Digital Sky Survey Research Articles

A radio and mid-infrared survey of northern bright-rimmed clouds

We have carried out an archival radio, optical and infrared wavelength imaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA Sky Survey (NVSS) archive, images from the Digitised Sky Survey (DSS) and the Midcourse Space eXperiment (MSX). The data characterise the physical properties of the Ionised Boundary Layer (IBL) of the BRCs. We have classified the radio detections as: that associated with the ionised cloud rims; that associated with possible embedded Young Stellar Objects (YSOs); and that unlikely to be associated with the clouds at all. The stars responsible for ionising each cloud are identified and a comparison of the expected ionising flux to that measured at the cloud rims is presented. A total of 25 clouds display 20 cm radio continuum emission that is associated with their bright optical rims. The ionising photon flux illuminating these clouds, the ionised gas pressure and the electron density of the IBL are determined. We derive internal molecular pressures for 9 clouds using molecular line data from the literature and compare these pressures to the IBL pressures to determine the pressure balance of the clouds. We find three clouds in which the pressure exerted by their IBLs is much greater than that measured in the internal molecular material. A comparison of external pressures around the remaining clouds to a global mean internal pressure shows that the majority of clouds can be expected to be in pressure equilibrium with their IBLs and hence are likely to be currently shocked by photoionisation shocks. We identify one source which shows 20 cm emission consistent with that of an embedded high-mass YSO and confirm its association with a known infrared stellar cluster. This embedded cluster is shown to contain early-type B stars, implying that at least some BRCs are intimately involved in intermediate to high mass star formation.

Discovery of three optical open clusters in the Galaxy

We report the discovery of three optical open clusters in the Milky Way. Two clusters are in Scutum (Cluster 1 at � = 18.44 ◦ and b = −0.42 ◦ , and Cluster 2 at � = 19.60 ◦ and b = −1.02 ◦ ), thus projected not far from the Galactic center direction, and the other is in Canis Major (Cluster 3 at � = 235.61 ◦ and b = −4.10 ◦ ), near the anti-center direction. Cluster 3 is less populous than Clusters 1 and 2, but presents evidence of being a physical system. The objects were found optically by inspecting maps obtained from the Guide Star Catalogue and images from the Digitized Sky Survey. No previous identification of a cluster has been reported in any of these areas so far. The analysis was carried out with 2MASS photometry in J and H. For Cluster 1 we derive an age of t = 25 ± 5 Myr, a reddening E(B − V) = 2.18 ± 0.03 and a distance from the Sun d� = 1.64 ± 0.19 kpc; for Cluster 2, t = 500 ± 100 Myr, E(B − V) = 0.91 ± 0.03 and d� = 2.19 ± 0.21 kpc; finally for Cluster 3, t = 32−100 Myr, E(B − V) = 0.94 ± 0.03 and d� = 3.93 ± 0.35 kpc. Luminosity and mass functions are derived for Clusters 1 and 2 which, in turn, allow us to estimate their observed masses as 147 Mand 89 M� , respectively. Estimated total masses, by extrapolating the mass functions to 0.08 M� , amount to 382 Mand 614 Mfor the two clusters. Cluster 3 has an observed mass of ∼55 M� . The present results indicate that further searches in the optical might still reveal new open clusters, especially in the infrared bands.

Powerful, obscured active galactic nuclei among X-ray hard, optically dim serendipitousChandrasources

We present a small sample of Chandra X-ray sources selected from the fields of ACIS observations which probe fluxes around the break in the hard band source counts. The targets of these fields include nine nearby galaxy clusters, one distant cluster and two powerful radio galaxy fields. The follow-up of this serendipitous sample was biased towards X-ray hard and optically dim sources mostly not seen on the Digitized Sky Survey; for these, we present X-ray fluxes, optical and near-infrared photometry leading to 51 photometric redshifts in all and 18 independently measured spectroscopic redshifts. Few sources are associated with the target fields themselves. 56 of 58 sources imaged in the K-band are detected at K 20 with K median = 18, and of these, 38 have hard X-ray count ratios and 24 of these are significantly hard with most of the counts emerging about 2 keV. We find that almost all are active galactic nuclei (AGN) hosted in massive early-type host galaxies with a photometric redshift distribution peaking at z ∼ 1. Two type 2 quasars with intrinsic X-ray luminosity L 10 45 erg s −1 ,F e Kα emission lines and absorbing column density N H > 10 23 cm −2 ‐ and N H 10 24 cm −2 in one case ‐ are discussed in detail; the sample contains at least 12 potential type 2 quasars in all. We discuss various detection strategies for type 2 quasars and calculate their inferred space density. This combines and extends a number of results from subsamples already published by us.

