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1. A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE): XX. Star formation in the tidal tail of NGC 4254

   https://doi.org/10.1051/0004-6361/202659007  

   Boselli, A; Lupi, A; Serra, P; Andreani, P; Calura, F; Miville-Deschênes, M-A; Hensler, G; Boquien, M; Fossati, M; Boissier, S; et al (August 2026, Astronomy & Astrophysics)

   ALMA¹²CO(1-0) observations of 42 star-forming regions located outside the disc of the Virgo Cluster galaxy NGC 4254 within an H Igas tail produced during the galaxy’s interaction with another cluster member have revealed the presence of ten giant molecular clouds (GMCs) in four of these regions. All of the GMCs were resolved at the angular resolution of the observations (≃160 pc) and have molecular gas masses ofM(H₂)≃(0.8 − 2.0)×10⁶M_⊙. These ten clouds are characterised by gas column densities [Σ(H₂) ≃ 10 M_⊙pc⁻²] and velocity dispersions [σ_v(CO) ≃ 3–12 km s⁻¹] respectively lower and comparable to those encountered in similar GMCs in the Milky Way. They follow the relation between the gas column density and the star formation activity (Schmidt law) derived using similar data over the stellar disc of NGC 4254 and other local and Virgo cluster galaxies. With analytic calculations and tuned simulations, we show that these clouds are unstable and thus expected to dissolve on relatively short timescales (∼10–30 Myr). We show that they probably formed after the collapse of dense gas clouds in the H Igas tail stripped during the gravitational interaction that the galaxy suffered several hundreds millions of years ago. The clouds are short-lived and isolated given the low density of the surrounding intracluster medium, which cannot confine the gas expelled by stellar feedback. We discuss the implications of these results in the general context of the fate of stripped gas in hostile cluster environments. 

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2. The MeerKAT Fornax Survey: III. Ram-pressure stripping of the tidally interacting galaxy NGC 1427A in the Fornax cluster

   https://doi.org/10.1051/0004-6361/202450114  

   Serra, P; Oosterloo, T A; Kamphuis, P; Józsa, G_I G; de_Blok, W_J G; Bryan, G L; van_Gorkom, J H; Iodice, E; Kleiner, D; Loni, A; et al (October 2024, Astronomy & Astrophysics)

   We present MeerKAT Fornax Survey H Iobservations of NGC 1427A, a blue irregular galaxy with a stellar mass of ∼2 × 10⁹M_⊙located near the centre of the Fornax galaxy cluster. Thanks to the excellent resolution (1–6 kpc spatially, 1.4 km s⁻¹in velocity) and H Icolumn density sensitivity (∼4 × 10¹⁹to ∼10¹⁸cm⁻²depending on resolution), our data deliver new insights on the long-debated interaction of this galaxy with the cluster environment. We confirm the presence of a broad, one-sided, starless H Itail stretching from the outer regions of the stellar body and pointing away from the cluster centre. We find the tail to have 50% more H I(4 × 10⁸M_⊙) and to be 3 times longer (70 kpc) than in previous observations. In fact, we detect scattered H Iclouds out to 300 kpc from the galaxy in the direction of the tail – possibly the most ancient remnant of the passage of NGC 1427A through the intracluster medium of Fornax. Both the velocity gradient along the H Itail and the peculiar kinematics of H Iin the outer region of the stellar body are consistent with the effect of ram pressure given the line-of-sight motion of the galaxy within the cluster. However, several properties cannot be explained solely by ram pressure and suggest an ongoing tidal interaction. This includes: the close match between dense H Iand stars within the disturbed stellar body; the abundant kinematically anomalous H I; and the inversion of the H Ivelocity gradient near the base of the H Itail. We rule out an interaction with the cluster tidal field, and conclude that NGC 1427A is the result of a high-speed galaxy encounter or of a merger started at least 300 Myr ago, where ram pressure shapes the distribution and kinematics of the H Iin the perturbed outer stellar body and in the tidal tails. 

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3. MeerKAT HI imaging of the jellyfish galaxy ESO 137-001

   https://doi.org/10.1051/0004-6361/202451050  

   Ramatsoku, M; Serra, P; Sun, M; Smirnov, O M; Makhathini, S (February 2025, Astronomy & Astrophysics)

