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We investigate the environmental dependence of the local gas-phase metallicity in a sample of star-forming galaxies from the MaNGA survey. Satellite galaxies with stellar masses in the range are found to be ∼0.05 dex higher in metallicity than centrals of similar stellar mass. Within the low-mass satellite population, we find that the interstellar medium (ISM) metallicity depends most strongly on the stellar mass of the galaxy that is central to the halo, though there is no obvious difference in the metallicity gradients. At fixed total stellar mass, the satellites of high-mass (M* > 1010.5M⊙) centrals are ∼0.1 dex more metal-rich than the satellites of low-mass (M* < 1010M⊙) centrals, controlling for local stellar mass surface density and gas fraction. Fitting a gas regulator model to the spaxel data, we are able to account for variations in the local gas fraction, stellar mass surface density, and local escape velocity–dependent outflows. We find that the best explanation for the metallicity differences is the variation in the average metallicity of accreted gas between different environments that depends on the stellar mass of the dominant galaxies in each halo. This is interpreted as evidence for the exchange of enriched gas between galaxies in dense environments that is predicted by recent simulations. [ABSTRACT FROM AUTHOR]

We assemble an unbiased sample of 29 galaxies with [O ii] λ3727 and/or [O iii] λ5007 detections at z < 0.15 from the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) Pilot Survey (HPS). The HPS finds galaxies without preselection based on their detected emission lines via integral field spectroscopy. Sixteen of these objects were followed up with the second-generation, low-resolution spectrograph on the upgraded HET. Oxygen abundances were then derived via strong emission lines using a Bayesian approach. We find that most of the galaxies fall along the mass–metallicity relation derived from photometrically selected star-forming galaxies in the Sloan Digital Sky Survey (SDSS). However, two of these galaxies have low metallicity (similar to the very rare green pea galaxies in mass–metallicity space). The star formation rates (SFRs) of this sample fall in an intermediate space between the SDSS star-forming main sequence and the extreme green pea galaxies. We conclude that spectroscopic selection fills a part of the mass–metallicity–SFR phase space that is missed in photometric surveys with preselection like SDSS; i.e., we find galaxies that are actively forming stars but are faint in continuum. We use the results of this pilot investigation to make predictions for the upcoming unbiased, large spectroscopic sample of local line emitters from HETDEX. With the larger HETDEX survey, we will determine if galaxies selected spectroscopically without continuum brightness preselection have metallicities that fall on a continuum that bridges typical star-forming and rarer, more extreme systems like green peas. [ABSTRACT FROM AUTHOR]

We present a method of fitting optical spectra of galaxies using a basis set of six vectors obtained from principal-component analysis of a library of synthetic spectra of 40,000 star formation histories (SFHs). Using this library, we provide estimates of the resolved effective stellar mass-to-light ratio () for thousands of galaxies from the SDSS-IV/MaNGA integral-field spectroscopic survey. Using a testing framework built on additional synthetic SFHs, we show that the estimates of are reliable (as are their uncertainties) at a variety of signal-to-noise ratios, stellar metallicities, and dust attenuation conditions. Finally, we describe the future release of the resolved stellar mass-to-light ratios as an SDSS-IV/MaNGA Value-Added Catalog and provide a link to the software used to conduct this analysis. (The software can be found at https://github.com/zpace/pcay.) [ABSTRACT FROM AUTHOR]

A galaxy’s stellar mass is one of its most fundamental properties, but it remains challenging to measure reliably. With the advent of very large optical spectroscopic surveys, efficient methods that can make use of low signal-to-noise spectra are needed. With this in mind, we created a new software package for estimating effective stellar mass-to-light ratios that uses a principal component analysis (PCA) basis set to optimize the comparison between observed spectra and a large library of stellar population synthesis models. In Paper I, we showed that with a set of six PCA basis vectors we could faithfully represent most optical spectra from the Mapping Nearby Galaxies at APO (MaNGA) survey, and we tested the accuracy of our M/L estimates using synthetic spectra. Here, we explore sources of systematic error in our mass measurements by comparing our new measurements to data from the literature. We compare our stellar mass surface density estimates to kinematics-derived dynamical mass surface density measurements from the DiskMass Survey and find some tension between the two that could be resolved if the disk scale heights used in the kinematic analysis were overestimated by a factor of ∼1.5. We formulate an aperture-corrected stellar mass catalog for the MaNGA survey, and compare to previous stellar mass estimates based on multiband optical photometry, finding typical discrepancies of 0.1 dex. Using the spatially resolved MaNGA data, we evaluate the impact of estimating total stellar masses from spatially unresolved spectra, and we explore how the biases that result from unresolved spectra depend upon the galaxy’s dust extinction and star formation rate. Finally, we describe an SDSS Value-Added Catalog that will include both spatially resolved and total (aperture-corrected) stellar masses for MaNGA galaxies. [ABSTRACT FROM AUTHOR]