Tidal Remnants and Intergalactic H II Regions

We report the discovery of two small intergalactic H II regions in the loose group of galaxies around the field elliptical NGC 1490. The H II regions are located at least 100 kpc from any optical galaxy but are associated with a number of large H I clouds that are lying along an arc 500 kpc in length and that have no optical counterpart on the Digital Sky Survey. The sum of the H I masses of the clouds is almost 1010M⊙ and the largest H I cloud is about 100 kpc in size. Deep optical imaging reveals a very low surface brightness counterpart to this largest H I cloud, making this one of the H I richest optical galaxies known (MHI/LV ~ 200). Spectroscopy of the H II regions indicates that the abundance in these H II regions is only slightly sub-solar, excluding a primordial origin of the H I clouds. The H I clouds are perhaps remnants resulting from the tidal disruption of a reasonably sized galaxy, probably quite some time ago, by the loose group to which NGC 1490 belongs. Alternatively, they are remnants of the merger that created the field elliptical NGC 1490. The isolated H II regions show that star formation on a very small scale can occur in intergalactic space in gas drawn from galaxies by tidal interactions. Many such intergalactic small star formation regions may exist near tidally interacting galaxies.

Searching for unknown open clusters in the Tycho-2 catalog

We present 11 new open cluster candidates found in a systematic search for unknown star clusters using the astrometric and photometric data included in the Tycho 2 catalog. The possible existence of these stellar aggregates is supported by the analysis of proper motions, color–magnitude diagrams, stellar density distributions, and by the visual inspection of the Digitized Sky Survey (DSS) plates. With these tools we were able to determine mean absolute proper motions as well as preliminary reddenings, distances and ages for the majority of the candidates. We found that most of them are possibly nearby (closer than ~600 pc) open clusters never studied before.

TheIRASRevised Bright Galaxy Sample

IRAS flux densities, redshifts, and infrared luminosities are reported for all sources identified in the IRAS Revised Bright Galaxy Sample (RBGS), a complete flux-limited survey of all extragalactic objects with total 60 micron flux density greater than 5.24 Jy, covering the entire sky surveyed by IRAS at Galactic latitude |b| > 5 degrees. The RBGS includes 629 objects, with a median (mean) sample redshift of 0.0082 (0.0126) and a maximum redshift of 0.0876. The RBGS supersedes the previous two-part IRAS Bright Galaxy Samples, which were compiled before the final ("Pass 3") calibration of the IRAS Level 1 Archive in May 1990. The RBGS also makes use of more accurate and consistent automated methods to measure the flux of objects with extended emission. Basic properties of the RBGS sources are summarized, including estimated total infrared luminosities, as well as updates to cross-identifications with sources from optical galaxy catalogs established using the NASA/IPAC Extragalactic Database (NED). In addition, an atlas of images from the Digitized Sky Survey with overlays of the IRAS position uncertainty ellipse and annotated scale bars is provided for ease in visualizing the optical morphology in context with the angular and metric size of each object. The revised bolometric infrared luminosity function, phi(L_ir), for infrared bright galaxies in the local Universe remains best fit by a double power law, phi(L_ir) ~ L_ir^alpha, with alpha = -0.6 (+/- 0.1), and alpha = -2.2 (+/- 0.1) below and above the "characteristic" infrared luminosity L_ir ~ 10^{10.5} L_solar, respectively. (Abridged)

