Spatially Resolved Circumgalactic Medium around a Star-forming Galaxy Driving a Galactic Outflow at <i>z</i> ≈ 0.8

Abstract We report the small-scale spatial variation in cool ( T ∼ 10 4 K) Mg ii absorption detected in the circumgalactic medium (CGM) of a star-forming galaxy at z ≈ 0.8. The CGM of this galaxy is probed by a spatially extended bright background gravitationally lensed arc at z = 2.76. The background arc continuously samples the CGM of the foreground galaxy at a range of impact parameters between 54 and 66 kpc. The Mg ii absorption strengths vary by more than a factor of 2 within these ranges. A power-law fit to the fractional variation of absorption strengths yields a coherence length of 5.8 kpc within this range of impact parameters. This suggests a high degree of spatial coherence in the CGM of this galaxy. The host galaxy is driving a strong galactic outflow with a mean outflow velocity ≈ −179 km s −1 and mass outflow rate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mover accent="true"> <mml:mi>M</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> <mml:mi>out</mml:mi> </mml:msub> <mml:mspace width="0.25em"/> <mml:mo>≥</mml:mo> <mml:mspace width="0.25em"/> <mml:mn>6</mml:mn> <mml:msubsup> <mml:mn>4</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>27</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>31</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> M ⊙ yr −1 traced by blueshifted Mg ii and Fe ii absorption lines. The galaxy itself has a spatially extended emission halo with a maximum spatial extent of ≈33 kpc traced by [O ii ], [O iii ], and H β emission lines. The extended emission halo shows kinematic signatures of corotating halo gas with solar metallicity. Taken together, these observations suggest evidence of a baryon cycle that is recycling the outflowing gas to form the next generation of stars.