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1. Young, Blue, and Isolated Stellar Systems in the Virgo Cluster. II. A New Class of Stellar System

   https://doi.org/10.3847/1538-4357/ac7c6c  

   Jones, Michael G. ; Sand, David J. ; Bellazzini, Michele ; Spekkens, Kristine ; Karunakaran, Ananthan ; Adams, Elizabeth A. K. ; Battaglia, Giuseppina ; Beccari, Giacomo ; Bennet, Paul ; Cannon, John M. ; et al ( August 2022 , The Astrophysical Journal)

   We discuss five blue stellar systems in the direction of the Virgo cluster, analogous to the enigmatic object SECCO 1 (AGC 226067). These objects were identified based on their optical and UV morphology and followed up with Hiobservations with the Very Large Array (and Green Bank Telescope), Multi Unit Spectroscopic Explorer (on the Very Large Telescope) optical spectroscopy, and Hubble Space Telescope imaging. These new data indicate that one system is a distant group of galaxies. The remaining four are extremely low mass (M_*∼ 10⁵M_⊙), are dominated by young blue stars, have highly irregular and clumpy morphologies, are only a few kiloparsecs across, yet host an abundance of metal-rich,$12+\log (\mathrm{O}/\mathrm{H})>8.2$, Hiiregions. These high metallicities indicate that these stellar systems formed from gas stripped from much more massive galaxies. Despite the young age of their stellar populations, only one system is detected in Hi, while the remaining three have minimal (if any) gas reservoirs. Furthermore, two systems are surprisingly isolated and have no plausible parent galaxy within ∼30′ (∼140 kpc). Although tidal stripping cannot be conclusively excluded as the formation mechanism of these objects, ram pressure stripping more naturally explains their properties, inmore »particular their isolation, owing to the higher velocities, relative to the parent system, that can be achieved. Therefore, we posit that most of these systems formed from ram-pressure-stripped gas removed from new infalling cluster members and survived in the intracluster medium long enough to become separated from their parent galaxies by hundreds of kiloparsecs and that they thus represent a new type of stellar system.

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2. The Gas in Virgo’s “Red and Dead” Dwarf Elliptical Galaxies

   Hallenbeck, Gregory L. ; Koopmann, Rebecca A. ( January 2017 , American Astronomical Society ... meeting)

   As star-forming dwarf irregulars and faint spirals fall onto a cluster, their gas content is easily and quickly removed by ram-pressure stripping or other cluster forces. Residual signs of star formation cease within 100 Myr, and only after approximately 1 Gyr do their optical features transition to elliptical.Despite this, ALFALFA has uncovered a population of three “red and dead” dwarf ellipticals in the Virgo Cluster which still have detectable reservoirs of HI. These dwarf ellipticals are extremely gas-rich—as gas-rich as the cluster’s star-forming dwarf irregulars (Hallenbeck et al. 2012). Where does this gas come from? We consider two possibilities. First, that the gas is recently acquired, and has not yet had time to form stars. Second, that the gas is primordial, and has been disrupted from being able to form stars during the current epoch.We present deep optical (using CFHT and KPNO) and HI (Arecibo and VLA) observations of this sample to demonstrate that this gas is primordial. These observations show that all three galaxies have exponentially decreasing profiles characteristic of dwarf ellipticals and that their rotation velocities are extremely low. However, like more massive elliptical galaxies with HI, these dwarf galaxies show irregular optical morphology. For one target, VCCmore »190, we additionally observe an HI tail consistent with a recent interaction with the massive spiral galaxy NGC 4224.« less
3. HST viewing of spectacular star-forming trails behind ESO 137-001

   https://doi.org/10.1093/mnras/stad963  

   Waldron, William ; Sun, Ming ; Luo, Rongxin ; Laudari, Sunil ; Chatzikos, Marios ; Sivanandam, Suresh ; Kenney, Jeffrey D. P. ; Jáchym, Pavel ; Voit, G. Mark ; Donahue, Megan ; et al ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   We present the results from the HST WFC3 and ACS data on an archetypal galaxy undergoing ram pressure stripping (RPS), ESO 137-001, in the nearby cluster Abell 3627. ESO 137-001 is known to host a prominent stripped tail detected in many bands from X-rays, H α to CO. The HST data reveal significant features indicative of RPS such as asymmetric dust distribution and surface brightness as well as many blue young star complexes in the tail. We study the correlation between the blue young star complexes from HST, H ii regions from H α (MUSE), and dense molecular clouds from CO (ALMA). The correlation between the HST blue star clusters and the H ii regions is very good, while their correlation with the dense CO clumps are typically not good, presumably due in part to evolutionary effects. In comparison to the starburst99 + cloudy model, many blue regions are found to be young (<10 Myr) and the total star formation (SF) rate in the tail is 0.3–0.6 M⊙ yr−1 for sources measured with ages less than 100 Myr, about 40 per cent of the SF rate in the galaxy. We trace SF over at least 100 Myr and give a full picture of the recent SF history in the tail. We alsomore »demonstrate the importance of including nebular emissions and a nebular to stellar extinction correction factor when comparing the model to the broad-band data. Our work on ESO 137-001 demonstrates the importance of HST data for constraining the SF history in stripped tails.

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4. Catching Tidal Dwarf Galaxies at a Later Evolutionary Stage with ALFALFA

   https://doi.org/10.3847/1538-3881/acc4c6  

   Gray, Laurin M. ; Rhode, Katherine L. ; Leisman, Lukas ; Mancera Piña, Pavel E. ; Cannon, John M. ; Salzer, John J. ; Gault, Lexi ; Fuson, Jackson ; Józsa, Gyula I. G. ; Adams, Elizabeth A. K. ; et al ( April 2023 , The Astronomical Journal)

   We present deep optical imaging and photometry of four objects classified as “Almost-Dark” galaxies in the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have Himasses of 10⁷–10⁹M_⊙and distances of ∼9–100 Mpc. Observations with the WIYN 3.5 m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of ∼24–25$\mathrm{mag}{\mathrm{arcsec}}^{-2}$in thegband. We also present the results of Hisynthesis observations with the Westerbork Synthesis Radio Telescope. These Almost-Dark galaxies have been identified as possible tidal dwarf galaxies (TDGs) based on their proximity to one or more massive galaxies. We demonstrate that AGC 229398 and AGC 333576 likely have the low dark matter content and large effective radii representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. AGC 219369 is likely dark matter dominated, while AGC 123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDG candidates such as a traditional major merger scenariomore »and gas ejection from a high-velocity flyby. Blind Hisurveys like ALFALFA enable the detection of gas-rich, optically faint TDGs that can be overlooked in other surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle.

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5. 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 occursmore »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.« less