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Register a new userGASP XXXII. Measuring the diffuse ionized gas fraction in ram-pressure stripped galaxies
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The diffuse ionized gas (DIG) is an important component of the interstellar medium and it can be affected by many physical processes in galaxies. Measuring its distribution and contribution in emission allows us to properly study both its ionization and star formation in galaxies. Here, we measure for the first time the DIG emission in 38 gas-stripped galaxies in local clusters drawn from the GAs Stripping Phenomena in galaxies with MUSE survey (GASP). These galaxies are at different stages of stripping. We also compare the DIG properties to those of 33 normal galaxies from the same survey. To estimate the DIG fraction (C$_{DIG}$) and derive its maps, we combine attenuation corrected H$alpha$ surface brightness with $rm [SII]/Halpha$ line ratio. Our results indicate that we cannot use neither a single H$alpha$ or $rm [SII]/Halpha$ value, nor a threshold in equivalent width of H$alpha$ emission line to separate spaxels dominated by DIG and non-DIG emission. Assuming a constant surface brightness of the DIG across galaxies underestimates C$_{DIG}$. Contrasting stripped and non-stripped galaxies, we find no clear differences in C$_{DIG}$. The DIG emission contributes between 20% and 90% of the total integrated flux, and does not correlate with the galactic stellar mass and star-formation rate (SFR). The C$_{DIG}$ anti-correlates with the specific SFR, which may indicate an older ($>10^8$ yr) stellar population as ionizing source of the DIG. The DIG fraction shows anti-correlations with the SFR surface density, which could be used for a robust estimation of integrated C$_{DIG}$ in galaxies.
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The diffuse ionized gas (DIG) is an important component of the interstellar medium that can provide insights into the different physical processes affecting the gas in galaxies. We utilise optical IFU observations of 71 gas-stripped and control galaxies from the Gas Stripping Phenomena in galaxies (GASP) survey, to analyze the gas properties of the dense ionized gas and the DIG, such as metallicity, ionization parameter log(q), and the difference between the measured $log[OI]/Halpha$ and the value predicted by star-forming models, given the measured log[OIII]/H$beta$ ($Delta log[OI]/Halpha$). We compare these properties at different spatial scales, among galaxies at different gas-stripping stages, and between disks and tails of the stripped galaxies. The metallicity is similar between the dense gas and DIG at a given galactocentric radius. The log(q) is lower for DIG compared to dense gas. The median values of log(q) correlate best with stellar mass, and the most massive galaxies show an increase in log(q) toward their galactic centers. The DIG clearly shows higher $Delta log[OI]/Halpha$ values compared to the dense gas, with much of the spaxels having LIER/LINER like emission. The DIG regions in the tails of highly stripped galaxies show the highest $Delta log[OI]/Halpha$, exhibit high values of log(q) and extend to large projected distances from star-forming areas (up to 10 kpc). We conclude that the DIG in the tails is at least partly ionized by a process other than star-formation, probably by mixing, shocks and accretion of inter-cluster and interstellar medium gas.
Jellyfish galaxies in clusters are key tools to understand environmental processes at work in dense environments. The advent of Integral Field Spectroscopy has recently allowed to study a significant sample of stripped galaxies in the cluster environment at z$sim 0.05$, through the GAs Stripping Phenomena in galaxies with MUSE (GASP) survey. However, optical spectroscopy can only trace the ionized gas component through the H$_{alpha}$ emission that can be spatially resolved on kpc scale at this redshift. The complex interplay between the various gas phases (ionized, neutral, molecular) is however yet to be understood. We report here the detection of large amounts of molecular gas both in the tails and in the disks of 4 jellyfish galaxies from the GASP sample with stellar masses $sim 3.5times 10^{10}-3times 10^{11} M_{odot}$, showing strong stripping. The mass of molecular gas that we measure in the tails amounts to several $10^9 M_{odot}$ and the total mass of molecular gas ranges between 15 and 100 % of the galaxy stellar mass. The molecular gas content within the galaxies is compatible with the one of normal spiral galaxies, suggesting that the molecular gas in the tails has been formed in-situ. We find a clear correlation between the ionized gas emission $rm Halpha$ and the amount of molecular gas. The CO velocities measured from APEX data are not always coincident with the underlying $rm Halpha$ emitting knots, and the derived Star Formation Efficiencies appear to be very low.