New High Proper Motion Stars from the Digitized Sky Survey. II. Northern Stars with 0."5 yr-1< < 2."0 yr-1at High Galactic Latitudes

In a continuation of our systematic search for high proper motion stars in the Digitized Sky Survey, we have completed the analysis of northern sky fields at Galactic latitudes above 25°. With the help of our SUPERBLINK software, a powerful automated blink comparator developed by us, we have identified 1146 stars in the magnitude range 8 < r < 20 with proper motions 0500 yr-1 < μ < 2000 yr-1. These include 1080 stars previously listed in Luyten's proper-motion catalogs (the Luyten Half-Second catalog and the New Luyten Two-Tenths catalog), nine stars not previously listed in the Luyten catalogs but reported elsewhere in the literature (including one previously reported by our team), and 57 new objects reported here for the first time. This paper includes a list of positions, proper motions, magnitudes, and finder charts for all the new high proper motion stars. Combined with our previous study of low Galactic latitude fields (see Paper I), our survey now covers over 98% of the northern sky. We conclude that the Luyten catalogs were 90% complete in the northern sky for stars with 05 < μ < 20 down to magnitude r = 19. We discuss the incompleteness of the old Luyten proper-motion survey and estimate completeness limits for our new survey.

Comment on “Detection of small comets with a ground‐based telescope” by L. A. Frank and J. B. Sigwarth

Comment on “Detection of small comets with a ground‐based telescope” by L. A. Frank and J. B. Sigwarth

A study of 11 newly discovered and 11 poorly known open clusters in the solar vicinity

We present the first determination of fundamental parameters (positions, reddenings, distances, ages and kinematics) for a sample of 22 open clusters in the solar vicinity. A systematic search for unknown open clusters using the astrometric and photometric data included in the Tycho2 catalogue has resulted in the identification of several candidates. In this work, we introduce 11 of the most promising open cluster candidates. The reality of the stellar aggregates is supported by the analysis of proper motions, colour-magnitude diagrams, stellar density distributions, and by the visual inspection of the Digitised Sky Survey (DSS) plates. The proper motion data included in the Tycho2 catalogue has also allowed us to identify members in 11 open clusters for which no information, besides their positions and estimates of their apparent diameters, was previously available. Reddenings, distances and ages were determined for most objects using colour-magnitude diagrams for cluster members, built using the photometry included in Tycho2, complemented by spectral types found in the literature. We find that the majority of the aggregates are nearby (closer than ~500 pc) open clusters.

Extending the limits of globule detection

Small isolated clouds were discovered by the ISOPHOT 170 μm Serendipity Survey (ISOSS) as faint MJy sr-1 FIR sources. One of them is ISOSS J20246+6540, a cold ( K) pointlike ISOSS source without an IRAS pointsource counterpart. In the Digitized Sky Survey B band it is seen as a 3´size bipolar nebulosity with an average excess DSS blue band surface brightness of 8% of the background's photographical density. The CO column density distribution determined by multi-isotopic, multi-level CO measurements with the IRAM-30 m telescope agrees well with the optical appearance. An average hydrogen column density of ≈ 1021 cm-2 was derived from both the FIR and CO data. Using a kinematic distance estimate of 400 pc the NLTE modelling of the CO, HCO+, and CS measurements gives a peak density of ≈ 104 cm-3. The multiwavelength data characterise ISOSS 20246+6540 as a representative of a class of globules which has not been discovered so far due to their small angular size and low 100 μm brightness. A significant overabundance of 13CO is found COC18O. This is likely due to isotope selective chemical processes.

