Renbin Yan

Spatially resolved spectroscopic surveys enable the interstellar medium (ISM) of galaxies to be studied. However, observations of Galactic H α emission are contaminated by geocoronal H α emission. The latter is known to depend on the shadow altitude, a geometric parameter that relates the line of sight to Earth’s shadow cone. Using fibres on blank skys from the MaNGA Stellar Library (MaStar) project of the Sloan Digital Sky Survey IV (SDSS-IV), we establish an empirical relation between the geocoronal H α emission and the shadow altitude, with a root mean square fractional scatter of 24.41%. This relation can be used to predict geocoronal H α emission so that it can be removed from observed spectra. This removal method is advantageous when the observed targets are extensive in the sky, and it does not require a large velocity separation between the observed target and the local standard of rest. This will enable reliable studies of Galactic H α in intermediate spectral resolution integral field spectroscopic surveys. We also find tentative evidence for the dependence of geocoronal emission on solar activity and the distance between the Earth and the Sun.

Aims. The mass–metallicity relation (MZR) is essential for understanding the chemical evolution of galaxies. Whether the star formation rate (SFR) plays a role in setting the metallicity has long been debated. Using various metallicity calibrations can result in different conclusions for this fundamental yet unresolved issue. Methods. We applied a self-consistent metallicity calibration based on photoionization models to re-evaluate the resolved and integrated MZR. We utilized the integral field unit data from SDSS-IV/MaNGA, with ∼3.5 × 10 ⁶ spaxels and ∼4550 galaxies. We compared our preferred metallicity calibration with several strong-line calibrations in the literature and direct method metallicity. We analyzed the metallicity residual of MZR to evaluate the effects of SFR and applied the partial correlation coefficient to quantify the effects. Results. The metallicity calibration we used is most consistent with the direct method. We provide three equations for the resolved MZR and verify that the local SFR does not show significant correlation with metallicity. Considering the integrated properties, the (specific) SFRs do not present any correlation with the metallicity residuals. The results suggest that an equilibrium of inflow and outflow is favored, and the mass-metallicity relation does not exhibit a secondary dependence on the SFR.

The enrichment of heavy elements in the interstellar medium is central to understanding galaxy evolution. Oxygen, the most abundant element after H and He, provides the principal tracer of gas-phase metallicity, typically measured using nebular emission lines in star-forming regions. However, a long-standing issue—the abundance discrepancy factor (ADF)—produces a ~0.25 dex offset between metallicities derived from recombination lines (RLs) and collisionally excited lines (CELs). We present deep spectroscopic observations of the Orion Nebula (M42), and use them to assess the contribution of the often neglected continuum pumping process. We find that the continuum pumping effect is at least a factor of two stronger than previously expected. After accounting for the flux contribution from continuum pumping, the RL metallicity is consistent with the CEL metallicity, thus explaining the bulk of the ADF. Reassessing O/H measurements in five literature star-forming regions using the same method, we similarly find a systematic reduction of RL-based metallicities. These results indicate that RL metallicities, which have previously been considered more reliable than CELs, are subject to potentially large bias from continuum pumping. This study provides a foundation to correct this bias and establish accurate element abundance scales throughout cosmic time.

We present the stellar parameters for 59,266 high-quality spectra of 24,130 unique stars from the MaNGA Stellar Library (MaStar) in the Sloan Digital Sky Survey (SDSS) Data Release 17 (DR17). The median signal-to noise ratio per pixel of the spectra is 96. We derive four stellar parameters, effective temperature ( T _eff ), surface gravity ( log g ), metallicity ([M/H]), and α -enhancement ratio ([ α /M]), by comparing the data with BOSZ (ATLAS-9 based) and MARCS theoretical atmospheric models. We adopt a Bayesian method and use color and absolute magnitude derived from Gaia to select a subset of theoretical models for each star. We then perform full-spectrum fitting to estimate the likelihood of each model in the subset and then compute their likelihood-weighted mean parameters as the final parameters. We set stellar-parameter quality flags to facilitate the use of the derived stellar parameters. The MaStar stellar parameters derived herein span an effective temperature range of 2600 ≤ T _eff ≤ 29,861 K, a surface gravity range of 0 ≤ log g ≤ 5.5 , a metallicity range of −4.9 ≤ [M/H] ≤ 1.0, and an α -abundance range of −0.98 ≤ [ α /M] ≤ 1.0. We compare these parameters with those from APOGEE and Gaia for stars in common, finding general consistency within the uncertainties. However, some artifacts and systematic differences are present, and we discuss their potential causes. These new stellar parameters are available through the MaStar SDSS-IV DR17 value-added catalog ( https://www.sdss4.org/dr17/mastar/mastar-stellar-parameters/ ).

