The AGN-Starburst Connection in Submillimeter Galaxies

1. The AGN-Starburst Connection in Submillimeter Galaxies Josh Younger Institute for Advanced Study

2. Collaborators • Observations: Andy Biggs (ESO) Rob Ivison (UKATC, Edinburgh) • Theory: Chris Hayward, Desika Narayanan,Lars Hernquist, Patrik Jonsson (Harvard)T. J. Cox (Carnegie Observatories)

3. The AGN-Starburst Connection in the Local Universe • SMBHs are common in the nuclei of galaxies • Observations have established that their masses are correlated with the properties of their host galaxy • Indicative of a close and fundamental link between SMBHs and stellar populations • Recent theoretical models suggest this co-evolution is driven primarily by major mergers Younger et al. (2008)arXiv:0804.2672

4. A Merger-Driven Cosmic Cycle

5. The AGN-Starburst Connection in Submillimeter Galaxies • SMGs represent highly obscured, massive SB galaxies at the peak of cosmic SMBH growth and SF, and thus are an ideal laboratory for testing the AGN-SB connection • Theory suggests they are transition objects between SF- and AGN-dominated systems, with some observational support • If SMGs were powered primarily by obscured AGN, it would have important consequences for galaxy formation models and buildup of cosmic SMBH mass Hopkins, Younger, Hayward et al. (2009)arXiv:0911.1131 Alexander et al. (2008)

6. Searching for SMBHs with VLBI • Previous methods for detecting SMBHs in SMGs have their downsides • X-ray photometry is insensitive to the most heavily obscured (i.e., Compton-thick) objects • Interpreting mid-IR spectra is model dependant; e.g., power-law could arise via SB-heated dust, difficult to model PAH formation/destruction • High resolution radio imaging via VLBI can isolate ultracompact AGN cores from more diffuse star-forming regions • The optical depth at radio wavelengths is low • Natural upper limit to the brightness temperature from a starburst (TB<105 K: Condon 1992)

7. Observations • Utilize the European VLBI Network, with stations across Europe (and Urumqi) • Baselines ranging from 500 to 2000 km; synthesized beam 30 mas (200 pc at z=2) • Maps with 10 mJy rms flux at 18cm means any detection at full resolution has TB>105 K • 6 targets from Lockman Hole with existing MERLIN imaging with compact components (200 mas resolution); VLA fluxes 70-300 mJy Biggs, Younger, & Ivison (in prep)

8. Results • Of first three objects, two are detected at full resolution • Preliminary maps of three additional targets with no detection (<30 mJy at 3-s) • Radio emission is preferentially SF-dominated, even when there are compact MERLIN sources • Only continent-scale VLBI, with almost an order of magnitude higher resolution than previous measurements, can reliably isolate AGN cores

9. Actively Growing SMBHs in SMGs • Known scaling between radio luminosity and BH mass • SMGs with detected AGN cores have MBH ~ 1-2x109 Msun, non-detections have approximately MBH < 2-5x108 Msun • If the radio emission derives from a SB, then why are SMGs systematically below the FIR/radio correlation • They actually are on the local relation • Evolution with redshift/environment • Synchrotron bridges owing to interaction; ‘Taffy’ galaxies (e.g., Condon et al. 2002) Kovacs et al. (2006) Lacy et al. (2001)

10. Comparison to Models • Observations suggest that SMGs are driven by major mergers • Merger-driven model matches many of the observational constraints • Halo/stellar mass; median SED; CO kinematics/morphology • Use these models to place theoretical constraints on the SMBH content of SMGs, as well as their bolometric contribution in a merger-driven model Narayanan et al. (2009a,b)arXiv:0904.0004, 0905.2184

11. Overview • Two 5x1012 Msun halos, with embedded 80% gas disks on a parabolic orbit; 15 different orbital geometries (Cox et al. 2006) • Use PGadget2 for hydrodynamics • Star formation (tuned to local K-S law) • Subresolution ISM model • SMBH growth and feedback (tuned to local M-s relation) • Use Sunrise for RT • Public, polychromatic Monte Carlo RT code (Jonnsson 2006) • Iteratively solves for dust self-absorption/re-emission • Turn AGN contribution off/on

12. ISM Model • Assume a smooth (i.e., non-clumpy) ISM with Gas/Dust=50 by mass • Maximal optical depth to the center of the galaxy • Yields HyLIRGs that peak at approx. 5-7 mJy at 850mm • Support from observations • Molecular gas in local ULIRGs (Scoville et al. 1991; Downes & Solomon 1998) • NH from X-ray observations • Submm sizes Younger et al. (2008)arXiv:0807.2243

13. Results The SMG phase is not correlated with AGN bolometric contribution. SMGs will still be SMGs with or without AGN input. Younger, Hayward, Narayanan et al. (in prep)

14. Conclusions • High-redshift, heavily obscured HyLIRGs (i.e., SMGs) are a compelling place to look for a connection between the growth of SMBHs and stellar populations • VLBI can resolve out the stellar radio emission, isolating AGN cores • An initial search using the EVN has yielded two detections and four upper limits, suggesting the radio emission in SMGs is typically powered by SF rather than an AGN • These same models suggest that the SMG phase is not correlated with periods of high AGN activity, nor is the AGN important in driving the submm emission • SMGs are heavily obscured HyLIRGs powered primarily by SF, rather than an AGN