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The specific star formation rate (sSFR) is commonly used to describe the level of galaxy star formation (SF) and to select quenched galaxies. However, since it is a relative measure of the young-to-old population, an ambiguity in its interpretation may arise because a low sSFR can be due to either a substantial previous mass buildup or SF activity that is low. We show, using large samples spanning 0 < z < 2, that the normalization of the star formation rate (SFR) by the physical extent over which SF is taking place (i.e., the SFR surface density, ΣSFR) overcomes this ambiguity. ΣSFR has a strong physical basis, being tied to the molecular gas density and the effectiveness of stellar feedback, so we propose ΣSFR–M * as an important galaxy evolution diagram to complement (s)SFR–M * diagrams. Using the ΣSFR–M * diagram we confirm the Schiminovich et al. result that the level of SF along the main sequence today is only weakly mass-dependent—high-mass galaxies, despite their redder colors, are as active as blue, low-mass ones. At higher redshift, the slope of the “ΣSFR main sequence” steepens, signaling the epoch of bulge buildup in massive galaxies. We also find that ΣSFR based on the optical isophotal radius more cleanly selects both starbursting and spheroid-dominated (early-type) galaxies than the sSFR. One implication of our analysis is that the assessment of the inside-out versus outside-in quenching scenarios should consider both sSFR and ΣSFR radial profiles, because ample SF may be present in bulges with low sSFRs (red color).

In this work, we use ∼500 low-redshift (z ∼ 0.1) X-ray active galactic nuclei (AGNs) observed by XMM-Newton and the Sloan Digital Sky Survey (SDSS) to investigate the prevalence and nature of AGNs that apparently lack optical emission lines (“optically dull AGNs”). Although one quarter of spectra appear absorption-line dominated in visual assessment, line extraction with robust continuum subtraction from the MPA/JHU catalog reveals usable [O iii] measurements in 98% of the sample, allowing us to study [O iii]-underluminous AGNs together with more typical AGNs in the context of the L [O III]–L X relation. We find that “optically dull AGNs” do not constitute a distinct population of AGNs. Instead, they are the [O iii]-underluminous tail of a single, unimodal L [O III]–L X relation that has substantial scatter (0.6 dex). We find the degree to which an AGN is underluminous in [O iii] correlates with the specific star formation rate or D 4000 index of the host, which are both linked to the molecular gas fraction. Thus the emerging physical picture for the large scatter seems to involve the gas content of the narrow-line region. We find no significant role for previously proposed scenarios for the presence of optically dull AGNs, such as host dilution or dust obscuration. Despite occasionally weak lines in SDSS spectra, >80% of X-ray AGNs are identified as such with the Baldwin–Phillips–Terlevich diagram. More than 90% are classified as AGNs based only on [N ii]/Hα, providing more complete AGN samples when [O iii] or Hβ are weak. X-ray AGNs with LINER spectra obey essentially the same L [O III]–L X relation as Seyfert 2s, suggesting their line emission is produced by AGN activity.

The metallicity of galaxies, and its variation with galactocentric radius, provides key insights into the formation histories of galaxies and the physical processes driving their evolution. In this work, we analyze the radial metallicity gradients of star-forming galaxies in the EAGLE, Illustris, IllustrisTNG, and SIMBA cosmological simulations across broad mass (108.0 M⊙ ≤ M⋆ ≲ 1012.0 M⊙) and redshift (0 ≤ z ≤ 8) ranges. We find that all simulations predict strong negative (i.e., radially decreasing) metallicity gradients at early cosmic times, likely due to their similar treatments of relatively smooth stellar feedback not providing sufficient mixing to quickly flatten gradients. The strongest redshift evolution occurs in galaxies with stellar masses of 1010.0–1011.0 M⊙, while galaxies with stellar mass < 1010M⊙ and >1011M⊙ exhibit weaker redshift evolution. Our result of negative gradients at high redshift contrast with the many positive and flat gradients in the 1 < z < 4 observational literature. At z > 6, the negative gradients observed with JWST and the Atacama Large Millimeter/submillimeter Array are flatter than those in simulations, albeit with closer agreement than at lower redshift. Overall, we suggest that these smooth stellar feedback galaxy simulations may not sufficiently mix their metal content radially, and that either stronger stellar feedback or additional subgrid turbulent metal diffusion models may be required to better reproduce observed metallicity gradients.