   We present MeerKAT H Iobservations of ESO 137-001, a quintessential jellyfish galaxy with long multi-phase tails formed due to the interaction with the intra-cluster medium of its host galaxy cluster, ACO 3627. Our observations reveal the presence of H Iin both the disc and outer regions of the galaxy for the first time, with a total H Imass of (3.5 ± 0.4)×10⁸M_⊙. ESO 137-001 is at an advanced stage of gas stripping; it is extremely H Ideficient and seems to have lost 90% of its initial H Imass; about 2/3 of the surviving H Iis found at larger radius than expected for a normal H Idisc and forms ∼40 kpc tail coincident with the tail detected at other wavelengths. Only ∼10% of the surviving H Iis still found within the stellar disc, consistent with the expectation of an outside-in truncation due to ram pressure. Similarly to other jellyfish galaxies, ESO137-001 has a high star formation rate for the low amount of H Idetected. We measure an H Idepletion time of 0.29 Gyr. However, when taking into account the total gas (H I+ H₂) content, the depletion time is consistent with typical values measured in nearby spiral galaxies. This suggests that ESO 137-001 is at its current stage of ram pressure interaction characterised by an efficient H Istripping, rather than an enhanced conversion of H Ito H₂, which was recently observed in some other jellyfish galaxies. 

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4. The MeerKAT Fornax Survey: I. Survey description and first evidence of ram pressure in the Fornax galaxy cluster

   https://doi.org/10.1051/0004-6361/202346071  

   Serra, P.; Maccagni, F. M.; Kleiner, D.; Molnár, D.; Ramatsoku, M.; Loni, A.; Loi, F.; de Blok, W. J.; Bryan, G. L.; Dettmar, R. J.; et al (May 2023, Astronomy & Astrophysics)

   The MeerKAT Fornax Survey maps the distribution and kinematics of atomic neutral hydrogen gas (H  I ) in the nearby Fornax galaxy cluster using the MeerKAT telescope. The 12 deg 2 survey footprint covers the central region of the cluster out to ∼ R vir and stretches south-west out to ∼2 R vir to include the NGC 1316 galaxy group. The H  I column density sensitivity (3 σ over 25 km s −1 ) ranges from 5 × 10 19 cm −2 at a resolution of ∼10″ (∼1 kpc at the 20 Mpc distance of Fornax) down to ∼10 18 cm −2 at ∼1′ (∼6 kpc), and slightly below this level at the lowest resolution of ∼100″ (∼10 kpc). The H  I mass sensitivity (3 σ over 50 km s −1 ) is 6 × 10 5 M ⊙ . The H  I velocity resolution is 1.4 km s −1 . In this paper, we describe the survey design and H  I data processing, and we present a sample of six galaxies with long, one-sided, starless H  I tails (only one of which was previously known) radially oriented within the cluster and with measurable internal velocity gradients. We argue that the joint properties of the H  I tails represent the first unambiguous evidence of ram pressure shaping the distribution of H  I in the Fornax cluster. The disturbed optical morphology of all host galaxies supports the idea that the tails consist of H  I that was initially pulled out of the galaxies’ stellar body by tidal forces. Ram pressure was then able to further displace the weakly bound H  I and give the tails their current direction, length, and velocity gradient. 

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5. Black Hole Mass Measurements of Early-type Galaxies NGC 1380 and NGC 6861 through ALMA and HST Observations and Gas-dynamical Modeling*

   https://doi.org/10.3847/1538-4357/ac7a38  

   Kabasares, Kyle M.; Barth, Aaron J.; Buote, David A.; Boizelle, Benjamin D.; Walsh, Jonelle L.; Baker, Andrew J.; Darling, Jeremy; Ho, Luis C.; Cohn, Jonathan (August 2022, The Astrophysical Journal)

   Abstract We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 2 observations of CO(2–1) emission from the circumnuclear disks in two early-type galaxies, NGC 1380 and NGC 6861. The disk in each galaxy is highly inclined ( i ∼ 75°), and the projected velocities of the molecular gas near the galaxy centers are ∼300 km s −1 in NGC 1380 and ∼500 km s −1 in NGC 6861. We fit thin disk dynamical models to the ALMA data cubes to constrain the masses of the central black holes (BHs). We created host galaxy models using Hubble Space Telescope images for the extended stellar mass distributions and incorporated a range of plausible central dust extinction values. For NGC 1380, our best-fit model yields M BH = 1.47 × 10 8 M ⊙ with a ∼40% uncertainty. For NGC 6861, the lack of dynamical tracers within the BH’s sphere of influence due to a central hole in the gas distribution precludes a precise measurement of M BH . However, our model fits require a value for M BH in the range of (1–3) × 10 9 M ⊙ in NGC 6861 to reproduce the observations. The BH masses are generally consistent with predictions from local BH–host galaxy scaling relations. Systematic uncertainties associated with dust extinction of the host galaxy light and choice of host galaxy mass model dominate the error budget of both measurements. Despite these limitations, the measurements demonstrate ALMA’s ability to provide constraints on BH masses in cases where the BH’s projected radius of influence is marginally resolved or the gas distribution has a central hole. 

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