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Title: Source plane reconstruction of the giant gravitational arc in Abell 2667: a candidate Wolf-Rayet galaxy at z~1
Author: Shuo Cao, Giovanni Covone, Eric Jullo, Johan Richard, Luca Izzo, Zong-Hong Zhu

We present a new analysis of HST, Spitzer telescope imaging and VLT imaging and spectroscopic data of a bright lensed galaxy at z=1.0334 in the lensing cluster Abell~2667. Using this high-resolution imaging we present an updated lens model that allows us to fully understand the lensing geometry and reconstruct the lensed galaxy in the source plane. This giant arc gives a unique opportunity to peer into the structure of a high-redshift disk galaxy. We find that the lensed galaxy of Abell 2667 is a typical spiral galaxy with morphology similar to the structure of its counterparts at higher redshift z~2. The surface brightness of the reconstructed source galaxy in the z850 band reveals the central surface brightness I(0)=20.28±0.22mag arcsec-2 and the characteristic radius rs=2.01±0.16 kpc at redshift z~1. The morphological reconstruction in different bands shows obvious negative radial colour gradients for this galaxy. Moreover, the redder central bulge tends to contain a metal-rich stellar population, rather than being heavily reddened by dust due to high and patchy obscuration. We analyse the VIMOS/IFU spectroscopic data and find that, in the given wavelength range (~1800-3200 \AA), the combined arc spectrum of the source galaxy is characterised by a strong continuum emission with strong UV absorption lines (FeII and MgII) and shows the features of a typical starburst Wolf-Rayet galaxy NGC5253. More specifically, we have measured the EWs of FeII and MgII lines in the Abell 2667 spectrum, and obtained similar values for the same wavelength interval of the NGC5253 spectrum. Marginal evidence for CIII] 1909 emission at the edge of the grism range further confirms our expectation.

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The new Hubble observations, made by an international team of astronomers led by Luca Cortese of Cardiff University, United Kingdom, provide one of the best examples to date of this metamorphosis. While looking at galaxy cluster Abell 2667, astronomers found an odd- looking spiral galaxy (shown in the upper left hand corner of the image) that ploughs through the cluster after being accelerated to at least 3.5 million km/h by the enormous combined gravity of the cluster's dark matter, hot gas and hundreds of galaxies. As it speeds through, it rams into the hot gas that permeates the cluster. Its gas and stars are pulled away by the gravitational tidal forces exerted by the cluster, just as the forces exerted by our moon and sun pull the Earth's oceans.

Abell 2667
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Credit NASA

The unique galaxy is situated 3.2 billion light-years from the Earth. It has an extended stream of bright blue knots and diffuse wisps of young stars whose formation and evolution have been driven by both the cluster's tidal forces and a mechanism called "ram pressure stripping." Abell 2667's hot gas is composed of charged particles with a temperature of 10-100 million degrees. During the ram pressure stripping process, the charged particles strip and push away the infalling galaxy's gas, just as the solar wind of charged particles pushes ionised gas away from a comet to create a gas tail. For this reason the scientists have nicknamed the stretched spiral the "comet galaxy."

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A galaxy in the process of being dismembered has been imaged by the Hubble Space Telescope. The unfortunate victim is having its stars and gas stripped away as it plunges at great speed towards the centre of a massive galaxy cluster.
The galaxy has been nicknamed the 'Comet galaxy' because of its unusual shape, which includes a tail of stars and gas streaming out behind it. It was discovered in Hubble images of a galaxy cluster called Abell 2667 by a team of astronomers led by Luca Cortese of Cardiff University in the UK.
It is a spiral galaxy with about the same mass as our own Milky Way. But while the Milky Way resides in a relatively placid environment, the Comet galaxy is hurtling towards the core of a massive cluster of galaxies, drawn by the cluster's tremendous gravity.
The galaxy is being ripped apart by the pressure of hot gas within the cluster that it is speeding through, as well as so-called 'tidal' forces due to the uneven pull of the cluster's gravity on different parts of the galaxy.

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NASA's Hubble Space Telescope, in collaboration with several other ground- and space-based telescopes, has captured a galaxy being ripped apart by a galaxy cluster's gravitational field and harsh environment.
The finding sheds light on the mysterious process by which gas-rich spiral-shaped galaxies might evolve into gas-poor irregular- or elliptical-shaped galaxies over billions of years. The new observations also reveal one mechanism for forming the millions of "homeless" stars seen scattered throughout galaxy clusters.

This unique galaxy, situated 3.2 billion light-years from Earth, has an extended stream of bright blue knots and diffuse wisps of young stars driven away by the tidal forces and the ram pressure stripping of the hot dense gas” - Jean-Paul Kneib, a study collaborator from the Laboratoire d'Astrophysique de Marseille.

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