In this VERTICO early science paper we explore in detail how environmental mechanisms, identified in H i, affect the resolved properties of molecular gas reservoirs in cluster galaxies. The molecular gas is probed using ALMA ACA (+TP) observations of 12CO(2–1) in 51 spiral galaxies in the Virgo cluster (of which 49 are detected), all of which are included in the VIVA H i survey. The sample spans a stellar mass range of 9≤logM⋆/M⊙≤11 . We study molecular gas radial profiles, isodensity radii, and surface densities as a function of galaxy H i deficiency and morphology. There is a weak correlation between global H i and H2 deficiencies, and resolved properties of molecular gas correlate with H i deficiency: galaxies that have large H i deficiencies have relatively steep and truncated molecular gas radial profiles, which is due to the removal of low-surface-density molecular gas on the outskirts. Therefore, while the environmental mechanisms observed in H i also affect molecular gas reservoirs, there is only a moderate reduction of the total amount of molecular gas.

Recent observations with the Five-hundred-meter Aperture Spherical Telescope have revealed abundant reservoirs of neutral hydrogen (H I) in low-redshift poststarburst galaxies (PSBs), raising the question of why star formation ceases rapidly in these systems. In this study, we present a detailed analysis of the shape of the integrated H I spectra of 67 PSBs. We find that PSBs exhibit significantly higher H I spectral concentration values (K) compared to a matched sample from xGASS, and are more comparable to those of starburst galaxies. By extending our analysis to spatially resolved H I data from THINGS and Atlas3D, we show that both centrally concentrated H I distributions and dynamically unsettled H I can effectively increase K, while nonaxisymmetric structures only contribute to the scatter of the K distribution. Distinguishing between central concentration and dynamically unsettled gas as the origin of high K can be achieved by measuring the spectral asymmetry (AF), making the K–AF plane a powerful diagnostic tool for identifying galaxies with unsettled H I using integrated spectra alone. Based on their location in the K–AF plane, we find that most PSBs are not dominated by unsettled H I, but rather exhibit elevated central gas concentration. Both modes of gas redistribution in PSBs may eventually contribute to their quenching.

Integral field units have extended our knowledge of galactic properties to kiloparsec (or, sometimes, even smaller) patches of galaxies. These scales are where the physics driving galaxy evolution (feedback, chemical enrichment, etc.) take place. Quantifying the spatially resolved properties of galaxies, both observationally and theoretically, is therefore critical to our understanding of galaxy evolution. To this end, we investigate spatially resolved scaling relations within galaxies of M⋆ > 109.0 at z = 0 in IllustrisTNG. We examine both the resolved star formation main sequence (rSFMS) and the resolved mass–metallicity relation (rMZR) using 1 kpc × 1 kpc maps. We find that the rSFMS in IllustrisTNG is well described by a power law but is significantly shallower than the observed rSFMS. However, the disagreement between the rSFMS of IllustrisTNG and observations is likely driven by an overestimation of AGN feedback in IllustrisTNG for the higher-mass hosts. Conversely, the rMZR for IllustrisTNG has very good agreement with observations. Furthermore, we argue that the rSFMS is an indirect result of the Schmidt–Kennicutt law and local gas relation, which are both independent of host galaxy properties. Finally, we expand upon a localized leaky-box model to study the evolution of idealized spaxels and find that it provides a good description of these resolved relations. The degree of agreement, however, between idealized spaxels and simulated spaxels depends on the “net” outflow rate for the spaxel, and the IllustrisTNG scaling relations indicate a preference for a low net outflow rate.

Star formation quenching is one of the key processes that shape the evolution of galaxies. In this study, we investigate the changes in molecular gas and star formation properties as galaxies transit from the star-forming main sequence to the passive regime. Our analysis reveals that as galaxies move away from the main sequence toward the green valley the radial profile of specific star formation rate surface density (ΣsSFR) is suppressed compared with main-sequence galaxies out to a galactocentric radius of 1.5 R e(∼7 kpc for our sample). By combining radial profiles of gas fraction (f gas) and star formation efficiency (SFE), we can discern the underlying mechanism that determines ΣsSFR at different galactocentric radii. Analysis of relative contributions of f gas and SFE to ΣsSFR uncovers a diverse range of quenching modes. Star formation in approximately half of our quenching galaxies is primarily driven by a single mode (i.e., either f gas or SFE), or a combination of both. A collective analysis of all galaxies reveals that the reduction in star formation within the central regions (R < 0.5 R e) is primarily attributable to a decrease in SFE. Conversely, in the disk regions (R > 0.5 R e), both f gas and SFE contribute to the suppression of star formation. Our findings suggest that multiple quenching mechanisms may be at play in our sample galaxies, and even within a single galaxy. We also compare our observational outcomes with those from galaxy simulations and discuss the implications of our data.

