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1. A complete census of circumgalactic Mg  ii at redshift z ≲ 0.5

   https://doi.org/10.1093/mnras/stab360  

   Huang, Yun-Hsin ; Chen, Hsiao-Wen ; Shectman, Stephen A ; Johnson, Sean D ; Zahedy, Fakhri S ; Helsby, Jennifer E ; Gauthier, Jean-René ; Thompson, Ian B ( February 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT This paper presents a survey of Mg ii absorbing gas in the vicinity of 380 random galaxies, using 156 background quasi-stellar objects (QSOs) as absorption-line probes. The sample comprises 211 isolated (73 quiescent and 138 star-forming galaxies) and 43 non-isolated galaxies with sensitive constraints for both Mg ii absorption and H α emission. The projected distances span a range from d = 9 to 497 kpc, redshifts of the galaxies range from z = 0.10 to 0.48, and rest-frame absolute B-band magnitudes range from MB = −16.7 to −22.8. Our analysis shows that the rest-frame equivalent width of Mg ii, Wr(2796), depends on halo radius (Rh), B-band luminosity(LB), and stellar mass (Mstar) of the host galaxies, and declines steeply with increasing d for isolated, star-forming galaxies. At the same time, Wr(2796) exhibits no clear trend for either isolated, quiescent galaxies or non-isolated galaxies. In addition, the covering fraction of Mg ii absorbing gas 〈κ〉 is high with 〈κ〉 ≳ 60 per cent at <40 kpc for isolated galaxies and declines rapidly to 〈κ〉 ≈ 0 at d ≳ 100 kpc. Within the gaseous radius, the incidence of Mg ii gas depends sensitively on both Mstar and the specific star formation rate inferred from H α. Different from what is known for massive quiescent haloes, the observed velocity dispersion of Mg ii absorbing gas around star-forming galaxies is consistent with expectations from virial motion, which constrains individual clump mass to $m_{\rm cl} \gtrsim 10^5 \, \rm M_\odot$ and cool gas accretion rate of $\sim 0.7\!-\!2 \, \mathrm{ M}_\odot \, \rm yr^{-1}$. Finally, we find no strong azimuthal dependence of Mg ii absorption for either star-forming or quiescent galaxies. Our results demonstrate that multiple parameters affect the properties of gaseous haloes around galaxies and highlight the need of a homogeneous, absorption-blind sample for establishing a holistic description of chemically enriched gas in the circumgalactic space. 

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2. A complex multiphase DLA associated with a compact group at z  = 2.431 traces accretion, outflows, and tidal streams

   https://doi.org/10.1093/mnras/stac1824  

   Nielsen, Nikole M. ; Kacprzak, Glenn G. ; Sameer ; Murphy, Michael T. ; Nateghi, Hasti ; Charlton, Jane C. ; Churchill, Christopher W. ( July 2022 , Monthly Notices of the Royal Astronomical Society)

   As part of our program to identify host galaxies of known z = 2–3 Mg ii absorbers with the Keck Cosmic Web Imager (KCWI), we discovered a compact group giving rise to a z = 2.431 DLA with ultrastrong Mg ii absorption in quasar field J234628+124859. The group consists of four star-forming galaxies within 8–28 kpc and v ∼ 40–340 km s−1 of each other, where tidal streams are weakly visible in deep HST imaging. The group geometric centre is D = 25 kpc from the quasar (D = 20–40 kpc for each galaxy). Galaxy G1 dominates the group (1.66L*, SFRFUV = 11.6 M⊙ yr−1) while G2, G3, and G4 are less massive (0.1–0.3L*, SFRFUV = 1.4–2.0 M⊙ yr−1). Using a VLT/UVES quasar spectrum covering the H i Lyman series and metal lines such as Mg ii, Si iii, and C iv, we characterized the kinematic structure and physical conditions along the line of sight with cloud-by-cloud multiphase Bayesian modelling. The absorption system has a total $\log (N({{{\rm H}\,\rm{\small I}}})/{\rm cm}^{-2})=20.53$ and an $N({{{\rm H}\,\rm{\small I}}})$-weighted mean metallicity of log (Z/Z⊙) = −0.68, with a very large Mg ii linewidth of Δv ∼ 700 km s−1. The highly kinematically complex profile is well modelled with 30 clouds across low- and intermediate-ionization phases with values ${13\lesssim \log (N({{{\rm H}\,\rm{\small I}}})/{\rm cm}^{-2})\lesssim 20}$ and −3 ≲ log (Z/Z⊙) ≲ 1. Comparing these properties to the galaxy properties, we infer a wide range of gaseous environments, including metal-rich outflows, metal-poor IGM accretion, and tidal streams from galaxy–galaxy interactions. This diversity of structures forms the intragroup medium around a complex compact group environment at the epoch of peak star formation activity. Surveys of low-redshift compact groups would benefit from obtaining a more complete census of this medium for characterizing evolutionary pathways.

