UV sky surveys

I review the development of UV and EUV astronomy, covering the spectral range from 5 to 300 nm, with emphasis on sky surveys for discrete sources. I discuss studies which resulted in lists of sources observed by imaging and deliberately omit most spectroscopic studies. Technical issues, such as detector and telescope developments, are treated separately from descriptions of specific missions and their results, which contributed to the understanding of the UV sky. The missions are compared in terms of their ``survey power'', a variable which combines sky coverage and survey depth. I use the existing knowledge of UV sources to predict views of the UV sky, which I then compare with those actually detected. Finally, UV missions which will detect fainter sources and will fly in the near future are described, and a wish list for low-cost ventures, which could advance considerably our knowledge of the UV sky is presented.Comment: 53 pages, 11 figures, accepted by Experimental Astronomy, uses crckapb.sty (style file from Kluwer

Galaxy interactions in the Hickson Compact Group 88

I present observations of the Hickson Compact Group 88 (HCG88) obtained during the commissioning of a new 28-inch telescope at the Wise Observatory. This galaxy group was advertised to be non-interacting, or to be in a very early interaction stage, but this is not the case. The observations reported here were done using a "luminance" filter, essentially a very broad R filter, reaching a low surface brightness level of about 26 mag per square arcsec. Additional observations were obtained in a narrow spectral band approximately centered on the rest-frame H-alpha line from the group. Contrary to previous studies, my observations show that at least two of the major galaxies have had significant interactions in the past, although probably not between themselves. I report the discovery of a faint extended tail emerging from the brightest of the group galaxies, severe isophote twisting and possible outer shells around another galaxy, and map the HII regions in all the galaxies.Comment: 17 pages, 3 figures. MNRAS, in pres

Thinking about Archeoastronomy

I discuss various aspects of archeoastronomy concentrating on physical artifacts (i.e., not including ethno-archeoastronomy) focusing on the period that ended about 2000 years ago. I present examples of artifacts interpreted as showing the interest of humankind in understanding celestial phenomena and using these to synchronize calendars and predict future celestial and terrestrial events. I stress the difficulty of identifying with a high degree of confidence that these artifacts do indeed pertain to astronomy and caution against the over-interpretation of the finds as definite evidence. With these in mind, I point to artifacts that seem to indicate a human fascination with megalithic stone circles and megalithic alignments starting from at least 11000 BCE, and to other items presented as evidence for Neolithic astronomical interests dating to even 20000 BCE or even before. I discuss the geographical and temporal spread of megalithic sites associated with astronomical interpretations searching for synchronicity or for a possible single point of origin. A survey of a variety of artifacts indicates that the astronomical development in antiquity did not happen simultaneously at different locations, but may be traced to megalithic stone circles and other megalithic structures with possible astronomical connections originating in the Middle East, specifically in the Fertile Crescent area. The effort of ancient societies to erect these astronomical megalithic sites and to maintain a corpus of astronomy experts does not appear excessive.Comment: 28 pages, three figures, extended contribution to the archeoastronomy workshop held at IMeRA, Marseille on November 30, 201

Grey Milky Way Extinction from SDSS Stellar Photometry

We report results concerning the distribution and properties of galactic extinction at high galactic latitudes derived from stellar statistics using the Sloan Digital Sky Survey (SDSS). We use the classical Wolf diagram method to identify regions with extinction, and derive the extinction and the extinction law of the dust using all five SDSS spectral bands. We estimate the distance to the extinguishing medium using simple assumptions about the stellar populations in the line of sight. We report the identification of three extinguishing clouds, each a few tens of pc wide, producing 0.2-0.4 mag of g'-band extinction, located 1-2 kpc away or 0.5-1 kpc above the galactic plane. All clouds exhibit grey extinction, i.e., almost wavelength-independent in the limited spectral range of the SDSS. We discuss the implication of this finding on general astrophysical questions.Comment: 30 pages, 13 figures. Accepted for publication in MNRA

