arXiv subjects

L. M. Buson

Publications and source records attributed to L. M. Buson.

23 records · Page 2Linked to original sources

arXiv2005

NGC 7679: an anomalous, composite Seyfert 1 galaxy whose, X-ray luminous AGN vanishes at optical wavelengths

Morphological disturbances and gas kinematics of the SB0 galaxy NGC 7679=Arp 216 are investigated to get clues to the history of this highly composite object, where AGN and starburst signatures dominate each other in the X-ray and optical/IR regime, respectively. Perturbations of the ionized gas velocity field appear quite mild within 15'' (~5 kpc) from the center, so as it can be straightforwardly modeled as a circularly rotating disk. On the contrary, outside that radius, significant disturbances show up. In particular, the eastern distorted arm as well as the huge neutral hydrogen bridge connecting NGC 7679 with the nearby Seyfert spiral NGC 7682 unambiguously represent the vestige of a close encounter of the two objects dating back ~500 Myr ago. The relationship of such past event with the much more recent, centrally located starburst (not older than 20 Myr) cannot be easily established. Altogether, the classification of NGC 7679, turns out to be less extreme than that proposed in the past, being simply a (disturbed) galaxy where starburst and AGN activity cohexist with a starburst dominating the bolometric luminosity.

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arXiv2005

New Candidate EHB Stars in the Open Cluster NGC 6791: Looking Locally into the UV-upturn Phenomenon

Relying on U,B imagery at the Italian Telescopio Nazionale Galileo (TNG), we report here the discovery of a sample of 13 new UV-bright post-HB candidate stars in the field of the galactic open cluster NGC 6791. Owing to its super-solar metal content ([Fe/H]>0.2 dex) and estimated age (t> 8 Gyr), this cluster represents the nearest and ideal stellar aggregate to match the distinctive properties of the evolved stellar populations possibly ruling the UV-upturn phenomenon in elliptical galaxies and bulges of spirals. Our ongoing spectroscopic follow-up of this unique UV-bright sample will allow us to assess - once cluster membership of the candidates is properly checked - the real nature (e.g. SdB, SdO, AGB-manque' or EHB stars) of these hot sources, and their link with the ultraviolet excess emerging from low-mass, metal-rich evolutionary environments of external galaxies.

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arXiv2004

Is the Giant Elliptical Galaxy NGC5018 a Post-Merger Remnant?

NGC5018, one of the weakest UV emitters among giant ellipticals (gE) observed with IUE, appears to consist of an optical stellar population very similar to that of the compact, dwarf elliptical M32, which is several magnitudes fainter in luminosity than NGC5018 and whose stellar population is know to be ~3 Gyr old. Here we show that the mid-UV spectra of these two galaxies are also very similar down to an angular scale hundreds times smaller than the IUE large aperture (as probed by HST/FOS UV spectra obtained through 0.86 arcsec apertures). This implies a reasonably close match of the populations dominating their mid-UV light (namely, their main-sequence turnoff stars). These data indicate that NGC5018 has, in its inner regions, a rather uniform dominance of a ~3 Gyr-old stellar population, probably a bit different in metallicity from M32. Combined with the various structures that indicate that NGC5018 is the result of a recent major merger, it appears that almost all of stars we see in its center regions were formed about 3 Gyr ago, in that merger event. NGC5018 is likely the older brother of NGC7252, the canonical gE-in-formation merger. As such, NGC5018 is perhaps the best galaxy which can tell us how a merger works, after the fireworks subside, to form a gE galaxy today. For this reason alone, the stellar populations in NGC5018 at all radii are worth studying in detail.

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arXiv2002

Multiwavelength evidence for a 15-year periodic activity in the symbiotic nova V1016 Cygni

The ~15.1 years period found in the long-term UBV photoelectric and photographic photometry of the symbiotic nova V1016 Cyg is detected also in the (J-K) colour index and in the UV continuum and emission line fluxes from IUE and HUT spectra. It could be interpreted either as the effect of recurrent enhanced mass loss episodes from the Mira type variable companion to a hot component along its ultra-wide orbit (proposed from recent HST observations) or the true orbital period of the inner, unresolved binary of a triple system. A 410-day delay of the maximum of UV emission lines fluxes with respect to the maximum of continuum was found. The pulsation period of the Mira type variable was improved to 474+/-6 days.

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arXiv1992

Coordinated UV-optical observations of quasars: the evolution of the Lyman absorption

The average flux decrement shortward the Ly$_α$ emission, due to the well-known ``forest'' of absorptions, has been measured in the spectra of 8 quasars. Quasi-simultaneous optical and IUE observations of the two low redshift quasars PKS 0637--75 (z=0.654) and MC 1104+16 (z=0.632) have been carried out, obtaining relatively high S/N, spectrophotometrically calibrated data on their energy distribution from the rest frame H$_β$ to the Lyman continuum. Six more quasars in the redshift range 2.5-3.4 have been observed in the optical domain. For all the quasars the ``intrinsic'' continuum slope and normalization have been estimated longward the Ly$_α$ emission and extrapolated towards the Lyman continuum to measure the average depressions, which have been compared with the model statistics of the Ly$_α$ clouds. When all the known classes of absorbers are taken into account with plausible values for their equivalent width distribution and evolution, a good agreement is obtained with the observations. The results for the observed continuum decrement at $z \sim 0.65$ are identical to those predicted by the evolution with redshift of the number of Ly$_α$ forest systems including the HST data and within $2σ$ of the predicted value using the ``standard'' Ly$_α$ evolution (as determined only at high z).

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