Elchanan Almoznino

High-rate active galaxy monitoring at the wise observatory. III, The broad-line region of NGC 4151

Results of an AGN monitoring program for the Seyfert galaxy NGC 4151 are presented. The space and velocity distribution of the broad large region (BLR) gas in this AGN are analyzed. Through cross-correlation, it is found that both Hα and Hβ lag the continuum variations by 9 ± 12 days. Through Fourier and maximun entropy deconvolutions of the continuum and emission-line curves, the approximate transfert function of the BLR in this object is recovered.

Are interactions the primary triggers of star formation in dwarf galaxies

We investigate the assumption that the trigger of star formation in dwarf galaxies is interactions with other galaxies, in the context of a search for a ‘primary’ trigger of a first generation of stars. This is cosmologically relevant because the galaxy formation process consists not only of the accumulation of gas in a gravitational potential well but also of the triggering of star formation in this gas mass, and also because some high-z potentially primeval galaxy blocks look like nearby star-forming dwarf galaxies. We review theoretical ideas proposed to account for the tidal interaction triggering mechanism and present a series of observational tests of this assumption using published data. We also show results of a search in the vicinity of a composite sample of 96 dwarf late-type galaxies for interaction candidates showing star formation. The small number of possible perturbing galaxies identified in the neighbourhood of our sample galaxies, along with similar findings from other studies, supports the view that tidal interactions may not be relevant as primary triggers of star formation. We conclude that interactions between galaxies may explain some forms of star formation triggering, perhaps in central regions of large galaxies, but they do not seem to be significant for dwarf galaxies and, by inference, for first-time galaxies forming at high redshifts. Intuitive reasoning, based on an analogy with stellar dynamics, shows that conditions for primary star formation triggering may occur in gas masses oscillating in a dark-matter gravitational potential. We propose this mechanism as a plausible primary trigger scenario, which would be worth investigating theoretically.

High-rate spectroscopic active galactic nucleus monitoring at the wise observatory. II: NGC 5548

Les observations spectroscopiques de la galaxie de Seyfert de type 1, NGC 5548, sont analysees. Les fonctions de correlation croisee entre le continuum et les raies Hα et Hβ sont etudiees. La taille de la region de raies larges est determinee

Late-type dwarf irregular galaxies in the Virgo cluster — I. Hα and red continuum data

We present H-alpha and red continuum observations for a sample of late-type low surface brightness (LSB) dwarf irregular galaxies(DIGs), consisting of all the m_B<17.2 ImIV and V galaxies in the Virgo cluster (VC), and compare them with a representative sample of VC high surface brightness (HSB) DIGs. Line fluxes and equivalent widths are listed for individual HII regions and total H-alpha emission is measured for the entire DIG. The HII regions we identified do not produce all the line emission. For LSBs with line emission we find star formation rates (SFR) of 6.9-43.0 10^{-3} M_sol/yr, ~10x weaker than for HSB objects. About 2 HII regions are detected per LSB galaxy; they are smaller and fainter than in HSB galaxies in the VC, have line equivalent widths ~50% of those in HSBs, and cover similar fractions of the galaxies. The line-emitting regions of LSB galaxies are preferentially located at the periphery of a galaxy, while in HSBs they tend to be central. The line strength of an HII region is correlated with the red continuum light underneath the region. No fundamental differences were identified in the star formation properties of the LSB and HSB dwarf galaxies we studied and infer that these galaxies must be similar, with the difference being the intensity of the present star-formation burst.

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 H I and blue total photometry, for which we collected Hα data at the Wise Observatory. We find that dwarf galaxies do not show the tight correlation of the surface brightness of Hα (a star formation indicator) with the H I 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.

