Kinematics of the OVI Circumgalactic Medium: Halo Mass Dependence and Outflow Signatures

TL;DR

This study investigates how the kinematics of OVI absorbers in the circumgalactic medium depend on galaxy halo mass, revealing mass-driven differences in velocity dispersion and outflow signatures through detailed spectral analysis.

Contribution

It provides the first detailed analysis of halo mass dependence on OVI absorber kinematics using pixel-velocity correlation functions and absorber-galaxy velocity comparisons.

Findings

OVI absorber velocity dispersion correlates strongly with halo mass.

Edge-on minor axis gas shows signatures of bipolar outflows.

Lower mass galaxies exhibit smaller velocity dispersions.

Abstract

We probe the high-ionization circumgalactic medium by examining absorber kinematics, absorber-galaxy kinematics, and average absorption profiles of 31 OVI absorbers from the "Multiphase Galaxy Halos" Survey as a function of halo mass, redshift, inclination, and azimuthal angle. The galaxies are isolated at $0.12<z_{\rm gal}<0.66$ and are probed by a background quasar within $D\approx 200$ kpc. Each absorber-galaxy pair has Hubble Space Telescope images and COS quasar spectra, and most galaxy redshifts have been accurately measured from Keck/ESI spectra. Using the pixel-velocity two-point correlation function (TPCF) method, we find that OVI absorber kinematics have a strong halo mass dependence. Absorbers hosted by $\sim L^{\ast}$ galaxies have the largest velocity dispersions, which we interpret to be that the halo virial temperature closely matches the temperature at which the collisionally ionized OVI fraction peaks. Lower mass galaxies and group environments have smaller velocity dispersions. Total column densities follow the same behavior, consistent with theoretical findings. After normalizing out the observed mass dependence, we studied absorber-galaxy kinematics with a modified TPCF and found non-virialized motions due to outflowing gas. Edge-on minor axis gas has large optical depths concentrated near the galaxy systemic velocity as expected for bipolar outflows, while face-on minor axis gas has a smoothly decreasing optical depth distribution out to large normalized absorber-galaxy velocities, suggestive of decelerating outflowing gas. Accreting gas signatures are not observed due to "kinematic blurring" in which multiple line-of-sight structures are observed. These results indicate that galaxy mass dominates OVI properties over baryon cycle processes.

