The fourth generation of the Sloan Digital Sky Survey (SDSS-IV) began observations in 2014 July. It pursues three core programs: the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2), Mapping Nearby Galaxies at APO (MaNGA), and the Extended Baryon Oscillation Spectroscopic Survey (eBOSS). As well as its core program, eBOSS contains two major subprograms: the Time Domain Spectroscopic Survey (TDSS) and the SPectroscopic IDentification of ERosita Sources (SPIDERS). This paper describes the first data release from SDSS-IV, Data Release 13 (DR13). DR13 makes publicly available the first 1390 spatially resolved integral field unit observations of nearby galaxies from MaNGA. It includes new observations from eBOSS, completing the Sloan Extended QUasar, Emission-line galaxy, Luminous red galaxy Survey (SEQUELS), which also targeted variability-selected objects and X-ray-selected objects. DR13 includes new reductions of the SDSS-III BOSS data, improving the spectrophotometric calibration and redshift classification, and new reductions of the SDSS-III APOGEE-1 data, improving stellar parameters for dwarf stars and cooler stars. DR13 provides more robust and precise photometric calibrations. Value-added target catalogs relevant for eBOSS, TDSS, and SPIDERS and an updated red-clump catalog for APOGEE are also available. This paper describes the location and format of the data and provides references to important technical papers. The SDSS web site, http://www.sdss.org, provides links to the data, tutorials, examples of data access, and extensive documentation of the reduction and analysis procedures. DR13 is the first of a scheduled set that will contain new data and analyses from the planned ∼6 yr operations of SDSS-IV.

Existing models of galaxy formation have not yet explained striking correlations between structure and star formation activity in galaxies, notably the sloped and moving boundaries that divide star-forming from quenched galaxies in key structural diagrams. This paper uses these and other relations to “reverse engineer” the quenching process for central galaxies. The basic idea is that star-forming galaxies with larger radii (at a given stellar mass) have lower black hole (BH) masses due to lower central densities. Galaxies cross into the green valley when the cumulative effective energy radiated by their BH equals ∼4× their halo gas-binding energy. Because larger-radii galaxies have smaller BHs, one finds that they must evolve to higher stellar masses in order to meet this halo energy criterion, which explains the sloping boundaries. A possible cause of radii differences among star-forming galaxies is halo concentration. The evolutionary tracks of star-forming galaxies are nearly parallel to the green-valley boundaries, and it is mainly the sideways motions of these boundaries with cosmic time that cause galaxies to quench. BH scaling laws for star-forming, quenched, and green-valley galaxies are different, and most BH mass growth takes place in the green valley. Implications include the radii of star-forming galaxies are an important second parameter in shaping their BHs; BHs are connected to their halos but in different ways for star-forming, quenched, and green-valley galaxies; and the same BH–halo quenching mechanism has been in place since z ∼ 3. We conclude with a discussion of BH–galaxy coevolution and the origin and interpretation of BH scaling laws.

The radiation mechanism of the prompt emission of gamma-ray bursts (GRBs) remains an open question. Although their spectra are usually well fitted with the empirical Band function, which is widely believed to be fully nonthermal and interpreted as an optically thin synchrotron emission, accumulating evidence shows that a thermal component actually exists. In this paper, a multicolor blackbody (mBB) model is proposed for the time-integrated spectrum of GRB 081221 by assuming a power-law distribution of the thermal luminosities with temperature, which manifests photospheric emissions from a different radius and/or angle. The effects of the minimum temperature kT _min, the maximum temperature kT _max, and the power-law index m of the luminosity distribution of an mBB are discussed. The fitting to the time-integrated spectrum during the bright phase (from 20 to 30 s since the trigger) of GRB 081221 by the mBB model yields kT _min = 4.4 ± 0.3 keV, , and . When the time bin is small enough, the time-resolved spectra of GRB 081221 are well fitted with a series of single-temperature blackbodies. Our results imply the prompt emission of GRB 081221 is dominated by the photosphere emission and its time-integrated spectrum is a superposition of pure blackbody components at different times, indicating that some empirical Band spectra may be interpreted as mBB if the temperature is widely distributed.