    

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3. The MOSDEF survey: the mass–metallicity relationship and the existence of the FMR at z ∼ 1.5

   https://doi.org/10.1093/mnras/stab1793  

   Topping, Michael W ; Shapley, Alice E ; Sanders, Ryan L ; Kriek, Mariska ; Reddy, Naveen A ; Coil, Alison L ; Mobasher, Bahram ; Siana, Brian ; Freeman, William R ; Shivaei, Irene ; et al ( July 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We analyse the rest-optical emission-line ratios of z ∼ 1.5 galaxies drawn from the Multi-Object Spectrometer for Infra-Red Exploration Deep Evolution Field (MOSDEF) survey. Using composite spectra, we investigate the mass–metallicity relation (MZR) at z ∼ 1.5 and measure its evolution to z = 0. When using gas-phase metallicities based on the N2 line ratio, we find that the MZR evolution from z ∼ 1.5 to z = 0 depends on stellar mass, evolving by $\Delta \rm log(\rm O/H) \sim 0.25$ dex at M*< $10^{9.75}\, \mathrm{M}_{\odot }$ down to $\Delta \rm log(\rm O/H) \sim 0.05$ at M* ≳ $10^{10.5}\, \mathrm{M}_{\odot }$. In contrast, the O3N2-based MZR shows a constant offset of $\Delta \rm log(\rm O/H) \sim 0.30$ across all masses, consistent with previous MOSDEF results based on independent metallicity indicators, and suggesting that O3N2 provides a more robust metallicity calibration for our z ∼ 1.5 sample. We investigated the secondary dependence of the MZR on star formation rate (SFR) by measuring correlated scatter about the mean M*-specific SFR and M*−$\log (\rm O3N2)$ relations. We find an anticorrelation between $\log (\rm O/H)$ and sSFR offsets, indicating the presence of a M*−SFR−Z relation, though with limited significance. Additionally, we find that our z ∼ 1.5 stacks lie along the z = 0 metallicity sequence at fixed μ = log (M*/M⊙) − 0.6 × $\log (\rm SFR / M_{\odot } \, yr^{-1})$ suggesting that the z ∼ 1.5 stacks can be described by the z = 0 fundamental metallicity relation (FMR). However, using different calibrations can shift the calculated metallicities off of the local FMR, indicating that appropriate calibrations are essential for understanding metallicity evolution with redshift. Finally, understanding how [N ii]/H α scales with galaxy properties is crucial to accurately describe the effects of blended [N ii] and H α on redshift and H α fiux measurements in future large surveys utilizing low-resolution spectra such as with Euclid and the Roman Space Telescope. 

