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Register a new userRam pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438
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B. Vollmer
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Ram pressure stripping of the multiphase ISM is studied in the perturbed Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction ~100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA radio continuum observations at 6 and 20 cm are presented. We detect prominent extraplanar emission to the west of the galactic center, which extends twice as far as the other tracers of extraplanar material. The spectral index of the extraplanar emission does not steepen with increasing distance from the galaxy. This implies in situ re-acceleration of relativistic electrons. The comparison with multiwavelength observations shows that the magnetic field and the warm ionized interstellar medium traced by Halpha emission are closely linked. The kinematics of the northern extraplanar Halpha emission, which is ascribed to star formation, follow those of the extraplanar CO emission. In the western and southern extraplanar regions, the Halpha measured velocities are greater than those of the CO lines. We suggest that the ionized gas of this region is excited by ram pressure. The spatial and velocity offsets are consistent with a scenario where the diffuse ionized gas is more efficiently pushed by ram pressure stripping than the neutral gas. We suggest that the recently found radio-deficient regions compared to 24 mum emission are due to this difference in stripping efficiency.
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It has been shown that the Virgo spiral galaxy NGC 4330 shows signs of ongoing ram pressure stripping in multiple wavelengths: at the leading edge of the interaction, the Halpha and dust extinction curve sharply out of the disk; on the trailing side, a long Halpha/UV tail has been found which is located upwind of a long HI tail. We complete the multiwavelength study with IRAM 30m HERA CO(2-1) and VLA 6 cm radio continuum observations of NGC 4330. The data are interpreted with the help of a dynamical model including ram pressure and, for the first time, star formation. Our best-fit model reproduces qualitatively the observed projected position, radial velocity of the galaxy, the molecular and atomic gas distribution and velocity field, and the UV distribution in the region where a gas tail is present. However, the observed red UV color on the windward side is currently not reproduced by the model. Based on our model, the galaxy moves to the north and still approaches the cluster center with the closest approach occurring in ~100 Myr. In contrast to other Virgo spiral galaxies affected by ram pressure stripping, NGC 4330 does not show an asymmetric ridge of polarized radio continuum emission. We suggest that this is due to the relatively slow compression of the ISM and the particular projection of NGC 4330. The observed offset between the HI and UV tails is well reproduced by our model. Since collapsing and starforming gas clouds decouple from the ram pressure wind, the UV-emitting young stars have the angular momentum of the gas at the time of their creation. On the other hand, the gas is constantly pushed by ram pressure. The reaction (phase change, star formation) of the multiphase ISM (molecular, atomic, ionized) to ram pressure is discussed in the framework of our dynamical model.
VIVA HI observations of the Virgo spiral galaxy NGC 4501 are presented. The HI disk is sharply truncated to the southwest, well within the stellar disk. A region of low surface-density gas, which is more extended than the main HI disk, is discovered northeast of the galaxy center. These data are compared to existing 6cm polarized radio continuum emission, Halpha, and optical broad band images. We observe a coincidence between the western HI and polarized emission edges, on the one hand, and a faint Halpha emission ridge, on the other. The polarized emission maxima are located within the gaps between the spiral arms and the faint Halpha ridge. Based on the comparison of these observations with a sample of dynamical simulations with different values for maximum ram pressure and different inclination angles between the disk and the orbital plane,we conclude that ram pressure stripping can account for the main observed characteristics. NGC 4501 is stripped nearly edge-on, is heading southwest, and is ~200-300 Myr before peak ram pressure, i.e. its closest approach to M87. The southwestern ridge of enhanced gas surface density and enhanced polarized radio-continuum emission is due to ram pressure compression. It is argued that the faint western Halpha emission ridge is induced by nearly edge-on ram pressure stripping. NGC 4501 represents an especially clear example of early stage ram pressure stripping of a large cluster-spiral galaxy.
