During this evolution, a galaxy experiences a wide range of inter- actions that are dependent on its location relative to other galaxies in the Universe, or known more commonly as the galaxies’ envi- ronment. Dense environments, such as clusters, expose galaxies to interactions such as tidal stripping (Read et al. 2006), galaxy harass- ment (Farouki & Shapiro 1981) or even strangulation of gas from neighbours (Larson, Tinsley & Caldwell 1980). Galaxies that reside under denser regions, such as voids however, remain largely un- touched, accreting gas from the intergalactic medium. This diverse range of evolutionary processes should affect the galaxies proper- ties in different ways. Yet, until the discovery of the morphology– density relation (Oemler 1974; Dressler 1980), the importance of environment on galaxy evolution was poorly understood. Since then,
Figure 7 shows the light-weighted age and metallicity gradients for central (grey) and satellite (red) galaxies, as a function of local en- vironment. Table 4 shows the numerical results of this analysis. First, Figure 7 shows that stellarpopulationgradients are independent of environmental density, as no correlation is evident between stellarpopulation gradient and local density. This can be quantitively described by fitting a line through the stellarpopulation gradient-environment plane in each panel plot. We find that luminosity and mass-weighted stellarpopulationgradients generally do not correlate with local environment neither for central nor satellite galaxies. We further do not detect any evidence for a difference in gradients between satellite and central galaxies (see also Table 4). We conclude that the galaxyenvironment, whether measured as local environmental density or through central/satellite classification, does not appear to have any significant effect on age and metallicity gradients in galaxies. This result agrees well with a recent IFU study of nearby massive galaxies as part of the MASSIVE survey, where it is found that even at large radius, internal properties matter more than environment in determining star formation history (Greene et al. 2015).
The advent of large-scale Integral-Field-Unit (IFU) surveys of the local galaxypopulation are providing the next step forward in these studies. IFU spectroscopy, pioneered by the SAURON and ATLAS-3D projects (Bacon et al. 2001; Davies et al. 2001; Cappellari et al. 2011), allows spatially resolved studies of stellar populations at unprecedented detail. The new generation of such surveys, CALIFA (Sánchez et al. 2012), SAMI (Croom et al. 2012), and MaNGA (Bundy et al. 2015) are providing IFU data for large galaxy samples for the first time, allowing for statistically meaningful studies of spatially resolved stellarpopulation properties in the multivariate space of galaxy parameters to be conducted. Studies from the CALIFA survey, for example, have shown a dependence of stellarpopulation properties with central velocity dispersion (Sánchez-Blázquez et al. 2014) and concluded that galaxy morphology is more important than mass in shaping stellarpopulationgradients (González Delgado et al. 2015) using a sample of 300 galaxies (73 early-types and 227 late-types). In this paper, we report on the first year data collected by the MaNGA survey and analyse radial gradients in stellarpopulation properties using a statistically superior galaxy sample of 721 galaxies. We also reconstruct full star formation histories as a function of galaxy mass and type for both early and late-type galaxies. The MaNGAgalaxy sample is also large enough to conduct an unbiased investigation into the dependence of stellarpopulationgradients on galaxyenvironment, and the results of this study are presented in an accompanying paper (Goddard et al. 2016, hereafter Paper 2).
Warps in stellar discs may result from multiple stellar compo- nents, suggesting they result from mergers. Several studies have argued that early-type galaxies are assembled through dry mergers, and the majority have had a major merger in their recent history (van Dokkum 2005; Bell et al. 2006) which naturally explains distinct kinematic components in the remnant. Since early-type galaxies dominate the galaxypopulation in the high-mass red sequence (e.g. Nakamura et al. 2003), we expect the majority of stellar Type-OB profiles to fall in this regime if they do in fact reflect multiple com- ponent stellar systems, which is hinted at in Fig. 13. It has also been argued that more distant tidal interactions can sustain galaxy warps (e.g. Weinberg & Blitz 2006). Combining environmental in- formation with estimates of gas content and metallicity can help constrain the importance of cosmic accretion versus merging and tidal interactions in driving gas and stellar warps.
