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Star formation and AGN regulation in early-type galaxies

Star formation and AGN regulation in early-type galaxies. Sugata Kaviraj Hertfordshire Heidelberg 14 July 2014 With: Stas Shabala, Richard Ellis, Adam Deller, Enno Middelberg, Kevin Schawinski, Sukyoung Yi. Key points. Massive early-type galaxies have widespread star formation (not ‘dead’)

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Star formation and AGN regulation in early-type galaxies

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  1. Star formation and AGN regulation in early-type galaxies Sugata Kaviraj Hertfordshire Heidelberg 14 July 2014 With: Stas Shabala, Richard Ellis, Adam Deller, Enno Middelberg, Kevin Schawinski, Sukyoung Yi

  2. Key points • Massive early-type galaxies have widespread star formation (not ‘dead’) • Star formation largely driven by minor mergers, adds 20-30% of galaxy stellar mass after z~1 • Not strongly regulated by AGN feedback

  3. UV colours: GALEX and SDSSEvidence for widespread star formation • Tight optical colour relation (as expected) • But NUV colour shows a spread of 6 mags • Strong UV sources present in nearby ETGs • UV is driven by star formation (UV from old stars insufficient) SK +07, ApJS, 173, 619 Yi +05, ApJ, 619, L111

  4. Rest frame UV colours at 0.5<z<1: CDF-S Persistent star formation in ETGs since z~1 Low z SK +08, MNRAS, 388, 67

  5. What drives the star formation? • Stellar mass loss at z~0 not enough to produce blue UV colours (SK +07) • Gas and stellar kinematics decoupled, suggests external accretion (Sarzi+06, Young+11, Davis+11) • Dust masses typically exceed maximum stellar mass loss (e.g. Merluzzi 98, Knapp+89, Rowlands+12, SK +12) • 70%+ of ETGs morphologically disturbed (van Dokkum+05, Yi +12) • Young, kinematically decoupled cores in ETGs – signs of recent mergers (e.g. McDermid 2006)

  6. What drives the star formation?Are blue ETGs morphologically disturbed? Relaxed ETGs SK +11, MNRAS, 411, 2148

  7. What drives the star formation?Are blue ETGs morphologically disturbed? Relaxed ETGs Disturbed ETGs (~35% of the ETG population) Disturbed ETGs (30% of the ETG population) SK +11, MNRAS, 411, 2148

  8. Minor-merger-driven star formation at z<1 • Star formation is merger driven • But major merger rate (e.g. Lin et al. 04, Conselice et al. 07)too low to satisfy fraction of disturbed ETGs • At least 60% (and up to 90%) of events are minor mergers [Rest-frame NUV-g] SK+ 11, MNRAS, 411, 2148

  9. Summary of star formation in ETGs (0<z<1) • Widespread star formation in ETGs since z~1, which adds 20-30% of the stellar mass after z~1 • Star formation is driven by minor mergers (minor mergers likely drive 50%+ of the local SF budget (SK 14, MNRAS, 437, L41 and SK 14, MNRAS, 440, 2944) • Do AGN regulate this formation?

  10. GALEX + SDSS + radio VLBI (mJIVE-20) • AGN identification can be difficult: • Nuclear activity can be obscured, emission-line (BPT) selection does not necessarily trace the jet • Radio is best (no obscuration) but FIRST/NVSS do not resolve galaxy cores, contributions from SF and AGN hard to disentangle • VLBI can identify AGN unambiguously, high resolution requires temperatures of order 106 K for a detection, only reached in non-thermal sources • mJIVE is using VLBA filler time, 20k+ FIRST sources observed, 4k+ VLBI detections • Is there evidence for AGN quenching in VLBI-detected ETGs (mainly tracing ‘cold-mode’ AGN because SF is merger-driven)?

  11. Do AGN regulate minor-merger-driven SF? • VLBI-detected galaxies overwhelmingly on the red sequence • Blue to red transit times (>1 Gyr) much longer than AGN lifetimes (a few 107 yr) • AGN are not prompt (c.f. Schawinski +10 using SWIFT-BAT) SK in prep.

  12. Do AGN regulate minor-merger-driven SF? • VLBI-detected galaxies overwhelmingly on the red sequence • Blue to red transit times (>1 Gyr) much longer than AGN lifetimes (a few 107 yr) • AGN are not prompt (c.f. Schawinski +10 using SWIFT-BAT) SK in prep.

  13. Do AGN regulate minor-merger-driven SF? • AGN (optical and radio) triggered after several dynamical timescales • Gas reservoir significantly depleted before AGN really switches on • Cold-mode AGN do not regulate star formation (unlike hot mode AGN) SK in prep.

  14. Summary • There are no truly passive galaxies • Widespread star formation in ETGs at late epochs, adds 30% of stellar mass after z~1 • Driven by minor mergers (a process that plausibly drives half the SF budget at low redshift) • Cold-mode AGN (those fuelled by mergers) do not appear to regulate star formation (although most hot-mode AGN probably do)

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