The role of gas accretion in galaxy evolution is still a matter of debate. The presence of inflows of metal-poor gas that trigger star formation bursts of low metallicity has been proposed as an explanation for the local anticorrelation between star formation rate (SFR) and gas-phase metallicity (Zg) found in the literature. In the present study, we show how the anticorrelation is also present as part of a diversified range of behaviors for a sample of more than 700 nearby spiral galaxies from the SDSS-IV MaNGA survey. We have characterized the local relation between SFR and Zg after subtracting the azimuthally averaged radial profiles of both quantities. Of the analyzed galaxies, 60% display an SFR–Zg anticorrelation, with the remaining 40% showing no correlation (19%) or positive correlation (21%). Applying a random forest machine-learning algorithm, we find that the slope of the correlation is mainly determined by the average gas-phase metallicity of the galaxy. Galaxy mass, g − r colors, stellar age, and mass density seem to play a less significant role. This result is supported by the performed second-order polynomial regression analysis. Thus, the local SFR–Zg slope varies with the average metallicity, with the more metal-poor galaxies presenting the lowest slopes (i.e., the strongest SFR–Zg anticorrelations), and reversing the relation for more metal-rich systems. Our results suggest that external gas accretion fuels star formation in metal-poor galaxies, whereas in metal-rich systems, the gas comes from previous star formation episodes. [ABSTRACT FROM AUTHOR]

The late assembly of massive galaxies is thought to be dominated by stellar accretion in their outskirts (beyond two effective radii Re) due to dry, minor galaxy mergers. We use observations of 1010 passive early-type galaxies (ETGs) within z < 0.15 from MaNGA (Mapping Nearby Galaxies at Apache Point Observatory) to search for evidence of this accretion. The outputs from the stellar population fitting codes FIREFLY, pPXF, and Prospector are compared to control systematic errors in stellar metallicity (Z) estimation. We find that the average radial log Z/Z⊙ profiles of ETGs in various stellar mass (M*) bins are not linear. As a result, these profiles are poorly characterized by a single gradient value, explaining why weak trends reported in previous work can be difficult to interpret. Instead, we examine the full radial extent of stellar metallicity profiles and find them to flatten in the outskirts of M* ≳ 1011M⊙ ETGs. This is a signature of stellar accretion. Based on a toy model for stellar metallicity profiles, we infer the ex situ stellar mass fraction in ETGs as a function of M* and galactocentric radius. We find that ex situ stars at R ∼ 2Re make up 20% of the projected stellar mass of M* ≲ 1010.5M⊙ ETGs, rising up to 80% for M* ≳ 1011.5M⊙ ETGs. [ABSTRACT FROM AUTHOR]

We use data from 1222 late-type star-forming galaxies in the SDSS IV Mapping Nearby Galaxies at Apache Point Observatory survey to identify regions in which the gas-phase metallicity is anomalously low compared to expectations from the tight empirical relation between metallicity and stellar surface mass density at a given stellar mass. We find anomalously low-metallicity (ALM) gas in 10% of the star-forming spaxels and in 25% of the galaxies in the sample. The incidence rate of ALM gas increases strongly with both global and local measures of the specific star formation rate and is higher in lower mass galaxies and in the outer regions of galaxies. The incidence rate is also significantly higher in morphologically disturbed galaxies. We estimate that the lifetimes of the ALM regions are a few hundred Myr. We argue that the ALM gas has been delivered to its present location by a combination of interactions, mergers, and accretion from the halo, and that this infusion of gas stimulates star formation. Given the estimated lifetime and duty cycle of such events, we estimate that the time-averaged accretion rate of ALM gas is similar to the star formation rate in late-type galaxies over the mass range –1010M⊙. [ABSTRACT FROM AUTHOR]