Normal Galaxies - INES Guide No. 2

This guide presents, in a uniform manner, all the information collected by the IUE satellite on normal galaxies. It contains information on 274 galaxies and it supersedes the previous IUE guide to normal galaxies (Longo & Capaccioli 1992). The data shown here are restricted to galaxies defined as ``normal'' by the observer, and entered as such in the IUE data file header. The information is also restricted to low-dispersion spectra obtained through the large apertures of IUE. For the first time, we provide spectral information from well-defined and identifiable locations in the target galaxies. These are mostly located close to the photocenter of each object, although there are few exceptions. Each representative spectrum of a galaxy consists of a short-wave (SW) and a long-wave (LW) IUE low-dispersion spectrum (where available) combined into a single spectrum covering the wavelength range 1150 A to 3350 A. We selected the two spectra to be combined so as to be, preferably, the deepest exposures available in the INES archive. Each representative spectrum is accompanied by two images of the galaxy, on which the locations of the SW entrance aperture and the LW entrance aperture are marked. The guide is available at (http://wise-iue.tau.ac.il/~lili/Fnet/Fnet.html)Comment: 36 pages, 7 figures. INES Access Guide N.2-ESA SP-1239 2000. Noordwijk:ESA Publ. Dep. The full atlas is available on line at http://wise-iue.tau.ac.il/~lili/Fnet/Fnet.htm

A Simple Isolation Criterion based on 3D Redshift Space Mapping

We selected a sample of galaxies, extremely isolated in 3D redshift space, based on data from NED and the ongoing ALFALFA HI (21cm) survey. A simple selection criterion was employed: having no neighbors closer than 300 km/s in 3D redshift space. The environments of galaxies, selected using this criterion and NED data alone, were analyzed theoretically using a constrained simulation of the local Universe, and were found to be an order of magnitude less dense than environments around randomly selected galaxies. One third of the galaxies selected using NED data alone did not pass the criterion when tested with ALFALFA data, implying that the use of unbiased HI data significantly improves the quality of the sample.Comment: 4 pages, 2 figures. To be published in the proceedings of the conference "Galaxies in Isolation: Exploring Nature vs. Nurture", Granada, 12-15 May 200

Many binaries among NEAs

The number of binary asteroids in the near-Earth region might be significantly higher than expected. While Bottke and Melosh (1996) suggested that about 15% of the NEAs are binaries, as indicated from the frequency of double craters, and Pravec and Harris (2000) suggested that half of the fast-rotating NEAs are binaries, our recent study of Aten NEA lightcurves shows that the fraction of binary NEAs might be even higher than 50%. We found two asteroids with asynchronous binary characteristics such as two additive periods and fast rotation of the primary fragment. We also identified three asteroids with synchronous binary characteristics such as amplitude higher than one magnitude, U-shaped lightcurve maxima and V-shaped lightcurve minima. These five binaries were detected out of a sample of eight asteroids observed, implying a 63% binarity frequency. Confirmation of this high binary population requires the study of a larger representative sample. However, any mitigation program that requires the deflection or demise of a potential impactor will have to factor in the possibility that the target is a binary or multiple asteroid system.Comment: 5 pages, 2 figures, invited paper for NASA's "Near-Earth Object Detection, Characterization, and Threat Mitigation" workhop, Colorado, June 200

Quest for truly isolated galaxies

I describe attempts to identify and understand the most isolated galaxies starting from my 1983 Leiden PhD thesis, continuing through a string of graduate theses on various aspects of this topic, and concluding with an up-to-date account of the difficulty to find really isolated objects. The implication of some of the findings revealed on the way and presented here is that the nearby Universe may contain many small dark-matter haloes, and that some such haloes may possibly be accreting intergalactic gas to form dwarf galaxies.Comment: Contribution for the "Galaxies in Isolation" conferenc

Star Formation in Dwarf Galaxies

We explore mechanisms for the regulation of star formation in dwarf galaxies. We concentrate primarily on a sample in the Virgo cluster, which has HI and blue total photometry, for which we collected H$\alpha$ data at the Wise Observatory. We find that dwarf galaxies do not show the tight correlation of the surface brightness of H$\alpha$ (a star formation indicator) with the HI surface density, or with the ratio of this density to a dynamical timescale, as found for large disk or starburst galaxies. On the other hand, we find the strongest correlation to be with the average blue surface brightness, indicating the presence of a mechanism regulating the star formation by the older (up to 1 Gyr) stellar population if present, or by the stellar population already formed in the present burst.Comment: 15 pages (LATEX aasms4 style) and three postscript figures, accepted for publication in the Astrophysical Journa

Late-Type Dwarf Galaxies in the Virgo Cluster: I. The Samples

We selected samples of late-type dwarf galaxies in the Virgo cluster with HI information. The galaxies were observed at the Wise-Observatory using several broad-band and H$\alpha$ bandpasses. UV measurements were carried out with the IUE Observatory from VILSPA, and with the FAUST shuttle-borne UV telescope. We describe our observations in detail, paying particular attention to the determination of measurement errors, and present the observational results together with published data and far-infrared information from IRAS. The sample will be analyzed in subsequent papers, in order to study star formation mechanisms.Comment: Modified version, accepted for publication in MNRAS, 26 page