2025A&A...694A..67B

We present the design, rationale, properties, and catalogues of the MusE Gas FLOw and Wind survey (MEGAFLOW) of cool gaseous halos of z ≃ 1.0 galaxies, using low-ionisation Mg II absorption systems. The survey consists of 22 quasar fields selected from the Sloan Digital Sky Survey (SDSS), including multiple (≥3) strong Mg II absorption lines over the redshift range of 0.3 < z < 1.5. Each quasar was observed with the Multi-Unit Spectroscopic Explorer (MUSE) and the Ultraviolet and Visual Echelle Spectrograph (UVES), for a total of 85 hours and 63 hours, respectively. The UVES data resulted in 127 Mg II absorption lines over 0.25 < z < 1.6, with a median rest-frame equivalent width (REW) 3σ limit of ≈0.05 Å. The MUSE data resulted in ∼2400 galaxies, of which 1403 are characterised by a redshift confidence of ZCONF > 1; this amounts to more than 60 galaxies per arcmin². They were identified using a dual detection algorithm based on both continuum and emission line objects. The achieved [O II] 50% completeness is 3.7_–0.6^+0.8 × 10^–18 erg s^–1 cm^–2 (corresponding to an SFR of > 0.01 M_⊙ yr^–1 at z = 1) using realistic mock [O II] emitters and the 50% completeness is m_F775W ≈ 26 AB magnitudes for continuum sources. We find that (i) the fraction of [O II] emitting galaxies that have no continuum is ∼15%; (ii) the success rate in identifying at least one galaxy within 500 km s^–1 and 100 kpc is ≈90% for Mg II absorptions with $ W_{\mathrm{r}}^{2796}\gtrsim0.5 $ Å; (iii) the mean number of galaxies per Mg II absorption is 2.9 ± 1.6 within the MUSE field-of-view; (iv) of the 80 Mg II systems at 0.3 < z < 1.5, 40 (20) have 1 (2) galaxies within 100 kpc, respectively; and, finally, (v) all but two host galaxies have stellar masses of M_⋆ > 10⁹ M_⊙ and star formation rates of > 1 M_⊙ yr^–1.