Exchange Report by Kevin Corneilus Harrington

 

Introduction

My research focuses on the brightest galaxies roughly 9-12​ Gyr in the past. Corresponding to redshifts, z, of about​ ​1.5-3.5, this is when the peak of the co-moving​ ​starformation rate density in the universe takes place. This​ ​therefore motivates observers to pin down the physical conditions of the neutral, molecular and ionized interstellar​ ​medium (ISM) at these high-z’s. The most popular tool to​ ​constrain the molecular ISM conditions (stars are believed​ ​to form within collapsing molecular gas clouds), is to​ ​observe multiple CO emission lines. The most important transition to determine the bulk molecular gas mass across​ ​all of cosmic time is the ground state CO(1-0) line​ ​emission, as it has traditionally been the most well-calibrated to total molecular hydrogen gas mass. Unfortunately​ ​this line cannot yet be observed with ALMA at these​ ​redshifts because the receiver capabilities have not yet been​ ​made available, but in any galaxy this transition also​ ​happens to have a faint line emission compared to the​ ​higher-J lines. Observational constraints on the total molecular​ ​gas mass in a star-forming galaxy have thus been limited​ ​to higher-J CO lines.

The Planck and Herschel satellites have recently discovered​ ​many z > 1 galaxies that are extremely luminous(L​IR > 10​13-14L​SUN) and all having strong magnification from​ ​gravitational lensing. The advantage of observing these strongly​ ​lensed galaxies lies in the shorter integration times required​ ​to observe emission lines, while the challenge is in the​ ​unique lens modeling of each system. Alongside 8 of the most​ ​luminous galaxies observed in the sky (Harrington & Yun et al.​ 2016), identified by Planck and Herschel, and observed with​ ​the Large Millimeter Telescope, another 15-20 lensed candidates​ ​were selected via Planck and detected with the LMT (Yun et al. prep.). I have since led two follow-up campaigns to observe​ ​the CO(1-0) line emission in all of these luminous IR​ ​galaxies at high-z.

Experiences

University of Massachusetts (UMass), Amherst, MA:

Upon arriving to UMass in August the campus was seemingly quiet, although it slowly increased in volume as more and more students arrived to begin the semester in September. I arrived a week and a half before the JWST proposal deadline. Prof. Dr. J. Lowenthal from nearby Smith College has recently led a successful 22 orbit HST program to obtain H band optical images of these gravitationally lensed galaxies, and he was the Principal Investigator (PI) for the JWST proposal. J. Lowenthal, UMass Prof. Dr. Q. D. Wang, Prof Dr. M. Yun, his grad student P. Kamieneski and myself were meeting regularly to discuss the science cases and compare (on-the-fly-in-real-time) our sensitivity estimates we had been calculating for the NIRSPEC and MIRI instruments on the JWST. Some of my text was eventually included in the final proposal to observe strong optical emission lines in the NIR and characterize the PAH features in the MIR. I was also able to learn a great deal about the software tools for JWST.

Cornell University, Ithaca, NY

The autumn season in Ithaca, NY was the perfect time of the year. The change of color on the leaves and the waterfalls all around Ithaca were very inspiring. While working at Cornell I spent a lot of time with Prof. Dr. D. Riechers and also Prof. Dr. G. Stacey. I greatly benefited from hearing Prof. Dr. Stacey’s talk about the instrumentation for the new CCAT telescope in Chile, and I also gained a strong professional connection as he and I talked about future proposals with the new ZEUS-2 spectrometer. He provided insights to enable me to make estimates of the CII, NII, OI and OIII FIR fine-structure lines for the Planck galaxies I am targeting. What stood out for me while at Cornell was all of the guidance I received for my writing from Prof. Dr. Riechers. I focused on wrapping up a final draft of a manuscript presenting the results of 7 Planck galaxies and their CO(1-0) line detections from the Green Bank Telescope (GBT). After some final revisions I was able to submit this paper to the Monthly Notices of the Royal Astronomical Society Journal by early October, and after a minor revision from the referee this paper has recently been accepted for publication in November 2017. As well, I received strong feedback on an IRAM 30m telescope proposal that I was the PI for. In leading this proposal, I requested about 90 hours of observing time to fully characterize the CO spectral line energy distributions in the Planck sample of high-z, lensed galaxies. In November this proposal was awarded A-rating, and I will be observing with the IRAM 30m this December, 2017 and March, 2018. Again, I am grateful for the excellent feedback and support from Prof. Dr. Riechers, as well as his graduate students.

Green Bank Observatory (GBO), Green Bank, WV:

The Green Bank Observatory is home to the 100-m Green Bank Telescope (GBT). For me this was the pinnacle of peace and quiet after the exciting two months of travel prior to going to Green Bank. No WIFI, no microwaves, no cell phones. I was thankfully in the National Radio Quiet Zone. The program I was a PI for was accepted for C-rating, and the receiver I requested was only scheduled to be installed and on the GBT for about one month. There is a significant boost in the probability to get scheduled if the observer is in Green Bank --- which is why I was motivated to be present to lead the observations from the control room there. The data reduction process was discussed frequently with staff scientist, Dr. D. Frayer. The Ka band receiver hardware is difficult to manufacture in order to maintain a stable baseline beyond 1 GHz across the 26-40 GHz regime. The systematic features introduced to the observed spectra require an expert-eye to understand what is real and what is for example a standing wave, for example, and which spectra to drop before averaging. I benefited from direct feedback and assistance from Dr. Frayer, and he was able to reassure me that my reduction steps were correct. He also commented that if I were not present I wouldn’t have gotten any data most likely--even though I ended up receiving 35/45hr requested and detecting 15 CO(1-0) emission lines from new Planck targets. The successful observing run and confirmation of the LMT results will lead to a VLA proposal to resolve the single-dish observed CO(1-0) emission I recently obtained, and additionally those 7 galaxies presented in Harrington et al. (2017). The hands-on observing and rapid data reduction enabled me to significantly increase the known population of galaxies at z > 1 with direct CO(1-0) line measurements. I was also scheduled to observe on Halloween with the GBT and I dressed up as a frog from another planet.

With each talk I gave, at UMass, Cornell and then at the GBO, I felt my confidence growing and my knowledge of the material becoming more refined. I look forward to where this experience will lead me. I certainly recommend other doctoral students to benefit from this experience!