Dust obscured radio-quiet quasars at high redshifts

Mark Lacy, NAASC/NRAOAndreea Petric (SSC), Susan Ridgway

(CTIO), Tanya Urrutia (SSC), Anna Sajina (Haverford), Alejo Martinez-Sansigre

(Oxford/Portsmouth)

The co-evolution of black holes and massive galaxies

• Models can reproduce observed relation if galaxy mergers trigger quasar activity, and subsequent AGN feedback stops star formation.

• But only about 1/3 quasar hosts show signs of mergers.– Different timescales?– Star formation quickly

suppressed in quasars?– Selection effects?

Tremaine et al 2002

IR-selected quasars• We have used

Spitzer to select samples of quasars based on their mid-infrared dust continua.

Blue continuum

Red continuum

Continuum+PAH

Spectroscopy• Follow up with optical/IR

spectroscopy.• Classify optical spectra

as: – type-1 (normal quasar)– type-2 (high-ionization

narrow lines only)– red type-1 (1R)– starburst/LINER

• Based on broad lines, BPT diagrams, [NeV] emission, high-ionization UV emission lines.

Evolution of obscured quasars

Host galaxies of dust obscured quasars

• Hosts out to z~1 easy to image with HST/ACS.

• All show some signs of interaction/merger.

Stacking at 160mu• Stack all z>2 type-1s (17) and obscured

quasars (37). Type-1s have lower SFRs.• Mean luminosity ~LIRG/ULIRG transition

SEDs• Two z>2 obscured

quasars detected with MAMBO (1.3mm), corresponding to HLIRG luminosities.

• z=4.27 object also detected in CO.

Summary• A population of radio-quiet, luminous, dust

obscured quasars exists out to at least z~4, whose numbers exceed those of normal quasars.

• Tentative evidence that the true peak of luminous quasar activity occurred at higher redshifts than currently believed.

• Dust obscured quasars seem to show higher star formation rates than their unobscured counterparts.