Wei Zhang

HI 1225+01 is a nearby large H I complex on the outskirts of the Virgo cluster and one of the most discussed dark galaxy candidates in the local Universe. It consists of a northeastern (NE) clump with the only known optical counterpart, a metal-poor dwarf galaxy, a southwestern (SW) clump with no detected stellar component, and a connecting H I bridge. In our previous work, we reported a faint blue source projected onto the bridge as a possible optical counterpart. Here we present Keck/LRIS long-slit spectroscopy of the NE dwarf and the bridge source, supplemented by DESI DR1 spectra, Subaru/HSC imaging, and archival GALEX data. For the NE counterpart, we measure Tₑ([O III]) = 16,310⁺²³⁰₋₂₄₀ K, 12 + log(O/H) = 7.65 ± 0.02, log(N/O) = −1.47 ± 0.01, and star formation rates of (1.6–2.7) × 10⁻² M⊙ yr⁻¹ from Hα, FUV, and NUV. The bridge candidate is a background emission-line galaxy at z ≃ 0.048. A multi-line stack shows no detectable emission at the H I velocity at the same slit position. Together with the HSC g + r + i coadd limit of μ ≃ 28–29 mag arcsec⁻² for the SW clump, these results leave two of the three H I components without detected optical counterparts.

We present a stacking study of the neutral atomic hydrogen (H i ) content in and around 36 local galaxy clusters at z < 0.07, using a combination of the FAST All Sky H i survey (FASHI) and the extensive spectroscopic catalog mainly from the Dark Energy Spectroscopic Instrument (DESI). We employ spectral stacking techniques to probe the average H i mass and HI-to-stellar mass ratio ( M _HI / M _* ) for member galaxies down to stellar masses of M _* ∼ 10 ⁹ M _⊙ , spanning a projected cluster-centric distance of up to 5 R ₂₀₀ . Our analysis reveals a pronounced environmental effect; both M _HI and M _HI / M _* decrease steadily toward the cluster center, dropping by ∼0.5 dex on average from the outskirts to the core. Crucially, we find that M _HI / M _* of galaxies remain lower than the field galaxies even at the 5 R ₂₀₀ . This provides direct, statistical evidence for substantial gas stripping and preprocessing in the cluster outskirts, likely occurring in infalling groups and large-scale filaments. By further splitting the sample by g − r color, we show that the H i deficiency persists at fixed galaxy color; even the bluest cluster members exhibit ∼0.5 dex lower M _HI / M _* than field galaxies of similar color, reflecting environmental effects on the cold gas reservoir prior to full optical transformation. The total H i mass within clusters and their outskirts agrees broadly with predictions from cosmological simulations. Our results underscore the critical role of the extended cluster environment in quenching galaxies by depleting their cold gas reservoirs well before they enter the dense cluster core.

Based on LAMOST Medium-Resolution Spectroscopic Survey of Nebulae (MRS-N) data and the Wide-field Infrared Survey Explorer Galactic H II region catalog, we construct a sample of 280 Galactic H II regions and candidates in the outer Galaxy (80° ≲ l ≲ 220°). Using MRS-N optical spectra, we measure four emission lines (H α , [N II ] λ 6584, [S II ] λλ 6717, 6731) and use line ratios to spectroscopically confirm 255 H II regions, including 90 previously “Known” H II regions and 165 newly classified ones. We measure their T _e , n _e , and oxygen abundance and determine distances via associated OB stars and the kinematic method. The sample spans R _gal from 8.16 to 15.36 kpc, enabling investigation of radial gradients in physical properties. We find that [N II ]/H α and [S II ]/H α decrease with increasing R _gal , while [S II ]/[N II ] remains nearly flat; these trends are quite different from diffuse ionized gas (DIG). We derive the T _e gradient of 344.530 ± 78.083 K kpc ⁻¹ and the log n e gradient of −0.143 ± 0.041 cm ⁻³ kpc ⁻¹ . Oxygen abundance shows a steep slope of −0.044 ± 0.010 dex kpc ⁻¹ in the inner disk and a shallow slope of −0.016 ± 0.005 dex kpc ⁻¹ in the outer disk, with a global slope of −0.014 ± 0.005 dex kpc ⁻¹ . We also examine the two-dimensional distributions of T _e , n _e , and oxygen abundance and find that the gradients vary with azimuth. There is no obvious difference between spiral arm and interarm regions, and no trend appears along individual arms. From the [N II ]/H α [S II ] λ 6717/H α diagram, H II regions have a S ⁺ /S ratio (0.32) lower than that of DIG (0.43); however, heavy overlap prevents clear separation from this diagram alone.

