Space Telescope Imaging Spectrograph Ultraviolet/Optical Spectroscopy of “Warm” Ultraluminous Infrared Galaxies

Abstract

We present high spatial resolution ultraviolet and optical spectroscopy,obtained using the Space Telescope Imaging Spectrograph on board theHubble Space Telescope, of nuclear structures within four ``warm''ultraluminous infrared galaxies (ULIRGs). We find an active galacticnucleus (AGN) in at least three and probably all four in our sample,hosted in a compact, optically luminous ``knot.'' In three cases theseknots were previously identified as a putative AGN from multibandoptical imaging. Three objects of the sample also harbor a starburst inone or more knots, suggesting that the optically luminous knots seen inlocal ULIRGs are the most likely sites of the dust-shrouded starburstand AGN activity that power the infrared emission. The four AGNs have adiverse range of properties: two are classical narrow-line AGNs, oneshows both broad and narrow lines and evidence for lines of sight fromthe narrow- to the broad-line regions, and one is plausibly an FeLoBALAGN. The probable presence in one object of an FeLoBAL AGN, which areextremely rare in the QSO population, supports the idea that LoBAL AGNsmay be youthful systems shrouded in gas and dust rather than AGNs viewedalong a certain line of sight. The three starbursts for which detailedconstraints are possible show a smaller range in properties; all threebursts are young, with two having ages of ~4 Myr and the third having anage of 20 Myr, suggesting that ULIRGs undergo several bursts of starformation during their lifetimes. None of the starbursts show evidencefor initial mass function slopes steeper than about 3.3. Themetallicities of the knots for which metallicities can be derived areall at least 1.5 Z_{solar}. The properties of one furtherstarburst knot are consistent with it being the forming core of anelliptical galaxy. Our results suggest that detailed studies of theknots seen in ULIRGs can give important insights into the most violentstarburst and AGN activity at both low and high redshift.