Periodic Reporting for period 4 - GASP (GAs Stripping Phenomena in galaxies)
Período documentado: 2023-12-01 hasta 2024-05-31
"GASP" is an ERC Advanced Grant program that have studied 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 how galaxies form and evolve is fundamental for our comprehension of the Universe, of our own Milky Way and of the ingredients and birth of our Solar System and Earth. Some of the mechanisms that can remove gas from a galaxy depend 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 lose their gas and sometimes long tails of stripped gas can be observed at different wavelengths. The most spectacular such cases are named "jellyfish galaxies". Other mechanisms are more efficient in galaxy groups, such as the one where our Milky Way is located, or in galaxy filaments that together with clusters form the so called “cosmic web”. GASP has revealed what happens to galaxies in clusters, groups and filaments and has employed state-of-the-art observations and simulations unveiling the physical conditions that drive the evolution of galaxies, determining their gas content, star formation activity, colors, morphologies, chemical abundances and other properties.
GASP has used some of the most advanced instrumentation in the world, both on the ground and in space. Some of the scientific highlights of GASP have been:
1) the characterization of the star formation rate in the galactic disks and in the stripped tails;
2) understanding the effects of ram pressure stripping on the galaxy structure;
3) studying the relation between different gas phases (molecular, neutral and ionized gas, that have different densities and temperatures, as well as very hot X-ray emitting gas);
4) revealing the interaction between the galactic and tail gas and the surrounding intergalactic medium;
5) measuring the chemical composition of the gas;
6) extending ram pressure studies to a significant sample of galaxies hosted in clusters seen as they were 4-5 billion years ago;
7) investigating the nuclear activity of GASP galaxies, which is related to the rate at which the central supermassive black hole in galaxies accretes material becoming an Active Galactic Nucleus;
8) observing direct evidence for an evolutionary link between ram pressure stripping and post-starburst galaxies;
9) providing an assessment of the various physical processes at work in each galaxy in groups, filaments and isolated galaxies;
10) performing cutting edge hydrodynamical cosmological simulations to interpret the observational results.
To summarize the GASP results in one sentence, we can state that ram pressure is the most important physical mechanism affecting galaxies in clusters both in the local Universe and in distant clusters and that we have developed a method that can be applied to larger samples in the future in different environmental conditions. Thanks to GASP we have made a leap forward in understanding how these physical processes happen and why they happen, and what are the consequences for galaxy evolution.
1) the characterization of the star formation rate in the galactic disks and in the stripped tails;
2) understanding the effects of ram pressure stripping on the galaxy structure;
3) studying the relation between different gas phases (molecular, neutral and ionized gas, that have different densities and temperatures, as well as very hot X-ray emitting gas);
4) revealing the interaction between the galactic and tail gas and the surrounding intergalactic medium;
5) measuring the chemical composition of the gas;
6) extending ram pressure studies to a significant sample of galaxies hosted in clusters seen as they were 4-5 billion years ago;
7) investigating the nuclear activity of GASP galaxies, which is related to the rate at which the central supermassive black hole in galaxies accretes material becoming an Active Galactic Nucleus;
8) observing direct evidence for an evolutionary link between ram pressure stripping and post-starburst galaxies;
9) providing an assessment of the various physical processes at work in each galaxy in groups, filaments and isolated galaxies;
10) performing cutting edge hydrodynamical cosmological simulations to interpret the observational results.
To summarize the GASP results in one sentence, we can state that ram pressure is the most important physical mechanism affecting galaxies in clusters both in the local Universe and in distant clusters and that we have developed a method that can be applied to larger samples in the future in different environmental conditions. Thanks to GASP we have made a leap forward in understanding how these physical processes happen and why they happen, and what are the consequences for galaxy evolution.
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. We have obtained 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. We have shown that their masses and stellar mass densities are compatible with them being new dwarf galaxies formed without dark matter. The exquisite spatial resolution of the Hubble Space Telescope images has allowed us to investigate the morphologies of the star forming clumps, confirming the “fireballs effect” by which the remaining ionized gas is confined to the side of the clumps which is further away from the galaxy.
For the first time, we have 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, highlighting the capability of ram pressure to funnel gas towards the galaxy center.
For the first time, GASP has measured the magnetic field from polarization data in the long stripped tail finding that the magnetic field is well ordered and strongly aligned with the stripping direction. 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 the first catalog of unwinding arm galaxies in low redshift clusters.
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 observationally measured the gas metallicity scaling relations and metallicity gradients in ram pressure stripped galaxies for the first time and has discovered the imprint of mixing of ICM-stripped gas from the metallicity gradient in stripped tails.
Being the only sufficiently large sample of galaxies in many different galaxy clusters, GASP has been able to characterize the orbits of ram pressure galaxies within clusters and the effects of the cluster dynamical state.
A long standing question has been whether the large population of lenticular galaxies that dominate galaxy clusters today can originate from the quenching of star formation in spiral galaxies. GASP has shown how this is possible and related to ram pressure.
On the simulation side, novel GASP results include the comprehension of the physical processes that push gas towards the center originating the AGN in ram pressure stripped galaxies, and the unexpected result that star formation in the disk does not aid ram pressure stripping nor enhances the rate at which the gas is stripped. GASP hydrodynamical simulations have also shown that the usually employed analytic approximation underpredict the stripping rate of neutral gas and overpredict that of ionized gas.
Finally, we have assessed the success rate of optical imaging in finding ram pressure stripped galaxies, which is important for ongoing and future very large imaging surveys of the sky such as those obtained by Euclid and the Vera Rubin Telescope.
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. We have obtained 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. We have shown that their masses and stellar mass densities are compatible with them being new dwarf galaxies formed without dark matter. The exquisite spatial resolution of the Hubble Space Telescope images has allowed us to investigate the morphologies of the star forming clumps, confirming the “fireballs effect” by which the remaining ionized gas is confined to the side of the clumps which is further away from the galaxy.
For the first time, we have 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, highlighting the capability of ram pressure to funnel gas towards the galaxy center.
For the first time, GASP has measured the magnetic field from polarization data in the long stripped tail finding that the magnetic field is well ordered and strongly aligned with the stripping direction. 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 the first catalog of unwinding arm galaxies in low redshift clusters.
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 observationally measured the gas metallicity scaling relations and metallicity gradients in ram pressure stripped galaxies for the first time and has discovered the imprint of mixing of ICM-stripped gas from the metallicity gradient in stripped tails.
Being the only sufficiently large sample of galaxies in many different galaxy clusters, GASP has been able to characterize the orbits of ram pressure galaxies within clusters and the effects of the cluster dynamical state.
A long standing question has been whether the large population of lenticular galaxies that dominate galaxy clusters today can originate from the quenching of star formation in spiral galaxies. GASP has shown how this is possible and related to ram pressure.
On the simulation side, novel GASP results include the comprehension of the physical processes that push gas towards the center originating the AGN in ram pressure stripped galaxies, and the unexpected result that star formation in the disk does not aid ram pressure stripping nor enhances the rate at which the gas is stripped. GASP hydrodynamical simulations have also shown that the usually employed analytic approximation underpredict the stripping rate of neutral gas and overpredict that of ionized gas.
Finally, we have assessed the success rate of optical imaging in finding ram pressure stripped galaxies, which is important for ongoing and future very large imaging surveys of the sky such as those obtained by Euclid and the Vera Rubin Telescope.