Partial Lyman Limit Systems (pLLSs)

• pLLSs are quasar absorption-line systems defined by neutral hydrogen column densities between 10^16 and 10^17.2 cm⁻², placing them between the Lyα forest and denser absorbers.
• They serve as key tracers of the cool, predominantly ionized circumgalactic medium and evidence for metal-poor gas accretion, with bimodal metallicity at low redshift and a broader unimodal distribution at high redshift.
• High-quality observations using HST/COS and ground-based spectrographs enable precise measurements of H I column densities and multiphase ionization states, informing models of gas accretion and feedback in galaxy halos.

Partial Lyman Limit Systems (pLLSs) are quasar absorption-line systems that occupy the neutral-hydrogen column-density interval immediately below the classical Lyman-limit threshold, generally with $16.0 \lesssim \log N_{\mathrm{H\,I}} < 17.2$, while classical Lyman Limit Systems (LLSs) occupy $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$; some low-redshift surveys adopt $16.1$ or $16.2$ as the lower pLLS boundary, but the physical role is the same across these definitions (Lehner, 2016). They are partially optically thick at the Lyman limit, with $\tau_{\mathrm{LL}}$ somewhat below unity, and they lie at the interface between the highly ionized Ly$\alpha$ forest and the denser gas traced by sub-damped Ly$\alpha$ systems and damped Ly$\alpha$ absorbers. Because they are selected by H I opacity yet require substantial ionization corrections, pLLSs have become a principal observational tracer of the cool, dense, predominantly ionized circumgalactic medium (CGM), of dense intergalactic structures, and of the metal-poor gas reservoirs that are widely interpreted as evidence for cold accretion onto galaxies (Lehner, 2016).

1. Definition and placement within the H I absorber hierarchy

In the literature summarized here, pLLSs are defined by their neutral hydrogen column density rather than by prior metallicity or galaxy information. Following Lehner et al. (2013), pLLSs have $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$, while LLSs span $$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$; Wotta et al. adopt $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$0 for low-$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$1 pLLSs, and the COS CGM Compendium uses $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$2 (Lehner, 2016, Wotta et al., 2016, Wotta et al., 2018). In practice, pLLSs are “partially” optically thick, with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$3 below unity, whereas LLSs satisfy $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$4 (Lehner, 2016).

On a cosmological density scale, their $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$5 values and implied total hydrogen densities place them between the diffuse intergalactic medium (IGM) and the denser, more neutral absorbers at higher column density. The review literature describes pLLSs and LLSs as occupying the interface between the Ly$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$6 forest and the virialized gas traced by SLLSs and DLAs, with implied densities $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$7 to $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$8 via the Schaye (2001) prescription (Lehner, 2016). At low redshift, pLLSs probe cool gas with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$9, $16.1$0, and baryonic overdensities $16.1$1–$16.1$2, characteristic of galaxy halos (Wotta et al., 2016).

This intermediate position in column density is astrophysically consequential. Higher-$16.1$3 absorbers at $16.1$4, especially SLLSs and DLAs, show mostly $16.1$5 and unimodal metallicity distributions peaked around $16.1$6 to $16.1$7, whereas pLLSs and LLSs probe a distinct regime in which low-metallicity and metal-rich gas coexist over a broad dynamic range (Berg et al., 2022).

2. Observational identification and measurement of $16.1$8

Large, unbiased pLLS samples became feasible through two observational developments. At low redshift, the installation of COS on HST enabled direct measurements of $16.1$9 in G140L mode and metal-line spectroscopy with the higher-resolution G130M/G160M gratings; at high redshift, Keck/HIRES and VLT/UVES archives, together with surveys such as KODIAQ and HD-LLS, provided high-S/N spectra of the full Lyman series and weak metal lines (Lehner, 2016). These instrumental developments turned pLLSs from isolated case studies into statistically tractable absorber classes.

