Sugata KavirajHertfordshire

Heidelberg14 July 2014

With: Stas Shabala, Richard Ellis, Adam Deller, Enno Middelberg, Kevin Schawinski, Sukyoung Yi

Star formation and AGN regulation in early-type galaxies

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

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, 619Yi +05, ApJ, 619, L111

Rest frame UV colours at 0.5<z<1: CDF-S

Low zSK +08, MNRAS, 388, 67

Persistent star formation in ETGs since z~1

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)

What drives the star formation? Are blue ETGs morphologically disturbed?

Relaxed ETGs

SK +11, MNRAS, 411, 2148

Disturbed ETGs (30% of the ETG population)

What drives the star formation? Are blue ETGs morphologically disturbed?

Relaxed ETGs

Disturbed ETGs (~35% of the ETG population)

SK +11, MNRAS, 411, 2148

Minor-merger-driven star formation at z<1

[Rest-frame NUV-g]

• 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

SK+ 11, MNRAS, 411, 2148

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?

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)?

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.

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.

• 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.

Do AGN regulate minor-merger-driven SF?

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)