Tables

Table 1. Table 1   O vi Absorber–Galaxy Properties

(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)
Field	$z_{\mathrm{gal}}$	Ref aaGalaxy redshift references: (1) Kacprzak et al. (2019), (2) Muzahid et al. (2015), (3) Johnson et al. (2013), (4) Pointon et al. (2019), (5) Chen et al. (2001), (6) Prochaska et al. (2011), (7) Kacprzak et al. (2012), (8) Guillemin & Bergeron (1997), and (9) this work.	$z_{\mathrm{abs}}$	$W_r\left(1031\right)$	$\Phi$	$i$	$M_r$bbGalaxy absolute $r$-band magnitude in the AB system. The magnitudes are converted into Vega mags with $M_r-5\log h=M_r\left(AB\right)-5\log \left(0.7\right)-0.1429$, which are then used in the halo abundance matching method (for details, see Churchill et al., 2013).	$\log \left(M_h/M_{\odot }\right)$	$V_c\left(D\right)$	$D$	$R_{\mathrm{vir}}$
				(Å)	(deg)	(deg)	(AB)		($\mathrm{km}{s}^{-1}$)	(kpc)	(kpc)
$\mathrm{J012528}-000555$	$0.398525$	2	$0.399090$	$0.817$	${73.4}_{-4.7}^{+4.6}$	${63.2}_{-2.6}^{+1.7}$	-21.99	${12.51}_{-0.15}^{+0.16}$	${242.9}_{-53.6}^{+68.8}$	$163.0\pm 0.1$	${285.4}_{-32.0}^{+37.3}$
$\mathrm{J035128}-142908$	$0.356992$	1	$0.356825$	$0.396$	${4.9}_{-40.2}^{+33.0}$	${28.5}_{-12.5}^{+19.8}$	-20.86	${12.00}_{-0.19}^{+0.29}$	${174.8}_{-50.6}^{+90.9}$	$72.3\pm 0.4$	${190.8}_{-25.9}^{+47.9}$
$\mathrm{J040748}-121136$	$0.3422$	3	$0.342042$	$0.056$	${48.1}_{-0.9}^{+1.0}$	${85.0}_{-0.4}^{+0.1}$	-19.77	${11.62}_{-0.21}^{+0.42}$	${107.5}_{-36.9}^{+83.8}$	$172.0\pm 0.1$	${142.5}_{-21.6}^{+53.9}$
$\mathrm{J040748}-121136$	$0.495164$	4	$0.495101$	$0.229$	${21.0}_{-3.7}^{+5.3}$	${67.2}_{-7.5}^{+7.6}$	-19.73	${11.41}_{-0.21}^{+0.45}$	${97.5}_{-33.7}^{+82.6}$	$107.6\pm 0.4$	${124.4}_{-18.2}^{+51.6}$
$\mathrm{J045608}-215909$	$0.381511$	1	$0.381514$	$0.219$	${63.8}_{-2.7}^{+4.3}$	${57.1}_{-2.4}^{+19.9}$	-20.87	${12.00}_{-0.19}^{+0.29}$	${167.7}_{-48.4}^{+86.8}$	$103.4\pm 0.3$	${192.4}_{-26.0}^{+48.0}$
$\mathrm{J091440}+282330$	$0.244312$	1	$0.244098$	$0.333$	${18.2}_{-1.0}^{+1.1}$	${39.0}_{-0.2}^{+0.4}$	-20.55	${11.88}_{-0.20}^{+0.33}$	${153.2}_{-46.9}^{+91.5}$	$105.9\pm 0.1$	${170.7}_{-23.9}^{+49.4}$
$\mathrm{J094331}+053131$	$0.353052$	1	$0.353286$	$0.220$	${8.2}_{-5.0}^{+3.0}$	${44.4}_{-1.2}^{+1.1}$	-19.88	${11.66}_{-0.21}^{+0.41}$	${125.9}_{-42.6}^{+95.4}$	$96.5\pm 0.3$	${146.8}_{-22.0}^{+54.1}$