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4. The MOSDEF survey: a comprehensive analysis of the rest-optical emission-line properties of z ∼ 2.3 star-forming galaxies

   https://doi.org/10.1093/mnras/stab119  

   Runco, Jordan N ; Shapley, Alice E ; Sanders, Ryan L ; Topping, Michael W ; Kriek, Mariska ; Reddy, Naveen A ; Coil, Alison L ; Mobasher, Bahram ; Siana, Brian ; Freeman, William R ; et al ( February 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT We analyse the rest-optical emission-line spectra of z ∼ 2.3 star-forming galaxies in the complete MOSFIRE Deep Evolution Field (MOSDEF) survey. In investigating the origin of the well-known offset between the sequences of high-redshift and local galaxies in the [O iii]λ5008/Hβ versus [N ii]λ6585/Hα (‘[N ii] BPT’) diagram, we define two populations of z ∼ 2.3 MOSDEF galaxies. These include the high population that is offset towards higher [O iii]λ5008/Hβ and/or [N ii]λ6585/Hα with respect to the local SDSS sequence and the low population that overlaps the SDSS sequence. These two groups are also segregated within the [O  iii]λ5008/Hβ versus [S ii]λλ6718,6733/Hα and the [O iii]λλ4960,5008/[O ii ]λλ3727,3730 (O32) versus ([O  iii]λλ4960,5008+[O ii]λλ3727,3730)/Hβ (R23) diagrams, which suggests qualitatively that star-forming regions in the more offset galaxies are characterized by harder ionizing spectra at fixed nebular oxygen abundance. We also investigate many galaxy properties of the split sample and find that the high sample is on average smaller in size and less massive, but has higher specific star formation rate (SFR) and SFR surface density values and is slightly younger compared to the low population. From Cloudy+BPASS photoionization models, we estimate that the high population has a lower stellar metallicity (i.e. harder ionizing spectrum) but slightly higher nebular metallicity and higher ionization parameter compared to the low population. While the high population is more α-enhanced (i.e. higher α/Fe) than the low population, both samples are significantly more α-enhanced compared to local star-forming galaxies with similar rest-optical line ratios. These differences must be accounted for in all high-redshift star-forming galaxies – not only those ‘offset’ from local excitation sequences. 

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5. Origin of Weak Mg ii and Higher-ionization Absorption Lines in Outflows from Intermediate-redshift Dwarf Galaxies

   https://doi.org/10.3847/1538-4357/abe2a4  

   Fujita, Akimi ; Misawa, Toru ; Charlton, Jane C. ; Meiksin, Avery ; Low, Mordecai-Mark Mac ( March 2021 , The Astrophysical Journal)

   Abstract Observations at intermediate redshifts reveal the presence of numerous compact, weak Mg ii absorbers with near to supersolar metallicities, often surrounded by extended regions that produce C iv and/or O vi absorption, in the circumgalactic medium at large impact parameters from luminous galaxies. Their origin and nature remain unclear. We hypothesize that undetected satellite dwarf galaxies are responsible for producing some of these weak Mg ii absorbers. We test our hypothesis using gas dynamical simulations of galactic outflows from a dwarf galaxy with a halo mass of 5 × 10 9 M ⊙ , as might be falling into a larger L * halo at z = 2. We find that thin, filamentary, weak Mg ii absorbers (≲100 pc) are produced in two stages: (1) when shocked core-collapse supernova (SN II)–enriched gas descending in a galactic fountain gets shock compressed by upward flows driven by subsequent SN II and cools (phase 1) and, later, (2) during an outflow driven by Type Ia supernovae that shocks and sweeps up pervasive SN II–enriched gas, which then cools (phase 2). The Mg ii absorbers in our simulations are continuously generated by shocks and cooling with moderate metallicity ∼0.1–0.2 Z ⊙ but low column density <10 12 cm −2 . They are also surrounded by larger (0.5–1 kpc) C iv absorbers that seem to survive longer. Larger-scale (>1 kpc) C iv and O vi clouds are also produced in both expanding and shocked SN II–enriched gas. Observable ion distributions from our models appear well converged at our standard resolution (12.8 pc). Our simulation highlights the possibility of dwarf galactic outflows producing highly enriched multiphase gas. 

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