It has been suggested for some time that there are multiple channels by which galaxies can quench. Broadly speaking, there has been some consensus in the litera- ture to divide processes into two channels, those depen- dent on stellar mass and those that rely on environ- ment (Silk 1977; Rees & Ostriker 1977; Peng et al. 2010b; Mendel et al. 2013; Schawinski et al. 2014; Smethurst et al. 2015; Belfiore et al. 2016, 2017). Mass-quenching refers to the mechanisms that shut down star formation due to the intrinsic properties of the galaxy, such as radio-mode feed- back from AGN, morphological quenching, bar quenching and halo-shock heating (Bower et al. 2006; Schawinski et al. 2007; Masters et al. 2011; Fabian 2012; Page et al. 2012; Heckman & Best 2014; Gavazzi et al. 2015; Belfiore et al. 2016, 2017). Environmental-quenching refers to the mecha- nisms related to the extrinsic properties of a galaxy, these in- clude ram pressure stripping, tidal stripping, galaxy harass- ment and strangulation (Gunn & Gott 1972; Abadi et al. 1999; Balogh et al. 2000; Lewis et al. 2002; Font et al. 2008; McCarthy et al. 2008; van den Bosch et al. 2008; Bialas et al. 2015; Peng et al. 2015; Gupta et al. 2017).
The broader nature of ionized gas in early-types has also been the subject of papers by Belfiore et al. (2016) and Belfiore et al. (2017) following up on the analysis of a small sample observed with the MaNGA prototype instrument in Belfiore et al. (2015). By using spatially resolved maps of nebular diagnostics and stellar popula- tion ages, this work has added substantial support to the notion that evolved stellar populations provide the ion- ization source for a galaxy class that arguably should be renamed from LINER (Low Ionization Nuclear Emission Region) galaxies to LIER galaxies. LIERs, it turns out, are ubiquitous in both quiescent galaxies and in the cen- tral regions of galaxies where star formation takes place at larger radii. The study of Belfiore et al. (2016) and Belfiore et al. (2017) have put the occurrence of the LIER phenomenon into a physically relevant framework that can be directly tied to the diversity of the galaxy popu- lation as a whole. Specifically, they identify two classes of galaxies as extended LIER (eLIER) and central LIER (cLIER), respectively, and study their kinematics and stellarpopulation properties. cLIERs turn out to be mostly late type galaxies located around the green val- ley, while eLIERs are morphologically early types and are indistinguishable from passive galaxies devoid of line emission in terms of their stellar populations, morphol- ogy and central stellar velocity dispersion.
there has been a sharp increase in the star formation rate over a short timescale (∼ 10 7 years). These spectra are identified by their unusually weak Balmer absorption lines (low PC2), strong UV– blue continua, and weak 4000Å breaks (low PC1) i.e. spectra dom- inated by light from O/B stars. These spaxels lie in the lower left of Fig. 1. As the starburst ages to a few 10 8 yrs, the Balmer absorp- tion lines increase in strength as the galaxy passes into the post- starburst phase (Dressler & Gunn 1983; Couch & Sharples 1987), i.e. A/F star light dominates the spectrum for up to 1 Gyr following a starburst. These spaxels with stronger Balmer absorption lines lie to the top of Figure 1. Through comparison with population synthesis models, using simple toy model star formation histories or more complex histories derived from simulations, Wild et al. (2007, 2009) showed that the shape of the left hand side of the distribution in Fig. 1 describes the evolutionary track of a starburst, with time since the starburst increasing from bottom to top, and burst strength increasing from right to left. The spaxels lying at the outermost edge of the distribution have undergone the strongest recent bursts of star formation in the entire sample. At these low- redshifts, these starbursts are not strong; Bayesian fits to spectral synthesis models imply typical burst mass fractions (i.e. fraction of stellar mass formed in the burst) of ∼10% (Wild et al. 2010). The models show that if spaxels that had undergone stronger bursts ex- isted, they would lie to the left of the distribution at intermediate starburst ages, where no spaxels are observed.
misaligned gas-star kinematics or tidal features supports the idea that a merger has triggered the gas inflow in the ma- jority of cases. The globally young mass-weighted age indi- cates a violent process has mixed the stars throughout the galaxy, and the significant decrease in stellar v/σ compared to the control sample again implies a violent interaction or merger has occurred. It seems unlikely that less violent pro- cesses, such as misaligned gas accretion from a neighbour- ing dwarf or the cosmic web could contribute significantly to the population. The weak W(Hα) throughout the disk implies subsequent galaxy-wide quenching of the star forma- tion, which could be caused by complete gas exhaustion, ex- pulsion or some additional heating mechanism. While AGN feedback has been postulated as a plausible mechanism for global quenching, it is unclear whether it can cause such galaxy wide quenching in the relatively low mass PSB galax- ies present in our local Universe sample.
galaxies each. These groups, or bins (i) sample each of three “principal components” defining galaxy populations – stellar mass, SFR and environment; (ii) divide each “dimension” into 6 bins, sufficient to distinguish the functional form of trends across each dimension; and finally (iii) contain adequate counting statistics (galaxies) such that differences in mean properties between bins can be detected at the 5 sigma level even when the measurement pre- cision for individual galaxies is comparable to this difference. This optimization dovetails MaNGA’s scientific goals for statistical analyses of resolved galaxy samples, and complements existing, smaller data sets such as ATLAS3D (Cappellari et al. 2011), DiskMass (Bershady et al. 2010), and CAL- IFA (S´ anchez et al. 2012), as well as forthcoming data from instruments such as MUSE (Bacon et al. 2010) and KCWI (Martin et al. 2010) capable of producing even higher fidelity data for more mod- est samples.
