Erica's Potential Research Projects

TIP Last Update: JeffMangum - 2020-07-24


Start with the introduction to this project. Contains a summary of the science behind this project and what the potential analysis steps will be to turn this research project into a published article. This document also includes a reading list with articles that in many cases are meant to provide an introduction to the broader topic of the study of star formation in galaxies. Sample image (below) is HCN 1-0 integrated intensity. NGC253 HCN 1-0 Integrated IntensityCheck out the Overleaf document containing analysis items done so-far (which will serve as a starting point for the published article for this project).

IC860 Continuum

This project will combine a number of VLA and VLBI imaging measurements of the continuum emission from IC860 with a goal toward deriving a definitive model of the radio/millimeter/submillimeter continuum emission in this enigmatic object. Combines data from the following observing projects: IC860 Radio Continuum
  • IC860 C- and K-band VLA and IC860 Q-band VLA: C-, K-, and Q-band A-array imaging observed in July 2015.
    • Four-panel image figure (right) showing 5 (C-band), 7 (C-band), 19 (K-band), and 45 (Q-band) GHz continuum images. Green ellipse in each panel is the synthesized beam, while the linear scale for each image is also shown.
    • Additional Q-band VLA measurements acquired in October 2019 (project 19A-436) available in SRDP archive. Should be added to existing Q-band measurements (from July 2015) to produce a higher signal-to-noise Q-band image.
  • IC860 Ka-band VLA: Ka-band A-array imaging to be observed in Fall/Winter 2020/2021.
  • ALMA Bands 3, 6, and 7 (and possibly Bands 5 and 9): Susanne Aalto (Chalmers) has these data. Shown in Aalto etal 2019, A&A, 627, A147, "The hidden heart of the luminous infrared galaxy IC 860. I. A molecular inflow feeding opaque, extreme nuclear activity", Figure 1.
  • Merlin (VLBI) C- and L-band: Erskil Varenius (Chalmers) has these data.

A rough outline/to-do list for this project:
  • By way of introduction, it might be instructive to review our latest successful VLA proposal (project 20B-174) science case (from above) for further continuum (and spectral line) measurements of this galaxy.
  • Gather/reduce all data.
    • Some reduction/analysis complete (VLA C-, K-, part of Q-band, ALMA Bands 3, 6, and 7), while others not (second half of VLA Q-band, new Ka-band, and ALMA Bands 5 and 9).
    • Second half of VLA Q-band (project 19A-436) is in NRAO SRDP archive.
  • Assess spatial structure differences between various continuum images.
    • Most images are simple either point or minimally-resolved gaussian sources
    • To do synthesized analysis of flux as a function of frequency will need to convolve to same spatial resolution
    • Check relative spatial frequency sampling for all measurements (will dictate how well one can compare measurements made with different instruments and configurations within a given instrument)
  • Multi-spectral component fit to 5 to 900(?) GHz emission distribution.
    • Multiple components are due to multiple emission processes possible over the wide frequency range. Comprised of (at least) synchrotron (S_nu propto nu^{-0.7}), free-free (S_nu propto nu^{-0.1}), and dust (S_nu propto nu^2).
    • Note that power laws for emission processes have possible ranges (i.e. are not fixed).
    • Analyze emission generation mechanisms (i.e. cosmic ray sources, young stars, dust) needed to produce resultant spectral components derived above. This should result is a very well-constrained model of the continuum emission from this galaxy.
  • With this model of the continuum emission in-hand, we will be able to do a better job of modeling/understanding the spectral line emission (which is often in absorption) from this object. Absolutely crucial to understand the continuum emission from this galaxy in order to make progress on its global properties.

ALCHEMI Science Project 9.0: Comparison of Physical and Chemical Conditions in the Central Molecular Zones in the Milky Way and NGC253

ALCHEMI science project PIs: Kunihiko Tanaka and Jeff Mangum

Note: This project is the least well-developed of the three projects currently listed, so may not be appropriate as a Master's Thesis project. Should certainly be considered for a more long-term project such as for one's PhD.