Two complementary methods are used to determine $16.2$0. One is direct measurement of the Lyman-limit optical depth,

$16.2$1

with $16.2$2 at $16.2$3; the other is curve-of-growth or Voigt-profile fitting of higher-order Lyman-series lines (Wotta et al., 2016, Shull et al., 2017). In the HST/COS ultraviolet survey of 102 AGN sight lines, pLLSs were recognized through the “Lyman-comb” of higher-order Lyman lines and, when $16.2$4, by the continuum decrement at the redshifted Lyman edge. Combining line-series fitting and Lyman-limit measurements yielded column densities accurate to $16.2$5 over $16.2$6 (Shull et al., 2017).

Incidence measurements place pLLSs among the common strong H I absorbers at low redshift. Over a total pathlength $16.2$7 at $16.2$8, 54 pLLSs yielded $16.2$9 at $\tau_{\mathrm{LL}}$0, while the maximum-likelihood column-density distribution was fitted as

$\tau_{\mathrm{LL}}$1

with $\tau_{\mathrm{LL}}$2 and $\tau_{\mathrm{LL}}$3 for $\tau_{\mathrm{LL}}$4 (Shull et al., 2017). Interpreting this incidence in terms of halo cross-sections gives characteristic gaseous halo radii of $\tau_{\mathrm{LL}}$5–$\tau_{\mathrm{LL}}$6 for $\tau_{\mathrm{LL}}$7–$\tau_{\mathrm{LL}}$8 galaxies, consistent with direct CGM measurements (Shull et al., 2017).

Beyond absorber statistics, pLLSs are also significant contributors to far-UV opacity. Integrating over $\tau_{\mathrm{LL}}$9 yields

$\alpha$0

implying mean LyC optical depths $\alpha$1–$\alpha$2 toward sources at $\alpha$3–$\alpha$4 (Shull et al., 2017).

3. Ionization modeling and abundance inference

pLLSs are highly ionized systems, with $\alpha$5 in low-redshift samples, so metallicity determinations require explicit ionization corrections (Berg et al., 2022). The standard metallicity definition is

$\alpha$6

and the relevant control parameter in photoionization modeling is

$\alpha$7

or equivalently $\alpha$8 in the COS CGM Compendium formalism (Berg et al., 2022, Wotta et al., 2018).

Most modern analyses use Cloudy grids for a uniform plane-parallel slab photoionized by an extragalactic UV background. The review literature summarizes modeling with Haardt–Madau UV backgrounds, including HM05 and HM12; BASIC used Cloudy plane-parallel slabs with the HM96 background and Bayesian MCMC; CCC employed Bayesian inference with emcee over grids in $\alpha$9, redshift, $\alpha$0, $\alpha$1, and $\alpha$2 (Lehner, 2016, Berg et al., 2022, Wotta et al., 2018). Where ionic constraints are sparse, CCC imposed Gaussian priors on $\alpha$3 and, in carbon-only cases, on $\alpha$4 (Wotta et al., 2018).

Low-resolution abundance methods have also been developed for large samples. Wotta et al. used an empirically derived $\alpha$5 distribution together with Cloudy v13.03 models to compute ionization-correction factors for Mg II and showed that this approach reproduces metallicities to $\alpha$6 when tested against 22 previously modeled pLLSs/LLSs (Wotta et al., 2016). In BASIC, typical statistical errors on $\alpha$7 were $\alpha$8–$\alpha$9, while systematic uncertainties of $\alpha$0–$\alpha$1 associated with the UV background shape were explicitly recognized (Berg et al., 2022).

The inference machinery is not merely procedural; it controls the physical interpretation of pLLSs. By coupling Cloudy-derived $\alpha$2 and $\alpha$3 with observed $\alpha$4, one can infer absorber sizes and total hydrogen columns through $\alpha$5 (Lehner, 2016). A central result of subsequent high-S/N work is that single-phase solutions can be misleading: pLLSs can contain kinematically aligned low- and high-ionization gas that would masquerade as a single cloud if only integrated columns were modeled (Cooper et al., 2021).

