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NGC4435
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Title: The Star Formation History of the Virgo early-type galaxy NGC4435: the Spitzer Mid Infrared view
Authors: P. Panuzzo (1), O. Vega (2), A. Bressan (1,2,4), L. Buson (1), M. Clemens (1), R. Rampazzo (1), L. Silva (3), J. R. Valdes (2), G. L. Granato (1,4), L. Danese (4) ((1) INAF Padova, (2) INAOE, (3) INAF Trieste, (4) SISSA)

We present a population synthesis study of NGC4435, an early-type Virgo galaxy interacting with NGC4438. We combine new spectroscopic observations obtained with the Spitzer Space Telescope IRS instrument with IRAC archival data and broad band data from the literature. The IRS spectrum shows prominent PAH features, low ionisation emission lines and H_2 rotational lines arising from the dusty circumnuclear disk characterising this galaxy. The central SED, from X-ray to radio, is well fitted by a model of an exponential burst superimposed on an old simple stellar population. From the lack of high excitation nebular lines, the (NeIII)15.5/(NeII)12.8 ratio, the temperature of molecular hydrogen, and the fit to the full X-ray to radio SED we argue that the present activity of the galaxy is driven by star formation alone. The AGN contribution to the ionising flux is constrained to be less than 2%. The age of the burst is found to be around 190 Myr and it is fully consistent with the notion that the star formation process has been triggered by the interaction with NGC4438. The mass involved in the rejuvenation episode turns out to be less than 1.5% of the stellar galaxy mass sampled in a 5" central aperture. This is enough to render NGC4435 closely similar to a typical interacting early-type galaxy with inverted CaII(H+K) lines that will later turn into a typical cluster E+A galaxy and enforces the notion that these objects are the result of a recent rejuvenation episode rather than a genuine delayed formation.

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