arXiv subjects

C. Balkowski

Publications and source records attributed to C. Balkowski.

At least 55 records · Page 3Linked to original sources

arXiv2003

A study of HI-selected galaxies in the Hercules cluster

The present study is aimed at a sample of 22 galaxies detected in the blind VLA HI survey of the Hercules cluster by Dickey (1997), 18 of which were selected on an HI line width smaller than 270 km/s and 4 others with only tentative optical counterparts on the Palomar Sky Survey. Sensitive single-dish HI line spectra were obtained for 20 of them, and for one (47-154) the VLA detection was not confirmed. Optical surface photometry was obtained of 10 objects, for 8 of which optical spectroscopy was obtained as well. Based on various selection criteria, two (ce-143 and ne-204) can be classified as dwarfs. The objects of which optical observations were made show star formation properties similar to those of published samples of actively star forming galaxies, and approximately half of them have properties intermediate between those of dwarf galaxies and low-luminosity disc galaxies. No optical redshifts could be obtained for two of the galaxies (sw-103 and sw-194) and their physical association with the HI clouds detected at their positions therefore remains uncertain. Unique among the objects is the Tidal Dwarf Galaxy ce-061 in a tail of the IC 1182 merger system.

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arXiv2003

Gas Kinematics of Three Hickson Compact Groups: The Data

We present Fabry Perot observations of three Hickson Compact Groups, HCG 88, HCG 89 and HCG 100. We detect ionized gas in 15 group members, three of which were previously uncatalogued objects, two in HCG 89 and one in HCG 100. We were able to derive 2D velocity, monochromatic and continuum maps and rotation curves for a total of 12 giant late-type galaxies and two dwarf galaxies. Even with this small sample of three groups, we can clearly see a trend of kinematic evolution and the different evolutionary stages of the groups. The members of HCG 88 show almost no signs of previous or current interactions, while HCG 100 contains at least two merging and one strongly-interacting galaxies. HCG 89 shows members with signs of interactions and galaxies with normal kinematics. We therefore classify HCG 88, HCG 89 and HCG 100 respectively as an unevolved group, a mildly interacting group and a system in the final stage of evolution.

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arXiv2002

Non-confirmation of reported HI clouds without optical counterparts in the Hercules Cluster

21 cm HI line observations were made with the Arecibo Gregorian telescope of 9 HI clouds in the Hercules Cluster which were reported as tenative detections in a VLA HI study of the cluster (Dickey 1997) and for which our deep CCD imaging failed to find any optical counterparts. No sensitive observations could be made of one of these (sw-174) due to the presence of a close-by strong continuum source. The other 8 tentative HI detections were not reconfirmed by the Arecibo HI measurements. The CCD images did reveal faint, low surface brightness counterparts near the centres of two other VLA HI sources invisible on the Palomar Sky Survey, sw-103 and sw-194.

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arXiv2002

From Spirals to Low Surface Brightness galaxies

We show that simple models of the chemical and spectrophotometric evolution of galaxies can be used to explore the properties of present-day galaxies and especially the causes of the observed variety among disc galaxies. We focus on the link between ``classical'' spirals and Low Surface Brightness galaxies.

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arXiv2002

The properties of Low Surface Brightness galaxies

A description is given of the samples of Low Surface Brightness galaxies (LSBs) used for comparison with models of their chemical and spectro-photometric evolution (Boissier et al., this Volume). These samples show the large variation and scatter in observed global properties of LSBs, some of which cannot be modeled without adding starbursts or truncations to their star formation history.

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arXiv2002

Unusual magnetic fields in the interacting spiral NGC 3627

By observing the interacting galaxy NGC 3627 in radio polarization we try to answer the question to which degree the magnetic field follows the galactic gas flows. We obtained total power and polarized intensity maps at 8.46 GHz and 4.85 GHz using the VLA in its compact D-configuration. In order to overcome the zero-spacing problems, the interferometric data were combined with single-dish measurements obtained with the Effelsberg 100-m radio telescope. The observed magnetic field structure in NGC 3627 suggests that two field components are superposed. One component smoothly fills the interarm space and shows up also in the outermost disk regions, the other component follows a symmetric S-shaped structure. In the western disk the latter component is well aligned with an optical dust lane, following a bend which is possibly caused by external interactions. However, in the SE disk the magnetic field crosses a heavy dust lane segment, apparently being insensitive to strong density-wave effects. We suggest that the magnetic field is decoupled from the gas by high turbulent diffusion, in agreement with the large \ion{H}{i} line width in this region. We discuss in detail the possible influence of compression effects and non-axisymmetric gas flows on the general magnetic field asymmetries in NGC 3627. On the basis of the Faraday rotation distribution we also suggest the existence of a large ionized halo around this galaxy.

