Abstract
Multi-Unit Spectroscopic Explorer (MUSE) observations of the cluster of galaxies CGr32 (M200 ≃ 2 × 1014 M⊙) at z = 0.73 reveal the presence of two massive star-forming galaxies with extended tails of diffuse gas detected in the [O II]λλ3727–3729 Å emission-line doublet. The tails, which have a cometary shape with a typical surface brightness of a few 10−18 erg s−1 cm−2 arcsec−2, extend up to ≃100 kpc (projected distance) from the galaxy discs, and are not associated with any stellar component. All this observational evidence suggests that the gas was removed during a ram-pressure stripping event. This observation is thus the first evidence that dynamical interactions with the intracluster medium were active when the Universe was only half its present age. The density of the gas derived using the observed [O II]λ3729/[O II]λ3726 line ratio implies a very short recombination time, suggesting that a source of ionisation is necessary to keep the gas ionised within the tail.
Highlights
Since the seminal work of Dressler (1980), it has become evident that the environment plays a major role in shaping galaxy evolution
The derived density, mass, and recombination time of ionised gas stripped during the interaction, as well as the shape and size of the tail associated to the two galaxies ID 345 and ID 473, are very similar to those observed in the massive spiral NGC 4569 in the Virgo cluster, or ESO 137
If the gas keeps the dynamical imprint of the region from where it has been stripped and is decelerated by the interaction with the hot ICM, as observed in local edge-on galaxy UGC 6697 in A1367 (Consolandi et al 2017), Fig. 4 can be interpreted as follows: in the northern galaxy ID 473 the gas is decelerated in the tail with respect to the galaxy, which means that the trajectory of the galaxy within the cluster is not on the plane of the sky, but rather on the line of sight, the galaxy is crossing the cluster from the background
Summary
Since the seminal work of Dressler (1980), it has become evident that the environment plays a major role in shaping galaxy evolution. Another ∼ fifty objects with similar characteristics have been found in deep Hubble Space Telescope (HST) images of clusters at 0.3 ≤ z ≤ 0.7 (Owers et al 2012; Ebeling et al 2014; McPartland et al 2016) Since it is selected in the optical bands, where the emission is dominated by stars of intermediate age, a jellyfish morphology is not direct proof of an ongoing ram-pressure stripping episode (Cortese et al 2007). We detected two star-forming galaxies in the COSMOS cluster 32 (Knobel et al 2012) at z = 0.73 with extended (up to 100 kpc in projected distance) tails of ionised gas, without any stellar counterpart in the deep optical images This observation is the first evidence of an ongoing ram-pressure stripping event at z > 0.5.
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