We present a deep VLA search for HI emission from the low-luminosity Virgo Cluster elliptical galaxy NGC 4476, which contains 1.1 x 10^8 M_sun of molecular gas in an undisturbed disk in regular rotation. No HI was detected. The rms noise in the final image corresponds to a 3 sigma column density sensitivity of 1.2 x 10^20 cm^{-2} at the position of NGC 4476, averaged over the 4 kpc beam. The total HI mass is less than 1.5 x 10^7 M_sun. If we compare our HI upper limit to the H_2 content, we find that NGC 4476 is extremely deficient in HI compared to other galaxies detected in these two species. The H_2/HI mass ratio for NGC 4476 is > 7, whereas typical H_2/HI ratios for elliptical galaxies detected in both HI and H_2 are <~2. Based on this extreme HI deficiency and the intra-cluster medium (ICM) density at the projected distance from M87 we argue that either NGC 4476 has undergone ram-pressure stripping while traveling through the Virgo cluster core or its average molecular gas density is larger and its interstellar UV field is smaller than in typical spiral galaxies. NGC 4476 is located 12 in projection from M87, which causes extreme continuum confusion problems. We also discuss in detail the techniques used for continuum subtraction. The spectral dynamic range of our final image is 50,000 to 1.
NGC 4330 is one of the Virgo galaxies whose UV distribution shows a tail structure. An associated tail structure is also observed in the HI and H$alpha$ emission distributions. Previous dynamical modeling showed that the galaxy is approaching the cluster center and is therefore undergoing increasing ram pressure stripping. Recent stellar population fitting of deep optical spectra together with multiband photometry lead to the determination of the time when star formation was quenched in the galactic disk. We introduce a new version of the dynamical model that includes the diffuse ionized gas and aim to reproduce the HI, H$alpha$, UV distributions together with the star formation histories of the outer gas-free parts of the galactic disk. The results of 50 simulations with five different Lorentzian temporal ram-pressure profiles and five different delays between the simulation onset and peak ram pressure are presented. The inclusion of diffuse gas stripping changes significantly the HI, UV, and H$alpha$ emission distributions. The simulations with diffuse gas stripping naturally lead to vertical low surface density filaments in the downwind region of the galactic disk. These filaments occur less frequently in the simulations without diffuse gas stripping. The simulations with diffuse gas stripping lead to better joint fits to the SEDs and optical spectra. The HI, NUV, and H$alpha$ morphologies of the model snapshots which best reproduce the SEDs and optical spectra are sufficiently different to permit a selection of best-fit models. We conclude that the inclusion of diffuse gas stripping significantly improves the resemblance between the model and observations. Our preferred model yields a time to peak ram pressure of 140 Myr in the future. The spatial coincidence of the radio continuum and diffuse H$alpha$ tails suggests that both gas phases are stripped together.
We study galaxies undergoing ram pressure stripping in the Virgo cluster to examine whether we can identify any discernible trend in their star formation activity. We first use 48 galaxies undergoing different stages of stripping based on HI morphology, HI deficiency, and relative extent to the stellar disk, from the VIVA survey. We then employ a new scheme for galaxy classification which combines HI mass fractions and locations in projected phase space, resulting in a new sample of 365 galaxies. We utilize a variety of star formation tracers, which include g - r, WISE [3.4] - [12] colors, and starburstiness that are defined by stellar mass and star formation rates to compare the star formation activity of galaxies at different stripping stages. We find no clear evidence for enhancement in the integrated star formation activity of galaxies undergoing early to active stripping. We are instead able to capture the overall quenching of star formation activity with increasing degree of ram pressure stripping, in agreement with previous studies. Our results suggest that if there is any ram pressure stripping induced enhancement, it is at best locally modest, and galaxies undergoing enhancement make up a small fraction of the total sample. Our results also indicate that it is possible to trace galaxies at different stages of stripping with the combination of HI gas content and location in projected phase space, which can be extended to other galaxy clusters that lack high-resolution HI imaging.
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