A persistent misaligned gas disc in an early-type galaxy

Transcription

1 A persistent misaligned gas disc in an early-type galaxy Freeke van de Voort UC Berkeley & ASIAA

2 observed gas-star misalignment SDSS NGC7465: early-type galaxy (stars in orange) with misaligned molecular gas disc (blue) Alatalo et al. 2013, Davis et al. 2013, CARMA

3 observed gas-star misalignment About 50% of local field ETGs with gas (25% of all ETGs) have kinematically misaligned gas and stars. can be due to stellar rest is gas brought in by mass loss mergers? Davis et al. 2011, ATLAS 3D

4 relaxation Stellar potential is not spherical. Gas feels torques from the stars, causing precession. This precession happens faster closer to the centre. Cloud-cloud collisions with (differentially precessing) gas on non-spherical orbits. Dissipates angular momentum in direction perpendicular to stellar angular momentum. Gas and stars align, the most stable configuration. Relaxation time depends on precession timescale (dynamical timescale and eccentricity of the potential). trelax = tdyn / ɛ (tens of Myr in central kpc)

5 observed gas-star misalignment About 50% of local field ETGs with gas (25% of all ETGs) have kinematically misaligned gas and stars Why so many gas discs misaligned? Why so few gas discs counter-rotating (180 o )? Davis et al. 2011, ATLAS 3D

6 early-type galaxy in group (400 kpc) 3 image

7 simulated gas-star misalignment Mgas = 3x10 9 Msun Mstar = Msun gas particle LOS velocities stellar contours in white stellar halfmass radius ~3.4 kpc. (stars in edge-on projection) (20 kpc) 3 image

8 gas disc evolution pre-merger merger post-merger rotation warp realignment

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10 misalignment angle Misaligned gas z=0 merger z=0.5 disc persists for about 2 Gyr, much longer than tdyn. Misalignment angle 120 o. Should become counterrotating, but does not. time van de Voort et al. 2015b

11 misalignment angle Gas just z=0 merger z=0.5 outside the disc has similar angle for most of its evolution. Gas accretion drives ang. mom. evolution for ~1.5 Gyr. Halo gas still misaligned at z=0. time van de Voort et al. 2015b

12 accretion rate High gas accretion rates during & before the merger. Two gas z=0 merger z=0.5 Accretion of gas from merging galaxy accretion peaks after merger. Lower accretion at tlookback<1.5 Gyr. time van de Voort et al. 2015b

13 metallicity evolution Gas metallicity decreases steadily after merger (accretion dominates). Increases at tlookback<1.7 Gyr (star formation dominates). z=0 merger z=0.5 time van de Voort et al. 2015b

14 relaxation time Total trelax does not scale with the tdyn: evolution is dominated by gas accretion. trelax below 50 degrees (where gas accretion rate low) is ~ 6 tdyn. van de Voort et al. 2015b

15 relaxation time Total trelax does not scale with the tdyn: evolution is dominated by gas accretion. trelax below 50 degrees (low accretion rate) is ~ 6 tdyn, resulting in a temporary warp. van de Voort et al. 2015b

16 conclusions Misaligned gas discs, commonly seen in observations, can be long-lived (2 Gyr in our case study). This is due to the continuous accretion of gas. Once accretion has slowed, stellar torques dominate the realignment, which causes the gas disc to warp. Understanding the origin of misaligned discs can help us get constraints on the galaxy merger rate and cosmological gas accretion rate.