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arXiv2001

VLA HI Imaging of the brightest spiral galaxies in Coma II. The HI Atlas and deep continuum imaging of selected early type galaxies

In the first paper of this series we used HI observations of the 19 brightest spirals in Coma to analyze the dynamical state of the cluster. In this paper we present the detailed HI distribution and kinematics of the spirals that were detected in HI, and radio continuum data for a sample of star forming and post starburst galaxies in Coma. We discuss the importance of ICM-ISM interactions to explain the observed HI morphology. A rough comparison of observed HI sizes with predicted HI sizes from simulations by Abadi et al. (1999) gives reasonable agreement. We use the results on radio continuum emission to estimate the star formation rate in the PSB galaxies we pointed at. The radio continuum emission in the 11 so called post starburst galaxies, identified by Caldwell et al. (1993) in the cluster, is weak. Eight of the 11 were not detected down to a 3 sigma upper limit of 0.6 mJy. This sets an upper limit to the star formation rate in these galaxies of less than 0.2 solar masses per year. The three detected post starburst galaxies have a star formation rate of less than one solar mass per year. Thus none of the post starburst galaxies in Coma are dust enshrouded starbursts.

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arXiv2001

Ram pressure stripping and galaxy orbits: The case of the Virgo cluster

We investigate the role of ram pressure stripping in the Virgo cluster using N-body simulations. Radial orbits within the Virgo cluster's gravitational potential are modeled and analyzed with respect to ram pressure stripping. The N-body model consists of 10000 gas cloud complexes which can have inelastic collisions. Ram pressure is modeled as an additional acceleration on the clouds located at the surface of the gas distribution in the direction of the galaxy's motion within the cluster. We made several simulations changing the orbital parameters in order to recover different stripping scenarios using realistic temporal ram pressure profiles. We investigate systematically the influence of the inclination angle between the disk and the orbital plane of the galaxy on the gas dynamics. We show that ram pressure can lead to a temporary increase of the central gas surface density. In some cases a considerable part of the total atomic gas mass (several 10^8 M_solar) can fall back onto the galactic disk after the stripping event. A quantitative relation between the orbit parameters and the resulting HI deficiency is derived containing explicitly the inclination angle between the disk and the orbital plane. The comparison between existing HI observations and the results of our simulations shows that the HI deficiency depends strongly on galaxy orbits. It is concluded that the scenario where ram pressure stripping is responsible for the observed HI deficiency is consistent with all HI 21cm observations in the Virgo cluster.

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arXiv2001

12CO(1-0) observations of NGC 4848: a Coma galaxy after stripping

We study the molecular gas content and distribution in the Coma cluster spiral galaxy NGC 4848. Plateau de Bure interferometric CO(1-0) observations reveal a lopsided H_2 distribution with an off-center secondary maximum coincident with the inner part of the HI. NGC 4848 is not at all deficient in molecular gas as it contains M_H_2~4x10^9 M_solar. At the interface between the CO and HI emission regions, about 8 kpc NW of the center, however, strong star formation is present as witnessed by Halpha and radio continuum emission. This is the region in which earlier Fabry-Perot observations revealed a double-peaked Halpha line, indicating gas at two different velocities at the same sky position. In order to understand these observations, we present the results of numerical simulations of the ISM-ICM interaction. We suggest that NGC 4848 already passed through the center of the cluster about 4x10^8 years ago. At the observed stage ram pressure has no more direct dynamical influence on the galaxy's ISM. We observe the galaxy when a fraction of the stripped gas is falling back onto the galaxy. Ram pressure is thus a short-lived event with longer-lasting consequences. The combination of ram-pressure and rotation results in gas at different velocities colliding where the double-peaked Halpha line is observed. Ram-pressure can thus result, after re-accretion, in displaced molecular gas without the H_2 itself being pushed efficiently by the ICM. A scenario where two interactions take place simultaneously is also consistent with the available data but less probable on the basis of our numerical simulations.

