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1. A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE): IV. A tail of ionised gas in the merger remnant NGC4424

   https://doi.org/10.1051/0004-6361/201833914  

   Boselli, A.; Fossati, M.; Consolandi, G.; Amram, P.; Ge, C.; Sun, M.; Anderson, J. P.; Boissier, S.; Boquien, M.; Buat, V.; et al (December 2018, Astronomy & Astrophysics)
2. A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE): III. Star formation in the stripped gas of NGC 4254★

   https://doi.org/10.1051/0004-6361/201732410  

   Boselli, A.; Fossati, M.; Cuillandre, J. C.; Boissier, S.; Boquien, M.; Buat, V.; Burgarella, D.; Consolandi, G.; Cortese, L.; Côté, P.; et al (July 2018, Astronomy & Astrophysics)
3. A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE): I. Introduction to the survey★

   https://doi.org/10.1051/0004-6361/201732407  

   Boselli, A.; Fossati, M.; Ferrarese, L.; Boissier, S.; Consolandi, G.; Longobardi, A.; Amram, P.; Balogh, M.; Barmby, P.; Boquien, M.; et al (June 2018, Astronomy & Astrophysics)
4. A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE): VI. Environmental quenching on HII-region scales

   https://doi.org/10.1051/0004-6361/201937310  

   Boselli, A.; Fossati, M.; Longobardi, A.; Boissier, S.; Boquien, M.; Braine, J.; Côté, P.; Cuillandre, J. C.; Epinat, B.; Ferrarese, L.; et al (February 2020, Astronomy & Astrophysics)

   The Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE) is a blind narrow-band H α + [NII] imaging survey of the Virgo cluster carried out with MegaCam at the Canada-French-Hawaii telescope. We use a new set of data extracted from VESTIGE to study the impact of the hostile cluster environment on the star formation process down to the scale of HII regions (∼50 pc). Here, HII regions are identified and their parameters measured using the HII PHOT code on a sample of 114 late-type galaxies spanning a wide range in morphological type (Sa-Sd, Im, BCD), stellar mass (10 6.5  ≤  M star  ≤ 10 11 M ⊙ ), and star formation activity (10 −3  ≤ SFR ≤ 10 M ⊙ yr −1 ). Owing to the exquisite average resolution of the VESTIGE data (0.65 arcsec), we detect 11302 HII regions with an H α luminosity L (H α ) ≥ 10 37 erg s −1 . We show that the typical number of HII regions in gas-stripped objects is significantly lower than in healthy late-types of similar stellar mass. We also show that in these gas-stripped galaxies the number of HII regions significantly drops outside the effective radius, suggesting that the quenching process occurs outside-in, in agreement with other multifrequency observations. These new results consistently confirm that the main mechanism responsible for the decrease of the star formation activity observed in cluster galaxies is ram pressure, allowing us to discard other milder processes such as starvation or strangulation, which are unable to reproduce the observed radially truncated profiles. 

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5. Early Science from POSSUM: Shocks, turbulence, and a massive new reservoir of ionised gas in the Fornax cluster

   https://doi.org/10.1017/pasa.2021.4  

   Anderson, C. S.; Heald, G. H.; Eilek, J. A.; Lenc, E.; Gaensler, B. M.; Rudnick, Lawrence; Van Eck, C. L.; O’Sullivan, S. P.; Stil, J. M.; Chippendale, A.; et al (January 2021, Publications of the Astronomical Society of Australia)

   null (Ed.)

   Abstract We present the first Faraday rotation measure (RM) grid study of an individual low-mass cluster—the Fornax cluster—which is presently undergoing a series of mergers. Exploiting commissioning data for the POlarisation Sky Survey of the Universe’s Magnetism (POSSUM) covering a $${\sim}34$$ square degree sky area using the Australian Square Kilometre Array Pathfinder (ASKAP), we achieve an RM grid density of $${\sim}25$$ RMs per square degree from a 280-MHz band centred at 887 MHz, which is similar to expectations for forthcoming GHz-frequency $${\sim}3\pi$$ -steradian sky surveys. These data allow us to probe the extended magnetoionic structure of the cluster and its surroundings in unprecedented detail. We find that the scatter in the Faraday RM of confirmed background sources is increased by $$16.8\pm2.4$$ rad m −2 within 1 $$^\circ$$ (360 kpc) projected distance to the cluster centre, which is 2–4 times larger than the spatial extent of the presently detectable X-ray-emitting intracluster medium (ICM). The mass of the Faraday-active plasma is larger than that of the X-ray-emitting ICM and exists in a density regime that broadly matches expectations for moderately dense components of the Warm-Hot Intergalactic Medium. We argue that forthcoming RM grids from both targeted and survey observations may be a singular probe of cosmic plasma in this regime. The morphology of the global Faraday depth enhancement is not uniform and isotropic but rather exhibits the classic morphology of an astrophysical bow shock on the southwest side of the main Fornax cluster, and an extended, swept-back wake on the northeastern side. Our favoured explanation for these phenomena is an ongoing merger between the main cluster and a subcluster to the southwest. The shock’s Mach angle and stand-off distance lead to a self-consistent transonic merger speed with Mach 1.06. The region hosting the Faraday depth enhancement also appears to show a decrement in both total and polarised radio emission compared to the broader field. We evaluate cosmic variance and free-free absorption by a pervasive cold dense gas surrounding NGC 1399 as possible causes but find both explanations unsatisfactory, warranting further observations. Generally, our study illustrates the scientific returns that can be expected from all-sky grids of discrete sources generated by forthcoming all-sky radio surveys. 

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