$\mathrm{J094331}+053131$	$0.548494$	1	$0.548769$	$0.275$	${67.2}_{-1.0}^{+0.9}$	${58.8}_{-1.1}^{+0.6}$	-21.30	${11.96}_{-0.18}^{+0.26}$	${150.3}_{-41.3}^{+70.2}$	$150.9\pm 0.6$	${190.9}_{-24.9}^{+42.9}$
$\mathrm{J095000}+483129$	$0.211866$	1	$0.211757$	$0.211$	${16.6}_{-0.1}^{+0.1}$	${47.7}_{-0.1}^{+0.1}$	-21.73	${12.37}_{-0.16}^{+0.18}$	${237.0}_{-55.3}^{+74.7}$	$93.6\pm 0.2$	${246.9}_{-29.0}^{+36.1}$
$\mathrm{J100402}+285535$	$0.1380$	5	$0.137724$	$0.117$	${12.4}_{-2.9}^{+2.4}$	${79.1}_{-2.1}^{+2.2}$	-17.05	${10.87}_{-0.22}^{+0.63}$	${69.9}_{-27.6}^{+89.3}$	$56.7\pm 0.2$	${76.3}_{-11.7}^{+47.3}$
$\mathrm{J100902}+071343$	$0.227855$	1	$0.227851$	$0.576$	${89.6}_{-1.3}^{+1.3}$	${66.3}_{-0.9}^{+0.6}$	-20.19	${11.76}_{-0.21}^{+0.37}$	${149.0}_{-48.2}^{+101.2}$	$64.0\pm 0.8$	${154.5}_{-22.5}^{+50.9}$
$\mathrm{J104116}+061016$	$0.442173$	1	$0.441630$	$0.368$	${4.3}_{-1.0}^{+0.9}$	${49.8}_{-5.2}^{+7.4}$	-21.36	${11.99}_{-0.18}^{+0.26}$	${173.8}_{-47.1}^{+79.1}$	$56.2\pm 0.3$	${193.1}_{-24.9}^{+42.2}$
$\mathrm{J111908}+211918$	$0.1383$	6	$0.138521$	$0.074$	${34.4}_{-0.4}^{+0.4}$	${26.4}_{-0.4}^{+0.8}$	-21.45	${12.24}_{-0.17}^{+0.21}$	${204.3}_{-51.4}^{+76.5}$	$138.0\pm 0.2$	${219.0}_{-27.1}^{+38.8}$
$\mathrm{J113327}+032719$	$0.154599$	4	$0.153979$	$0.252$	${56.1}_{-1.3}^{+1.7}$	${23.5}_{-0.2}^{+0.4}$	-19.84	${11.64}_{-0.21}^{+0.41}$	${139.8}_{-47.5}^{+107.1}$	$55.6\pm 0.1$	${138.7}_{-20.9}^{+51.6}$
$\mathrm{J113910}-135043$	$0.204194$	1	$0.204297$	$0.231$	${5.8}_{-0.5}^{+0.4}$	${83.4}_{-0.5}^{+0.4}$	-19.99	${11.69}_{-0.21}^{+0.40}$	${133.0}_{-44.3}^{+98.0}$	$93.2\pm 0.3$	${146.2}_{-21.6}^{+52.4}$
$\mathrm{J113910}-135043$	$0.212259$	1	$0.212237$	$0.137$	${80.4}_{-0.5}^{+0.4}$	${85.0}_{-0.6}^{+5.0}$	-20.09	${11.73}_{-0.21}^{+0.39}$	${119.2}_{-39.2}^{+84.4}$	$174.8\pm 0.1$	${150.3}_{-22.0}^{+51.7}$
$\mathrm{J113910}-135043$	$0.219724$	4	$0.219820$	$0.021$	${44.9}_{-8.1}^{+8.9}$	${85.0}_{-8.5}^{+5.0}$	-17.67	${11.04}_{-0.21}^{+0.60}$	${66.8}_{-25.8}^{+80.5}$	$122.0\pm 0.2$	${88.7}_{-13.5}^{+52.0}$
$\mathrm{J113910}-135043$	$0.319255$	1	$0.319167$	$0.255$	${39.1}_{-1.7}^{+1.9}$	${83.4}_{-1.1}^{+1.4}$	-20.48	${11.86}_{-0.20}^{+0.34}$	${157.0}_{-48.5}^{+96.1}$	$73.3\pm 0.4$	${170.4}_{-23.9}^{+50.7}$
$\mathrm{J121920}+063838$	$0.1241$	6	$0.124103$	$0.424$	${67.2}_{-91.4}^{+39.8}$	${22.0}_{-21.8}^{+18.7}$	-20.50	${11.87}_{-0.20}^{+0.34}$	${156.8}_{-48.4}^{+95.6}$	$93.4\pm 5.3$	${163.1}_{-22.9}^{+48.2}$
