The Star Formation History of the Virgo Early-Type Galaxy NGC 4435: The Spitzer Mid-Infrared View

We present a population synthesis study of NGC 4435, an early-type Virgo galaxy interacting with NGC 4438. We combine new spectroscopic observations obtained with the Spitzer Space Telescope IRS instrument with IRAC archival data and broadband data from the literature. The IRS spectrum shows prominent polycyclic aromatic hydrocarbon (PAH) features, low-ionization emission lines, and H₂ rotational lines arising from the dusty circumnuclear disk characterizing this galaxy. The central spectral energy distribution (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 [Ne III] λ15.5/[Ne II] λ12.8 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 active galactic nucleus contribution to the ionizing flux is constrained to be less than 2%. The age of the burst is found to be around 190 Myr, and this is fully consistent with the notion that the star formation process was triggered by the interaction with NGC 4438. 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 NGC 4435 closely similar to a typical interacting early-type galaxy with inverted Ca II [H+K] lines that will later turn into a typical cluster E+A galaxy and reinforces the notion that these objects are the result of a recent rejuvenation episode rather than a genuine delayed formation.