Space telescope imaging spectrograph ultraviolet/optical spectroscopy of "warm'' ultraluminous infrared galaxies

We present high spatial resolution ultraviolet and optical spectroscopy, obtained using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, of nuclear structures within four "warm'' ultraluminous infrared galaxies (ULIRGs). We find an active galactic nucleus (AGN) in at least three and probably all four in our sample, hosted in a compact, optically luminous "knot.'' In three cases these knots were previously identified as a putative AGN from multiband optical imaging. Three objects of the sample also harbor a starburst in one or more knots, suggesting that the optically luminous knots seen in local ULIRGs are the most likely sites of the dust-shrouded starburst and AGN activity that power the infrared emission. The four AGNs have a diverse range of properties: two are classical narrow-line AGNs, one shows both broad and narrow lines and evidence for lines of sight from the narrow- to the broad-line regions, and one is plausibly an FeLoBAL AGN. The probable presence in one object of an FeLoBAL AGN, which are extremely rare in the QSO population, supports the idea that LoBAL AGNs may be youthful systems shrouded in gas and dust rather than AGNs viewed along a certain line of sight. The three starbursts for which detailed constraints are possible show a smaller range in properties; all three bursts are young, with two having ages of similar to 4 Myr and the third having an age of 20 Myr, suggesting that ULIRGs undergo several bursts of star formation during their lifetimes. None of the starbursts show evidence for initial mass function slopes steeper than about 3.3. The metallicities of the knots for which metallicities can be derived are all at least 1.5 Z(circle dot). The properties of one further starburst knot are consistent with it being the forming core of an elliptical galaxy. Our results suggest that detailed studies of the knots seen in ULIRGs can give important insights into the most violent starburst and AGN activity at both low and high redshift.