The large Integral Field Spectroscopy surveys have allowed the classification of ionizing sources of emission lines on sub-kiloparsec scales. In this work, we define two non-parametric parameters, quiescence (Fq) and its concentration (Cq), to quantify the strength and the spatial distribution of the quenched areas, respectively, traced by the LI(N)ER regions with low EW(Hα). With these two measurements, we classify MaNGA galaxies into inside-out and outside-in quenching types according to their locations on the Fq versus Cq plane and we measure the fraction of inside-out (outside-in) quenching galaxies as a function of halo mass. We find that the fraction of galaxies showing inside-out quenching increases with halo mass, irrespective of stellar mass or galaxy type (satellites versus centrals). In addition, high-stellar-mass galaxies exhibit a greater fraction of inside-out quenching compared to low-stellar-mass ones in all environments. In contrast, the fraction of outside-in quenching does not depend on halo mass. Our results suggest that morphological quenching may be responsible for the inside-out quenching seen in all environments. On the other hand, the flat dependence of the outside-in quenching on halo mass could be a mixed result of ram pressure stripping and galaxy mergers. Nevertheless, for a given environment and stellar mass, the fraction of inside-out quenching is systematically greater than that of outside-in quenching, suggesting that inside-out quenching is the dominant quenching mode in all environments. [ABSTRACT FROM AUTHOR]

A new class of quiescent galaxies harboring possible AGN-driven winds has been discovered using spatially resolved optical spectroscopy from the ongoing SDSS-IV MaNGA survey. These galaxies, termed “red geysers,” constitute 5%–10% of the local quiescent population and are characterized by narrow bisymmetric patterns in ionized gas emission features. Cheung et al. argued that these galaxies host large-scale AGN-driven winds that may play a role in suppressing star formation at late times. In this work, we test the hypothesis that AGN activity is ultimately responsible for the red geyser phenomenon. We compare the nuclear radio activity of the red geysers to a matched control sample with similar stellar mass, redshift, rest-frame NUV − r color, axis ratio, and presence of ionized gas. We have used the 1.4 GHz radio continuum data from the VLA FIRST survey to stack the radio flux from the red geyser and control samples. In addition to a three times higher FIRST detection rate, we find that red geysers have a 5σ higher level of average radio flux than control galaxies. After restricting to rest-frame NUV − r color >5 and checking mid-IR WISE photometry, we rule out star formation contamination and conclude that red geysers are associated with more active AGNs. Red geysers and a possibly related class with disturbed Hα emission account for 40% of all radio-detected red galaxies with log (M⋆/M⊙) < 11. Our results support a picture in which episodic AGN activity drives large-scale, relatively weak ionized winds that may provide a feedback mechanism for many early-type galaxies. [ABSTRACT FROM AUTHOR]

A large population of ultra-diffuse galaxies (UDGs) was recently discovered in the Coma cluster. Here we present optical spectra of three such UDGs, DF 7, DF 44, and DF 17, which have central surface brightnesses of μg ≈ 24.4–25.1 mag arcsec−2. The spectra were acquired as part of an ancillary program within the SDSS-IV MaNGA Survey. We stacked 19 fibers in the central regions from larger integral field units (IFUs) per source. With over 13.5 hr of on-source integration, we achieved a mean signal-to-noise ratio in the optical of 9.5 Å−1, 7.9 Å−1, and 5.0 Å−1, respectively, for DF 7, DF 44, and DF 17. Stellar population models applied to these spectra enable measurements of recession velocities, ages, and metallicities. The recession velocities of DF 7, DF 44, and DF 17 are km s−1, km s−1, and km s−1, spectroscopically confirming that all of them reside in the Coma cluster. The stellar populations of these three galaxies are old and metal-poor, with ages of Gyr, Gyr, and Gyr, and iron abundances of [Fe/H] , , and , respectively. Their stellar masses are (3–6) × 108M⊙. The UDGs in our sample are as old or older than galaxies at similar stellar mass or velocity dispersion (only DF 44 has an independently measured dispersion). They all follow the well-established stellar mass–stellar metallicity relation, while DF 44 lies below the velocity dispersion-metallicity relation. These results, combined with the fact that UDGs are unusually large for their stellar masses, suggest that stellar mass plays a more important role in setting stellar population properties for these galaxies than either size or surface brightness. [ABSTRACT FROM AUTHOR]

