We investigate the effect of metallicity calibrations, AGN classification, and aperture covering fraction on the local mass-metallicity (M-Z) relation using 27,730 star-forming galaxies from the SDSS ...Data Release 4. We analyze the SDSS M-Z relation with 10 metallicity calibrations, including theoretical and empirical methods. We show that the choice of metallicity calibration has a significant effect on the shape and y-intercept image of the M-Z relation. The absolute metallicity scale (y-intercept) varies up to image dex, depending on the calibration used, and the change in shape is substantial. These results indicate that it is critical to use the same metallicity calibration when comparing different luminosity-metallicity or M-Z relations. We present new metallicity conversions that allow metallicities that have been derived using different strong-line calibrations to be converted to the same base calibration. These conversions facilitate comparisons between different samples, particularly comparisons between galaxies at different redshifts for which different suites of emission lines are available. Our new conversions successfully remove the large 0.7 dex discrepancies between the metallicity calibrations, and we reach agreement in the M-Z relation to within 0.03 dex on average. We investigate the effect of AGN classification and aperture covering fraction on the M-Z relation. We find that different AGN classification methods have negligible effect on the SDSS M-Z relation. We compare the SDSS M-Z relation with nuclear and global relations from the NFGS. The turnover of the M-Z relation at image depends on the aperture covering fraction. We find that a lower redshift limit of image is insufficient for avoiding aperture effects in fiber spectra of the highest stellar mass galaxies.
ABSTRACT
Galaxy mergers are a major evolutionary transformation whose effects are borne out by a plethora of observations and numerical simulations. However, most previous simulations have used ...idealized, isolated, binary mergers and there has not been significant progress on studying statistical samples of galaxy mergers in large cosmological simulations. We present a sample of 27 691 post-merger (PM) galaxies (0c ≤ z ≤ 1) identified from IllustrisTNG: a cosmological, large box, magnetohydrodynamical simulation suite. The PM sample spans a wide range of merger and galaxy properties (M⋆, μ, fgas). We demonstrate that star-forming (SF) PMs exhibit enhanced star formation rates (SFRs) on average by a factor of ∼2, while the passive PMs show no statistical enhancement. We find that the SFR enhancements: (1) show no dependence on redshift, (2) anticorrelate with the PM’s stellar mass, and (3) correlate with the gas fraction of the PM’s progenitors. However, SF PMs show stronger enhancements which may indicate other processes being at play (e.g. gas phase, feedback efficiency). Although the SFR enhancement correlates mildly with the merger mass ratio, the more abundant minor mergers (0.1 ≤ μ < 0.3) still contribute ${\sim}50{{\ \rm per\ cent}}$ of the total SFR enhancement. By tracing the PM sample forward in time, we find that galaxy mergers can drive significant SFR enhancements which decay over ∼0.5 Gyr independent of the merger mass ratio, although the decay time-scale is dependent on the simulation resolution. The strongest merger-driven starburst galaxies evolve to be passive/quenched on faster time-scales than their controls.
ABSTRACT
We investigate how star formation quenching proceeds within central and satellite galaxies using spatially resolved spectroscopy from the SDSS-IV MaNGA DR15. We adopt a complete sample of ...star formation rate surface densities (ΣSFR), derived in Bluck et al. (2020), to compute the distance at which each spaxel resides from the resolved star forming main sequence (ΣSFR − Σ* relation): ΔΣSFR. We study galaxy radial profiles in ΔΣSFR, and luminosity weighted stellar age (AgeL), split by a variety of intrinsic and environmental parameters. Via several statistical analyses, we establish that the quenching of central galaxies is governed by intrinsic parameters, with central velocity dispersion (σc) being the most important single parameter. High mass satellites quench in a very similar manner to centrals. Conversely, low mass satellite quenching is governed primarily by environmental parameters, with local galaxy overdensity (δ5) being the most important single parameter. Utilizing the empirical MBH − σc relation, we estimate that quenching via AGN feedback must occur at $M_{\rm BH} \ge 10^{6.5-7.5} \, \mathrm{M}_{\odot }$, and is marked by steeply rising ΔΣSFR radial profiles in the green valley, indicating ‘inside-out’ quenching. On the other hand, environmental quenching occurs at overdensities of 10–30 times the average galaxy density at z∼ 0.1, and is marked by steeply declining ΔΣSFR profiles, indicating ‘outside-in’ quenching. Finally, through an analysis of stellar metallicities, we conclude that both intrinsic and environmental quenching must incorporate significant starvation of gas supply.
