Search

We have undertaken high-resolution follow-up of a sample of high velocity $\ion{H}{i}$clouds apparently associated with M 31. Our sample was chosen from the population of high-velocity clouds (HVCs) detected out to 50 kpc projected radius of the Andromeda Galaxy by Thilker et al. (2004, ApJ, 601, L39) with the Green Bank Telescope. Nine pointings were observed with the Westerbork Synthesis Radio Telescope to determine the physical parameters of these objects and to find clues to their origin. One additional pointing was directed at a similar object near M 33.
At $2'$resolution we detect 16 individual HVCs around M 31and 1 HVC near M 33with typical $\ion{H}{i}$masses of a few times $10^5 \, M_{\odot}$and sizes of the order of 1 kpc. Estimates of the dynamical and virial masses of some of the HVCs indicate that they are likely gravitationally dominated by additional mass components such as dark matter or ionised gas. Twelve of the clouds are concentrated in an area of only $1^{\circ} \times 1^{\circ}$at a projected separation of less than 15 kpc from the disk of M 31. This HVC complex has a rather complicated morphological and kinematical structure and partly overlaps with the giant stellar stream of M 31, suggesting a tidal origin. Another detected feature is in close proximity, in both position and velocity, with NGC 205, perhaps also indicative of tidal processes. Other HVCs in our survey are isolated and might represent primordial, dark-matter dominated clouds.

We have used the Westerbork array to carry out an unbiased wide-field survey of H iemission features, achieving an rms sensitivity of about 18 mJy/Beam at a velocity resolution of 17 km s-1over 1800 deg2and between $-1000

We have used the Westerbork array to carry out an unbiased wide-field survey for H?i emission features, achieving an rmssensitivity of about 18 mJy/Beam at a velocity resolution of 17 km?s-1over 1800 deg2and between $-1000 < V_{\rm Hel} <+6500$km?s-1. The primary data consists of auto-correlation spectra with an effective angular resolution of 49' FWHM, although cross-correlation data were also acquired. The survey region is centered approximately on the position of Messier 31 and is Nyquist-sampled over $60\times 30^\circ$in RA?$\times$?Dec. More than 100 distinct features are detected at high significance in each of the two velocity regimes (negative and positive LGSR velocities). In this paper we present the results for our H?i detections of external galaxies at positive LGSR velocity. We detect 155 external galaxies in excess of 8sin integrated H?i flux density. Plausible optical associations are found within a 30' search radius for all but one of our H?i detections in DSS images, although several are not previously cataloged or do not have published red-shift determinations. Our detection without a DSS association is at low galactic latitude. Twenty-three of our objects are detected in H?i for the first time. We classify almost half of our detections as “confused”, since one or more companions is cataloged within a radius of 30' and a velocity interval of 400 km?s-1. We identify a handful of instances of significant positional offsets exceeding 10 kpc of unconfused optical galaxies with the associated H?i centroid, possibly indicative of severe tidal distortions or uncataloged gas-rich companions. A possible trend is found for an excess of detected H?i flux in unconfused galaxies within our large survey beam relative to that detected previously in smaller telescope beams, both as function of increasing distance and increasing gas mass. This may be an indication for a diffuse gaseous component on 100 kpc scales in the environment of massive galaxies or a population of uncataloged low mass companions. We use our galaxy sample to estimate the H?i mass function from our survey volume. Good agreement is found with the HIPASS BGC results, but only after explicit correction for galaxy density variations with distance.?

Aims. This paper reports on a search for new classical nova candidates in the M?81 galaxy based on archival, as well as recent, new images.Methods. We used images from 1999?2007 to search for optical transients in M?81. The positions of the identified classical nova candidates were used to study their spatial distribution. Kolmogorov-Smirnov test (KS) and bottom-to-top (BTR) ratio diagnostic were used to analyze the nova candidate distribution and differentiate between the disk and the bulge populations.Results. In total, 49?classical nova candidates were discovered. In this study, we present the precise positions and photometry of these objects, plus the photometry of an additional 9?classical nova candidates found by Neill & Shara (2004, AJ, 127, 816). With our large sample, we find a different spatial distribution of classical nova candidates when compared to the results of earlier studies. Also, an extraordinarily bright nova was found and studied in detail.

We present deep H? and [S II] emission-line images of three galaxies, the Sab spiral M81, the Sc spiral M51, and its companion, NGC 5195, in our ongoing study of the diffuse ionized gas (DIG) in spiral galaxies. M81 and M51 show widespread DIG with the characteristic enhanced [S II]/H? line intensities also seen in other galaxies. A large fraction of the H? emission in these systems, 0.40-0.50, is contributed by DIG, which is in agreement with results obtained for other disk galaxies. In spite of a drastic difference in the star formation rate per unit disk area and the difference in Hubble types, the overall properties of the DIG appear similar in both galaxies. In detail, however, differences do appear. There is some indication that the [S II]/H? ratio is systematically lower in M51 than it is in M81. There is also a radial dependence on the [S II]/H? ratio in M51 that is not seen in M81. The H? emission from the general bulge area of M81, while diffuse in nature, has a distinctly higher [S II]/H? intensity ratio than disk DIG elsewhere in M81. This ionized gas in the bulge is most likely not photoionized by massive stars, the dominant source of ionization for DIG in galactic disks. Overall, our results reinforce the important role of DIG in the ISM of spiral galaxies in general. We detect significant emission from the companion to M51, NGC 5195. We find clumps of H? emission in an 800 × 800 pc region offset from the nucleus and spectacular kiloparsec-long filaments reminiscent of the outflow cones in starburst galaxies. We attribute these structures to the recent passage of NGC 5195 through the disk plane of M51, which led to a burst of star formation some 70 million years ago.

Aims. We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M 31, determined using a recent deep, full-disk 21-cm imaging survey smoothed to 466 pc resolution.Methods. We fit a tilted ring model to the HI data from 8 to 37 kpc and establish conclusively the presence of a dark halo and its density distribution via dynamical analysis of the rotation curve.Results. The disk of M 31 warps from 25 kpc outwards and becomes more inclined with respect to our line of sight. Newtonian dynamics without a dark matter halo provide a very poor fit to the rotation curve. In the framework of modified Newtonian dynamic (MOND) however the 21-cm rotation curve is well fitted by the gravitational potential traced by the baryonic matter density alone. The inclusion of a dark matter halo with a density profile as predicted by hierarchical clustering and structure formation in a ΛCDM cosmology makes the mass model in newtonian dynamic compatible with the rotation curve data. The dark halo concentration parameter for the best fit is C= 12 and its total mass is 1.2 $\times$1012$M_\odot$. If a dark halo model with a constant-density core is considered, the core radius has to be larger than 20 kpc in order for the model to provide a good fit to the data. We extrapolate the best-fit ΛCDM and constant-density core mass models to very large galactocentric radii, comparable to the size of the dark matter halo. A comparison of the predicted mass with the M 31 mass determined at such large radii using other dynamical tracers, confirms the validity of our results. In particular the ΛCDM dark halo model which best fits the 21-cm data well reproduces the mass of M 31 traced out to 560 kpc. Our best estimate for the total mass of M 31 is 1.3 $\times$1012$M_\odot$, with 12% baryonic fraction and only 6% of the baryons in the neutral gas phase.

Aims. We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M 31, determined using a recent deep, full-disk 21-cm imaging survey smoothed to 466 pc resolution.