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GAs Stripping Phenomena in galaxies

Periodic Reporting for period 1 - GASP (GAs Stripping Phenomena in galaxies)

Reporting period: 2019-06-01 to 2020-11-30

"""GASP"" is an ERC Advanced Grant program started in 2019 to study why and how galaxies evolve. In particular, the goal of GASP is to understand the physical processes that can influence the gas, which is a fundamental ingredient of galaxies because it allows new stars to form. When a galaxy runs out of gas, is unable to form new stars and continues its evolution becoming redder and dimmer. Understanding the process of star formation, and its observed overall decline in the Universe with time, is the central question for astrophysics today. Some of the mechanisms that can remove gas from a galaxy depends on the environmental conditions in which the galaxy resides. Clusters of galaxies, which are the largest gravitationally bound structures in the Universe, are the site of a mechanism called ""ram pressure stripping"", which is the interaction between the hot gas filling the space between galaxies and the gas within the disks of galaxies. Due to ram pressure stripping, galaxies quickly lose their gas in clusters and sometimes long tails of stripped gas can be observed at different wavelengths. The most spectacular such cases are named ""jellyfish galaxies"", because they have long tentacles of shining gas that make them resemble the animal jellyfish. Until recently, only a few such galaxies were known and studied. With the GASP program, we are studying the first large sample of jellyfish galaxies in clusters, and compare them with galaxies in galaxy groups and filaments that are affected by similar or different mechanisms. Jellyfish galaxies are not only fascinating objects, they are also a key to understand the physical phenomena regulating the evolutionary histories of galaxies. GASP is unveiling the physics that regulates the inflow and outflow of gas from galaxies and understand how the building blocks of the Universe -- galaxies -- form and evolve."
In spite of the difficulties and delays due to the Covd19 pandemic, in the first one and a half year the project has reached some fundamental scientific milestones, publishing 14 refereed publications and 1 invited review paper. In person before Covid-19 and online since March 2020, GASP results were presented at 9 international conferences, and 4 invited colloquia of the PI at institutes around the world. The scientific highlights for this period were:
Objective 1): The MUSE data have been analysed for the whole GASP sample, yielding results on the star formation rate in the stripped tails (Gullieuszik et al. 2020), the unwinding of spiral arms due to ram pressure stripping (Bellhouse et al. 2020, ERC PostDoc), the diffuse ionized gas (Tomicic et al. 2020), the gas metallicity (Franchetto et al. 2020) and the multi-wavelength analysis of jellyfish JW100 (see figures) (Poggianti et al. 2019). The ALMA data (Moretti et al. 2020a, 2020b) combined with HI data (Ramatsouku et al. 2020, Deb et al. 2020), revealed a large amount of molecular gas and the efficient conversion of neutral to molecular gas. Moreover, the HST observations proposed were approved and data have been collected already for 3 GASP galaxies (see below). As promised, we proposed for JWST observations and are waiting for the outcome.
Objective 2) The first measurement of the magnetic field in a jellyfish tail was the subject of a GASP publication in Nature Astronomy (Mueller et al. 2020). New radio observations for GASP with were granted (ATCA data already taken, JVLA proposed) to measure the magnetic field in other jellyfish galaxies. Regarding AGN, we were able to identify the signature of AGN feedback across the 8kpc central region of a jellyfish galaxy (George et al. 2019). A GASP X-Shooter proposal was approved to study the outflow regions of other jellyfish galaxies with X-Shooter@VLT and we are waiting for the data to arrive. Moreover, a PhD student (see below) has started to analyse the whole GASP sample and all available literature for jellyfish galaxies to ascertain the AGN fraction among this type of galaxies.
Objective 3) The spatially resolved properties of GASP post-starburst galaxies were published in Vulcani et al. 2020a. Werle (ERC PostDoc) has modified the SINOPSIS spectrophotometric code and has applied it to post-starburst galaxies in distant clusters (paper in preparation). Another important achievement has been the approval of a GASP proposal for 300ksec with UVIT@ASTROSAT for other 15 GASP clusters.
Objective 4) The spatially resolved star formation-stellar mass relation of non-stripped galaxies was investigated, leading to an interpretation for the integrated star formation-mass relation (Vulcani et al. 2019). This was compared with the spatially resolved relation for stripped galaxies (Vulcani et al. 2020b), revealing an enhanced star formation at all galactic radii and for all galaxy stellar masses. A global view of the MUSE data for GASP galaxies in groups, filaments and isolated, with an assessment of the various physical processes at work in each galaxy, is presented in a paper submitted to the Astrophysical Journal and currently at the referee stage.
Objective 5) One of the ERC PostDoc (Kulier) started the analysis of the EAGLES simulations: she has identified ram pressure stripped galaxies and is analysing the impact of stellar and AGN feedback on the stripping process and will proceed with the quantification of the gas fraction being stripped as a function of time and all parameters.
Moreover, we have expanded the GASP project to clusters at higher redshift: thanks to an agreement with the GTO MUSE team, we are investigating jellyfish and post-starburst galaxies in 10 clusters at z=0.3-0.45 (Moretti et al. in prep, Werle et al. in prep.). Their phase-space diagram and the relation with X-ray and weak lensing maps will be presented in Bellhouse et al. (in prep.).
A summary of the GASP progress at the time of writing is presented in Poggianti et al. 2020b.
All the results presented above are novel results that go beyond what was previously known. We expect to reach the goals outlined in the agreement, briefly: Obj.1): Kennicutt-Schmidt relation (Moretti, ERC PostDoc Bacchini), propose for more ALMA data to be obtained (Moretti). HST reduction and analysis (ERC phD student Giunchi and Gullieuszik). Comparison of the gas in different phases, gas metallicity, diffuse ionized gas, X-ray and radio continuum emission in tails. Distant galaxy clusters from the GTO MUSE. Obj. 2): AGN frequency and properties (ERC PhD student Peluso), photoionization models, AGN from X-ray data. Molecular gas AGN outflows from ALMA. New ATCA data reduced. Obj.3): Post-starburst galaxies (ERC PostDoc(Werle) and SINOPSIS spin-off to derive the past and future spatially resolved multi-wavelength images. Statistics of stripping and quenching, also with respect to cluster substructure. Publiction of whole GASP cluster sample (Poggianti). Obj.4): Presentation of the whole non-cluster GASP sample (Vulcani). Machine learning method to identify jellyfish galaxies and ram pressure stripped galaxies in any environment (ERC PostDoc Bellhouse). Obj.5): Cosmological hydrodynamical simulations of clusters (ERC PostDoc Kulier), simulations of a cloud embedded in the intracluster medium, simulations of a disk with ram pressure stripping and magnetic field to investigate the occurrence of AGN activity.
Jellyfish galaxy JW100 (HST): three color image
Jellyfish galaxy JW100 (MUSE): contours are stellar disk, blue is the emission from ionized gas