WALLABY Pilot Survey: First Look at the Hydra I Cluster and Ram Pressure Stripping of ESO 501-G075

Abstract

We present results from neutral atomic hydrogen (H ) observations of Hydra I, the first cluster observed by the Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY) on the Australian Square Kilometre Array Pathfinder. For the first time we show that WALLABY can reach its final survey sensitivity. Leveraging the sensitivity, spatial resolution and wide field of view of WALLABY, we identify a galaxy, ESO 501−G075, that lies near the virial radius of Hydra I and displays an H tail. ESO 501−G075 shows a similar level of morphological asymmetry as another cluster member, which lies near the cluster centre and shows signs of experiencing ram pressure. We investigate possible environmental processes that could be responsible for producing the observed disturbance in the H morphology of ESO 501−G075. We rule out tidal interactions, as ESO 501−G075 has no nearby neighbours within ∼ 0.34 Mpc. We use a simple model to determine that ram pressure can remove gas from the disc at radii 𝑟 25 kpc. We conclude that, as ESO 501−G075 has a typical H mass compared to similar galaxies in the field and its morphology is compatible with a ram pressure scenario, ESO 501−G075 is likely recently infalling into the cluster and in the early stages of experiencing ram pressure.

NGC 3314a/b and NGC 3312: Ram pressure stripping in Hydra I cluster substructure

“…Fundamental plane measurements for 11 E/S0 Hydra cluster galaxies give a redshift-independent distance of 56.6 ± 4 Mpc (Jorgensen et al 1996). While a pure luminosity distance of D = 59 Mpc, taking into account bulk motions relative to the 3K CMB, was adopted by Reynolds et al (2021b) and is consistent with that reported in the NASA Extragalactic Database (NED) 2 . Thus, our assumed value of 58.6 Mpc is consistent with the best recent estimates.…”

“…The signatures of ram pressure are seen in X-ray (Sun et al 2006), optical (Poggianti et al 2016;Roberts & Parker 2020), Hα (Kenney & Koopmann 1999;Sun et al 2007;Fumagalli et al 2014), dust (Cortese et al 2010;Abramson & Kenney 2014;Kenney et al 2015;Abramson et al 2016), CO (Kenney et al 1990;Vollmer et al 2001a, 2008a), H i (Kenney et al 2004Oosterloo & van Gorkom 2005;Reynolds et al 2021b), radio continuum (Gavazzi et al 1995;Chen et al 2020;Roberts et al 2021), polarized emission (Vollmer et al 2008b), andcombinations thereof (e.g. Crowl et al 2005;Vollmer et al 2009;Abramson et al 2011;Ramatsoku et al 2019;Longobardi et al 2020; see also Boselli et al 2022 for a review).…”

“…Instead, it was suggested that NGC 3312, NGC 3314a, and a gas-rich ring galaxy, LEDA 753342 (first noted by Wirth & Gallagher 1980) may be part of a foreground cluster substructure: NGC 3312 and NGC 3314a have almost the same systemic velocity, and tidal interactions could be responsible for their disturbed outer H i morphology. Recent studies as part of WALLABY (Koribalski et al 2020) early science operations, have quantified the amount of strippable gas in NGC 3312 (Wang et al 2021), or argue based on the galaxy's position close to the center in phase space that it is most likely undergoing ram pressure stripping (Reynolds et al 2021b), but the H i morphology of the ASKAP maps are still inconclusive.…”

“…In the first instance, we adopt the same central density as McMahon et al (1992) of ρ 0 = 5.0 × 10 −3 cm −3 and assume a value of β = 0.67 and r 500 = 823 kpc estimated from ROSAT observations (Eckert et al 2011; r 500 = 0.19r c ). In the second instance, we use the same model adopted by Reynolds et al (2021b) to model ram pressure stripping in ESO 501-G075 at the outskirts of Hydra. In this case, ρ 0 = 1.1 × 10 −2 cm −3 , β = 0.69, and r c = 90.9 kpc based on XMM-Newton data from Hayakawa et al (2006).…”