$\mathrm{J123304}-003134$	$0.318757$	4	$0.318609$	$0.439$	${17.0}_{-2.3}^{+2.0}$	${38.7}_{-1.8}^{+1.6}$	-20.62	${11.91}_{-0.20}^{+0.32}$	${159.2}_{-48.3}^{+92.5}$	$88.9\pm 0.2$	${176.5}_{-24.7}^{+49.7}$
$\mathrm{J124154}+572107$	$0.205267$	1	$0.205538$	$0.519$	${77.6}_{-0.4}^{+0.3}$	${56.4}_{-0.5}^{+0.3}$	-19.83	${11.64}_{-0.21}^{+0.41}$	${145.0}_{-49.3}^{+114.4}$	$21.1\pm 0.1$	${140.2}_{-21.1}^{+52.3}$
$\mathrm{J124154}+572107$	$0.217905$	4	$0.218043$	$0.366$	${63.0}_{-2.1}^{+1.8}$	${17.4}_{-1.6}^{+1.4}$	-19.77	${11.62}_{-0.21}^{+0.42}$	${124.0}_{-42.5}^{+96.7}$	$94.6\pm 0.2$	${138.7}_{-21.0}^{+52.4}$
$\mathrm{J124410}+172104$	$0.5504$	5	$0.550622$	$0.447$	${20.1}_{-19.1}^{+16.7}$	${31.7}_{-4.8}^{+16.2}$	-20.97	${11.82}_{-0.19}^{+0.31}$	${144.4}_{-42.3}^{+79.0}$	$21.2\pm 0.3$	${171.2}_{-23.1}^{+45.5}$
$\mathrm{J131956}+272808$	$0.6610$	7	$0.660670$	$0.311$	${86.6}_{-1.2}^{+1.5}$	${65.8}_{-1.2}^{+1.2}$	-21.70	${12.15}_{-0.15}^{+0.19}$	${183.6}_{-41.6}^{+59.8}$	$103.9\pm 0.5$	${223.9}_{-24.9}^{+34.7}$
$\mathrm{J132222}+464546$	$0.214431$	1	$0.214320$	$0.354$	${13.9}_{-0.2}^{+0.2}$	${57.9}_{-0.2}^{+0.1}$	-21.18	${12.13}_{-0.18}^{+0.25}$	${204.0}_{-54.7}^{+90.8}$	$38.6\pm 0.2$	${204.6}_{-26.1}^{+43.8}$
$\mathrm{J134251}-005345$	$0.227042$	1	$0.227196$	$0.373$	${13.2}_{-0.4}^{+0.5}$	${0.1}_{-0.1}^{+0.6}$	-21.77	${12.39}_{-0.16}^{+0.17}$	${239.3}_{-55.2}^{+73.7}$	$35.3\pm 0.2$	${251.7}_{-29.3}^{+35.9}$
$\mathrm{J155504}+362847$	$0.189201$	1	$0.189033$	$0.385$	${47.0}_{-0.8}^{+0.3}$	${51.8}_{-0.7}^{+0.7}$	-21.03	${12.07}_{-0.18}^{+0.27}$	${196.3}_{-54.2}^{+93.8}$	$33.4\pm 0.1$	${193.7}_{-25.2}^{+44.6}$
$\mathrm{J213135}-120704$	$0.4302$	8	$0.430164$	$0.385$	${14.9}_{-4.9}^{+6.0}$	${48.3}_{-3.7}^{+3.5}$	-21.47	${12.04}_{-0.18}^{+0.25}$	${181.3}_{-48.1}^{+79.0}$	$48.4\pm 0.2$	${199.7}_{-25.3}^{+41.8}$
$\mathrm{J225357}+160853$	$0.153718$	9	$0.153821$	$0.263$	${59.6}_{-1.8}^{+0.9}$	${33.3}_{-2.0}^{+2.7}$	-19.55	${11.55}_{-0.21}^{+0.45}$	${137.8}_{-48.1}^{+115.4}$	$31.8\pm 0.2$	${129.5}_{-19.6}^{+52.7}$
$\mathrm{J225357}+160853$	$0.352787$	1	$0.352708$	$0.381$	${88.7}_{-4.8}^{+4.6}$	${36.7}_{-4.6}^{+6.9}$	-20.67	${11.93}_{-0.20}^{+0.32}$	${138.1}_{-41.6}^{+78.4}$	$203.2\pm 0.5$	${180.3}_{-25.2}^{+49.5}$
$\mathrm{J225357}+160853$	$0.390013$	9	$0.390705$	$0.173$	${24.2}_{-1.2}^{+1.2}$	${76.1}_{-1.2}^{+1.1}$	-21.25	${12.16}_{-0.18}^{+0.24}$	${160.4}_{-42.5}^{+69.0}$	$276.3\pm 0.2$	${217.2}_{-27.5}^{+44.9}$

