A QSO survey via optical variability and zero proper motion in the M92 field.III. Narrow emission line galaxies

We study a sample of 23 narrow-emission line galaxies (NELGs) which were selected by their strong variability as QSO candidates in the framework of a variability-and-proper motion QSO survey on digitised Schmidt plates. In previous work, we have shown that variability is an efficient method to find AGNs. The variability properties of the NELGs are however significantly different from those of the QSOs. The main aim of this paper is to clarify the nature of this variability and to estimate the fraction of AGN-dominated NELGs in this sample. New photometric and spectroscopic observations are presented, along with revised data from the photographic photometry. The originally measured high variability indices could not be confirmed. The diagnostic line-ratios of the NELG spectra are consistent with HII region-like spectra. No AGN could be proved, yet we cannot rule out the existence of faint low-luminosity AGNs masked by HII regions from intense star formation.Comment: 10 pages, 8 figures, accepted for publication in Astronomy & Astrophysic

Flat-Cored Dark Matter in Cuspy Clusters of Galaxies

Sand, Treu, & Ellis (2002) have measured the central density profile of cluster MS2137-23 with gravitational lensing and velocity dispersion and removed the stellar contribution with a reasonable M/L. The resulting dark matter distribution within r<50 kpc was fitted by a density cusp of r^{-beta} with beta=0.35. This stands in an apparent contradiction to the CDM prediction of beta~1, and the disagreement worsens if adiabatic compression of the dark matter by the infalling baryons is considered. Following El-Zant, Shlosman & Hoffman (2001), we argue that dynamical friction acting on galaxies moving within the dark matter background counters the effect of adiabatic compression by transfering the orbital energy of galaxies to the dark matter, thus heating up and softening the central density cusp. Using N-body simulations of massive solid clumps moving in clusters we show that indeed the inner dark matter distribution flattens (with beta approx 0.35 for a cluster like MS2137-23) when the galaxies spiral inward. We find as a robust result that while the dark matter distribution becomes core-like, the overall mass distribution preserves its cuspy nature, in agreement with X-ray and lensing observations of clusters.Comment: 7 pages, 3 figures, to be published in Astrophysical Journal Letter

A QSO survey via optical variability and zero proper motion in the M92 field. IV. More QSOs due to improved photometry

We continue the QSO search in the 10 square degrees Schmidt field around M92 based on variability and proper motion (VPM) constraints. We have re-reduced 162 digitised B plates with a time-baseline of more than three decades and have considerably improved both the photometric accuracy and the star-galaxy separation at B>19. QSO candidates are selected and marked with one out of three degrees of priority based on the statistical significance of their measured variability and zero proper motion. Spectroscopic follow-up observations of 84 new candidates with B>19 revealed an additional 37 QSOs and 7 Seyfert1s. In particular, all 92 high-priority candidates are spectroscopically classified now; among them are 70 QSOs and 9 Seyfert1s (success rate 86%). We expect that 87% (55%) of all QSOs with B<19.0 (19.8) are contained in this high-priority subsample. For the combined sample of high-priority and medium-priority objects, a completeness of 89% is estimated up to B_lim=19.5. The sample of all AGNs detected in the framework of the VPM search in the M92 field contains now 95 QSOs and 14 Seyfert1s with B<19.9. Although the VPM QSOs were selected by completely different criteria, their properties do not significantly differ from those of QSOs found by more traditional optical survey techniques. In particular, the spectra and the optical broad band colours do not provide any hints on a substantial population of red QSOs up to the present survey limit.Comment: 13 pages, 12 figures, accepted for publication in Astronomy and Astrophysic

Photometric and kinematic studies of open star clusters. III. NGC 4103, NGC 5281, and NGC 4755

We present CCD photometry and proper motion studies of the three open star clusters NGC 4103, NGC 5281, and NGC 4755 (kappa Cru). By fitting isochrones to the colour magnitude diagrams, we found that all three objects are young open star clusters with ages of at most t=45 Myr. They are located at distances from approx. 1600 pc to 2200 pc, derived from distance moduli (m-M)_0 ranging from 11 mag to 12 mag. We combined membership determinations based on proper motions and statistical field star subtraction to derive the initial mass function (IMF) of the clusters. The shape of the IMFs could be represented by power laws with exponents of Gamma=-1.46 +/- 0.22 for NGC 4103, Gamma=-1.60 +/- 0.50 for NGC 5281, and Gamma=-1.68 +/- 0.14 for NGC 4755, when - as a reference - Salpeter's (1955) value would be Gamma=-1.35. These results agree well with other IMF studies of open star clusters.Comment: 16 pages, 14 figures, accepted for publication in Astronomy & Astrophysic

The History of the Mysterious Eclipses of KH 15D II. Asiago, Kiso, Kitt Peak, Mt. Wilson, Palomar, Tautenburg and Rozhen Observatories, 1954-97