Green Pea and Blueberry galaxies are compact, low-metallicity starbursts that provide nearby analogs of high-redshift star-forming systems. We investigate whether their weak CO emission reflects genuinely small molecular reservoirs or CO-dark gas where independent tracers are available. We compiled 60 strong O iii łambda5007 compact emission-line galaxies with partially overlapping gas-tracer coverage: 12 CO(1--0) observations from the Delingha 13.7,m telescope, H, 21,cm data from Arecibo, GBT, and FAST (ten detections and 41 upper limits), and Herschel/PACS ,łambda158,μm data (five detections and four upper limits after excluding merger systems). Quantitative conclusions are drawn only from detected or physically constraining subsets. Molecular masses were estimated using a metallicity-dependent CO-to-H_2 conversion factor and the -to-H_2 calibration for low-metallicity dwarf galaxies. Two CO upper limits are deep enough to constrain the expected molecular reservoir. We also removed major mergers and examined the large-scale environment using the SDSS DR7 VoidFinder catalog. i C , ii C , ii For the five C , ii -detected sources, we find M_ ̊m H_2 ^ ̊m CII ∼ 10^ 9.0 --10^ 9.7 ,M_⊙ and a median molecular gas depletion time of ̊m dep,H_2 ∼ 0.16,Gyr, shorter than the ∼ 1,Gyr typical of main-sequence galaxies. H, -detected sources have a median atomic depletion time of τ_ i ̊m dep,HI ∼ 2.6,Gyr. The detected subsets show high gas fractions and short depletion times consistent with rapid gas cycling, but the C , ii result remains limited by small-number statistics. About 39% of the volume-matched sample lies in cosmic voids. CO alone misses a substantial fraction of the star-forming fuel in these low-metallicity starbursts. Combined H, , i C , ii , and deep CO constraints are needed to characterize their interstellar medium and CO-dark molecular component.

The size–velocity dispersion ( σ ) relation, while well established for giant H ii regions, remains uncertain for their smaller counterparts (physical radii R < 20 pc). Thanks to the Large Sky Area Multi-Object Fiber Spectroscopic Telescope MRS-N dataset’s large sky coverage and high spatial/spectral resolution, we examined this relationship using 10 isolated Galactic H ii regions with R < 20 pc. Our results reveal two key findings: (1) these small-size H ii regions remarkably follow the same size– σ relation as giant H ii regions, suggesting this correlation could serve as a novel distance indicator for Galactic H ii regions; and (2) we find distinct dynamical behaviors between younger and older H ii regions. Specifically, in younger (0.5 Myr), matter-bounded H ii regions, a clear correlation emerges, implying that expansion-driven processes begin to play a significant role in generating turbulence. We therefore propose an evolutionary transition in the primary turbulence mechanisms—from being dominated by stellar winds and radiation to being increasingly influenced by expansion-driven dynamics—during the evolution of H ii regions. Considering the small sample size used in this work—particularly the inclusion of only two young H ii regions, which also have large uncertainties in their expansion velocities—further confirmation of this interpretation will require higher-resolution 2D spectroscopy to resolve blended kinematic components along the line of sight for more accurate estimation of expansion velocities, along with an expanded sample that specifically includes more young H ii regions.

We report multifiber, medium-resolution spectroscopy of the Rosette Nebula with full spatial coverages, and present a table of the nebular parameters based on the spatially resolved measurements of emission lines. These new observations were conducted through the Medium-Resolution Spectroscopic Survey of Nebulae (MRS-N) on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Comprehensive analyses were performed on a total of 3854 high-quality nebular spectra; so far, the most extensive spectral data set available for this nebula that encompasses an area of 4.52 square degrees. Various physical parameters, including relative line intensities, radial velocities (RVs), and full width at half-maximums (FWHMs), were derived through measurements of the H α , [N ii ] λλ 6548,6584 and [S ii ] λλ 6716,6731 emission lines detected in the LAMOST MRS-N spectra. For the first time, we found a bow-shaped feature in the spatial distribution of RVs of the Rosette Nebula. Moreover, the spatial distributions of RVs and FWHMs, as well as additional parameters such as gas temperature and turbulent velocity in the vicinity of the nebula, indicate possible interaction between Rosette and the nearby supernova remnant, Monoceros Loop. Our new observations provide indispensable measurements of the Rosette Nebula. The parameter table in particular can be used as valuable constraint on the chemo-dynamical modeling of the nebula, which will enable deeper understanding of the characteristics of this H ii region.