4. Metallicity distributions and their evolution with redshift

The most prominent empirical result for pLLSs is the metallicity distribution function (MDF). At $\alpha$6, low-redshift H I-selected samples established a strongly bimodal MDF for pLLSs. In Wotta et al., the combined sample of 44 pLLSs showed a low-metallicity peak at $\alpha$7 and a high-metallicity peak at $\alpha$8, with a trough near $\alpha$9; the low-metallicity peak comprised $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$0 of the pLLSs (Wotta et al., 2016). CCC, using 82 pLLSs at $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$1, confirmed this pattern with peaks at $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$2 and $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$3, a deep dip near $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$4, and a very metal-poor fraction $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$5 of $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$6–$$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$7 (Wotta et al., 2018).

The statistical evidence for bimodality is strong. In the low-redshift samples, Gaussian-mixture modeling rejects unimodality at $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$8 and the dip test at $$10^{16}\,\mathrm{cm}^{-2} < N_{\mathrm{H\,I}} < 10^{17.2}\,\mathrm{cm}^{-2}$$9 in Wotta et al.; the review summarizes GMM and dip-test rejections of unimodality at $$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$0 confidence for a sample of $$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$1 pLLSs+LLSs (Wotta et al., 2016, Lehner, 2016). BASIC recast the low-$$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$2 bimodality in a simpler two-branch form, with a low-metallicity branch at $$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$3 and a high-metallicity branch at $$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$4, containing approximately $$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$5–$$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$6 and $$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$7–$$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$8 of the pLLSs, respectively (Berg et al., 2022).

At high redshift, the MDF changes qualitatively. In the KODIAQ Z sample of 31 pLLSs+LLSs at $$10^{17.2}\,\mathrm{cm}^{-2} \le N_{\mathrm{H\,I}} < 10^{19}\,\mathrm{cm}^{-2}$$9, the metallicity distribution is unimodal, with survival-analysis mean $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$00, dispersion $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$01, median $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$02, and range from $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$03 to $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$04 (Lehner et al., 2016). The review describes a similarly selected sample at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$05–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$06 as broad and unimodal, centered at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$07, with little evidence for a dip and with the bulk of systems in the interval $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$08 (Lehner, 2016). Very metal-poor gas remains common: $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$09–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$10 of the high-$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$11 pLLS+LLS sample has $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$12, while the fraction with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$13 is only $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$14–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$15, implying that the gas is rarely pristine (Lehner et al., 2016).

Relative to higher-column absorbers, pLLSs remain distinctive. At $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$16, SLLSs and DLAs have metallicities largely confined to $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$17 and contain almost no absorbers below $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$18; even LLSs show a lower fraction of very metal-poor systems than pLLSs (Wotta et al., 2016, Wotta et al., 2018). This column-density dependence is one of the main reasons pLLSs are regarded as especially sensitive tracers of metal-poor halo and interface gas (Berg et al., 2022).

5. Galaxy associations, CGM locations, and group-scale environments

Low-redshift pLLSs were initially established as CGM absorbers through absorber-galaxy associations at modest impact parameter. A widely cited example is the $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$19 pLLS toward PG 1630+377, which has $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$20, lies only $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$21 in projection from a $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$22 galaxy with near-solar ISM abundance, and has inferred physical conditions $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$23, $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$24, and $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$25 (Lehner, 2016). The review further notes that most $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$26 pLLSs have impact parameters $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$27–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$28 and that their covering fraction within $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$29 is $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$30–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$31 (Lehner, 2016).

