publications

2022

1. ALMA Reveals Extended Cool Gas and Hot Ionized Outflows in a Typical Star-forming Galaxy at z = 7.13

   Hollis B. Akins, Seiji Fujimoto, Kristian Finlator, and 9 more authors

   The Astrophysical Journal, Jul 2022

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   We present spatially resolved morphological properties of [C II] 158 μm, [O III] 88 μm, dust, and rest-frame ultraviolet (UV) continuum emission for A1689-zD1, a strongly lensed, sub-L* galaxy at z = 7.13, by utilizing deep Atacama Large Millimeter/submillimeter Array (ALMA) and Hubble Space Telescope (HST) observations. While the [O III] line and UV continuum are compact, the [C II] line is extended up to a radius of r ~ 12 kpc. Using multi-band rest-frame far-infrared continuum data ranging from 52 to 400 μm, we find an average dust temperature and emissivity index of \{T}_{}mathrm{dust}}={41}_{-14}^{+17} K and {}beta ={1.7}_{-0.7}^{+1.1} , respectively, across the galaxy. We find slight differences in the dust continuum profiles at different wavelengths, which may indicate that the dust temperature decreases with distance. We map the star formation rate (SFR) via IR and UV luminosities and determine a total SFR of 37 ± 1M ⊙yr-1 with an obscured fraction of 87%. While the [O III] line is a good tracer of the SFR, the [C II] line shows deviation from the local L [C II]-SFR relations in the outskirts of the galaxy. Finally, we observe a clear difference in the line profile between [C II] and [O III], with significant residuals (~5σ) in the [O III] line spectrum after subtracting a single Gaussian model. This suggests a possible origin of the extended [C II] structure from the cooling of hot ionized outflows. The extended [C II] and high-velocity [O III] emission may both contribute in part to the high L [O III]/L [C II] ratios recently reported in z \textgreater 6 galaxies.

2. Quenching and the UVJ Diagram in the SIMBA Cosmological Simulation

   Hollis B. Akins, Desika Narayanan, Katherine E. Whitaker, and 5 more authors

   The Astrophysical Journal, Apr 2022

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   Over the past decade, rest-frame color-color diagrams have become popular tools for selecting quiescent galaxies at high redshift, breaking the color degeneracy between quiescent and dust-reddened star-forming galaxies. In this work, we study one such color-color selection tool-the rest-frame U - V versus V - J diagram-by employing mock observations of cosmological galaxy formation simulations. In particular, we conduct numerical experiments assessing both trends in galaxy properties in UVJ space and the color-color evolution of massive galaxies as they quench at redshifts z ~ 1-2. We find that our models broadly reproduce the observed UVJ diagram at z = 1-2, including (for the first time in a cosmological simulation) reproducing the population of extremely dust-reddened galaxies in the top right of the UVJ diagram. However, our models primarily populate this region with low-mass galaxies and do not produce as clear a bimodality between star-forming and quiescent galaxies as is seen in observations. The former issue is due to an excess of dust in low-mass galaxies and relatively gray attenuation curves in high-mass galaxies, while the latter is due to the overpopulation of the green valley in SIMBA. When investigating the time evolution of galaxies on the UVJ diagram, we find that the quenching pathway on the UVJ diagram is independent of the quenching timescale, and instead dependent primarily on the average specific star formation rate in the 1 Gyr prior to the onset of quenching. Our results support the interpretation of different quenching pathways as corresponding to the divergent evolution of post-starburst and green valley galaxies.

2021

1. Quenching Timescales of Dwarf Satellites around Milky Way–mass Hosts

   Hollis B. Akins, Charlotte R. Christensen, Alyson M. Brooks, and 4 more authors

   The Astrophysical Journal, Mar 2021

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   Observations of the low-mass satellites in the Local Group have shown high fractions of gas-poor, quiescent galaxies relative to isolated dwarfs, implying that the host halo environment plays an important role in the quenching of dwarf galaxies. In this work, we present measurements of the quenched fractions and quenching timescales of dwarf satellite galaxies in the DC Justice League suite of four high-resolution cosmological zoom-in simulations of Milky Way–mass halos. We show that these simulations accurately reproduce the satellite luminosity functions of observed nearby galaxies, as well as the variation in satellite quenched fractions from M * ∼ 105 M ⊙ to 1010 M ⊙. We then trace the histories of satellite galaxies back to z ∼ 15 and find that many satellites with M * ∼ 106−108 M ⊙ quench within ∼2 Gyr of infall into the host halo, while others in the same mass range remain star-forming for as long as 5 Gyr. We show that this scatter can be explained by the satellite’s gas mass and the ram pressure it feels at infall. Finally, we identify a characteristic stellar mass scale of 108 M ⊙ above which infalling satellites are largely resistant to rapid environmental quenching.

2018

1. Imaging planets from imaginary worlds

   Hollis B. Akins, and Donald A. Smith

   The Physics Teacher, Oct 2018

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   The planets in our solar system are remarkable and captivating objects for beginning astronomers to study. While merely observing the planets is fairly easy, it can be difficult to capture the high-quality images beginners hope for. The methods used for deep-sky astrophotography are optimized for dim, fairly large objects using large apertures and long exposure times. These tools do not work nearly as well for the planets, which have the reverse properties: they are bright and very small. As such, the planets can serve as great teaching tools for capturing astronomical images, using software processing tools, and understanding the role of angular size in a flat image. In this paper, we will explain how to implement "Lucky Imaging," a simple but powerful process for photographing the planets. World-class facilities use Lucky Imaging in conjunction with adaptive optics, but we will show how you can achieve striking telescopic images with only a commonly available smartphone. We will show an example of how this tool can be combined with an image processing tool like PhotoShop to create imaginative images of what the planets might look like if Earth were in a different place. These tasks also enable the students to grapple with the implications of the fact that we perceive angular size rather than linear size.