Table 2. Table 2 TPCF Δ ​ v ​ ( 50 ) Δ 𝑣 50 \Delta v(50) and Δ ​ v ​ ( 90 ) Δ 𝑣 90 \Delta v(90) measurements

Subsample	$\#$ galaxies	Cut 1	Cut 2	$⟨\log \left(M_h/M_{\odot }\right)⟩$	$⟨z_{\mathrm{gal}}⟩$	$\Delta v\left(50\right)$	$\Delta v\left(90\right)$
		Figure 1: Absorber Kinematics				$v=v_{\mathrm{pix}}$ (km s-1)
Lower Mass	10		$\dots$	$11.62\pm 0.09$	$0.21\pm 0.03$	${80}_{-8}^{+5}$	${188}_{-19}^{+11}$
Higher Mass	21	$\log \left(M_h/M_{\odot }\right)\ge 11.7$	$\dots$	$12.00\pm 0.04$	$0.32\pm 0.03$	${108}_{-9}^{+6}$	${255}_{-22}^{+14}$
Group aaData from Pointon et al. (2017).	6	$\dots$	$\dots$	$\dots$	$0.19\pm 0.05$	${64}_{-7}^{+9}$	${153}_{-18}^{+21}$
		Figure 3: Absorber–Galaxy Kinematics				$v=v_{\mathrm{pix}-\mathrm{gal}}/V_c\left(D\right)$
Lower Mass	15		$\dots$	$11.64\pm 0.04$	$0.22\pm 0.03$	${0.45}_{-0.03}^{+0.03}$	${1.12}_{-0.09}^{+0.08}$
Higher Mass	16	$\log \left(M_h/M_{\odot }\right)\ge 11.88$	$\dots$	$12.06\pm 0.07$	$0.34\pm 0.05$	${0.34}_{-0.02}^{+0.02}$	${0.87}_{-0.07}^{+0.06}$
		Figure 4: Absorber–Galaxy Kinematics				$v=v_{\mathrm{pix}-\mathrm{gal}}/V_c\left(D\right)$
Lower-z	15		$\dots$	$11.73\pm 0.18$	$0.205\pm 0.016$	${0.40}_{-0.04}^{+0.04}$	${1.11}_{-0.12}^{+0.11}$
Higher-z	16	$z_{\mathrm{gal}}\ge 0.244$	$\dots$	$11.95\pm 0.05$	$0.39\pm 0.03$	${0.37}_{-0.02}^{+0.02}$	${0.92}_{-0.05}^{+0.04}$
		Not Plotted: Absorber–Galaxy Kinematics				$v=v_{\mathrm{pix}-\mathrm{gal}}/V_c\left(D\right)$
Face-on	$15$		$\dots$	$11.82\pm 0.14$	$0.32\pm 0.09$	${0.40}_{-0.04}^{+0.03}$	${1.03}_{-0.12}^{+0.09}$
Edge-on	$16$	$i\ge {51}^{\circ }$	$\dots$	$11.91\pm 0.08$	$0.24\pm 0.05$	${0.38}_{-0.03}^{+0.02}$	${0.96}_{-0.08}^{+0.07}$
Major Axis	$17$		$\dots$	$11.76\pm 0.16$	$0.23\pm 0.06$	${0.35}_{-0.03}^{+0.02}$	${0.90}_{-0.09}^{+0.06}$
Minor Axis	$14$	$\Phi \ge {45}^{\circ }$	$\dots$	$11.92\pm 0.06$	$0.33\pm 0.05$	${0.42}_{-0.03}^{+0.04}$	${1.07}_{-0.10}^{+0.10}$