Exploiting the data from the GAs Stripping Phenomena in galaxies with MUSE (GASP) program, we compare the integrated Star Formation Rate- Mass relation (SFR-M_ast) relation of 42 cluster galaxies undergoing ram pressure stripping (stripping galaxies) to that of 32 field and cluster undisturbed galaxies. Theoretical predictions have so far led to contradictory conclusions about whether ram pressure can enhance the star formation in the gas disks and tails or not and until now a statistically significant observed sample of stripping galaxies was lacking. We find that stripping galaxies occupy the upper envelope of the control sample SFR-M_ast relation, showing a systematic enhancement of the SFR at any given mass. The star formation enhancement occurs in the disk (0.2 dex), and additional star formation takes place in the tails. Our results suggest that strong ram pressure stripping events can moderately enhance the star formation also in the disk prior to gas removal.
We present the results of deep imaging and spectroscopic observations of very extended ionized gas (EIG) around four member galaxies of the Coma cluster of galaxies: RB199, IC4040, GMP2923 and GMP3071. The EIGs were serendipitously found in an H-alpha narrow band imaging survey of the central region of the Coma cluster. The relative radial velocities of the EIGs with respect to the systemic velocities of the parent galaxies from which they emanate increase almost monotonically with the distance from the nucleus of the respective galaxies, reaching -400 - -800 km/s at around 40 - 80 kpc from the galaxies. The one-sided morphologies and the velocity fields of the EIGs are consistent with the predictions of numerical simulations of ram pressure stripping. We found a very low-velocity filament (v_rel = -1300 km/s) at the southeastern edge of the disk of IC4040. Some bright compact knots in the EIGs of RB199 and IC4040 exhibit blue continuum and strong H-alpha emission. The equivalent widths of the H-alpha emission exceed 200 A, and are greater than 1000 A for some knots. The emission line intensity ratios of the knots are basically consistent with those of sub-solar abundance HII regions. These facts indicate that intensive star formation occurs in the knots. Some filaments, including the low velocity filament of the IC4040 EIG, exhibit shock-like emission line spectra, suggesting that shock heating plays an important role in ionization and excitation of the EIGs.
Previous studies have revealed a population of galaxies in galaxy clusters with ram pressure stripped (RPS) tails of gas and embedded young stars. We observed 1.4 GHz continuum and HI emission with the Very Large Array in its B-configuration in two fields of the Coma cluster to study the radio properties of RPS galaxies. The best continuum sensitivities in the two fields are 6 and 8 $mu$Jy per 4 beam respectively, which are 4 and 3 times deeper than those previously published. Radio continuum tails are found in 10 (8 are new) out of 20 RPS galaxies, unambiguously revealing the presence of relativistic electrons and magnetic fields in the stripped tails. Our results also hint that the tail has a steeper spectrum than the galaxy. The 1.4 GHz continuum in the tails is enhanced relative to their H$alpha$ emission by a factor of $sim$7 compared to the main bodies of the RPS galaxies. The 1.4 GHz continuum of the RPS galaxies is also enhanced relative to their IR emission by a factor of $sim$2 compared to star-forming galaxies. The enhancement is likely related to ram pressure and turbulence in the tail. We furthermore present HI detections in three RPS galaxies and upper limits for the other RPS galaxies. The cold gas in D100s stripped tail is dominated by molecular gas, which is likely a consequence of the high ambient pressure. No evidence of radio emission associated with ultra-diffuse galaxies is found in our data.
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