MusE GAs FLOw and Wind (MEGAFLOW) IV: A two sightline tomography of a galactic wind (1911.09342v1)

Abstract: Galactic outflows are thought to eject baryons back out to the circum-galactic medium (CGM). Studies based on metal absorption lines (MgII in particular) in the spectra of background quasars indicate that the gas is ejected anisotropically, with galactic winds likely leaving the host in a bi-conical flow perpendicular to the galaxy disk. In this paper, we present a detailed analysis of an outflow from a z = 0.7 "green-valley" galaxy (log($M_*$/$\mathrm{M}\odot$) = 9.9; SFR = 0.5 $\mathrm{M}\odot\,\mathrm{yr}^{-1}$) probed by two background sources part of the MUSE Gas Flow and Wind (MEGAFLOW) survey. Thanks to a fortuitous configuration with a background quasar (SDSSJ1358+1145) and a bright background galaxy at $z = 1.4$, both at impact parameters of $\approx 15\,\mathrm{kpc}$, we can - for the first time - probe both the receding and approaching components of a putative galactic outflow around a distant galaxy. We measure a significant velocity shift between the MgII absorption from the two sightlines ($84\pm17\,\mathrm{km}\,\mathrm{s}^{-1}$), which is consistent with the expectation from our simple fiducial wind model, possibly combined with an extended disk contribution.

• The paper presents a dual sightline observation that measures an 84±17 km/s velocity shift in a bi-conical galactic wind.
• It utilizes MUSE and UVES instruments to analyze Mg II absorption, confirming a mass-loading factor of approximately 4 and precise CGM kinematics.
• The study underscores the importance of multi-sightline data in unraveling galactic wind dynamics and their impact on galaxy evolution.

Analysis of Galactic Outflows in the MEGAFLOW Survey: A Two-Sightline Approach

The paper of galactic outflows and their interactions with the circum-galactic medium (CGM) is a pivotal aspect of contemporary astrophysics. The paper "MusE GAs FLOw and Wind (MEGAFLOW) IV: A two sightline tomography of a galactic wind" explores the complex mechanisms of baryon expulsion from galaxies using dual sightline observations. This research utilizes the fortuitous alignment of a quasar and a background galaxy to probe a $z=0.7$ galaxy, which presents a unique opportunity to dissect the CGM's kinematics and morphology.

Observational Strategy and Methodology

The MEGAFLOW survey, an ambitious project aimed at cataloging galactic winds via strong metal absorption lines, provides the observational backbone of this paper. The authors capitalize on the alignment of two background light sources at impact parameters of approximately 15 kpc from the galaxy of interest. This configuration allows for the unprecedented simultaneous observation of both the receding and approaching components of a potential bi-conical galactic wind.

In terms of instrumentation, the use of the MUSE (Multi Unit Spectroscopic Explorer) and UVES (Ultraviolet and Visual Echelle Spectrograph) on the Very Large Telescope (VLT) provides a comprehensive spectral analysis. The MUSE data offer moderate resolution across a wide field, while UVES provides high-resolution spectra crucial for resolving fine details in velocity structures. This dual-faceted approach ensures a robust examination of absorption features, particularly focusing on the Mg II doublet, a well-established tracer of the cool, photo-ionized CGM.

Key Findings and Numerical Results

The analysis reveals a significant velocity shift of $84\pm17$ km/s between the absorption lines of the two sightlines, a result consistent with a bi-conical outflow model. This velocity offset is a vital clue to the wind's dynamics, suggesting outflow velocities around 100 km/s. The spectral data show saturated absorption profiles, indicating a substantial coverage of CGM gas along the sightlines, and providing critical insights into the anisotropic distribution of the outflow material.

Moreover, the paper estimates the mass outflow rate, energy, and momentum outflow rates from the galaxy. These calculations reveal a mass-loading factor, $\eta$, of approximately 4, which aligns with those derived from other empirical and simulation studies of similar systems. Such consistency underscores the potential universality of the galactic wind mechanisms across different environments and redshifts.

Theoretical and Practical Implications

Theoretical models in this work describe a plausible mechanism through which galaxies mediate mass and energy transfer with their surrounding medium, impacting the galaxy's evolution and star formation rates. The bi-conical outflow model serves as an essential framework for interpreting the anisotropic movement of gas, suggesting a structured and significant feedback mechanism influencing galaxy evolution.

Practically, these findings have significant bearing on our understanding of the CGM's role in cosmic baryon cycles. The research highlights the necessity of multi-sightline observations for unraveling the complexities of galactic winds, thus advocating for future surveys and instruments designed to exploit such configurations.

Future Directions and Challenges

This paper raises several intriguing questions for further research. The observed structures invite more detailed investigations into the coherence scale of the CGM and the nature of high-velocity absorptions, akin to the Milky Way's high-velocity clouds. Future work could extend the methodology to higher redshifts and more diverse galactic environments, potentially utilizing next-generation facilities to enhance resolution and sensitivity.

Additionally, there remains a challenge in definitively separating the influences of disk rotation and outflow components in the absorption data. High-resolution simulations aligned with rich observational datasets could provide a pathway to disentangle these components. As methodologies become more sophisticated, the MEGAFLOW approach serves as a crucial template, encouraging innovation in observational astrophysics to explore the multifaceted interactions within and beyond galaxies.