Large spectroscopic galaxy surveys, such as the Sloan Digital Sky Survey (York et al. 2000), and the Two- degree Field Galaxy Redshift Survey (Colless et al. 2001), have revolutionized the way we study galaxy evolution. The huge statistical power brought in by targeting a large number of galaxies using the same instrument with ex- cellent calibration enabled huge progress. Not only have these efforts quantified accurately with great precision those trends and scaling relations that were previously known, such as the color-bimodality (Strateva et al. 2001; Baldry et al. 2004), the color-density relation (Hogg et al. 2003; Blanton et al. 2005), the mass-metallicity rela- tion for gas (Tremonti et al. 2004) and stars (Thomas et al. 2010; Johansson et al. 2012), and the Fundamen- tal Plane (Bernardi et al. 2003), they have also discov- ered many new relations and trends, such as the depen- dence of star formation history on stellar mass (Kauff- mann et al. 2003), the star formation rate vs. stellar mass relation (Brinchmann et al. 2004; Salim et al. 2007; Wuyts et al. 2011), the strong mass dependence of the radio-loud AGN fraction (Best et al. 2005), large scale galactic conformity (Kauffmann et al. 2013), and many others. They also connected large scale structure stud- ies and galaxy evolution studies thanks to environmental measurements enabled by dense and uniform sampling of complete galaxy samples (see Blanton & Moustakas 2009 and references therein).
Therefore, in order to have a full understanding of star formation cessation in galaxies, one would need spatially resolved spectrosc- opy to measure the stellar and gaseous components for a large sample of galaxies covering a wide range of global properties ( e.g., stellar mass, color, star formation rate [ SFR ] , morphology, nuclear activity, etc. ) and probing environmental conditions over a wide range of spatial scales ( tidal interactions / mergers, central / satellite classi ﬁ cation, local density, dark halo mass, halo assembly history, large-scale structure con ﬁ guration, etc. ) . Resolved spectroscopy has become available only recently thanks to the integral ﬁ eld unit ( IFU ) surveys accomplished in the past decade: SAURON ( Bacon et al. 2001; de Zeeuw et al. 2002 ) , DiskMass ( Bershady et al. 2010 ) , ATLAS 3D ( Cappellari et al. 2011 ) , CALIFA ( Sánchez et al. 2012 ) , VENGA ( Blanc et al. 2013 ) , SLUGGS ( Brodie et al. 2014 ) , and MASSIVE ( Ma et al. 2014 ) . These IFU surveys have typically targeted tens or hundreds of local galaxies with different morphological types, enabling detailed studies of the spatially resolved stellar populations and kinematics of galaxies. Meanwhile, the KMOS 3D survey is extending the IFU efforts to higher redshifts, observing 600 galaxies at 0.7 < z < 2.7 using KMOS at the Very Large Telescope ( Wisnioski et al. 2015 ) .
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 H α complex with no optical counterpart that extends ≈ 8 ″ (≈ 6.3 kpc ) beyond the effective radius of a dusty, starbursting galaxy. This H α 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 H α extension could be diffuse ionized gas, a gas accretion event, or something else. We ﬁ nd 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 galaxystellar mass function.
Our work ﬁ ts into a broader conversation about the role of environment in establishing internal galaxy properties. Galaxy mass functions are a function of environment ( e.g., Binggeli et al. 1988 ) . Once stellar mass is controlled, however, there are only subtle remaining differences as a function of environment for many internal galaxy properties, including morphology and color ( e.g., Blanton & Moustakas 2009; Alpaslan et al. 2015 ) , star formation rates ( Wijesinghe et al. 2012 ) , stellar populations ( Thomas et al. 2010 ) , and gradients therein ( e.g., Greene et al. 2015; Goddard et al. 2017 ) . As the MaNGA survey progresses, the larger sample size will enable yet more sensitive searches for subtle trends between environment, stellar kinematics, gas content, and stellar populations.
