Observing Ram Pressure at Work in Intermediate Redshift Clusters with MUSE: The Case of Abell 2744 and Abell 370

Abstract

Ram pressure stripping has been proven to be effective in shaping galaxy properties in dense environments at low redshift. The availability of Multi Unit Spectroscopic Explorer (MUSE) observations of a sample of distant (z ∼ 0.3–0.5) clusters has allowed one to search for galaxies subject to this phenomenon at significant lookback times. In this paper we describe how we discovered and characterized 13 ram-pressure-stripped galaxies in the central regions of two intermediate redshift (z ∼ 0.3–0.4) clusters, A2744 and A370, using the MUSE spectrograph. Emission-line properties as well as stellar features have been analyzed to infer the presence of this gas-only stripping mechanism, that produces spectacular ionized gas tails (Hα and even more astonishing [O ii](3727, 3729)) departing from the main galaxy body. The inner regions of these two clusters reveal the predominance of such galaxies among blue star-forming cluster members, suggesting that ram pressure stripping was even more effective at intermediate redshift than in today’s universe. Interestingly, the resolved [O ii]/Hα line ratio in the stripped tails is exceptionally high compared to that in the disks of these galaxies, (which is comparable to that in normal low-z galaxies), suggesting lower gas densities and/or an interaction with the hot surrounding intracluster medium.

Post-starburst Galaxies in the Centers of Intermediate-redshift Clusters

“…We have identified two post-starbursts that, although lacking any emission lines in their stellar disks, display long tails of ionized gas; these are A2744-01 and SMACS 2131-06. We note that for A2744-01 these tails have already been identified by Owers et al (2012) and Moretti et al (2022). This intriguing feature is shown in Figure 9, where we plot HST F814W images of these two objects with the [O II]λ3727 flux in red.…”

“…As described by Moretti et al (2022), each datacube was visually inspected to find post-starbursts, as well as the stripped galaxies presented in that paper. Post-starburst galaxies are identified as those generally lacking emission lines but still presenting strong Balmer absorption features anywhere in the stellar disk.…”

“…In Figure 1, we show maps of the [O III]λ5007/Hβ line ratio for these galaxies, including spaxels with a signal-to-noise ratio (S/N) larger than 3 in both emission lines. The emission lines used here and throughout the paper were measured using the HIGHELF code as described in Moretti et al (2022). In the case of A2744-07 we are also able to measure the Hα and [N II]λ 6584/Hα lines, allowing the full characterization of the central spaxels in a Baldwin-Phillips-Terlevich diagram (Baldwin et al 1981).…”

“…The total number of valid spaxels in all 21 galaxies in our sample is 2230, ranging from 36 for MACS 0257-05 to 318 for MACS 1206-06 with an average of 106 per galaxy. The redshifts for each spaxel, which are required for running SINOPSIS, were determined using PPXF (Cappellari 2017) on Voronoi binned regions with S/N > 5 (as thoroughly described in Moretti et al 2022). For A2744-05, AS1063SW-01, and SMACS 2131-06, foreground objects were manually masked out.…”

Dust in Clusters of Galaxies

“…More recently, ram pressure has been recognized to be also responsible for forming jellyfish galaxies; see the review in [100]. Along with tidal stripping, ram pressure is a universally acting mechanism in mass exchange between the galaxies' ISM and the ICM; examples can be found, e.g., in [187][188][189][190]. A very picturesque example has been obtained by Boselli et al [191] for several galaxies in the Virgo cluster; see also the discussion in the review [98], two of them with stripped tails can be clearly seen in their Figure 9 [98].…”