Mg II Absorption Lines

• Mg II absorption lines are resonance doublet transitions from magnesium ions that trace cool, metal-enriched gas in galaxies and their halos.
• Automated surveys employ continuum normalization and matched-filter techniques to detect these lines with high completeness and measure key properties such as equivalent widths and doublet ratios.
• Studies leveraging Mg II absorbers yield insights into galaxy feedback, dust characteristics, and chemical evolution in the circumgalactic medium.

Magnesium II (Mg II) absorption lines are among the most widely used diagnostics of astrophysical gas in galaxies and the circumgalactic and intergalactic media. The Mg II λλ2796, 2803 Å resonance doublet and associated subordinate and triplet lines probe a wide range of physical processes, from the distribution and kinematics of cool gas around galaxies to chromospheric structure in stellar atmospheres and the properties of outflowing material in active galactic nuclei. Extensive surveys of Mg II absorption in large spectroscopic databases have enabled statistical studies of halo gas, cosmic metallicity, dust properties, galaxy feedback, quasar environments, and their evolution from low to high redshift.

1. Atomic Transitions, Physical Regimes, and Astrophysical Contexts

Mg II λλ(2796.3543, 2803.5315) Å are resonance lines corresponding to the 3s $^2$S$_{1/2}$ → 3p $^2$P$^\circ_{3/2,1/2}$ transitions, with oscillator strengths $f_{2796} \approx 0.61$ and $f_{2803} \approx 0.31$ (Raghunathan et al., 2016, Napolitano et al., 2023). The doublet is typically seen in absorption in both extragalactic and stellar sources, with doublet ratios (W$_0$(2796)/W$_0$(2803)) ranging from 2 (optically thin case) toward 1 (saturated regime).

Astrophysically, Mg II absorption arises in low-ionization (Mg$^+$) gas. In the extragalactic context, it traces neutral and mildly ionized hydrogen ($N$(HI) $\sim 10^{16}$–$10^{22}$ cm$^{-2}$), making it a sensitive probe of cool, metal-enriched gas in the circumgalactic medium (CGM) and interstellar/halo structures of galaxies across $z\approx0.3$–6 (Raghunathan et al., 2016, Zou et al., 2020, Napolitano et al., 2023). Mg II is also an important diagnostic in the solar chromosphere and other late-type stellar atmospheres, where additional subordinate and triplet transitions (e.g., 279.160, 279.875, 279.882 nm) on the wings of the stronger resonance h&k lines offer complementary constraints on temperature and density stratification (Pereira et al., 2015).

2. Detection, Measurement, and Statistical Incidence

Automated pipelines in SDSS, DESI, and other large surveys employ continuum normalization (via mean filters, nonnegative matrix factorization, or composite templates) and matched-filter or MCMC-based doublet detection algorithms to identify Mg II absorbers (Raghunathan et al., 2016, Napolitano et al., 2023, Zhu et al., 2012, Lawther et al., 2012). Candidate absorbers are flagged by the coincident detection of two troughs at the required rest-frame separation ($\Delta \lambda \approx 7.2$ Å), with minimum SNR thresholds and often confirmation from Fe II lines.

Key observed quantities include:

• Rest-frame equivalent width: $$W_r = (1/(1+z))\int [1-F(\lambda)/C(\lambda)] d\lambda$$
• Doublet ratio: $W_0$(2796)/$W_0$(2803), probing optical depth
• Velocity offset: $$v_{\rm abs} = c\,\frac{z_{\rm em}-z_{\rm abs}}{1+z_{\rm abs}}$$
• Redshift-path density: statistical completeness correction for varying SNR, coverage, and detection limits

Completeness and purity in recent catalogs exceed 90% for $W_0 \gtrsim 0.5$ Å in high-SNR spectra (Raghunathan et al., 2016, Napolitano et al., 2023). The largest SDSS surveys detect $\sim$40,000–800,000 Mg II absorbers over $0.3Zhu et al., 2012, Napolitano et al., 2023).

The equivalent width distribution for strong absorbers ($W_0\geq0.6$ Å) is exponential at all redshifts, $$\partial^2 N/\partial z\,\partial W_0 = [N^*(z)/W^*(z)]\exp[-W_0/W^*(z)]$$, with $W^*(z)$ peaking at $z\sim1.5$–1.7 (Lawther et al., 2012). Incidence rate $dN/dz$ especially for $W_0>1$ Å systems, closely tracks the cosmic star formation density, peaking at $z\sim2$ and declining at higher $z$ (Zhu et al., 2012).