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arXiv2001

HI deficiency in the galaxy cluster ACO 3627. ATCA observations in the Great Attractor region

ATCA 21 cm HI observations of the rich galaxy cluster ACO 3627 in the Great Attractor region are presented. Three fields of 30' diameter located within one Abell radius of ACO 3627 were observed with a resolution of 15'' and an rms noise of \sim 1 mJy/beam. Only two galaxies were detected in these fields. We compare their HI distribution to new optical R-band images and discuss their velocity fields. The first galaxy is a gas-rich unperturbed spiral whereas the second shows a peculiar HI distribution. The estimated 3-sigma HI mass limit of our observations is \sim 7 x 10^8 M_{\odot} for a line width of 150 km s^{-1}. The non-detection of a considerable number of luminous spiral galaxies indicates that the spiral galaxies are HI deficient. The low detection rate is comparable to the HI deficient Coma cluster (Bravo-Alfaro et al. 2000). ACO 3627 is a bright X-ray cluster. We therefore suspect that ram pressure stripping is responsible for the HI deficiency of the bright cluster spirals.

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arXiv2001

Candidate tidal dwarf galaxies associated with the Stephan's Quintet

We present kinematic and photometric evidence for the presence of seven candidate tidal dwarf galaxies in Stephan's quintet. The central regions of the two most probable parent galaxies, N7319 and N7318B, contain little or no gas whereas the intragroup medium, and particularly the optical tails that seem to be associated with N7318B are rich in cold and ionized gas. Two tidal-dwarf candidates may be located at the edge of a tidal tail, one within a tail and for four others there is no obvious stellar/gaseous bridge between them and the parent galaxy. Two of the candidates are associated with HI clouds, one of which is, in addition, associated with a CO cloud. All seven regions have low continuum fluxes and high H$α$ luminosity densities (F(H$α$) = 1 -- 60 $\times$ 10$^{-14}$ erg s$^{-1}$ cm$^{-2}$). Their magnitudes (M$_B =$ --16.1 to --12.6), sizes ($\sim$ 3.5 h$_{75}^{-1}$ kpc), colors (typically $B-R = 0.7$) and gas velocity gradients ($\sim$ 8 -- 26 h$_{75}$ km s$^{-1}$ kpc$^{-1}$) are typical for tidal dwarf galaxies. In addition the ratios between their star formation rates determined from H$α$ and from the B band luminosity are typical of other tidal dwarf galaxies. The masses of the tidal dwarf galaxies in Stephan's quintet range from $\sim$ 2 $\times$ 10$^8$ to 10$^{10}$ M$_\odot$ and the median value for their inferred mass-to-light ratios is 7 M$_\odot$/L$_\odot$.

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arXiv2000

The consequences of ram pressure stripping on the Virgo cluster spiral galaxy NGC 4522

New H-alpha Fabry-Perot interferometer observations of the Virgo cluster spiral galaxy NGC 4522 are presented. A velocity field up to a radius of 60'' was obtained. The observed rotation curve is symmetric. It is not perturbed in the inner disk. For radii >50'' it shows a solid body rotation. We compare these data with a numerical model which includes the effects of ram pressure stripping. The model can reproduce the main characteristics of the H-alpha emission distribution and velocity field. We therefore conclude that the stripping event which caused the HI deficiency and the distorted H-alpha distribution and velocity field has been due to ram pressure. The closest passage of the galaxy to the cluster center is estimated to be approx. 6.5 10^8 yr ago. The observed recent star formation is due to the re-accretion of gas clouds which were accelerated to velocities below the escape velocity during the stripping event. Furthermore, the gas which is located at radii >60'' is part of an asymmetric expanding ring of gas clouds. A substantial part of these clouds is located above the disk plane. We predict the HI gas distribution and velocity field as it would be observed by deep 21 cm observations.