The unusual pre-main-sequence binary star named KH 15D undergoes remarkably deep and long-lasting periodic eclipses. Some clues about the reason for these eclipses have come from the observed evolution of the system's light curve over the last century. Here we present UBVRI photometry of KH 15D based on photographic plates from various observatories, ranging in time from 1954 to 1997. The system has been variable at the ~1 mag level since at least 1965. There is no evidence for color variations, with a typical limit of Delta(B-V) < 0.2 mag. We confirm some previously published results that were based on a smaller sample of plates: from approximately 1965 to 1990, the total flux was modulated with the 48-day orbital period of the binary, but the maximum flux was larger, the fractional variations were smaller, and the phase of minimum flux was shifted by almost a half-cycle relative to the modern light curve. All these results are consistent with the recently proposed theory that KH 15D is being occulted by an inclined, precessing, circumbinary ring.Comment: 10 pages, 3 figures, 3 tables, accepted for publication in Astronomical Journa

A runaway collision in a young star cluster as the origin of the brightest supernova

Supernova 2006gy in the galaxy NGC 1260 is the most luminous one recorded \cite{2006CBET..644....1Q, 2006CBET..647....1H, 2006CBET..648....1P, 2006CBET..695....1F}. Its progenitor might have been a very massive ($>100$ \msun) star \cite{2006astro.ph.12617S}, but that is incompatible with hydrogen in the spectrum of the supernova, because stars $>40$ \msun are believed to have shed their hydrogen envelopes several hundred thousand years before the explosion \cite{2005A&A...429..581M}. Alternatively, the progenitor might have arisen from the merger of two massive stars \cite{2007ApJ...659L..13O}. Here we show that the collision frequency of massive stars in a dense and young cluster (of the kind to be expected near the center of a galaxy) is sufficient to provide a reasonable chance that SN 2006gy resulted from such a bombardment. If this is the correct explanation, then we predict that when the supernova fades (in a year or so) a dense cluster of massive stars becomes visible at the site of the explosion

The merger stage of the ultra-luminous infrared galaxy IRAS 03158+4227

We examine the merger stage of IRAS 03158+4227, one of the most luminous ULIRGs from the IRAS 2 Jy sample. Deep optical images are presented along with high-resolution NIR images and optical low-resolution spectra. IRAS 03158+4227 is confirmed as a component of an equal-luminosity binary galaxy with a nuclei separation of 47 / (h/75) kpc. A long lopsided tail emanating from the companion, which harbours an active nucleus, is the most prominent peculiar feature of the binary. The results of numerical simulations admit the interpretation of this structure as a product of the tidal interaction between the two components. If the infrared-activity of IRAS 03158+4227 is also dynamically triggered by this process, this would implicate that the ULIRG phenomenon is not restricted to the final stage of a binary merger. Alternatively, the system may be a multiple merger where the partner(s) has/have already coalesced.Comment: 9 pages, with 7 figures, to appear in Astronomy & Astrophysics, 379, 845 (Dec 2001), a full version is already available via http://www.edpsciences.fr/articles/aa/abs/2001/45/aa1671/aa1671.htm

Velocity Gradients in the Intracluster Gas of the Perseus Cluster

We report the results of spatially resolved X-ray spectroscopy of 8 different ASCA pointings distributed symmetrically around the center of the Perseus cluster. The outer region of the intracluster gas is roughly isothermal, with temperature ~ 6-7 keV, and metal abundance ~ 0.3 Solar. Spectral analysis of the central pointing is consistent with the presence of a cooling flow and a central metal abundance gradient. A significant velocity gradient is found along an axis at a position angle of \~135 deg, which is ~ 45 deg discrepant with the major axis of the X-ray elongation. The radial velocity difference is found to be greater than 1000 km/s/Mpc at the 90% confidence level. Simultaneous fittings of GIS 2 & 3 indicate that the velocity gradient is significant at the 95% confidence level and the F-test rules out constant velocities at the 99% level. Intrinsic short and long term variations of gain are unlikely (P < 0.03) to explain the velocity discrepancies.Comment: 19 pages, 6 figures, accepted for publication in the Astrophysical Journa

Galaxy Populations and Evolution in Clusters II: Defining Cluster Populations

This paper presents quantitative techniques for studying, in an unbiased manner, the photometric and structural properties of galaxies in clusters, including a means to identify likely background objects in the absence of redshift information. We develop self-consistent and reproducible measurements of fundamental properties of galaxies such as radius, surface brightness, concentration of light and structural asymmetry. We illustrate our techniques through an application to deep UBR images, taken with the WIYN 3.5m telescope, of the central ~173 arcmin^2 (or 0.3 Mpc * 0.3 Mpc) of the cluster Abell 0146 (Perseus). Our techniques allow us to study the properties of the galaxy population in the center of Perseus down to M_B = -11. Using these methods, we describe and characterize a well-defined relation between absolute magnitude and surface brightness for galaxy cluster members across the entire range of galaxy luminosity from M_B = -20 to M_B = -11. The galaxies that are assigned by our techniques to the background show no such tight relationship between apparent magnitude and surface brightness, with the exception of those we identify as being members of a background cluster of galaxies at z ~ 0.55. We, however, find that at the fainter magnitudes, M_B > -16, there is a large scatter about the underlying color--magnitude relation defined by the brighter galaxies. Our analysis also indicates that the vast majority of Perseus galaxies are `normal', with little evidence for features associated with evolution; we however discuss the detailed properties of a handful of unusual galaxies. Finally, the galaxy luminosity function of the Perseus cluster center is computed, with a derived faint end slope of alpha = -1.44+/-0.04, similar to values found in other nearby clusters.Comment: Accepted on Feb. 2, 2002 to the Astronomical Journa