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Discovery of High-Latitude CO in an H I Supershell in NGC 5775
We report the discovery of very high latitude molecular gas in theedge-on spiral galaxy NGC 5775. Emission from both the J=1-0 and 2-1lines of 12CO is detected up to 4.8 kpc away from themidplane of the galaxy. NGC 5775 is known to host a number of H Isupershells. The association of the molecular gas(MH2,F2=3.1×107Msolar) reported here with one of the H I supershells(labeled F2) is clear, which suggests that molecular gas may havesurvived the process that originally formed the supershell.Alternatively, part of the gas could have been formed in situ at highlatitude from shock compression of preexisting H I gas. The COJ=2-1/J=1-0 line ratio of 0.34+/-40% is significantly lower than unity,which suggests that the gas is excited subthermally, with gas density afew times 102 cm-3. The molecular gas is likely inthe form of cloudlets that are confined by magnetic and cosmic-raypressure. The potential energy of the gas at high latitude is found tobe 2×1056 ergs, and the total (H I+H2)kinetic energy is 9×1053 ergs. Based on the energeticsof the supershell, we suggest that most of the energy in the supershellis in the form of potential energy and that the supershell is on theverge of falling and returning the gas to the disk of the galaxy.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| Compact groups in the UZC galaxy sample
Applying an automatic neighbour search algorithm to the 3D UZC galaxycatalogue (Falco et al. \cite{Falco}) we have identified 291 compactgroups (CGs) with radial velocity between 1000 and 10 000 kms-1. The sample is analysed to investigate whether Tripletsdisplay kinematical and morphological characteristics similar to higherorder CGs (Multiplets). It is found that Triplets constitute lowvelocity dispersion structures, have a gas-rich galaxy population andare typically retrieved in sparse environments. Conversely Multipletsshow higher velocity dispersion, include few gas-rich members and aregenerally embedded structures. Evidence hence emerges indicating thatTriplets and Multiplets, though sharing a common scale, correspond todifferent galaxy systems. Triplets are typically field structures whilstMultiplets are mainly subclumps (either temporarily projected orcollapsing) within larger structures. Simulations show that selectioneffects can only partially account for differences, but significantcontamination of Triplets by field galaxy interlopers could eventuallyinduce the observed dependences on multiplicity. Tables 1 and 2 are onlyavailable in electronic at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/391/35
| NGC 5775: Anatomy of a disk-halo interface
We present the first high-resolution study of the disk-halo interface inan edge-on galaxy (NGC 5775) in which every component of theinterstellar medium is represented and resolved (though not all to thesame resolution). New single-dish CO J=2-1 and CO J=1-0 data, ROSATX-ray data, and HIRES IRAS data are presented along with HI data whichemphasizes the high latitude features. In conjunction with previouslypublished radio continuum (6 and 20 cm) and Hα data, we findspatial correlations between various ISM components in that allcomponents of the ISM are present in the disk-halo features (except forCO for which there is insufficient spatial coverage). The HI featuresextend to ~ 7 kpc above the plane, form loops in position-positionspace, in one case, form a loop in position-velocity space, and are alsoobserved over a large velocity range. This implies that the disk-halofeatures represent expanding supershells. However, the shells may beincomplete and partially open-topped, suggesting that we are observingthe breakup of the supershells as they traverse the disk-halo interface.There is some evidence for acceleration with z and both redshifted andblueshifted velocities are present, although the gas which is laggingwith respect to galactic rotation dominates. The radio continuumspectral index is flatter around the shell rims and we show that thiscannot be due to a contribution from thermal gas but rather is due tointrinsic flattening of the non-thermal spectral index, suggesting thatshocks may be important in these regions. The Hα emission islocated interior to the HI. For feature F3, the Hα emission formsthe interior ``skin" of the HI shell, yet there appears to be a minimumof in-disk star formation immediately below the feature. We present apicture of a ``typical" HI supershell which accelerates and breaks upthrough the disk-halo interface. Such a feature is likely internallygenerated via an energetic event in the disk.
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