The 1000 Brightest HIPASS Galaxies: Newly Cataloged Galaxies

The H I Parkes All-Sky Survey (HIPASS) is a blind 21 cm survey for extragalactic neutral hydrogen, covering the whole southern sky. The HIPASS Bright Galaxy Catalog (BGC) is a subset of HIPASS and contains the 1000 H I-brightest (peak flux density) galaxies. Here we present the 138 HIPASS BGC galaxies that had no redshift measured prior to the Parkes multibeam H I surveys. Of the 138 galaxies, 87 are newly cataloged. Newly cataloged is defined as having no optical (or infrared) counterpart in the NASA/IPAC Extragalactic Database. Using the Digitized Sky Survey, we identify optical counterparts for almost half of the newly cataloged galaxies, which are typically of irregular or Magellanic morphological type. Several H I sources appear to be associated with compact groups or pairs of galaxies rather than an individual galaxy. The majority (57) of the newly cataloged galaxies lie within 10° of the Galactic plane and are missing from optical surveys as a result of confusion with stars or dust extinction. This sample also includes newly cataloged galaxies first discovered by Henning et al. in the H I shallow survey of the zone of avoidance. The other 30 newly cataloged galaxies escaped detection because of their low surface brightness or optical compactness. Only one of these, HIPASS J0546-68, has no obvious optical counterpart, as it is obscured by the Large Magellanic Cloud. We find that the newly cataloged galaxies with |b| > 10° are generally lower in H I mass and narrower in velocity width compared with the total HIPASS BGC. In contrast, newly cataloged galaxies behind the Milky Way are found to be statistically similar to the entire HIPASS BGC. In addition to these galaxies, the HIPASS BGC contains four previously unknown H I clouds.

New High Proper Motion Stars from the Digitized Sky Survey. I. Northern Stars with 0[farcs]5 [CLC]yr[/CLC][TSUP]−1[/TSUP] &lt; μ &lt; 2[farcs]0 [CLC]yr[/CLC][TSUP]−1[/TSUP] at Low Galactic Latitudes

We have conducted a systematic search for high proper motion stars in the Digitized Sky Survey, in the area of the sky north of -2.8 degrees in declination and within 25 degrees of the galactic plane. Using the SUPERBLINK software, a powerful automated blink comparator developed by us, we have identified 601 stars in the magnitude range 9<r<20 with proper motions in the range 0.5 to 2.0 arcsec/yr which have not been observed with the HIPPARCOS satellite. Among those, we recovered 460 stars previously listed in Luyten's proper motion catalogs (LHS, NLTT), and discovered 141 new high proper motion stars, ranging in magnitude from R=13.0 to R=19.8. Only 9 stars from the Luyten catalogs that were not observed by HIPPARCOS could not be recovered with SUPERBLINK: 3 had proper motions larger than the search limit of the code (>2.0\arcsec/yr), and 5 were missed because they were either too bright for SUPERBLINK to handle or they are in the immediate proximity of very bright stars. Only one of Luyten's stars (LHS1657) could not be recovered at all, even by visual inspection of the POSS plates, and is now suspected to be bogus. The very high success rate in the recovery by SUPERBLINK of faint Luyten stars suggests that our new survey of high proper motion stars is at least 99 percent complete for stars with proper motions 0.5 to 2.0 arcsec/yr down to R=19. This paper includes a list of positions, proper motions, magnitudes, and finder charts for all the new high proper motion stars.

New Distant Companions to Known Nearby Stars. I. GJ 4047B, GJ 718B, GJ 747.2C, GJ 4100B, and GJ 4153B

In an ongoing survey for high proper motion stars at low Galactic latitudes (-25° < b < 25°) in the Digitized Sky Survey, we have identified five previously uncataloged common proper motion companions to stars listed in the Preliminary Version of the Third Catalogue of Nearby Stars (GJ stars). For each system, the relative proper motion between the components is less than 5% of the common proper motion of the pair. Spectra of the pairs have been obtained at the Lick 3 m Shane Telescope, confirming that the systems are indeed wide-separation binaries. The systems are classified as follows: GJ 4047AB = K5 V + M5 V, GJ 718AB = K5 V + M4.5 V, GJ 747.2ABC = (K7 V + K7 V) + M4 Ve, GJ 4100AB = M1 V + M4.5 Ve, and GJ 4153AB = M0.5 V + M3.5 V. The total area surveyed contains 346 Gliese stars, which suggests that ≈1.5% of the stars listed in the Preliminary Version of the Third Catalogue of Nearby Stars have unrecognized proper motion companions. We predict that ≈15–50 more new distant companions to GJ stars will be discovered in the Digitized Sky Survey.