		Figure 5: Absorber–Galaxy Kinematics				$v=v_{\mathrm{pix}-\mathrm{gal}}/V_c\left(D\right)$
Major Axis + Face-on	$10$			$12.00\pm 0.12$	$0.32\pm 0.05$	${0.35}_{-0.03}^{+0.02}$	${0.87}_{-0.09}^{+0.06}$
Major Axis + Edge-on	$7$		$i\ge {51}^{\circ }$	$11.69\pm 0.32$	$0.26\pm 0.07$	${0.38}_{-0.05}^{+0.06}$	${0.97}_{-0.17}^{+0.20}$
Minor Axis + Face-on	$5$	$\Phi \ge {45}^{\circ }$		$11.64\pm 0.17$	$0.18\pm 0.05$	${0.51}_{-0.06}^{+0.04}$	${1.24}_{-0.16}^{+0.10}$
Minor Axis + Edge-on	$9$	$\Phi \ge {45}^{\circ }$	$i\ge {51}^{\circ }$	$11.96\pm 0.13$	$0.32\pm 0.08$	${0.38}_{-0.03}^{+0.02}$	${0.96}_{-0.08}^{+0.07}$
		Figure 6: Absorber–Galaxy Kinematics				$v=v_{\mathrm{pix}-\mathrm{gal}}/V_c\left(D\right)$
Major Axis + Lower Mass	$7$			$11.66\pm 0.15$	$0.31\pm 0.09$	${0.40}_{-0.04}^{+0.04}$	${1.02}_{-0.16}^{+0.15}$
Major Axis + Higher Mass	$10$		$\log \left(M_h/M_{\odot }\right)\ge 11.88$	$12.09\pm 0.09$	$0.29\pm 0.06$	${0.33}_{-0.03}^{+0.02}$	${0.83}_{-0.09}^{+0.06}$
Minor Axis + Lower Mass	$8$	$\Phi \ge {45}^{\circ }$		$11.64\pm 0.05$	$0.20\pm 0.02$	${0.48}_{-0.04}^{+0.03}$	${1.17}_{-0.11}^{+0.07}$
Minor Axis + Higher Mass	$6$	$\Phi \ge {45}^{\circ }$	$\log \left(M_h/M_{\odot }\right)\ge 11.88$	$12.04\pm 0.14$	$0.41\pm 0.07$	${0.37}_{-0.03}^{+0.03}$	${0.92}_{-0.11}^{+0.10}$
Face-on + Lower Mass	$6$			$11.65\pm 0.09$	$0.22\pm 0.08$	${0.51}_{-0.05}^{+0.03}$	${1.23}_{-0.14}^{+0.08}$
Face-on + Higher Mass	$9$		$\log \left(M_h/M_{\odot }\right)\ge 11.88$	$12.00\pm 0.15$	$0.30\pm 0.06$	${0.33}_{-0.02}^{+0.03}$	${0.81}_{-0.06}^{+0.08}$
Edge-on + Lower Mass	$9$	$i\ge {51}^{\circ }$		$11.64\pm 0.10$	$0.23\pm 0.04$	${0.40}_{-0.03}^{+0.03}$	${0.99}_{-0.10}^{+0.11}$
Edge-on + Higher Mass	$7$	$i\ge {51}^{\circ }$	$\log \left(M_h/M_{\odot }\right)\ge 11.88$	$12.13\pm 0.08$	$0.39\pm 0.08$	${0.37}_{-0.03}^{+0.03}$	${0.93}_{-0.10}^{+0.09}$