3. Circumgalactic Gas around Galaxies: Covering Fraction, Profiles, and Host Dependence

Mg II reliably traces cool, metal-enriched gas halos. Surveys of quasar sightlines passing near galaxies show:

• Radial profiles: Mg II equivalent width declines steeply with galactocentric radius $b$, typically described by a singular isothermal sphere model or exponential (Bordoloi et al., 2011). The characteristic gas radius, $R_{\rm gas} \sim 110$ kpc for $L^*$ galaxies at $z\sim0.5$–1.
• Covering fraction: The probability of intercepting $W_0>0.6$ Å Mg II absorption is $f_c\sim0.1$–0.5 within 100 kpc for star-forming galaxies, declining to zero beyond (Lovegrove et al., 2011, Bowen et al., 2010, Bordoloi et al., 2011). Luminous red galaxies have much lower $f_c$ ($\sim10$–15%), compatible with hot virialized halos and an absence of star formation-driven outflows (Bowen et al., 2010).
• Host dependence: Blue/star-forming galaxies have much stronger and more extended Mg II absorption than red/quiescent galaxies, with $W_r$ scaling weakly with stellar mass for star-formers, $W_r(b<50\,{\rm kpc})_{\rm blue} \propto M_*^{0.36}$ (Bordoloi et al., 2011).
• Environment: Group environments do not enhance/suppress Mg II absorption beyond a simple superposition of individual galaxy halos (Bordoloi et al., 2011).
• Anisotropy: Strong azimuthal dependence near disk galaxies, with elevated $W_r$ along the minor axis within $b<50$ kpc (a factor $\sim2$ contrast), is interpreted as biconical outflows (wind-driven structures) (Bordoloi et al., 2011).

These findings establish Mg II as a principal tool for mapping cool CGM and probing the physics of feedback and accretion in galaxies.

4. Physical Conditions, Cloud Structure, and Metallicity Evolution

Stacked composite spectra reveal that individual Mg II absorbers represent a “foam” of compact, metal-rich clouds:

• Physical conditions: Photoionization modeling yields $n_{\rm H} \sim 0.3$ cm$^{-3}$, $T\sim2500$ K, and neutral fractions $n_{\rm HI}/n_{\rm H}\sim0.8$–0.9 (Lan et al., 2017).
• Sizes and masses: Characteristic line-of-sight size $l\sim30$ pc; baryonic mass $M_{\rm cloud}\sim10^3M_\odot$ (Lan et al., 2017).
• Ensemble morphology: To achieve the observed covering fractions (e.g., $f_c\sim0.5$ within 50 kpc), galaxy halos must host $\sim10^6$ such clouds (“foamy” CGM).
• Metallicity: Stacked spectra and ionization corrections give [Zn/H] evolving from $\sim-0.6$ dex at $z\sim2.5$ to $\sim$ solar at $z\sim0.5$, tracking the cumulative star formation and implying effective retention/enrichment of CGM gas (Lan et al., 2017).
• Cloud sizes: Direct constraints from multiepoch lensed quasar sightlines indicate individual absorber inhomogeneity and variances in $W_0$ over scales of $4$–$12$ kpc, supporting a patchy morphology (Okoshi et al., 2021).

Mg II absorption is thus directly sensitive to the small-scale structure and chemical evolution of galactic halos.

5. Associated, Intervening, and Quasar-driven Mg II Absorption

A key distinction is made between “associated” ($|v_{\rm abs}| \lesssim 2000$ km s$^{-1}$) and “intervening” absorbers. Associated systems, defined empirically via a strong $dn/d\beta$ and $f_c$ excess within $|v_{\rm abs}|<2000$ km s$^{-1}$, predominantly arise in outflows or the immediate CGM of active galactic nuclei (Zhi-Fu et al., 2018):

• Incidence: For Mg II NALs with $W_r>0.3$ Å, the associated fraction sharply rises within $|v_{\rm abs}|<2000$ km s$^{-1}$, and the evolutionary behavior of $dn/dz$ differs from that of intervening systems (Zhi-Fu et al., 2018).
• Physical effects: Broad absorption lines (BALs) in quasars show variable and even accelerated outflows, with measured $a\sim1$ cm s$^{-2}$ for Mg II/Al III, plausibly due to radiative acceleration from the central source (Lu et al., 2020).
• Host properties: The presence of Mg II associated absorption lines (AALs) does not correspond to higher star formation rates in quasar hosts, but is accompanied by redder continua, suggesting orientation (obscuration) rather than merger-driven phases (Chen et al., 12 Mar 2025).