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arXiv2000

Kinematics and morphology of ionized gas in Hickson Compact Group 18

We present new observations of $Hα$ emission in the Hickson Compact Group 18 (HCG 18) obtained with a scanning Fabry-Perot interferometer. The velocity field does not show motions of individual group members but, instead, a complex common velocity field for the whole group. The gas distribution is very asymmetric with clumps of maximum intensity coinciding with the optically brightest knots. Comparing $Hα$ and HI data we conclude that HCG 18 is not a compact group but instead a large irregular galaxy with several clumps of star formation.

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arXiv2000

VLA HI imaging of the brightest spiral galaxies in Coma

We have obtained 21cm images of 19 spiral galaxies in the Coma cluster, using the VLA in its C and D configurations. The sample selection was based on morphology, brightness, and optical diameters of galaxies within one Abell radius (1.2 degrees). The HI detected, yet deficient galaxies show a strong correlation in their HI properties with projected distance from the cluster center. The most strongly HI deficient (HI Def > 0.4) galaxies are located inside a radius of 30 arcmin (aprox 0.6 Mpc) from the center of Coma, roughly the extent of the central X-ray emission. These central galaxies show clear asymmetries in their HI distribution and/or shifts between the optical and 21cm positions. Seven so called blue disk galaxies in Coma were observed in HI and six were detected. We did a more sensitive search for HI from 11 of the 15 known post starburst galaxies in Coma. None were detected with typical HI mass limits between 3 and 7x10^7 solar masses. Our results present and enhance a picture already familiar for well studied clusters. HI poor galaxies are concentrated toward the center of the cluster. The HI morphology of the central galaxies, with optical disks extending beyond the HI disks is unique to cluster environments and strongly suggests an interaction with the IGM. A new result in Coma is the clumpy distribution of gas deficiency. In the cluster center the deficient galaxies are to the east while the non-detections are to the west. In the outer parts the gas rich galaxies are north of Coma, non-detected spirals are found in the NGC 4944 group to the east and NGC 4839 group to the SW. This supports recent findings that merging of groups is ongoing in the center of Coma, further out the NGC 4944 and NGC 4839 must have passed through the core, while the galaxies to the north have yet to fall in.

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arXiv1999

Kinematics of the anemic cluster galaxy NGC 4548 - Is stripping still active ?

We present new HI (20" resolution) and CO observations of NGC 4548, an anemic galaxy in the Virgo cluster. The atomic gas distribution shows a ring structure which is distorted at the northern edge. The overall rotation curve is derived with a tilted ring model. We compare our rotation curve with previous ones and discuss the differences. The velocity field of the CO pointings fit very well with the one of the atomic gas where they overlap. The CO emission permits to extend the rotation curve towards the galaxy centre. The molecular fraction is derived for the inner 120" x 120" centered on the galaxy. We compare the HI and CO emission to H_alpha line and optical blue emission maps. The bar in the centre favors star formation at the outer end of the bar. The HI intensity distribution and velocity field of the northern perturbation are deprojected with the help of a first order kinematical model. They are discussed in the framework of warps. It is concluded that the scenario of ram pressure stripping responsible for the gas removal and the northern perturbation is a very probable one. In this case the ram pressure which may have caused the galaxy's HI deficiency is now fading after the galaxy's close passage to the cluster centre or increasing again due to a second approach of the galaxy to the cluster centre.

