
Arp220 H2CO

Ammonia Measurements

The Green Bank Telescope (GBT) has been used to make single-pointing measurements of the (1,1) through (9,9) transitions of NH3 near 24 GHz toward starburst galaxies where we have detected H2CO. The major portion of this project has been completed. A sample of our NH3 measurements toward several starburst galaxies is shown in the following figures.

NGC253NH3J124578.jpg
NGC253 NH3 (1,1), (2,2), (4,4), (5,5), (7,7), and (8,8)
NGC253NH3J369.jpg
NGC253 NH3 (3,3), (6,6), and (9,9)

M83NH3.jpg
M83 NH3 (1,1), (2,2), and (4,4)
M82SWNH3.jpg
M82 NH3 (1,1), (2,2), and (4,4)

-- JeffMangum - 2012-02-18
Topic revision: r8 - 2013-03-08, JeffMangum
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