Ram pressure diagnostics using polarized emission

Bernd Vollmer, Observatoire de Strasbourg, France (PI)
Marian Soida, Jagiellonian University, Krakov, Poland
Marek Urbanik, Jagiellonian University, Krakov, Poland
Rainer Beck, Max-Planck-Institut für Radioastronomie, Bonn, Germany
Krzysztof T. Chyzy, Jagiellonian University, Krakov, Poland
Katarzyna Otmianowska-Mazur, Jagiellonian University, Krakov, Poland
Jeffrey Kenney, Yale University, New Haven, USA
Jacqueline van Gorkom, Columbia University, New York, USA
Aeree Chung, National Radio Astronomy Observatory, USA
Marek Wezgowiec, Ruhr Universitat Bochum, Germany

Ram pressure stripping in the Virgo cluster

The mapping of the gas content of spiral galaxies in the Virgo cluster showed that the HI disks of cluster spirals are disturbed and considerably  reduced (Cayatte et al. 1990, 1994). These observational results indicate that the gas removal due to the rapid motion of the galaxy within the intracluster medium (ICM) (ram pressure stripping; Gunn & Gott 1972) is responsible for the HI deficiency and the disturbed gas disks of the cluster spirals. Nevertheless, it is still an open question where and when these galaxies lose their gas. 
During a ram pressure stripping event the ISM of the galaxy is compressed by the ram pressure of the intracluster medium. After the galaxy's passage through the cluster core ram pressure decreases considerably and the ISM that has not been accelerated to the escape velocity possibly falls back onto the galactic disk. The backfalling gas causes shear motions. Both, compression and shear motions, cannot be easily detected in an HI velocity field. However, polarized radio continuum emission is very sensitive to these motions, because they generate anisotropic fields from isotropic random fields and they enhance locally the ordered magnetic field on large scales. Thus, whenever there is compression or shear the  polarized radio continuum emission lights up. This emission must be observed at a wavelength small enough to avoid significant Faraday depolarization.
The combination of the HI gas distribution, velocity field and the distribution of the polarized radio emission enables us to diagnostic if and when ram pressure affected the observed galaxy (see Kenney et al. 2004, Vollmer et al. 2004b, and Vollmer et al. 2006 for NGC 4522, Soida et al. 2006 for NGC 4654, and Vollmer et al. 2008 for NGC 4501).

Why polarized radio continuum emission

Polarized radio continuum emission (PI) is sensitive to the density of cosmic ray electrons and, most importantly, on the ordered magnetic field on large scales. In an exploring work (Otmianowska-Mazur & Vollmer 2003) we solved the induction equation on the velocity fields given by the above described numerical simulations to calculate the evolution of the ordered magnetic field. With an assumed distribution of cosmic ray electrons the evolution of the PI was determined. We showed that the PI increases whenever there is compression or shear. Both phenomenon occur during a ram pressure stripping event. During ram pressure maximum the gas is compressed; after the ram pressure maximum (when the galaxy is leaving the cluster core) the resettlement of the gas causes shear. The best example for the synergy between the VLA PI observations and our MHD modelling is NGC 4522. Kenney et al. (2004) detected high column density, extraplanar HI to the west of the almost edge-on galactic disk. With deep 6cm observations of the polarized emission (Vollmer et al. 2004) we directly detected the compression region on the opposite side of the galaxy. A dynamical and MHD modelling show that this galaxy is close to peak ram pressure despite its large distance from the cluster center (Vollmer et al. 2006). This implies that the intracluster gas is rapidly moving towards the location of NGC 4522. This intracluster gas is most probably associated with the infalling M49 group of galaxies.

NGC 4522. Left panel: greyscale: optical R band image. Contours:
HI gas distribution (from Kenney et al. 2004).
Right panel: greyscale: HI gas distribution.
Contours: polarized radio emission at 6cm (from Vollmer et al. 2004).

This kind of dynamical and MHD modelling was also applied to NGC 4654 (Soida et al. 2006).
The dynamical modelling of the HI data (Vollmer 2003) has shown that this galaxy underwent a tidal encounter ~500Myr ago and is now affected by a weak ram pressure stripping. The MHD modelling of the tidal interaction alone (GR model), ram pressure stripping alone (RPS model), and a mixed interaction (GRPS; Soida et al. 2006) confirmed the earlier result.

NGC 4654: Greyscale: HI distribution. Contours: polarized radio
continuum emission. GR model: gravitational interaction alone.
RPS model: ram pressure stripping alone. GRPS model: mixed
interaction. Right panel: VLA HI and 6cm continuum observations
(Soida et al. 2006).

For NGC 4501 only the 6cm polarized radio continuum observations and the MHD modelling show unambiguously that this galaxy undergoes active ram pressure stripping. The ridge of polarized 6cm radio continuum emission at the outer southwestern disk is a clear sign of compression. This compression was not detectable in the velocity field. The HI overdensity and sharp edge suggested a compression, but only the polarized 6~cm radio continuum emission could confirm it.

NGC 4501: Greyscale: HI distribution. Contours: polarized
radio continuum emission. Left panel: VLA observations.
Middle and right panel: ram pressure models with different
galaxy orbits (Vollmer et al. 2008).

Previous VLA polarized radio continuum observations

In a pilot study we observed 8 Virgo spiral galaxies at 20cm and 6cm in polarized emission.
All observed Virgo spiral galaxies (except NGC~4321) show a strongly asymmetric distribution of polarized radio continuum emission with elongated ridges located in the outer galactic disk. Such polarized emission ridges are also observed in two other Virgo spiral galaxies: NGC 4254 (Chyzy et al. 2007) and NGC 4522 (Vollmer et al. 2004). Based on this sample of 10 Virgo spiral galaxies observed with the VLA, we conclude that the 6cm polarized radio continuum emission distribution in Virgo spiral galaxies is different from that of field spiral galaxies (Beck 2005), in the sense that the distribution is strongly asymmetric with elongated ridges in the outer part of the galactic disks. Since all asymmetric emission ridges are located in the outer galactic disks, they are most likely due to external influences of the cluster environment on the galaxies. These influences can be of tidal or hydrodynamic nature. Thus the distribution of polarized radio continuum emission contains important information about the velocity distortions caused by the interaction of a spiral galaxy with its cluster environment. In addition, this information is complementary to that of CO or HI data cubes, because we can observe shear or compression caused by the velocity components in the plane of the sky. It is very difficult, if not impossible, to predict the distribution of polarized radio continuum emission on the basis of CO or HI observations, because of the complex evolution of the magnetic field (induction equation) and beam depolarization effects. It is therefore necessary to do detailed MHD modelling  of individual galaxies for direct comparison with observations.

6cm polarized intensities as contours and the vectors of the
magnetic field uncorrected for Faraday rotation on VLA HI
images. The size of the lines is proportional to the intensity
of the polarized emission (Vollmer et al. 2008).

Target selection

The target galaxies can be divided into 4 categories according to the following criteria:
(i) galaxy pairs,
(ii) galaxies which are likely affected by ram pressure (distance to M 87 D  < 3deg),
(iii) galaxies at the periphery (4deg < D < 8deg) of the cluster,
(iv) galaxies far away from the Virgo cluster (D > 8deg).

Virgo cluster X-ray emission (ROSAT) of the intracluster medium and HI
emission of the proposed sample of Virgo spiral galaxies
(from VIVA; image credit: A. Chung, NRAO).