A z$\sim$1 galactic-scale outflow transversally mapped to $\sim$50 kpc through gravitational-arc tomography

Abstract

We report spatially resolved measurements of cool gas traced by Mg II and Fe II absorption in the circumgalactic medium (CGM) of a star-forming galaxy at $z\sim1$ (G1). The fortuitous alignment of a background gravitational arc at z$\sim$2.4 provides seven closely spaced ($\sim$6 kpc) transverse sightlines along the minor axis of G1, probing its CGM out to $\sim$50 kpc. This geometry allows us to detect a galactic-scale outflow simultaneously in down-the-barrel and transverse directions, where blue-shifted Mg II absorption is detected along both types of sightlines, revealing a large-scale, collimated wind. We measure blue-shifted line-of-sight velocities of $v_{\mathrm{los}}$ $\sim$ 62 - 239 km s$^{-1}$ and line-of-sight velocity dispersions $σ_{\mathrm{los}}$ $\sim$ 53 - 133 km s$^{-1}$, suggesting a structure dominated by bulk motion. De-projection of $v_{\mathrm{los}}$ along the minor axis indicates that the outflow material barely approaches the escape velocity and is likely to be gravitationally bound to G1. We constrain an outflow opening angle $θ_c\sim$ 18$^\circ$ - 25$^\circ$, and a mass outflow rate of $ \dot{M}_{\mathrm{out}}$ $\gtrsim$ 0.06 $M_\odot$ yr$^{-1}$, corresponding to a mass loading factor $η$ $\gtrsim$ 0.004, estimated within $\sim$10 - 50 kpc ($\sim$ 0.05 - 0.3 $R_\text{vir}$) of the galaxy centre. Our measurements, combined with previous arc tomography data along the major axis, indicate that normalizing impact parameters by galaxy B-band luminosity substantially reduces scatter in the established anti-correlation between Mg II equivalent width and impact parameter, while also diminishing possible excess of Mg II equivalent width towards the minor axis.

Figures (9)

• Figure 1: Left: HST RGB (F160W, F125W, and F814W) image centred on the lensing cluster RCS 0224-0002 . Labels indicate the position of G1, and the lensed sources reported by 2017MNRAS.467.3306S, used for constraining the lens model. Redshifts of the lensed galaxies A, B, and C are $4.877$, $2.395$, and $5.498$, respectively. Centre: MUSE RGB image constructed by convolving the cube with the transmission curves of the broad-band SDSS i, r, g filters. The white square, centred on G1, shows the size of the rightmost panels. Top right: HST/F814W cutout centred on G1. Red, green, and blue contours show the $3\sigma$, $5\sigma$, and $7\sigma$ levels where its [O ii] emission is detected on the MUSE data. The typical PSF FWHM of 0.8 of the MUSE observations is represented by the diameter of the solid black circle. Bottom right: Magnification map centred on G1 showing the differential lensing effect at redshift of G1. Overlaid HST/F814W contours delineate the distribution of light from surrounding sources. Sources enclosing areas of higher magnification correspond to cluster member galaxies.
• Figure 2: Left: De-lensed F814W cutout centred on G1. Black contours show representative flux levels of the best-fit Sérsic model. Centre: Best-fit single-component Sérsic model obtained with GALFIT. Black contours indicate the same flux levels as in the left panel. Best-fit $R_e$, PA, $q$, and $n$ values are listed in Table \ref{['tab:G1_properties']}. Right: Flux residuals from the best-fit model.
• Figure 3: De-lensed (absorber-plane) centroid-velocity-map of the [O ii] emission from G1 and ERD model kinematics. Zero velocity is set at $z_\text{G1}$. Open rhomboids indicate native MUSE spaxels mapped into the absorber plane, with colours representing the measured line-of-sight [O ii] velocity centroid. The background colourmap shows the line-of-sight velocity field from the best-fit arctan disk model obtained with $\texttt{GalPak}^{\text{3D}}$. Dashed lines mark the kinematics major (red) and minor (blue) axes of G1. Grey contours trace the HST/F814W flux distribution. Note that the spaxel-based velocities are shown for illustrative purposes only: $\texttt{GalPak}^{\text{3D}}$ does not extract velocities per spaxel, but fits a synthetic data-cube—convolved with the instrumental line spread function and PSF—which is directly compared to the observed cube.
• Figure 4: Top left: Image-plane (i.e., as observed) MUSE pseudo-broad-band image (SDSS r) around G1 and the background gravitational arc B. Open squares show the $4\times4$ binned spaxels with detected Mg ii absorption. Spaxels with black (white) edges correspond to transverse (down-the-barrel) sightlines. Top right: Absorber-plane (de-lensed) distribution of the measured Mg ii equivalent widths $W_r(2796)$. Contours of the de-lensed HST/F814W image of the field are shown for spatial reference. Cluster galaxies and other non-lensed sources should not appear in the absorber plane, but we included them in the contours to provide a spatial reference. Bottom left: Absorber-plane distribution of the Mg ii line-of-sight velocities with respect to $z_\text{G1}$. The background colourmap shows the projected line-of-sight velocity of a rotating extended disk model for G1. The kinematic major and minor axes of G1 are indicated with the red and blue dashed lines, respectively. Black solid ellipses show a schematic model of a conical outflow along the minor axis of G1, with opening angle $\theta_c=17.8^\circ$ and inclination $i=65.5^\circ$. Bottom right: Absorber-plane distribution of the Mg ii line-of-sight velocity dispersion after subtracting the MUSE instrumental line spread function. Black curves and red and blue dashed lines indicate the same outflow model and kinematics axes of G1 as in the bottom left panel.
• Figure 5: Panel a: Rest-frame Mg ii equivalent width, $W_r(2796)$, as a function of impact parameter, $\rho$, to the absorber galaxy. Our arc tomography measurements are shown as blue stars. Black dots represent quasar-galaxy pairs at $z \lesssim 0.5$ from 2021MNRAS.502.4743H. The gray dashed line and shaded region correspond to the log-linear fit ($\pm1\sigma$) from 2013ApJ...776..114N based on the MAGIICAT sample at $0.07 \leq z \leq 1.1$. Coloured and semi-transparent symbols show individual measurements and upper limits, respectively, from gravitational arc systems from 2018Natur.554..493L, 2020MNRAS.491.4442L, 2021MNRAS.507..663T, and 2025ApJ...986..190S. Panel b: Comparison between the system presented in this work (probed primarily along the minor axis) and the system from 2020MNRAS.491.4442L (probed along the major axis). Symbols are coloured by azimuthal angle, $\phi$, of each sightline relative to the galaxy’s major axis. Overlaid are the results of statistical analyses, including the Energy test statistic and partial Pearson correlation coefficients between $W_r(2796)$ and $\rho$, and $W_r(2796)$ and $\phi$. Panel c: Same as Panel b, but with impact parameters scaled by galaxy B-band absolute magnitude using Eq. \ref{['eq:rho_scaled']}, following 2010ApJ...714.1521C