The galaxy integrated Hα star formation rate–stellar mass relation, or SFR(global)–M*(global) relation, is crucial for understanding star formation history and evolution of galaxies. However, many studies have dealt with SFR using unresolved measurements, which makes it difficult to separate out the contamination from other ionizing sources, such as active galactic nuclei and evolved stars. Using the integral field spectroscopic observations from SDSS-IV MaNGA, we spatially disentangle the contribution from different Hα powering sources for ∼1000 galaxies. We find that, when including regions dominated by all ionizing sources in galaxies, the spatially resolved relation between Hα surface density (ΣHα(all)) and stellar mass surface density (Σ*(all)) progressively turns over at the high Σ*(all) end for increasing M*(global) and/or bulge dominance (bulge-to-total light ratio, B/T). This in turn leads to the flattening of the integrated Hα(global)–M*(global) relation in the literature. By contrast, there is no noticeable flattening in both integrated Hα(H ii)–M*(H ii) and spatially resolved ΣHα(H ii)–Σ*(H ii) relations when only regions where star formation dominates the ionization are considered. In other words, the flattening can be attributed to the increasing regions powered by non-star-formation sources, which generally have lower ionizing ability than star formation. An analysis of the fractional contribution of non-star-formation sources to total Hα luminosity of a galaxy suggests a decreasing role of star formation as an ionizing source toward high-mass, high-B/T galaxies and bulge regions. This result indicates that the appearance of the galaxy integrated SFR–M* relation critically depends on their global properties (M*(global) and B/T) and relative abundances of various ionizing sources within the galaxies. [ABSTRACT FROM AUTHOR]

We study the role of cold gas in quenching star formation in the green valley by analyzing ALMA 12CO (1–0) observations of three galaxies with resolved optical spectroscopy from the MaNGA survey. We present resolution-matched maps of the star formation rate and molecular gas mass. These data are used to calculate the star formation efficiency (SFE) and gas fraction () for these galaxies separately in the central “bulge” regions and outer disks. We find that, for the two galaxies whose global specific star formation rate (sSFR) deviates most from the star formation main sequence, the gas fraction in the bulges is significantly lower than that in their disks, supporting an “inside-out” model of galaxy quenching. For the two galaxies where SFE can be reliably determined in the central regions, the bulges and disks share similar SFEs. This suggests that a decline in is the main driver of lowered sSFR in bulges compared to disks in green valley galaxies. Within the disks, there exist common correlations between the sSFR and SFE and between sSFR and on kiloparsec scales—the local SFE or in the disks declines with local sSFR. Our results support a picture in which the sSFR in bulges is primarily controlled by , whereas both SFE and play a role in lowering the sSFR in disks. A larger sample is required to confirm if the trend established in this work is representative of the green valley as a whole. [ABSTRACT FROM AUTHOR]

We present a Bayesian approach to the redshift classification of emission-line galaxies when only a single emission line is detected spectroscopically. We consider the case of surveys for high-redshift Lyα-emitting galaxies (LAEs), which have traditionally been classified via an inferred rest-frame equivalent width (EW; ) greater than 20 Å. Our Bayesian method relies on known prior probabilities in measured emission-line luminosity functions and EW distributions for the galaxy populations, and returns the probability that an object in question is an LAE given the characteristics observed. This approach will be directly relevant for the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), which seeks to classify ∼106 emission-line galaxies into LAEs and low-redshift emitters. For a simulated HETDEX catalog with realistic measurement noise, our Bayesian method recovers 86% of LAEs missed by the traditional > 20 Å cutoff over 2 < z < 3, outperforming the EW cut in both contamination and incompleteness. This is due to the method’s ability to trade off between the two types of binary classification error by adjusting the stringency of the probability requirement for classifying an observed object as an LAE. In our simulations of HETDEX, this method reduces the uncertainty in cosmological distance measurements by 14% with respect to the EW cut, equivalent to recovering 29% more cosmological information. Rather than using binary object labels, this method enables the use of classification probabilities in large-scale structure analyses. It can be applied to narrowband emission-line surveys as well as upcoming large spectroscopic surveys including Euclid and WFIRST. [ABSTRACT FROM AUTHOR]

