The Luminosity Function and Size Distribution of HII Regions in NGC 1365

We present the HII region luminosity function and size distribution obtained for the HII regions in NGC 1365. The diameters were corrected for the effects of seeing using a Wiener image restoration procedure. The luminosities and sizes were calculated using the Cepheid distance of 18±2 Mpc, to NGC 1365. We find good correlation between the H_α luminosities and other properties, such as absolute magnitudes and sizes of the HII regions within this galaxy. We find that the HII region luminosity function can be fitted by a power law of slope α=–2.29$plusmn;0.11, which is consistent with what is expected for its Hubble type. Also, the size distribution of the HII regions can be represented by an exponential form. This revised version was published online in July 2006 with corrections to the Cover Date.     

Strict Limits on the Ionizing Luminosity in NGC 1068 from Jet-Axis Molecular Clouds

The radio jet axis of NGC 1068 is characterised by energetic activity from x-ray to radio wavelengths. Detailed kinematic and polarization studies have shown that this activity is confined to bipolar cones centered on the AGN which intersect the plane of the disk. Thus, molecular clouds at 1 kpc distance along this axis are an important probe of the nuclear ionizing luminosity and spectrum. Extended 10.8μm emission coincident with the clouds is reasonably understood by dust heated to high temperatures by the nuclear radiation field. This model predicts that the nuclear spectrum is quasar-like (power law + blue excess) with a luminosity 2-5 times higher than inferred by Pier et al. (1994). Consequently, there is little or no polyaromatic hydrocarbon (PAH) emission associated with the radio-axis molecular clouds. We review this model in the light of new observations. A multi-waveband collage is included to illustrate the possible orientations of the double cones to our line of sight and the galaxian plane. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations and modelling of the interacting galaxies NGC 3395 and 3396

We present both observations and modelling of the atomic hydrogen in the closely interacting galaxies NGC 3395 and 3396. The observations were made with the VLA in both C- and D-arrays. We detect a large 'tail' of H  i extending to a projected distance of 63 kpc (10 arcmin) south-west of the pair, as well as two smaller galaxies, IC 2604, 14 arcmin to the south-west, and IC 2608, 14 arcmin to the south-east. However, these galaxies appear to have had at most a minor influence on the dynamics of NGC 3395/6. By means of N -body simulation we show that the tail is gas that has been stripped from NGC 3395 during a prograde encounter with NGC 3396, and that the pair has had one previous close approach. It is shown that the galaxies are within 5 × 10⁷ yr of their second perigalactic passage. Comparison of the time-scales for starburst activity with those from the simulations shows that the current starbursts are a result of the current close approach and not the first one. The interaction between NGC 3395 and 3396 has flattened the rotation curve of NGC 3396 owing to the parameters of the interaction. This naturally explains the more nucleated radio continuum structure observed in this galaxy, as significant infall and a subsequent central starburst would be expected in this scenario. The velocity structure and line profiles of the H  i are best explained if both cloud–cloud collisions between the two gas discs and tidal forces have been important.     

The Correlation between Optical Spectral Index and Continuum Luminosity Variation in the Seyfert 1 Galaxy NGC 5548

Using the archived optical spectra of NGC 5548 between 1989 and 2001, we derived the optical spectral index by fitting the spectra in wavelength windows unaffected by strong emission lines. We found that the index is anti-correlated with the continuum luminosity at 5100 A with a correlation coefficient of -0.8. Based on the standard thin accretion disk model, we investigated whether the correlation is related to the variations of the dimensionless accretion rate m (mass accretion rate in Eddington unit), or the inner radius of the accretion disk Rin, or both. The correlation can be modeled well using a co-variable mode of Rin/Rs = 12.5m-0.8 (Rs is Schwarzschild radius). As luminosity increases, m increases from 0.05 to 0.16 and at the same time Rin decreases from 133.9.RS to 55.5.RS, consistent with the prediction for a transition radius within which an ADAF structure exists. We concluded that the change of both inner accretion radius and the dimensionless accretion rate are key factors for the variation     

Stellar content of the interacting galaxies IC 1727 and NGC 672

Based on archival Hubble Space Telescope ACS/WFC images, we have performed stellar photometry for an isolated pair of interacting galaxies, IC 1727 and NGC 672, and constructed their Hertzsprung-Russell diagrams. The galaxy IC 1727 exhibits a strong asymmetry in the apparent distribution of young stars. The distribution of old stars is smoother and less asymmetric. In the galaxy NGC672, there is no noticeable asymmetry in the distribution of stars of different ages. Based on the TRGB method, we have determined an accurate distance to each galaxy for the first time: D = 7.14 ± 0.10 Mpc for IC 1727 and D = 7.22 ± 0.10 Mpc for NGC 672, confirming that these galaxies are closely spaced. The luminosity functions of red supergiants and peripheral AGB stars in both galaxies have positionally coincident local maxima, suggesting the simultaneous enhancement of star formation in the two galaxies occurred in the intervals 20–30 and 450–700 Myr ago. The results obtained point to an enhancement of star formation processes in the interacting galaxies.     