Subsequent galaxy-survey work complicated the earlier picture without overturning its CGM relevance. BASIC targeted 36 H I-selected pLLSs/LLSs at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$32 in 23 QSO fields and, in 11 IFU fields containing 19 pLLSs, identified 23 distinct associated galaxies, with an average of one associated galaxy per absorber (Berg et al., 2022). The adopted association criteria were $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$33 and $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$34 (Berg et al., 2022). Yet seven absorbers, all with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$35 solar metallicities, had no associated galaxies with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$36 within those cuts. The probability that a metal-poor system has a host galaxy with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$37 within $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$38 is $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$39, compared with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$40 for higher-metallicity absorbers (Berg et al., 2022).

BASIC therefore argued for a dual low-metallicity population. Approximately $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$41 of the metal-poor pLLSs appear associated with galaxy CGM, likely representing cold accretion onto $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$42 halos, while another population with no detected host within $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$43 or $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$44 probably traces dense IGM or filamentary gas with overdensities $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$45–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$46 (Berg et al., 2022). Using absorbers without identified galaxies, BASIC estimated an unweighted geometric-mean metallicity of the overdense IGM at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$47 of $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$48 (Berg et al., 2022).

pLLSs also arise in richer environments. The $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$49 pLLS toward HE1003+0149 has $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$50 from the Lyman-limit break and resides in a field where MUSE found three dwarf galaxies with stellar masses $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$51–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$52 at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$53–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$54, while wider spectroscopy identified 21 additional galaxies within projected separation $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$55 and $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$56, consistent with a loose galaxy group (Narayanan et al., 2021). The absorber was interpreted as tracing a multiphase, cool, photoionized intragroup medium (Narayanan et al., 2021).

6. Multiphase structure, O VI, and chemical inhomogeneity

Detailed component-level analyses show that pLLSs are not generally single-phase clouds. In CUBS, two pLLSs at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$57 and $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$58 with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$59–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$60 were resolved into multiple kinematic components spanning five ionization stages, from Mg II, C II, O II, and Si II through C III, Si III, N III, O III, O IV, N IV, and O VI (Cooper et al., 2021). Their inferred densities span more than two decades, from $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$61 to $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$62, while $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$63-element abundances vary from $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$64 to near solar (Cooper et al., 2021).

The distinctive point is that velocity alignment does not imply phase uniformity. In the CUBS systems, single-phase photoionization models could not reproduce the observed O II, O III, and O IV ratios, and two-phase decompositions were required even within individual velocity components (Cooper et al., 2021). If unresolved, these multiphase blends would masquerade as single clouds with ambiguous densities, metallicities, and sizes. The inferred environments were correspondingly diverse: one absorber lies $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$65 from a star-forming galaxy of $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$66, whereas the other lies in an overdense environment of 11 galaxies within $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$67, including a luminous red galaxy of $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$68 at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$69 (Cooper et al., 2021).

O VI provides a systematic diagnostic of this multiphase structure. In CCC V, the robust pLLS sample at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$70 contained 26 pLLSs, with a $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$71 O VI completeness limit of $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$72 (Sameer et al., 2024). The total O VI detection rate was $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$73, with nearly identical frequencies for low-metallicity and high-metallicity pLLSs: $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$74 for $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$75 and $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$76 for $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$77 (Sameer et al., 2024). Systems without O VI are consistent with a single cool photoionized phase, whereas systems with O VI require a distinct lower-density or collisionally ionized phase at $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$78–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$79 because single-phase models underpredict $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$80 by factors $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$81 (Sameer et al., 2024).

The O VI phenomenology is itself metallicity-dependent. In detected pLLSs, the mean $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$82 is $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$83 for low-metallicity systems and $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$84 for high-metallicity systems; the strongest absorbers with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$85 are nearly always associated with high-metallicity pLLSs and with broad O VI profiles characteristic of the CGM of star-forming galaxies (Sameer et al., 2024). By contrast, weak O VI occurs over the full metallicity range and is interpreted as tracing extended CGM or IGM, while pLLSs without O VI are likely to originate in the IGM (Sameer et al., 2024).