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arXiv1999

Discovery of Extreme Examples of Superclustering in Aquarius

We report the discovery of two highly extended filaments and one extremely high density knot within the region of Aquarius. The supercluster candidates were chosen via percolation analysis of the Abell and ACO catalogs and include only the richest clusters (R >= 1). The region examined is a 10x45 degree strip and is now 87% complete in cluster redshift measurements to mag_10 = 18.3. In all, we report 737 galaxy redshifts in 46 cluster fields. One of the superclusters, dubbed Aquarius, is comprised of 14 Abell/ACO clusters and extends 110h^-1Mpc in length only 7 degrees off the line-of-sight. On the near-end of the Aquarius filament, another supercluster, dubbed Aquarius-Cetus, extends for 75h^-1Mpc perpendicular to the line-of-sight. After fitting ellipsoids to both Aquarius and Aquarius-Cetus, we find axis ratios (long-to- midlength axis) of 4.3 for Aquarius and 3.0 for Aquarius-Cetus. We fit ellipsoids to all N>=5 clumps of clusters in the Abell/ACO measured-z cluster sample. The frequency of filaments with axis ratios >=3.0 (~20%) is nearly identical with that found among `superclusters' in Monte Carlo simulations of random and random- clumped clusters, however, so the rich Abell/ACO clusters have no particular tendency toward filamentation. The Aquarius filament also contains a `knot' of 6 clusters at Z ~0.11, with five of the clusters near enough togeteher to represent an apparent overdensity of 150 . There are three other R >= 1 cluster density enhancements similar to this knot at lower redshifts: Corona Borealis, the Shapely Concentration, and another grouping of seven clusters in Microscopium. All four of these dense superclusters appear near the point of breaking away from the Hubble Flow, and some may now be in collapse, but there is little evidence of any being virialized.

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arXiv1999

The ESO Slice Project (ESP) Galaxy Redshift Survey. VII. The Redshift and Real-Space Correlation Functions

We present analyses of the two-point correlation properties of the ESP galaxy redshift survey. From the redshift-space correlation function xi(s), we see positive clustering out to separations ~50/h Mpc, with a smooth break on larger scales and zero-crossing between 60 and 80/h Mpc. xi(s) is reasonably well described by a shallow power law with γ~1.5 between 3 and 50/h Mpc, while on smaller scales (0.2-2/h Mpc) it has a shallower slope (γ~ 1). We examine the full effect of redshift-space distortions through the two-dimensional correlation function xi(rp,pi), from which we project out the real-space xi(r) below 10/h Mpc. This function is well described by a power-law model (r/r_o)^{-γ}, with r_o=4.15^{+0.20}_{-0.21} h^{-1} Mpc and γ=1.67^{+0.07}_{-0.09}. Comparison to other redshift surveys shows a consistent picture in which clustering remains positive out to separations of 50/h Mpc or larger, in substantial agreement with the results obtained from angular surveys like the APM and EDSGC. Also the shape of the two-point correlation function is remarkably unanimous among these data sets, in all cases requiring more power above 5/h Mpc (a `shoulder'), than a simple extrapolation of the canonical xi(r)=(r/5)^{-1.8}. xi(s) for volume-limited subsamples shows evidence of luminosity segregation only for the most luminous sample with M_{b_J}\le -20.5. When redshift-space distortions are removed through projection of xi(rp,pi), however, a weak dependence on luminosity is seen at small separations also at fainter magnitudes. This effect is masked in redshift space, as the mean pairwise velocity dispersion experiences a parallel increase, basically erasing the effect of the clustering growth on xi(s).

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arXiv1998

The ESO Slice Project (ESP) galaxy redshift survey: VI Groups of Galaxies

In this paper we identify objectively and analyze groups of galaxies in the recently completed ESP survey. We find 231 groups above the number overdensity threshold delta(rho)/rho=80 in the redshift range 5000<cz <60000 km/s. These groups contain 1250 members, 40.5% of the 3085 ESP galaxies within the same redshift range. The median velocity dispersion (corrected for measurement errors and computed at the redshift of the group) is sigma_{ESP,median} = 194 km/s. We show that our result is reliable in spite of the particular geometry of the ESP survey (two rows of tangent circular fields of radius 15 arcmin), which causes most systems to be only partially surveyed. In general, we find that the properties of ESP groups are consistent with those of groups in shallower (and wider) catalogs (e.g. CfA2N and SSRS2). As in shallower catalogs, ESP groups trace very well the geometry of the large scale structure. Our results are of particular interest because the depth of the ESP survey allows us to sample group properties over a large number of structures. We also compare luminosity function and spectral properties of galaxies that are members of groups with those of isolated galaxies. We find that galaxies in groups have a brighter M* with respect to non--member galaxies; the slope αis the same, within the errors, in the two cases. We find that 34% of ESP galaxies with detectable emission lines are members of groups. The fraction of galaxies without detectable emission lines in groups is significantly higher: 45%. More generally, we find a gradual decrease of the fraction of emission line galaxies among members of systems of increasing richness. This result confirms that the morphology-density relation found for clusters also extends toward systems of lower density.

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