A Blind H [CSC]i[/CSC] Survey of the M81 Group

Results are presented of the first blind H I survey of the M81 Group of galaxies. The data were taken as part of the H I Jodrell All-Sky Survey (HIJASS). The survey reveals several new aspects to the complex morphology of the H I distribution in the group. All four of the known dwarf irregular (dIrr) galaxies close to M81 can be unambiguously seen in the HIJASS data. Each forms part of the complex tidal structure in the area. We suggest that at least three of these galaxies may have formed recently from the tidal debris in which they are embedded. The structure connecting M81 to NGC 2976 is revealed as a single tidal bridge of mass 2.1 × 108 M☉ and projected spatial extent 80 kpc. Two spurs of H I projecting from the M81 complex to lower declinations are traced over a considerably larger spatial and velocity extent than by previous surveys. The dwarf elliptical (dE) galaxies BK5N and Kar 64 lie at the spatial extremity of one of these features and appear to be associated with it. We suggest that these may be the remnants of dIrr's that have been stripped of gas and transmuted into dE's by close gravitational encounters with NGC 3077. The nucleated dE galaxy Kar 61 is unambiguously detected in H I for the first time and has an H I mass of ~108 M☉, further confirming it as a dE/dIrr transitional object. HIJASS has revealed one new possible group member, HIJASS J1021+6842. This object contains 2 × 107 M☉ of H I and lies 105' from IC 2574. It has no optical counterpart on the Digital Sky Survey.

New Wolf-Rayet central stars of planetary nebulae identified on the AAO/UKST HαSurvey

We have carried out an archival radio, optical and infrared wavelength imaging survey of 44 Bright-Rimmed Clouds (BRCs) using the NRAO/VLA Sky Survey (NVSS) archive, images from the Digitised Sky Survey (DSS) and the Midcourse Space eXperiment (MSX). The data characterise the physical properties of the Ionised Boundary Layer (IBL) of the BRCs. A total of 25 clouds display 20 cm radio continuum emission that is associated with their bright optical rims. The ionising photon flux illuminating these clouds, the ionised gas pressure and the electron density of the IBL are determined. We derive internal molecular pressures for 9 clouds using molecular line data from the literature and compare these pressures to the IBL pressures to determine the pressure balance of the clouds. We find three clouds in which the pressure exerted by their IBLs is much greater than that measured in the internal molecular material. A comparison of external pressures around the remaining clouds to a global mean internal pressure shows that the majority of clouds can be expected to be in pressure equilibrium with their IBLs and hence are likely to be currently shocked by photoionisation shocks. We identify one source which shows 20 cm emission consistent with that of an embedded high-mass YSO and confirm its association with a known infrared stellar cluster. This embedded cluster is shown to contain early-type B stars, implying that at least some BRCs are intimately involved in intermediate to high mass star formation.

Soft X-ray properties of a spectroscopically selected sample of interacting and isolated Seyfert galaxies

We present a catalogue of ROSAT detected sources in the sample of spectroscopically selected Seyfert 1 and Seyfert 2 galaxies of Rafanelli et al. (1995). The catalogue contains 102 Seyfert 1 and 36 Seyfert 2 galaxies. The identification is based on X-ray contour maps overlaid on optical images taken from the Digitized Sky Survey. We have derived the basic spectral and timing properties of the X-ray detected Seyfert galaxies. For Seyfert 1 galaxies a strong correlation between photon index and X-ray luminosity is detected. We confirm the presence of generally steeper X-ray continua in narrow-line Seyfert 1 galaxies (NLS1s) compared to broad-line Seyfert 1 galaxies. Seyfert 2 galaxies show photon indices similar to those of NLS1s. Whereas a tendency for an increasing X-ray luminosity with increasing interaction strength is found for Seyfert 1 galaxies, such a correlation is not found for Seyfert 2 galaxies. For Seyfert 1 galaxies we found also a strong correlation for increasing far-infrared luminosity with increasing interaction strength. Both NLS1s and Seyfert 2 galaxies show the highest values of far-infrared luminosity compared to Seyfert 1 galaxies, suggesting that NLS1s and Seyfert 2 galaxies host strong (cirumnuclear) starformation. For variable Seyfert galaxies we present the X-ray light curves obtained from the ROSAT All-Sky Survey and from ROSAT PSPC and HRI pointed observations. Besides the expected strong short- and long-term X-ray variability in Seyfert 1 galaxies, we find indications for X-ray flux variations in Seyfert 2 galaxies.