A conceptually similar multiphase decomposition was required for the $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$86 intragroup pLLS toward HE1003+0149. There, low-ionization models reproduced O II and O III but underproduced O IV and O V in all three velocity components, so a separate higher-ionization phase was necessary, with predicted H I columns $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$87–$17.2 \le \log N_{\mathrm{H\,I}} < 19.0$88 dex lower than those of the low-ionization phase (Narayanan et al., 2021).

7. Physical interpretation, simulations, and direct size constraints

The dominant interpretation of metal-poor pLLSs is that they trace gas accretion, although the precise location can vary between CGM halos, dense IGM structures, and group environments. The review literature states that the low metallicity of cold gaseous streams is the key discriminator relative to metal-rich outflows or recycled gas and argues that metal-poor pLLSs/LLSs are among the best observational evidence for cold, metal-poor gas accretion onto galaxies (Lehner, 2016). At $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$89, simulations predict cold streams with $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$90, metallicities $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$91 to $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$92, densities $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$93 to $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$94, and ionization parameters $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$95 to $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$96, often yielding $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$97 in the pLLS/LLS range (Lehner, 2016).

The mass budget implied by the low-redshift pLLS population is non-negligible. Using characteristic sizes $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$98 of a few to $17.2 \le \log N_{\mathrm{H\,I}} < 19.0$99, the review estimates gas masses per absorber of $16.1$00–$16.1$01 and an aggregate cool CGM reservoir of $16.1$02–$16.1$03 after integrating over the observed $16.1$04 at $16.1$05 (Lehner, 2016). For fiducial inflow conditions, a continuous infall rate of order $16.1$06–$16.1$07 may be sufficient to fuel ongoing star formation in $16.1$08 galaxies over Gyr timescales (Lehner, 2016).

Comparisons with simulations remain mixed. CCC found that FIRE zoom simulations predict a nearly unimodal, relatively metal-rich PDF and strongly underproduce absorbers with $16.1$09, while EAGLE HiRes yields a broader PDF with more low-metallicity gas but a stronger redshift evolution than observed and similar pLLS/LLS PDF shapes not seen in the data (Wotta et al., 2018). The dedicated EAGLE metallicity study likewise reported broad, continuous pLLS metallicity distributions and explicitly did not recover the observed trough near $16.1$10; at $16.1$11–$16.1$12, EAGLE finds $16.1$13 of pLLSs below $16.1$14 and $16.1$15 above, in broad agreement with the observed overall fraction above and below $16.1$16 but not with the strong low-$16.1$17 bimodality (Rahmati et al., 2017). This suggests that current feedback and CGM-resolution prescriptions may overmix metals and underresolve low-metallicity structures (Wotta et al., 2018).

Recent tomography has added direct geometrical information that was previously inferred only from ionization modeling. Using the multiply imaged Sunburst Arc at $16.1$18, HST/WFC3 UVIS G280 spectra identified two pLLSs at $16.1$19 and $16.1$20 with $16.1$21 and $16.1$22, respectively, and found consistent H I column densities across $16.1$23 (Berg et al., 8 Jul 2025). The reported de-lensed cloud-length measurements are $16.1$24 and $16.1$25, with average H I masses of $16.1$26 and a total range of $16.1$27–$16.1$28 (Berg et al., 8 Jul 2025). Two of the absorbers show strong C IV and are therefore likely in the CGM of foreground galaxies, while the metal-free system is most consistent with an IGM location (Berg et al., 8 Jul 2025).

Taken together, the observational record defines pLLSs as a transition population in column density but not as a single physical class. They include metal-rich recycled or outflowing halo gas, very metal-poor CGM clouds consistent with cold accretion, dense IGM or filamentary structures without luminous hosts, and cool intragroup media. Their low-redshift metallicity bimodality, high-redshift unimodal metal-poor distribution, frequent multiphase structure, and increasingly direct environmental constraints make them one of the most stringent empirical probes of baryon cycling across the IGM–CGM interface (Lehner, 2016).