We present spatially resolved spectroscopic observations of 10 isolated Galactic H II regions using data from the LAMOST Medium-Resolution Spectroscopic Survey of Nebulae (LAMOST MRS-N). The high spatial resolution of the data allows us to investigate the 1D radial profiles of emission-line fluxes (Hα, [S II], and [N II]), flux ratios ([N II]/Hα, [S II]/Hα, and [S II]/[N II]), and radial velocities of these three emission lines. Among these regions, two are ionization bounded, while the remaining eight are matter bounded. The matter-bounded H II regions exhibit shallower slopes in their radial flux profiles compared to the ionization-bounded ones. In most cases, the [N II]/Hα and [S II]/Hα ratios increase with distance from the center of the H II regions, consistent with model predictions that low-ionization emissions dominate the outer zones of these regions. The two ionization-bounded H II regions have kinematic ages t of 0.2 and 0.3 Myr, while the matter-bounded regions span ages from 1 to 12 Myr. For the matter-bounded H II regions, the optical emission flux decreases continuously beyond the photodissociation region (PDR), extending to approximately 1–4 times the radius of the PDR (r _PDR). The escape fraction f _esc of ionizing photons, derived from 1D Hα radial flux profiles, is ∼0% for ionization-bounded H II regions, while it ranges from 50% to 90% for the matter-bounded H II regions. The correlation between f _esc and t suggests that evolved H II regions (with t > 1 Myr) contribute more significantly to ionizing the surrounding diffuse ionized gas compared to younger, newly formed H II regions.

We present spatially-resolved spectroscopic observations of 10 isolated Galactic HII regions using data from the LAMOST Medium-Resolution Spectroscopic Survey of Nebulae (LAMOST MRS-N). The high spatial resolution of the data allows us to investigate the 1D radial profiles of emission line fluxes (Ha, [S II] and [N II]), flux ratios ([N II]/Ha, [S II]/Ha and [S II]/[N II]), and radial velocities of these three emission lines. Among these regions, two are ionization-bounded, while the remaining eight are matter-bounded. The matter-bounded HII regions exhibit shallower slopes in their radial flux profiles compared to the ionization-bounded ones. In most cases, the [N II]/Ha and [S II]/Ha ratios increase with distance from the center of the HII regions, consistent with model predictions that low-ionization emissions dominate the outer zones of these regions. The two ionization-bounded HII regions have kinematic ages (t) of 0.2 and 0.3 Myr, while the matter-bounded regions span ages from 1 to 12 Myr. For the matter-bounded HII regions, the optical emission flux decreases continuously beyond the photodissociation region (PDR), extending to approximately 1-4 times the radius of the PDR (r_PDR). The escape fraction f_esc of ionizing photons, derived from 1D Ha radial flux profiles, is ~ 0% for ionization-bounded HII regions, while it ranges from 50% to 90% for the matter-bounded HII regions. The correlation between f_esc and t suggests that evolved HII regions (with t > 1 Myr) contribute more significantly to ionizing the surrounding diffuse ionized gas compared to younger, newly formed HII regions.

Using data from the Large Area Multi-Object fiber Spectroscopic Telescope Medium-Resolution Spectroscopic Survey of Nebulae, we create a sample of 17,821 diffuse ionized gas spectra in the anticenter region of the Milky Way by excluding fibers in the directions of H ii regions and supernova remnants. We then analyze the radial and vertical distributions of three line ratios ([N ii]/Hα, [S ii]/Hα, and [S ii]/[N ii]), as well as the oxygen abundance. [N ii]/Hα and [S ii]/Hα do not exhibit a consistent, monotonic decrease with increasing Galactocentric distance (R _gal). Instead, they show enhancement within the interarm region, positioned between the Local Arm and the Perseus Arm. [S ii]/[N ii] has a radial gradient of 0.1415 ± 0.0646 kpc⁻¹ for the inner disk (8.34 < R _gal < 9.65 kpc) and remains nearly flat for the outer disk (R _gal > 9.65 kpc). In the vertical direction, [N ii]/Hα, [S ii]/Hα, and [S ii]/[N ii] increase with increasing Galactic disk height (∣z∣) in both southern and northern disks. Based on the N2S2Hα method, which combines [S ii]/[N ii] and [N ii]/Hα, we estimate the oxygen abundance. The oxygen abundance exhibits a consistent radial gradient with R _gal, featuring a slope of −0.0559 ± 0.0209 dex kpc⁻¹ for the inner disk and a similar slope of −0.0429 ± 0.0599 dex kpc⁻¹ for the outer disk. A single linear fitting to the entire disk yields a slope of −0.0317 ± 0.0124 dex kpc⁻¹. In the vertical direction, the oxygen abundance decreases with increasing ∣z∣ in both southern and northern disks.