Mapping the local and distant Universe is key to our understanding of it. For decades, the Sloan Digital Sky Survey (SDSS) has made a concerted effort to map millions of celestial objects to constrain the physical processes that govern our Universe. The most recent and fifth generation of SDSS (SDSS-V) is organized into three scientific “mappers”: the Milky Way Mapper, which aims to chart the various components of the Milky Way and constrain its formation and assembly; the Black Hole Mapper, which focuses on understanding supermassive black holes in distant galaxies across the Universe; and the Local Volume Mapper, which uses integral field spectroscopy to map the ionized interstellar medium in the Local Group. This paper describes the scope and content for the nineteenth data release (DR19) of SDSS, which is the most substantial to date in SDSS-V. DR19 is the first to contain data from all three mappers. Additionally, we also describe nine value-added catalogs that enhance the science that can be conducted with the SDSS-V data. Finally, we discuss how to access SDSS DR19 and provide illustrative examples and tutorials.

The host galaxies of tidal disruption events (TDEs) show enhanced central stellar concentration and are preferentially found in poststarburst and green valley populations. This connection has led to the proposal that TDE host galaxies likely have gone through recent mergers. We conduct a new morphological study of 14 TDE host galaxies, using the r -band images from the Sloan Digital Sky Survey (SDSS), Dark Energy Camera Legacy Survey, and Ultraviolet Near-Infrared Optical Northern Survey, with the images from the latter two surveys having much higher depth and resolution than SDSS. We examine galaxy structures using conventional methods and also apply diagnostics of merger activity from a suite of machine learning models. Consistent with previous studies, our results show that TDE host galaxies are ∼16% more centrally concentrated when compared to non-TDE-host controls. However, surprisingly, TDE hosts lack any indication of significant recent merger activity from both morphological analysis and the machine learning merger classifier. Instead, our results reveal that TDE host galaxies in the green valley are approximately 1.5–3 times more likely to have bar-like or ringlike structures compared to their controls. Based on these results, we propose that bar-driven secular evolution, instead of mergers, likely dominates the recent evolution of the TDE hosts found in the green valley, which can simultaneously explain their distinctive nuclear properties and enhanced TDE rates.

Host galaxies of tidal disruption events (TDEs) show enhanced central stellar concentration and are preferentially found in post-starburst and green valley populations. This connection has led to the proposal that TDE host galaxies likely have gone through recent mergers. We conduct a new morphological study of 14 TDE host galaxies, using the r-band images from Sloan Digital Sky Survey (SDSS), Dark Energy Camera Legacy Survey (DECaLS), and Ultraviolet Near-Infrared Optical Northern Survey (UNIONS), with the images from the latter two surveys having much higher depth and resolution than SDSS. We examine galaxy structures using conventional methods and also apply diagnostics of merger activity from machine learning models. Consistent with previous studies, our results show that TDE host galaxies are ~16% more centrally concentrated when compared to non-TDE-host controls. However, surprisingly, TDE hosts lack any indication of recent merger activity from both morphological analysis and our machine learning merger classifier. Instead, our results reveal that TDE host galaxies are approximately 1.5 to 2.5 times more likely to have bar-like or ring-like structures than their controls. This enhancement is even more prominent for TDEs in the green valley, with the factor reaching almost 3. Based on these results, we propose that bar-driven secular evolution, instead of merger, likely dominates the recent evolution of TDE hosts found in the green valley, which can simultaneously explain their distinctive nuclear properties and enhanced TDE rates.

