A New Twist to an Old Story: HE 0450-2958 and the ULIRG → Optically Bright QSO Transition Hypothesis

Abstract

We report on interferometric imaging of the CO J = 1-0 and J = 3-2 line emission from the controversial QSO/galaxy pair HE 0450-2958. The detected CO J = 1-0 line emission is found associated with the disturbed companion galaxy, not the luminous QSO, and implies M_gal(H₂) ~ (1-2) × 10¹⁰ M_⊙, which is gtrsim30% of the dynamical mass in its CO-luminous region. Fueled by this large gas reservoir, this galaxy is the site of an intense starburst with SFR ~ 370 M_⊙ yr ^-1, placing it firmly on the upper gas-rich/star-forming end of ultra luminous infrared galaxies (ULIRGs, L_IR > 10¹² L_⊙). This makes HE 0450-2958 the first case of extreme starburst and powerful QSO activity, intimately linked (triggered by a strong interaction) but not coincident. The lack of CO emission toward the QSO itself renews the controversy regarding its host galaxy by making a gas-rich spiral (the typical host of narrow-line Seyfert 1 AGNs) less likely. Finally, given that HE 0450-2958 and similar IR-warm QSOs are considered typical ULIRG → optically bright QSO transition candidates, our results raise the possibility that some may simply be gas-rich/gas-poor (e.g., spiral/elliptical) galaxy interactions which "activate" an optically bright unobscured QSO in the gas-poor galaxy, and a starburst in the gas-rich one. We argue that such interactions may have gone largely unnoticed even in the local universe because the combination of tools necessary to disentangle the progenitors (high-resolution and S/N optical and CO imaging) became available only recently.