We present a catalog of bulge, disk, and total stellar mass estimates for ~660,000 galaxies in the Legacy area of the Sloan Digital Sky Survey Data (SDSS) Release 7. These masses are based on a ...homogeneous catalog of g- and r-band photometry described by Simard et al., which we extend here with bulge+disk and Sersic profile photometric decompositions in the SDSS u, i, and z bands. We discuss the methodology used to derive stellar masses from these data via fitting to broadband spectral energy distributions (SEDs), and show that the typical statistical uncertainty on total, bulge, and disk stellar mass is ~0.15 dex. Despite relatively small formal uncertainties, we argue that SED modeling assumptions, including the choice of synthesis model, extinction law, initial mass function, and details of stellar evolution likely contribute an additional 60% systematic uncertainty in any mass estimate based on broadband SED fitting. We discuss several approaches for identifying genuine bulge+disk systems based on both their statistical likelihood and an analysis of their one-dimensional surface-brightness profiles, and include these metrics in the catalogs. Estimates of the total, bulge and disk stellar masses for both normal and dust-free models and their uncertainties are made publicly available here.
In order to investigate the effects of galaxy mergers throughout the interaction sequence, we present a study of 10 800 galaxies in close pairs and a smaller sample of 97 post-mergers identified in ...the Sloan Digital Sky Survey. We find that the average central star formation rate (SFR) enhancement (×3.5) and the fraction of starbursts (20 per cent) peak in the post-merger sample. The post-mergers also show a stronger deficit in gas phase metallicity than the closest pairs, being more metal-poor than their control by −0.09 dex. Combined with the observed trends in SFR and the time-scales predicted in merger simulations, we estimate that the post-mergers in our sample have undergone coalescence within the last few hundred Myr. In contrast with the incidence of star-forming galaxies, the frequency of active galactic nuclei (AGN) peaks in the post-mergers, outnumbering AGN in the control sample by a factor of 3.75. Moreover, amongst the galaxies that host an AGN, the black hole accretion rates in the closest pairs and post-mergers are higher by a factor of ∼3 than AGN in the control sample. These results are consistent with a picture in which star formation is initiated early on in the encounter, with AGN activity peaking post-coalescence.
ABSTRACT
Large galaxy surveys have demonstrated that galaxy–galaxy mergers can dramatically change the morphologies, star formation rates (SFRs), and metallicities of their constituents. However, ...most statistical studies have been limited to the measurement of global quantities, through large fibres or integrated colours. In this work, we present the first statistically significant study of spatially resolved star formation and metallicity profiles using integral field spectroscopy, using a sample of ∼20 000 spaxels in 36 visually selected post-merger galaxies from the Mapping Nearby Galaxies at Apache Point Observatory survey. By measuring offsets from SFR and metallicity scaling relations on a spaxel-by-spaxel basis, we are able to quantify where in the galaxy these properties are most affected by the interaction. We find that the SFR enhancements are generally centrally peaked, by a factor of 2.5 on average, in agreement with predictions from simulations. However, there is considerable variation in the SFR behaviour in the galactic outskirts, where both enhancement and suppression are seen. The median SFR remains enhanced by 0.1 dex out to at least 1.9 Re. The metallicity is also affected out to these large radii, typically showing a suppression of ∼−0.04 dex.
Abstract
The tight correlation between total galaxy stellar mass and star formation rate (SFR) has become known as the star-forming main sequence. Using ∼487 000 spaxels from galaxies observed as ...part of the Sloan Digital Sky Survey Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, we confirm previous results that a correlation also exists between the surface densities of star formation (ΣSFR) and stellar mass (Σ⋆) on kpc scales, representing a ‘resolved’ main sequence. Using a new metric (ΔΣSFR), which measures the relative enhancement or deficit of star formation on a spaxel-by-spaxel basis relative to the resolved main sequence, we investigate the SFR profiles of 864 galaxies as a function of their position relative to the global star-forming main sequence (ΔSFR). For galaxies above the global main sequence (positive ΔSFR) ΔΣSFR is elevated throughout the galaxy, but the greatest enhancement in star formation occurs at small radii (<3 kpc, or 0.5Re). Moreover, galaxies that are at least a factor of 3 above the main sequence show diluted gas phase metallicities out to 2Re, indicative of metal-poor gas inflows accompanying the starbursts. For quiescent/passive galaxies that lie at least a factor of 10 below the star-forming main sequence, there is an analogous deficit of star formation throughout the galaxy with the lowest values of ΔΣSFR in the central 3 kpc. Our results are in qualitative agreement with the ‘compaction’ scenario in which a central starburst leads to mass growth in the bulge and may ultimately precede galactic quenching from the inside-out.
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