FAUST observations in the Fourth Galactic Quadrant

We analyse UV observations with FAUST of four sky fields in the general direction of the Fourth Galactic Quadrant, in which we detect 777 UV sources. This is ∼50 per cent more than detected originally by Bowyer et al. We discuss the source detection process and the identification of UV sources with optical counterparts. For the first time in this project we use ground-based objective-prism information for two of the fields, to select the best-matching optical objects with which to identify the UV sources. Using this, and correlations with existing catalogues, we present reliable identifications for ∼75 per cent of the sources. Most of the remaining sources have assigned optical counterparts but, lacking additional information, we offer only plausible identifications. We discuss the types of objects found, and compare the observed population with predictions of our UV Galaxy model.

FAUST Observations of Ultraviolet Sources toward the Virgo Cluster

We analyze three UV images covering a ~100 square degree field toward the Virgo cluster, obtained by the FAUST space experiment. We detect 191 sources to a signal-to-noise ratio of 4.4 and identify 94% of them. Most sources have optical counterparts in existing catalogs, and about half are identified as galaxies. Some sources with no listed counterpart were observed at the Wise Observatory. We present the results of low-resolution visible spectrophotometry and discuss the foreground 101 stellar sources and the 76 detected galaxies, both in the cluster and in the foreground or background. We derive conclusions on star formation properties of galaxies and on the total UV flux from discrete and diffuse sources in the cluster. We test for the presence of intracluster dust, determine the clustering properties of UV emitting galaxies, and derive the UV luminosity function of Virgo galaxies.

Hidden subluminous stars among the FAUST UV sources towards Ophiuchus

We present results of an analysis of a UV image in the direction of Ophiuchus, obtained with the FAUST instrument. The image contains 228 UV sources. Most of these are identified as normal early-type stars through correlations with catalogued objects. For the first time in this project we identify UV sources as such stars by selecting suitable candidates in crowded fields as the bluest objects in colour–colour diagrams using observations from the Wise Observatory. These candidates are then studied using low-resolution spectroscopy, which allows the determination of spectral types to an accuracy of about one-half class, for 60 stars. Synthetic photometry of spectral data is performed in order to predict the expected UV emission, on the basis of the photometric information. These results are used along with the Hipparcos/Tycho information, to search for subluminous stars. The comparison of the predicted emission with the FAUST measured magnitudes allows us to select 12 stars as highly probable evolved hot stars. High signal-to-noise spectra are obtained for nine of these stars, and Balmer line profiles are compared with the prediction of atmosphere models and with the spectrum of real stellar atmospheres. Among the nine candidates, six are classified as previously unrecognized sdB stars, and two as white dwarfs. Our result indicates that indeed more bright subluminous stars are still unrecognized in the existing samples.

Ground-Based Leonid Imaging in the UV

During the 1999 Leonid meteor storm, a camera with a UV sensitive image intensifier and CCD readout (ICCD), was operated at the Wise Observatory in Mitzpe Ramon, Israel. The photocathode spectral response, together with the transmittance of the atmosphere, limited the sensitivity to a narrow band around 320 nm. The aim was to obtain quantitative information on emitted radiance in this band, for comparison with recordings of the same tracks in other wavelengths. The field of view of 7°.2 × 5°.7 was pointed to the East at about 60° elevation and the video recorded from 21 until 03 UT, Nov. 18, 1999. At least five clear images of meteor tracks were recorded, and compared with the signals from known stars. It was concluded that the brightest track gave an estimated monochromatic magnitude at 320 nm peaking at ~ −4. Some of the peculiarities of astronomical observation with a photon-counting imager are discussed.

Morphological aspects of star formation in dwarf galaxies

We studied the morphology of star formation in dwarf irregular galaxies and found that, in general, this takes place on one side of a galaxy and far from the center. This is mainly true for low surface brightness galaxies; in high surface brightness dwarf irregulars the star formation tends to be more centrally concentrated, as well as being more intense. We discuss possible star formation triggers in dwarf irregular galaxies, and evaluate the reasons for the peculiar distribution of star forming regions of these galaxies. Stochastic star formation, interactions with external gas, and tidal interactions appear to be ruled out as responsible for the asymmetric pattern of star formation. We conclude that asymmetry of a dark matter halo or of an asymmetric underlying stellar distribution may trigger an asymmetric pattern of star formation.