We investigate the recent star formation history (SFH) in the inner region of 57 nearly face-on spiral galaxies selected from the Calar Alto Legacy Integral Field Area (CALIFA) survey. For each galaxy, we use the integral field spectroscopy from CALIFA to obtain two-dimensional maps and radial profiles of three parameters that are sensitive indicators of the recent SFH: the 4000 Å break (D _n (4000)), and the equivalent width of Hδ absorption ( ) and Hα emission (EW(Hα)). We have also performed photometric decomposition of bulge/bar/disk components based on SDSS optical image. We identify a class of 17 “turnover” galaxies for which the central region presents a significant drop in D _n (4000), and most of them correspondingly show a central upturn in and EW(Hα). This indicates that the central region of the turnover galaxies has experienced star formation in the past 1–2 Gyr, which makes the bulge younger and more star-forming than surrounding regions. We find that almost all (15/17) of the turnover galaxies are barred, while only half of the barred galaxies in our sample (15/32) are classified as a turnover galaxies. This finding provides strong evidence in support of the theoretical expectation that the bar may drive gas from the disk inward to trigger star formation in the galaxy center, an important channel for the growth/rejuvenation of pseudobulges in disk galaxies.

We investigate radial gradients in the recent star formation history (SFH) of 1917 galaxies with 0.01 < z < 0.14 from the Mapping Nearby Galaxies at Apache Point Observatory project. For each galaxy, we obtain two-dimensional maps and radial profiles for three spectroscopically measured parameters that are sensitive to the recent SFH: D _n (4000) (the 4000 Å break), EW(Hδ _A ), and EW(Hα) (the equivalent width of the Hδ absorption and the Hα emission line). The majority of the spaxels are consistent with models of a continuously declining star formation rate, indicating that starbursts occur rarely in local galaxies with regular morphologies. We classify the galaxies into three classes: fully star-forming (SF), partly quenched (PQ), and totally quenched (TQ). The galaxies that are less massive than 10¹⁰ M _⊙ present at most weak radial gradients in the diagnostic parameters. In contrast, massive galaxies with a stellar mass above 10¹⁰ M _⊙ present significant gradients in the three diagnostic parameters if they are classified as SF or PQ but show weak gradients in D _n (4000) and EW(Hδ _A ) and no gradients in EW(Hα) if they are in the TQ class. This implies the existence of a critical stellar mass (∼10¹⁰ M _⊙) above which the star formation in a galaxy is shut down from the inside out. Galaxies tend to evolve synchronously from the inner to the outer regions before their mass reaches the critical value. We have further divided the sample at a fixed mass by both bulge-to-total luminosity ratio and morphological type, finding that our conclusions hold regardless of these factors; it appears that the presence of a central dense object is not a driving parameter but rather a by-product of the star formation cessation process.

We quantitatively investigate the dependence of central galaxy Hi mass (M _Hi ) on the stellar mass (M _*), halo mass (M _h ), star formation rate (SFR), and central stellar surface density within 1 kpc (Σ₁), taking advantage of the Hi spectra stacking technique using both the Arecibo Fast Legacy ALFA Survey and the Sloan Digital Sky Survey. We find that the shapes of M _Hi –M _h and M _Hi –M _* relations are remarkably similar for both star-forming and quenched galaxies, with massive quenched galaxies having constantly lower Hi masses of around 0.6 dex. This similarity strongly suggests that neither halo mass nor stellar mass is the direct cause of quenching, but rather the depletion of the Hi reservoir. While the Hi reservoir for low-mass galaxies of M _* < 10^10.5 M _☉ strongly increases with M _h , more massive galaxies show no significant dependence of M _Hi with M _h , indicating that the main effect of halo is to determine the smooth cold gas accretion. We find that the star formation and quenching of central galaxies are directly regulated by the available Hi reservoir, with an average relation of SFR ∝ M H I 2.75 / M * 0.40 , implying a quasi-steady state of star formation. We further confirm that galaxies are depleted of their Hi reservoir once they drop off the star formation main sequence and there is a very tight and consistent correlation between M _Hi and Σ₁ in this phase, with M H I ∝ Σ 1 − 2 . This result is consistent with the compaction-triggered quenching scenario, with galaxies going through three evolutionary phases of cold gas accretion, compaction and post-compaction, and quenching.