Equations

$$v_{\mathrm{pix}-\mathrm{gal}}=\left|v_{\mathrm{pix}}+c\frac{z_{\mathrm{abs}}-z_{\mathrm{gal}}}{1+z_{\mathrm{gal}}}\right|.$$

Full text

1. Introduction

The prodigious reserves of gas surrounding galaxies in the circumgalactic medium (CGM) play an important role in galaxy evolution (see review by Tumlinson et al., 2017). This gas is primarily derived from the intergalactic medium (IGM, e.g., Putman et al., 2012; Cooper et al., 2015; Glidden et al., 2016), from cannibalizing satellite galaxies (e.g., Cole et al., 2000; Cox et al., 2008; Qu et al., 2011; Lambas et al., 2012a, b; Kaviraj, 2014; Ownsworth et al., 2014; Gómez-Guijarro et al., 2018), and from galactic feedback (e.g., Strickland & Heckman, 2009; Schaye et al., 2015; van de Voort, 2017; Butler et al., 2017; Correa et al., 2018). The general accepted picture of how a typical galaxy evolves includes the accretion of relatively metal-poor gas from the CGM onto the galactic disk (see review by Kacprzak, 2017), which is used to fuel star formation. Gas is then driven out of the galactic disk in outflows when massive stars explode as supernovae and produce metal-enriched winds (e.g., Shen et al., 2012; Lehner et al., 2013; Ford et al., 2014; Muzahid et al., 2015). The velocities of the outflowing gas do not usually exceed the escape velocity of the galaxy (e.g., Tumlinson et al., 2011; Bouché et al., 2012; Stocke et al., 2013; Mathes et al., 2014; Bordoloi et al., 2014), thus the gas is recycled back onto the galaxy and could fuel further episodes of star formation (e.g., Oppenheimer et al., 2010; Ford et al., 2014; van de Voort, 2017). This paints the picture of the baryon cycle within the galaxy virial radius.

The Ovi $\lambda\lambda 1031,1037$ absorption doublet is a common tracer of the CGM, particularly in the high-temperature regime of $T\sim 10^{5}\rm{\,K}$ (e.g., Prochaska et al., 2011; Tumlinson et al., 2011; Stocke et al., 2013; Savage et al., 2014; Churchill et al., 2015; Johnson et al., 2015; Kacprzak et al., 2015; Werk et al., 2016). Oppenheimer et al. (2016) employed the EAGLE simulations to investigate the presence and role of different oxygen species in the CGM, assuming that Ovi is collisionally ionized. They found that Ovi is not the dominant oxygen species in the CGM, and that the column densities for Ovi peak for $L_{*}$ galaxies, while dropping for lower mass halos and group halos. This is thought to be due to the Ovi ionization fraction strongly tracing the virial temperature of the galaxy, where the associated virial temperature for $L_{*}$ galaxies provides the optimal conditions for the presence of Ovi. For less massive galaxies, the virial temperature would be too cool for strong Ovi presence, whereas the virial temperature would be too high for group environments as a large fraction of Ovi is ionized out to higher ionization species. Nelson et al. (2018) found similar trends in the Ovi column density with halo mass, but attributed them to black hole feedback (also see Oppenheimer et al., 2018). Several other works have also shown this trend in both observations (Bielby et al., 2019; Zahedy et al., 2019) and with gaseous halo models (Qu & Bregman, 2018).

Using a sample of quasar absorption-line spectra from HST/COS identified as part of the “Multiphase Galaxy Halos” Survey, Kacprzak et al. (2015) found that Ovi has an azimuthal angle preference, where Ovi tends to reside along the projected major axis ($0^{\circ}\leq\Phi\leq 20^{\circ}$) and/or along the projected minor axis ($60^{\circ}\leq\Phi\leq 90^{\circ}$). They also found a very weak dependence of the Ovi absorption on the galaxy inclination, where the covering fraction of the Ovi gas is roughly constant over all inclination angles except for $i>70^{\circ}$, as the high inclination minimizes the geometrical cross-section of gas flows. Moreover, the mean equivalent widths of Ovi in lower inclination ($i<45^{\circ}$) galaxies and higher inclination ($i>45^{\circ}$) galaxies are consistent with each other.