Host Galaxies and Circumgalactic Environment of “Narrow Line” Seyfert 1 Nuclei

This paper presents a statistical study of the host galaxies and environment for a sample of 27 narrow line Seyfert 1 (NLSy1) nuclei, based on a computer-aided search of companions in the Digitized Sky Survey. The sample includes all objects listed as NLSy1 in the Veron-Cetty & Veron catalog (8th edition) with redshift in the range 0.010 ≤ z ≤ 0.061 and Galactic latitude |b| ≥ 30°. We compared the environments of NLSy1's with those of Seyfert 1 and Seyfert 2 galaxies using the method and the Seyfert samples employed in 1999 by Dultzin-Hacyan et al. The frequency of physical companions was measured within a search radius equal to 3 times the diameter DS of each NLSy1 and up to a projected linear distance of ≈100 kpc. No statistical difference was found between the environments of NLSy1's and those of Seyfert 1 galaxies. Our results further suggest that NLSy1 hosts may show fewer companions and may be preferentially farther away from bright companion galaxies than normal galaxies with similar morphological type, diameter, and redshift distribution. The results for host galaxies and environments help to disprove the hypothesis that NLSy1 objects have characteristics similar to those of Seyfert 2 galaxies: whereas Seyfert 2 galaxies show a highly significant excess of bright companions, Seyfert 1's and NLSy1's do not. The morphological distribution of NLSy1's and Seyfert 1 objects is similar, but NLSy1 nuclei are hosted in galaxies smaller than Seyfert 1 and nonactive galaxies, to a statistical confidence level of 95%. These results support the interpretation of NLSy1 nuclei (suggested by X-ray observations) as Seyfert 1 nuclei with higher Eddington ratio, since smaller diameter galaxies may host central black holes of lower mass.

Accurate optical positions for 2978 objects from the Second Byurakan Survey (SBS) with the Digitized Sky Survey

Optical positions of 2978 objects listed in the Second Byurakan Survey (SBS) were obtained using the Digitized Sky Survey (DSS), and are given with an rms uncertainty arcsec in each coordinate.

An image database

This paper presents a method for extracting a catalogue of galaxy candidates from the Digitized Sky Survey (DSS). The method is based on a functional analysis applied on each individual plate. The standard deviation of pixel optical densities versus the inverse of surface area leads to a diagram in which extended and star-like objects are well separated. This diagram is used for a preliminary recognition. Then, a filtering process is applied using a Neural Network method associated with a training sample built with well identified objects. The main catalogue gives coordinates, total magnitude, isophotal diameter, axis ratio, position angle for galaxy candidates. The method favors the detection of normal galaxies. This creates a bias against compact high surface brightness galaxies.

Box- and peanut-shaped bulges

We present a classification for bulges of a complete sample of ~ 1350 edge-on disk galaxies derived from the RC3 ( Third Reference Catalogue of Bright Galaxies , de Vaucouleurs et al. 1991). A visual classification of the bulges using the Digitized Sky Survey (DSS) in three types of b/p bulges or as an elliptical type is presented and supported by CCD images. NIR observations reveal that dust extinction does almost not influence the shape of bulges. There is no substantial difference between the shape of bulges in the optical and in the NIR. Our analysis reveals that 45% of all bulges are box- and peanut-shaped (b/p). The frequency of b/p bulges for all morphological types from S0 to Sd is > 40%. In particular, this is for the first time that such a large frequency of b/p bulges is reported for galaxies as late as Sd. The fraction of the observed b/p bulges is large enough to explain the b/p bulges by bars.