We utilize the ALMA-MaNGA QUEnch and STar formation (ALMaQUEST) survey to investigate the kpc-scale scaling relations, presented as the resolved star-forming MS (rSFMS: ΣSFR versus Σ*), the resolved Schmidt–Kennicutt relation (rSK: ΣSFR versus ΣH2 ), and the resolved molecular gas MS (rMGMS: ΣH2 versus Σ*), for 11,478 star-forming and 1414 retired spaxels (oversampled by a factor of ∼20) located in 22 GV and 12 MS galaxies. For a given galaxy type (MS or GV), the retired spaxels are found to be offset from the sequences formed by the star-forming spaxels on the rSFMS, rSK, and rMGMS planes, toward lower absolute values of sSFR, SFE, and fH2 by ∼1.1, 0.6, and 0.5 dex. The scaling relations for GV galaxies are found to be distinct from that of the MS galaxies, even if the analyses are restricted to the star-forming spaxels only. It is found that, for star-forming spaxels, sSFR, SFE, and fH2 in GV galaxies are reduced by ∼0.36, 0.14, and 0.21 dex, respectively, compared to those in MS galaxies. Therefore, the suppressed sSFR/SFE/f gas in GV galaxies is associated with not only an increased proportion of retired regions in GV galaxies but also a depletion of these quantities in star-forming regions. Finally, the reduction of SFE and fH2 in GV galaxies relative to MS galaxies is seen in both bulge and disk regions (albeit with larger uncertainties), suggesting that, statistically, quenching in the GV population may persist from the inner to the outer regions.

Statistically, green valley (GV) galaxies exhibit lower molecular gas fractions (fgas) and reduced star formation efficiency (SFE) compared to star-forming galaxies. However, it remains unclear whether quenching is primarily driven by one factor or results from a combination of mechanisms in individual GV galaxies. In this study, we address this question by examining the spatial distributions of star formation and molecular gas in 28 GVs selected from the ALMaQUEST survey and additional literature samples. For each galaxy, we identify regions with suppressed specific star formation rate (sSFR) and measure Δfgas and ΔSFE—offsets from the resolved scaling relations of the star-forming main-sequence galaxies. By comparing the fraction of regions with negative Δfgas and ΔSFE, we classify 35.7% ± 13.2% (57.1% ± 17.9%) of GV galaxies as fgas driven, 39.3% ± 14.0% (39.3% ± 14.0%) as SFE driven, and 25.0% ± 10.6% (3.6% ± 3.6%) as mixed mode when adopting a fixed (variable) CO-to-H2 conversion factor (αCO). These results indicate that GVs undergo quenching through multiple pathways. As sSFR decreases from the main sequence to the GV, we observe a transition toward predominantly SFE-driven quenching, possibly linked to internal processes such as morphological quenching or active galactic nucleus activity. We further estimate the quenching timescale (τdecay), defined as the time from the peak star formation rate to 1 e–1 (approximately 37%) of its value, using integrated MaNGA spectra. SFE-driven quenching is typically associated with short τdecay, while fgas-driven quenching shows a broader range. Overall, 75% of GVs exhibit τdecay shorter than 1 Gyr, suggesting that quenching in most GVs proceeds rapidly, challenging purely slow-quenching scenarios like starvation.

This paper investigates C iv absorption in the circumgalactic medium (CGM) of L⋆ galaxies and its relationship with galaxy star formation rates. We present new observations from the C iv in L⋆ galaxies survey (PID#17076) using the Hubble Space Telescope/Cosmic Origins Spectrograph. By combining these measurements with archival C iv data (46 observations total), we estimate detection fractions for star-forming (sSFR > 10−11 yr−1) and passive galaxies (sSFR ≤ 10−11 yr−1 ) to be 72 −18+14 % [21/29] and 23 −15+27 % [3/13], respectively. This indicates a significant dichotomy in C iv presence between L⋆ star-forming and passive galaxies, with over 99% confidence. This finding aligns with J. Tumlinson et al., which noted a similar dichotomy in O vi absorption. Our results imply a substantial carbon reservoir in the CGM of L⋆ galaxies, suggesting a minimum carbon mass of ≳3.03 × 106 M⊙ out to 120 kpc. Together, these findings highlight a strong connection between star formation in galaxies and the state of their CGM, providing insight into the mechanisms governing galaxy evolution.