Previous kinematics studies examined the absorber velocity dispersions of Ovi with pixel-velocity two-point correlation functions (TPCFs) to characterize the absorber velocity dispersions for isolated galaxies (Nielsen et al., 2017). The authors found that there was no dependence of Ovi kinematics on the inclination angle, azimuthal angle, and/or galaxy color, which indirectly suggests a lack of dependence on current star formation activity. They attribute this to Ovi absorbers having ample time to mix and form a kinematically uniform halo surrounding the galaxies. This is consistent with Ford et al. (2014), who found that Ovi in simulations likely traces gas that originates from ancient outflows. These results are in contrast to Mgii kinematics, which depends strongly on galaxy color, redshift, inclination, and azimuthal angle (Nielsen et al., 2015, 2016).

Pointon et al. (2017) examined Ovi kinematics using TPCFs for galaxy group environments and found that the Ovi absorption profiles for galaxy group environments are narrower compared to isolated galaxies. They posit that the virial temperature of the CGM in galaxy group environments (with more massive halos) is hot enough to ionize a larger fraction of Ovi to higher order species to result in a lower Ovi ionization fraction compared to isolated galaxies, consistent with the findings of Oppenheimer et al. (2016). This suggests that halo mass needs to be considered when studying the absorber kinematics of Ovi.

Focusing on Ovi absorber–galaxy kinematics, Tumlinson et al. (2011) found that Ovi absorber–galaxy velocities rarely exceed the host galaxy escape velocity, indicating that the gas is bound. Mathes et al. (2014) found similar results, but noted that the fraction of gas that exceeds host galaxy escape velocities decreases with increasing halo mass. The authors suggested that wind recycling is increasingly important as the halo mass increases, consistent with simulations (Oppenheimer et al., 2010). Most recently, Kacprzak et al. (2019) related Ovi absorber kinematics to host galaxy rotation curves. They found that along the projected galaxy major axis, where accretion is expected, Ovi does not correlate with galaxy rotation kinematics like Mgii (e.g., Steidel et al., 2002; Kacprzak et al., 2010, 2011; Ho et al., 2017). For gas observed along the projected galaxy minor axis, Ovi absorbers best match models of decelerating outflows. Combined with simulations, the authors suggest that Ovi is not an ideal probe of gas accretion or outflows, but rather traces the virial temperature of the host halo.

The work presented here will address both the halo mass dependence of Ovi absorber kinematics, and how Ovi gas flows relative to the host galaxies by examining the absorber–galaxy kinematics, using a subset of Ovi absorbers from the “Multiphase Galaxy Halos” Survey. We employ two TPCF methods: (1) absorber kinematics, which is the approach employed by Nielsen et al. (2017), and (2) absorber–galaxy kinematics. In constructing the TPCFs for absorber–galaxy kinematics (method 2), we apply the velocity offset between the absorber redshift and the galaxy redshift. We also normalize the absorber–galaxy velocities with respect to the circular velocity at the observed impact parameter, $V_{\rm c}(D)$, to take into consideration the range of halo masses in the sample (similar to the normalization done in Nielsen et al., 2016). Average absorption profiles are presented to complement the TPCFs by providing information about the optical depth.

In Section 2, we present the sample and elaborate on how the kinematics are quantified, namely with the TPCFs and average absorption profiles. In Section 3, we present the mass dependence of absorber kinematics, comparing our sample to the group environment sample published in Pointon et al. (2017) and the simulated aperture column densities presented by Oppenheimer et al. (2016). Section 4 presents new absorber–galaxy kinematics for various subsamples segregated by galaxy redshift, $z_{\rm gal}$, inclination, $i$, azimuthal angle, $\Phi$, and halo mass, $\log(M_{\rm h}/M_{\odot})$. In Section 5, we discuss the halo mass dependence of Ovi absorber kinematics and non-virialized motions in the form of outflows. Finally, we conclude in Section 6. Throughout we assume a $\Lambda$CDM cosmology ($H_{0}=70{\rm\,km\,s^{-1}\,Mpc^{-1}},\Omega_{M}=0.3,\Omega_{\Lambda}=0.7$).