Early-type galaxies in UV

UV observations with the GALEX satellite have shown that over 30% of spiral galaxies possess previously undetected UV-bright extensions of their optical disks. This is also the case of some lenticular galaxies surrounded by an extended UV-emitting ring lying well beyond their optical image.
In particular we studied an UV ring around the galaxy NGC 4262.  HST/ACS imaging of NGC 4262 shows it is a typical SB0 galaxy that has a bright, nearly spherical bulge, a bar, and a lens extending well outside the bar radius

web fig1x

We want to ask the question whether the (strict) coincidence of the UV knots and HI ring reflects a past interaction event. The origin of the UV-bright ring is by necessity controversial. It could have formed  with the galaxy itself, especially considering that tidal stellar and gas arms are often offset. On the other hand, the knotting of the ring is similar to the similar instability seen in the Cartwheel galaxy, or shell early-type galaxies, commonly thought to have undergone accretion and/or merging phenomena
web fig2From these UV data the ring is composed by several knots (not visible at optical wavelengths) whose colors suggest that they are hot star clusters (see Fig. 1 and 2). In this respect, having clustered UV-bright sources in its outer parts, NGC 4262 could be classified as a Type 1 extended ultraviolet disk (XUV). Together with this, the observed inner decoupling of gas and stars velocity fields suggest that the origin of such a structure can be ascribed to the onset of cold gas, then stabilized in rings with the subsequent formation of young star. This gas may derives from the surrounding environment or from an interaction with other galaxies (see Fig 3). As a consequence, a past major interaction episode, undergone by NGC 4262, is responsible for the UV-bright ring we see today. The geometrical and kinematical analysis of the data indicate a strong inclined (or polar) gas ring, both clues of an external origin.

web fig3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

People: D. Bettoni, L. M. Buson

Collaboration  G. Galletta (Padova Univ.)

Recent Publications Bettoni et al 2010, A&A, 519, A72;  Buson, et al. (2011) Ap&SS 335,231


 The innermost regions of the NGC 1023 group. A View from GALEX

The NGC 1023 group is one of the most studied nearby groups. The most peculiar feature of NGC 1023 is its proximity to the (likely tidally interacting) fainter galaxy NGC1023A. The latter galaxy appears as a small companion located at the east end of NGC 1023. It appears as a low-luminosity condensation partially embedded in the disc of the larger galaxy. The separation (and different stellar population) of the two objects is also assured today by the ultraviolet (UV) images of the GALEX satellite.  In particular, NGC 1023A appears to be an irregular galaxy close to the SB0 NGC 1023.
web fig1aOur aim is to give an insight into the evolution of the innermost region of this group by means of ultraviolet observations and proper models. We use far-ultraviolet and near-ultraviolet GALEX archival data as well as a large set of smoothed particle hydrodynamics simulations with chemo-photometric implementation.

From the UV observations we find that several, already known, dwarf galaxies very close to NGC1023 are also detected in the ultraviolet (see Fig. 1) and two more objects (with no optical counterpart) can be added to the group. Using these data, we construct exhaustive models to account for their formation. We find that the whole spectral energy distribution of NGC 1023 (see Fig. 2) and its global properties are well matched by a simulation that provides a minor merger with a companion system 5 times less massive. The strong interaction phase started 7.7 Gyr ago and the final merger 1.8 Gyr ago.

web fig2a

 

 

 

 

 

 

 

 

 

 

 

 

 

 

People: L. Buson, D. Bettoni, P. Mazzei

Collaboration: G. Galletta (Padova Univ.)

Recent Publications: Bettoni et al. (2012), MNRAS 423, 2957

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