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GAs Stripping Phenomena in galaxies

Periodic Reporting for period 3 - GASP (GAs Stripping Phenomena in galaxies)

Reporting period: 2022-06-01 to 2023-11-30

"GASP" is an ERC Advanced Grant program that studies why and how galaxies evolve. 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. 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. 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.
The scientific highlights so far are:
1): The MUSE data have been analysed for the whole GASP sample, yielding results on the star formation rate in the stripped tails, the unwinding of spiral arms due to ram pressure stripping (RPS), the diffuse ionized gas, the gas metallicity and the multi-wavelength analysis of jellyfish JW100 (see figures). The ALMA data combined with HI data, revealed a large amount of molecular gas and the efficient conversion of neutral to molecular gas. We have shown the existence of a correlation between the X-ray and the Halpha surface brightness, suggesting an intermediate temperature mixed layer of stripped gas and intracluster medium responsible for both the X-ray emission and the unusual optical emission line ratios. We have received 30 orbits data of the Hubble Space Telescope for 6 GASP galaxies. A new GASP MUSE@VLT proposal has been approved to observe a sample of galaxies with unwinding arms and a Chandra Large Program (220ksec) has been approved to study draping, condensation and cooling of hot gas in jellyfish tails using the GASP jellyfish JO206.
2) The first measurement of the magnetic field in a jellyfish tail was the subject of a GASP publication in Nature Astronomy. New radio observations for GASP were granted to measure the magnetic field in other jellyfish galaxies. Regarding Active Galactic Nuclei (AGN), we were able to identify the signature of AGN feedback across the 8kpc central region of a jellyfish galaxy. Moreover, we have presented all the AGN cases in the GASP sample and analyzed the AGN frequency both in GASP and in the largest possible sample of RPS galaxies from the literature, finding an excess of AGN activity when ram pressure is at work, especially linked with long tails of stripped gas and high galaxy masses.
3) The spatially resolved properties of GASP post-starburst galaxies were analyzed. Another important achievement has been the approval of a GASP proposal with the ASTROSAT satellite for other 15 GASP clusters. A complete census of RPS in local galaxy clusters based on optical imaging alone has required addressing the importance of the unwinding phenomenon, concluding that a very large fraction (>75%) of the infalling blue star-forming spirals undergo a RPS phase visible at optical wavelengths at some point.
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. This was compared with the spatially resolved relation for stripped galaxies, 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 has been published.
5) We have carried out an analysis of hydrodynamical cosmological simulations identifying ram pressure stripped galaxies and analysing the impact of stellar and AGN feedback on the stripping process.
Moreover, we have expanded the GASP project to more distant clusters: we are now investigating jellyfish and post-starburst galaxies in 10 clusters 4-5 billion years ago.
GASP has provided the first large sample of confirmed ram-pressure stripped galaxies in clusters with a wide range of galaxy masses and cluster masses, in various stages and degrees of stripping. GASP also represents a tremendous step forward in terms of multi-wavelength coverage: this allows to study simultaneously the gas in different phases, the stellar content and the non-thermal processes.
Among the major GASP breakthroughs, there is the characterization of the star formation activity, both in the disks and in the tails. We have been able to study the star-forming regions in the ram pressure stripped disks and tails at a great level of detail, and to derive the global and spatially resolved star formation rate-mass relation (and how the latter gives origin to the first former).
For the first time, we have clearly shown that in jellyfish galaxies the large amount of molecular gas must originate from an efficient conversion of neutral into molecular gas.
Another GASP discovery has been the high incidence of AGN in ram pressure stripped galaxies, which was totally unexpected.
For the first time, GASP has also measured the magnetic field from polarization data in the long tail of a jellyfish galaxy: it turns out such magnetic field is well ordered and strongly aligned with the stripping direction, a result that has important consequences for the possibility to form new stars in the tails and for the interaction between the intracluster medium and the stripped gas.
Another unexpected GASP discovery has been that unwinding of spiral arms can be caused by ram pressure stripping. Following up on this, we have published a catalog of unwinding arm galaxies in low redshift clusters, to complement the sample of other stripping candidates.
Thanks to the large statistics, GASP has also provided the first and only complete census of ram pressure signatures visible from optical imaging in low redshift clusters, showing how common this phenomenon must be among infalling gas-rich galaxies.
GASP has discovered a correlation between the Halpha and the X-ray surface brightness, pointing to the existence of a hot plasma originating from the cooling of the intracluster medium (ICM) or mixing stripped gas-ICM.
GASP has also observationally measured the gas metallicity scaling relations and metallicity gradients in ram pressure stripped galaxies and has discovered the imprint of mixing of ICM-stripped gas from the metallicity gradient in stripped tails.
Thanks to GASP Hubble Space Telescope multi-wavelength images we have characterized the first statistically significant sample of star-forming clumps in the tails of stripped gas, measuring their luminosities, sizes, stellar masses, stellar ages and main scaling relations.
Jellyfish galaxy JW100 (HST): three color image
Jellyfish galaxy JW100 (MUSE): contours are stellar disk, blue is the emission from ionized gas