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- ItemA Kiloparsec-scale Molecular Wave in the Inner Galaxy: Feather of the Milky Way?(London : Institute of Physics Publ., 2021) Veena, V.S.; Schilke, P.; Sánchez-Monge, Á.; Sormani, M.C.; Klessen, R.S.; Schuller, F.; Colombo, D.; Csengeri, T.; Mattern, M.; Urquhart, J. S.We report the discovery of a velocity coherent, kiloparsec-scale molecular structure toward the Galactic center region with an angular extent of 30° and an aspect ratio of 60:1. The kinematic distance of the CO structure ranges between 4.4 and 6.5 kpc. Analysis of the velocity data and comparison with the existing spiral arm models support that a major portion of this structure is either a subbranch of the Norma arm or an interarm giant molecular filament, likely to be a kiloparsec-scale feather (or spur) of the Milky Way, similar to those observed in nearby spiral galaxies. The filamentary cloud is at least 2.0 kpc in extent, considering the uncertainties in the kinematic distances, and it could be as long as 4 kpc. The vertical distribution of this highly elongated structure reveals a pattern similar to that of a sinusoidal wave. The exact mechanisms responsible for the origin of such a kiloparsec-scale filament and its wavy morphology remains unclear. The distinct wave-like shape and its peculiar orientation makes this cloud, named as the Gangotri wave, one of the largest and most intriguing structures identified in the Milky Way.
- ItemPHANGS-JWST First Results: Spurring on Star Formation: JWST Reveals Localized Star Formation in a Spiral Arm Spur of NGC 628(London : Institute of Physics Publ., 2022) Williams, Thomas G.; Sun, Jiayi; Barnes, Ashley T.; Schinnerer, Eva; Henshaw, Jonathan D.; Meidt, Sharon E.; Querejeta, Miguel; Watkins, Elizabeth J.; Bigiel, Frank; Blanc, Guillermo A.; Boquien, Médéric; Cao, Yixian; Chevance, Mélanie; Egorov, Oleg V.; Emsellem, Eric; Glover, Simon C. O.; Grasha, Kathryn; Hassani, Hamid; Jeffreson, Sarah; Jiménez-Donaire, María J.; Kim, Jaeyeon; Klessen, Ralf S.; Kreckel, Kathryn; Kruijssen, J. M. Diederik; Larson, Kirsten L.; Leroy, Adam K.; Liu, Daizhong; Pessa, Ismael; Pety, Jérôme; Pinna, Francesca; Rosolowsky, Erik; Sandstrom, Karin M.; Smith, Rowan; Sormani, Mattia C.; Stuber, Sophia; Thilker, David A.; Whitmore, Bradley C.We combine JWST observations with Atacama Large Millimeter/submillimeter Array CO and Very Large Telescope MUSE Hα data to examine off-spiral arm star formation in the face-on, grand-design spiral galaxy NGC 628. We focus on the northern spiral arm, around a galactocentric radius of 3-4 kpc, and study two spurs. These form an interesting contrast, as one is CO-rich and one CO-poor, and they have a maximum azimuthal offset in MIRI 21 μm and MUSE Hα of around 40° (CO-rich) and 55° (CO-poor) from the spiral arm. The star formation rate is higher in the regions of the spurs near spiral arms, but the star formation efficiency appears relatively constant. Given the spiral pattern speed and rotation curve of this galaxy and assuming material exiting the arms undergoes purely circular motion, these offsets would be reached in 100-150 Myr, significantly longer than the 21 μm and Hα star formation timescales (both < 10 Myr). The invariance of the star formation efficiency in the spurs versus the spiral arms indicates massive star formation is not only triggered in spiral arms, and cannot simply occur in the arms and then drift away from the wave pattern. These early JWST results show that in situ star formation likely occurs in the spurs, and that the observed young stars are not simply the “leftovers” of stellar birth in the spiral arms. The excellent physical resolution and sensitivity that JWST can attain in nearby galaxies will well resolve individual star-forming regions and help us to better understand the earliest phases of star formation.
- ItemLYRA. III. The Smallest Reionization Survivors(London : Institute of Physics Publ., 2022) Gutcke, Thales A.; Pfrommer, Christoph; Bryan, Greg L.; Pakmor, Rüdiger; Springel, Volker; Naab, ThorstenThe dividing line between galaxies that are quenched by reionization ("relics") and galaxies that survive reionization (i.e., continue forming stars) is commonly discussed in terms of a halo mass threshold. We probe this threshold in a physically more complete and accurate way than has been possible to date, using five extremely high resolution (Mtarget = 4 M⊙) cosmological zoom-in simulations of dwarf galaxies within the halo mass range (1–4) × 109 M⊙. The employed LYRA simulation model features resolved interstellar medium physics and individual, resolved supernova explosions. Interestingly, two out of five of the simulated dwarf galaxies lie close to the threshold mass but are neither full reionization relics nor full reionization survivors. These galaxies initially quench at the time of reionization but merely remain quiescent for ∼500 Myr. At z ∼ 5 they recommence star formation in a synchronous way and remain star-forming until the present day. The parallel timing indicates consistent sound-crossing and cooling times between the halos. While the star formation histories we find are diverse, we show that they are directly related to the ability of a given halo to retain and cool gas. Whereas the latter is most strongly dependent on the mass (or virial temperature) of the host halo at the time of reionization, it also depends on its growth history, the UV background (and its decrease at late times), and the amount of metals retained within the halo.