Project Description: The curiously low star formation efficiency of the Milky Way’s galactic central region is a popular topic in recent star formation study; although the MW’s GC region shows chemical and physical characteristics similar to actively star-forming GMCs (high temperature, high density, and rich abundance of COMs), its SFE is 1 or 2 orders below the K-S relation.

The origin of this low SFE can be explored through comparison of the physical and chemical properties with GC of NGC253, which appears similar to MW’s GC in molecular maps (Sakamoto+11) but is a starburst. The rich molecular line data provided by ALCHEMI enables us to perform spatially-resolved measurement of gas kinetic temperature, gas volume density, and fractional abundance of molecules. I plan to use a hierarchical bayesian parameter inference method I developed for analysis of MW's GC data (Tanaka etal 2018, ApJS, 236, 40). This method is able to suppress unphysical solutions that are often problematic in flat-prior bayesian or maximum likelihood analysis, such as anti-correlation between column density and volume density. (Similar method to that used for dust SED fit; Kelly etal 2012, ApJ 752, 55)

Through this analysis, we will be able to diagnose the conditions of the GMCs in NGC253 and compare them with the result for MW’s GC; volume density and gas temperature are directly measured, and information on strength of turbulence is obtained from spatial variation of temperature and shock tracer abundances, and velocity widths. By using these value, we will search for the parameter sets that best distinguish NGC253 and MW's GCs.

Traditional analyses using LVG and 3D microturbulent modeling of a number of molecules, including the 23 P-branch (delta(J)=1) transitions of H2CO measured by ALCHEMI, will also be used to constrain a multi-scale model of the volume density and kinetic temperature in NGC253 on scales as small as 20 pc. This more traditional analysis will be compared to the hierarchical bayesian parameter inference method described above.

The H3O+/SO Ratio as a Tracer of Cosmic Ray Ionization and Heating in NGC253

This project is similar to the HCN/HNC ratio study above, but differs in that it focuses on an assessment of a different kind of dense gas heating (cosmic ray) in the NGC253 starburst. Derives from a compelling analysis from Mangum etal. (2019):
  1. Bayet et al. (2011) and Meijerink et al. (2011) modeled H3O+ (top right figure) and found that its abundance maintains a high level (of about 10^(−9) to 10^(−8), respectively) under a wide range of conditions, peaking at CR rates of ∼10^(−15) to 10^(−13) s^(−1) at solar metallicity when the H2 column densitybecomes high. Our measured abundance of H3O+ (∼10^(−9)) seems to be consistent with both the Bayet and Meijerink predictions.H3O+ Column Densities from Meijerink 2011
  2. Bayet et al. (2011) and Bayet et al. (2010) modeled SO (second and third right figure) and found that it is destroyed by CRs starting at 10^(−16) s^(−1). The regional SO abundance pattern we measure, with Region 5 showing the lowest abundance and Region 7 showing the highest, seems to be consistent with a higher concentration of CRs at the center of the NGC253 CMZ than in its outskirts. H3O+/SO Abundances and Ratio from Bayet 2011 H3O+/SO Abundances and Ratio from Bayet 2010
  3. As H3O+ abundances are enhanced by CRs, while SO is destroyed by CRs, we have made a direct comparison of the abundances of these two direct CR tracers for the different regions (see figure bottom right). Region 5 has an H3O+/SO abundance ratio more than 30% larger than that in Regions 3, 4, 6, and 7, suggestive of enhanced CR heating near the center of NGC253. Note, though, that this abundance ratio assumes optically thin emission from both molecules. Variations in the relative optical depth within the transitions measured to calculate this abundance ratio could at least partially explain this difference. Furthermore, our H3O+ and SO measurements were made with different tunings of the ALMA receiver system, making their abundance ratio susceptible to our estimated absolute amplitude calibration uncertainties of 10% and 15% at Bands 6 and 7, respectively. Factor of two differences could be explained by these two effects.
H3O+/SO Abundance Ratio in NGC253

A multi-transition analysis of both H3O+ and SO could be used to clear-up the potential issues with radiative transfer noted in the Mangum etal. (2019) analysis.

A rough outline/to-do list for this project:

-- JeffMangum - 2020-05-25
Topic revision: r12 - 2020-07-24, JeffMangum
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