Spectroscopic surveys allow spatially resolved spectroscopy of galaxies to study their interstellar medium (ISM). However, observations of Galactic H$α$ emission are contaminated by geocoronal H$α$ emission. The latter is known to depend on the shadow altitude, a geometric parameter relating the line of sight to Earth's shadow cone. Using fibres on blank skys from the SDSS-IV/MaStar survey, we established an empirical relation between the geocoronal H$α$ emission and the shadow altitude, with a root mean square fractional scatter of 23.52$\%$. This relation can be used to predict geocoronal H$α$ emission so that it can be removed from observed spectra. This removal method is advantageous when the observed targets are extensive in the sky, and it does not require a large velocity separation between the observed target and the local standard of rest. This will enable reliable studies of Galactic H$α$ in intermediate spectral resolution integral field spectroscopic surveys. We also find tentative evidences for the dependences of geocoronal emission on solar activity and the distance between the Earth and the Sun.

Aims . Gas-phase metallicity is a fundamental parameter that helps constrain the star-forming history and chemical evolution of a galaxy. Measuring the electron temperature through auroral-to-strong line ratios is a direct approach to deriving metallicity. However, there is a longstanding discrepancy between metallicity measured through the direct method and that based on the photoionization models. This paper aims to verify and understand the discrepancies. Methods . We binned ~1.5 million spaxels from SDSS-IV MaNGA according to metallicity and ionization parameters derived from theoretical strong-line calibrations. We stacked the spectra of spaxels within each bin and measured the flux of strong lines and faint auroral lines. Auroral lines for [O II ], [S II ], [O III ], and [S III ] were detected in the stacked spectra of most bins, and the [N II ] auroral line was detected in fewer bins. We applied an empirical method to correct dust attenuation, which makes more realistic corrections for low ionization lines. Results . We derived electron temperatures for these five ionic species and measured the oxygen and sulfur abundances using the direct method. We present the resulting abundance measurements and compare them with those model-calibrated strong-line abundances. The chemical abundances measured with the direct method are lower than those derived from the photoionization model, with a median of 0.09 dex. This discrepancy is smaller compared to the results based on other theoretical metallicity calibrations previously reported. However, we notice that the direct method could not account for the variation in ionization parameters, indicating that the precise calibration of metallicity using the direct method has yet to be fully realized. We report significant discrepancies between data and the photoionization model, which illustrates that the 1D photoionization model is incapable of representing the complexity of real situations, and cannot predict the increase in the auroral-to-strong line ratio of [O II ] at high metallicity.

Measuring gas-phase metallicity, the relative abundance of elements heavier than helium to the abundance of hydrogen, is essential for constraining the chemical evolution of galaxies. The electron temperature is a crucial parameter for the determination of metallicity as the strongest emission lines from metal ions are all collisionally excited, which depends sensitively on temperature. Electron temperatures can be measured by comparing the strengths of two emission lines of the same ion that originate from two different upper levels, which are usually referred to as auroral-to-strong line ratios. Low electron temperature is theoretically expected for metal-rich star-forming regions, as metal ions in high metallicity gas provide efficient cooling through collisional excitation and radiative de-excitation. In this work, we report the discovery that temperature, as measured through auroral-to-strong line ratios of O ⁺ , trends in reverse directions at supersolar metallicities. This trend remains consistent regardless of the emission line fitting method employed and is not attributable to contamination or dust attenuation correction. Notably, this phenomenon is not observed in other low-ionization ions, such as S ⁺ and N ⁺ , which also probe electron temperature. The results are verified in two independent datasets. This finding challenges the fundamental principles of the direct T _e method for metallicity measurement, warranting further investigation into its physical interpretation.