At high redshift ($z>2$), strong Mg II absorbers have broad velocity structures and are associated with sub-L$^*$, compact galaxies and disturbed environments, suggestive of outflows or superwinds (Zou et al., 2020).

6. Mg II Absorbers as Tracers of Dust, Dust Properties, and QSO Redshift Systematics

Mg II absorption systems are key probes of dust and its cosmic evolution:

• Extinction and reddening: The mean color excess for intervening systems is $\langle E(B-V) \rangle \sim0.04$ mag, rising strongly with equivalent width (EW) and for associated systems, with $E(B-V)$ reaching $\sim0.15$ mag for zero velocity offset absorbers (Napolitano et al., 19 Dec 2024).
• Redshift evolution: $E(B-V)$ decreases with increasing absorber redshift and is higher in associated systems, compatible with dust enrichment over time (Napolitano et al., 19 Dec 2024).
• Impact on quasar redshift fitting: Strong Mg II systems bias pipeline redshifts (e.g., in DESI), especially at $z>1.5$, by distorting QSO emission lines. Masking Mg II absorption alleviates these biases ($\Delta z \sim 0.005$) (Napolitano et al., 19 Dec 2024).
• 2175-Å dust bump: Rare but significant detection of the 2175-Å extinction feature in $\sim$1% of strong Mg II systems at $z\sim1$–$1.8$ indicates the presence of carbonaceous grains, with bump strengths similar to the LMC but weaker than the Milky Way (Jiang et al., 2011).
• Connection to cosmic chemical evolution: The EW and incidence evolution of Mg II absorbers, their $E(B-V)$, and the link to strong emission lines support a role as tracers of star formation, feedback, and metal/dust budgets in the universe.

7. Mg II Line Diagnostics in Stellar and Solar Physics

In the solar chromosphere, the subordinate Mg II triplet lines (279.160, 279.875, 279.882 nm) provide a diagnostic complementary to the stronger h&k resonance lines (Pereira et al., 2015):

• Line formation: Triplet absorption dominates in quiet-Sun columns where the source function decouples from the Planck function due to radiative losses. Emission occurs only for localized heating ($\Delta T>1500$ K, $n_e>10^{17-18}$ m$^{-3}$) at column masses $m_c \gtrsim 5\times10^{-4}$ g cm$^{-2}$.
• Diagnostics: The core-to-wing intensity ratio encodes the local chromospheric temperature rise; multi-peaked emission profiles reveal vertical structure. The lines are sensitive to height $z\sim0.6$–1.2 Mm (column masses $10^{-4}$–$10^{-1}$ g cm$^{-2}$), providing constraints on lower chromospheric heating not accessible to h&k (Pereira et al., 2015).
• 3D radiative-MHD simulations: Synthesis of Mg II triplet spectra in Bifrost runs matches IRIS observations, confirming the diagnostic value for mapping localized heating events and dynamic atmospheric structure.

• SDSS DR12 Mg II absorber catalog and statistical evolution (Raghunathan et al., 2016)
• DESI Mg II detection, completeness, and sample characteristics (Napolitano et al., 2023)
• Covering fraction, radial profiles, and environmental effects (Bordoloi et al., 2011, Lovegrove et al., 2011, Bowen et al., 2010)
• Cloud morphology, metallicity, and “foamy" CGM (Lan et al., 2017, Okoshi et al., 2021)
• Beam size, absorber scale constraints, and GRB–QSO discrepancy (Lawther et al., 2012)
• Associated NALs, velocity distribution, and quasar feedback (Zhi-Fu et al., 2018, Lu et al., 2020, Chen et al., 12 Mar 2025)
• Cosmic incidence and link to star formation history (Zhu et al., 2012, Lawther et al., 2012)
• Dust extinction, 2175-Å bump, and redshift bias (Napolitano et al., 19 Dec 2024, Jiang et al., 2011)
• Solar/stellar diagnostics with Mg II triplet (Pereira et al., 2015)
• High-redshift strong Mg II absorbers (Zou et al., 2020)

These studies establish Mg II absorption lines as precision probes of cool gas in galaxies, their halos, and between galaxies, providing a foundation for empirical, theoretical, and simulation-based investigations of galaxy evolution, CGM/IGM physics, and stellar atmospheric structure.