Neutral hydrogen gas in interacting galaxies: the NGC 1511 galaxy group

We present H  i line and 20-cm radio continuum observations of the NGC 1511 galaxy group obtained with the Australia Telescope Compact Array. The data reveal an extended, rather disturbed H  i distribution for the peculiar starburst galaxy NGC 1511 and a narrow bridge to its small companion galaxy, NGC 1511B, which has been severely distorted by the interaction/collision between the two galaxies. No stellar counterpart to the gaseous bridge has been detected. In addition, we find that the peculiar optical ridge to the east of NGC 1511 is probably the stellar remnant of a galaxy completely disrupted by interactions with NGC 1511. The slightly more distant neighbour, NGC 1511A, shows a regular H  i velocity field and no obvious signs of interactions.
Radio continuum emission from NGC 1511 reveals three prominent sources on top of a more diffuse, extended distribution. We derive an overall star formation rate of  7 M_⊙ yr⁻¹  . The most enhanced star formation is found in the south-eastern part of the disc, coincident with several bright H  ii regions, and closest to the peculiar optical ridge. No continuum emission was detected in the companions, but NGC 1511B appears to show an H  ii region at its faint western edge, closest to NGC 1511. The group displays a prime example of interaction-induced star formation activity.     

Mass distribution in the open cluster NGC 1664

We start with the hypotheses that the cluster space is limited by an ellipsoid with a semiprincipal axis directed to the observer and that the star density inside of concentric ellipsodal layers remains constant, although varying from layer to layer. It results that the mean star mass at the periphery of the cluster is 0.59 Sun masses smaller than that at the center.     

Companions and interactions in the NGC 6946 system

We report the results of a survey of the H  i environment of NGC 6946 in a search for gas-rich companions to the galaxy. Such gas-rich companions could include material left over from the galaxy assembly process which could persist into the current day around an isolated galaxy such as NGC 6946. NGC 6946 is prolifically forming stars, has a nuclear starburst, and has widespread high-velocity clouds associated with the disc. All of these features could be explained by the accretion of low-mass H  i clouds by NGC 6946. Our survey recovered two previously detected dwarf galaxies associated with NGC 6946, but otherwise found no signatures of interactions in the NGC 6946 system. The companions are small enough, and distant enough from NGC 6946 that they should have minimal effect on the main galaxy. Some tidal debris may be expected due to interaction between the two dwarf galaxies, but none is observed. This could be because it is at low column densities, or because the dwarf galaxies are more separated than they appear on the sky. This study of the system suggests that NGC 6946 is a gravitationally bound system with two dwarf galaxies in stable orbits about the larger primary galaxy.     

Nebula NGC7027 - as an evolved symbiotic system?

In the frame of the Starburst model, we show that the stellar processes expected to occur during the evolution of a metal rich massive stellar cluster can reproduce the observed optical light curves of the Seyfert 1 galaxies NGC 4151 and NGC 5548.     

Neutral hydrogen gas distribution and kinematics in NGC 7479

We have investigated the effects of a bar and an asymmetric spiral structure on the neutral hydrogen distribution and kinematics in the strongly barred spiral galaxy NGC 7479. The strongest 21-cm line emission at 1-kpc resolution comes from the western spiral arm which appears to be slightly inclined to the plane of the main disc. In contrast, the area within the radius of the bar is devoid of 21-cm line emission. The radio continuum emission at 21 cm follows the bar dust lanes, but beyond 3 kpc from the nucleus the radio continuum emission has a peculiar morphology, unlike that of optical and near-infrared images. We did not detect any low surface brightness gas-rich companions in the near neighbourhood of NGC 7479. This leads us to propose that the strong western spiral arm was created in a recent minor merger.     

Molecular gas distribution and dynamics in the luminous merger NGC 6240

We present the results of our subarcsecond resolution interferometricobservations of the 1.3 mm CO J = 21 line in the luminous merger NGC6240. Roughly half of the CO flux is contained in a rotating and highlyturbulent thick disk centered between the two radio and near-infrarednuclei. In this disk the molecular gas has velocity widths which reachFWZP line widths of up to 1000 km s^-1. The mass of this gasconcentration makes up between 30%–70% of the dynamical mass in thisregion. NGC 6240 may be in an earlier merging stage than typical ULIRGssuch as Arp 220. We compare these results from NGC 6240 with thoseof other luminous, gas-rich interacting galaxies and mergers.     

Nearby galaxies. II. Luminosity function and spatial distribution

A sample of nearby galaxies (Schmidt and Boller 1992, paper I) has been analysed with regard to the luminosity function and the spatial distribution. The main results are as follows: (i) a relation between the slope of the luminosity function of the members of a group of galaxies and the earliest morphological type in this group has been detected which possibly is a new kind of environmental effects; (ii) the virial masses of the groups of galaxies are, on the average, by a factor 3 to 4 greater, only, than the luminous masses derived from the individual galaxy masses; (iii) the nearby galaxies are concentrated in a thin disk-like layer around the supergalactic plane the thickness of which is a few hundred kiloparsecs, only; (iv) a population of field galaxies with nearly constant density (about 3 per cent of the number density in the supergalactic plane) is extended into the voids on both the supergalactic hemispheres.     

The distribution of interstellar matter in NGC 654

The young cluster NGC 654 is studied using UBV photographic photometry with a view to determining the distribution of interstellar matter in a region where star formation recently occurred.NGC 654 is found to be enclosed in a shell of interstellar matter of mass 1500M . The mass of all stars in the cluster is 4000M .     

Physical conditions in NGC 6164-5

From spectroscopic observations of the emission lines H, 4959 and 5007[Oiii], H, 6584[Nii], 6717 and 6731[Sii] and some interferometric data in H and [Nii] it was established that NGC 6164 and NGC 6165 have characteristics of normal Hii regions for Ne and Te. They are embedded in a very tenuous medium nearly coincident with the Strömgren sphere of the central star HD 148937, corresponding to the outer peripheral structure described by Westerlund. Variations of the relative abundances of N, O, and S are discussed.     

The distribution of Main-Sequence stars in the open cluster NGC 7789

By star counts on four plates with different limiting magnitudes, it was found that the relative frequency of Main-Sequence stars increases from the centre of the cluster to the boundary. It is shown that the stars of NGC 7789 are separated according to their masses.