We perform Jeans anisotropic modeling (JAM) on elliptical and spiral galaxies from the MaNGA DR13 sample. By comparing the stellar mass-to-light ratios estimated from stellar population synthesis and from JAM, we find a systematic variation of the initial mass function (IMF) similar to that in the earlier results. Early-type galaxies (elliptical and lenticular) with lower velocity dispersions within one effective radius are consistent with a Chabrier-like IMF, while galaxies with higher velocity dispersions are consistent with a more bottom-heavy IMF such as the Salpeter IMF. Spiral galaxies have similar systematic IMF variations, but with slightly different slopes and larger scatters, due to the uncertainties caused by the higher gas fractions and extinctions for these galaxies. Furthermore, we examine the effects of stellar mass-to-light ratio gradients on our JAM modeling, and we find that the trends become stronger after considering the gradients. [ABSTRACT FROM AUTHOR]

The nature of warm, ionized gas outside of galaxies may illuminate several key galaxy evolutionary processes. A serendipitous observation by the MaNGA survey has revealed a large, asymmetric Hα complex with no optical counterpart that extends ≈8″ (≈6.3 kpc) beyond the effective radius of a dusty, starbursting galaxy. This Hα extension is approximately three times the effective radius of the host galaxy and displays a tail-like morphology. We analyze its gas-phase metallicities, gaseous kinematics, and emission-line ratios and discuss whether this Hα extension could be diffuse ionized gas, a gas accretion event, or something else. We find that this warm, ionized gas structure is most consistent with gas accretion through recycled wind material, which could be an important process that regulates the low-mass end of the galaxy stellar mass function. [ABSTRACT FROM AUTHOR]

Galaxy proto-clusters at provide a direct probe of the rapid mass assembly and galaxy growth of present-day massive clusters. Because of the need for precise galaxy redshifts for density mapping and the prevalence of star formation before quenching, nearly all the proto-clusters known to date were confirmed by spectroscopy of galaxies with strong emission lines. Therefore, large emission-line galaxy surveys provide an efficient way to identify proto-clusters directly. Here we report the discovery of a large-scale structure at z = 2.44 in the Hobby Eberly Telescope Dark Energy Experiment (HETDEX) Pilot Survey. On a scale of a few tens of Mpc comoving, this structure shows a complex overdensity of Lyα emitters (LAE), which coincides with broadband selected galaxies in the COSMOS/UltraVISTA photometric and zCOSMOS spectroscopic catalogs, as well as overdensities of intergalactic gas revealed in the Lyα absorption maps of Lee et al. We construct mock LAE catalogs to predict the cosmic evolution of this structure. We find that such an overdensity should have already broken away from the Hubble flow, and part of the structure will collapse to form a galaxy cluster with by z = 0. The structure contains a higher median stellar mass of broadband selected galaxies, a boost of extended Lyα nebulae, and a marginal excess of active galactic nuclei relative to the field, supporting a scenario of accelerated galaxy evolution in cluster progenitors. Based on the correlation between galaxy overdensity and the z = 0 descendant halo mass calibrated in the simulation, we predict that several hundred proto-clusters with z = 0 mass of will be discovered in the 8.5 Gpc3 of space surveyed by the HETDEX. [ABSTRACT FROM AUTHOR]

We present an analysis of the data produced by the MaNGA prototype run (P-MaNGA), aiming to test how the radial gradients in recent star formation histories, as indicated by the 4000 Å break (Dn(4000)), Hδ absorption (EW(HδA)), and Hα emission (EW(Hα)) indices, can be useful for understanding disk growth and star formation cessation in local galaxies. We classify 12 galaxies observed on two P-MaNGA plates as either centrally quiescent (CQ) or centrally star-forming (CSF), according to whether Dn(4000) measured in the central spaxel of each datacube exceeds 1.6. For each spaxel we generate both 2D maps and radial profiles of Dn(4000), EW(HδA), and EW(Hα). We find that CSF galaxies generally show very weak or no radial variation in these diagnostics. In contrast, CQ galaxies present significant radial gradients, in the sense that Dn(4000) decreases, while both EW(HδA) and EW(Hα) increase from the galactic center outward. The outer regions of the galaxies show greater scatter on diagrams relating the three parameters than their central parts. In particular, the clear separation between centrally measured quiescent and star-forming galaxies in these diagnostic planes is largely filled in by the outer parts of galaxies whose global colors place them in the green valley, supporting the idea that the green valley represents a transition between blue-cloud and red-sequence phases, at least in our small sample. These results are consistent with a picture in which the cessation of star formation propagates from the center of a galaxy outward as it moves to the red sequence. [ABSTRACT FROM AUTHOR]