magnitude. At each i-band magnitude (and thus approximately in each stellar mass bin), the MaNGA primary and secondary samples are selected to be volume limited within a prescribed redshift range. Seventeen galaxy IFUs (ranging from 19-fibre IFU, 12 arcsec on sky diameter, to 127-fibre IFU, 32 arcsec on sky diameter) are observed simultaneously, together with a set of twelve 7-fibre mini- bundles used for flux calibration (Yan et al. 2016b) and 92 single fibres for sky subtraction. A three-point dithering pattern is used during observations to compensate for light loss and obtain a uni- form point spread function (PSF, Law et al. 2015). The MaNGA data were reduced using version v1_3_3 of the MaNGA reduction pipeline (Law et al. 2016). The wavelength calibrated, sky sub- tracted and flux calibrated MaNGA fibre spectra (error vectors and mask vectors) and their respective astrometric solutions are com- bined to produce final datacubes with pixel size set to 0.5 arcsec. The median PSF of the MaNGA datacubes is estimated to have a full width at half-maximum (FWHM) of 2.5 arcsec. The MaNGA sam- ple has been demonstrated to be well suited for studies of gas physics and stellarpopulation properties of nearby galaxies as shown in pre- liminary analysis of data from the MaNGA prototype instrument (Belfiore et al. 2015; Li et al. 2015; Wilkinson et al. 2015).
2014) for the first time. This capability is enabled by MaNGA’s 2D mapping of stellar rotation and velocity dispersion, as well as its long-wavelength coverage, which enables simultaneous measurements of widely distributed, intrinsically narrow (and therefore weak) absorption features. The near-IR wavelength range in MaNGA survey data will provide unique access to gravity-sensitive features (Conroy & van Dokkum 2012b) that constrain the faint end of the IMF (<0.5 M ), and hence the stel- lar mass-to-light ratio (ϒ*), while at the same time the extended spectral coverage provides strong constraints on the star forma- tion history, metallicity, and abundance of the stellarpopulation. For an illustration of MaNGA’s ability to address this issue, see Figure 2. Both the spectral and dynamical approaches currently employ rather restrictive assumptions in order to measure the stellar M/L, dark matter, and IMF. The spectral approach has currently been restricted to a few population components, with homogeneous chemical composition and IMF. The dynamical approach generally assumes simple geometries for the galaxy, as well as the shape and profile for the halo. The combination of both approaches, employing radically different assumptions, has the potential to break the degeneracies in the models to uniquely constrain the stellar M/L and dark matter content. This synergy can provide fundamental insights in our understanding of dark matter and stellar populations in galaxies.
plane, although the distribution of their central regions is strongly bimodal (Figure 13). This sequence closely follows the continuous star formation locus predicted by current stellarpopulation models, and covers a very sim- ilar area to the distribution of the large sample of galac- tic centers from SDSS (Figure 2). This relation suggests that, at least for the systems being studied here, galaxy growth has been a smooth process, and is likely regulated by a common set of physical drivers. This result is consis- tent with the ‘inside-out’ picture of galaxy growth, where the stellar mass assembly starts in the galactic center and gradually extends to the outer regions (White & Frenk 1991; Mo et al. 1998; Brook et al. 2006). In this picture, the shutdown of star formation also first occurs in the central region and slowly propagates out to ever larger radii. However, this result should not be overemphasized given the small size of our sample. The tight sequence on the D n (4000)–EW(Hδ A ) diagram might be just a re-
A complete picture of the way in which the star formation in galaxies gets shut down remains elusive. However, recent studies of the scaling relations of galaxy properties and their dependence on local environment have clearly established that, in addition to stellar mass, both internal structural properties and external environment are key indicators, or may even be drivers, of the star formation cessation processes in galaxies ( e.g., Kauffmann et al. 2006; Bell 2008; Franx et al. 2008; Peng et al. 2010; Thomas et al. 2010; Bell et al. 2012; Cheung et al. 2012; Fabello et al. 2012; Li et al. 2012b; Fang et al. 2013; Mendel et al. 2013; Zhang et al. 2013 ) . For instance, when studying the central galaxies in groups or clusters, the presence of a prominent bulge-like structure is found to be a necessary ( but not suf ﬁ cient ) condition for stopping star formation ( Bell 2008; Bell et al. 2012; Cheung et al. 2012; Fang et al. 2013 ) . In addition, studies of the relationship between galaxy morphology and color have revealed a signi ﬁ cant population of red-sequence galaxies with disk-dominated spirals at both low z ( e.g., Wolf et al. 2005, 2009; Bamford et al. 2009; Masters et al. 2010 ) and z ∼ 1 – 2 ( e.g., Bundy et al. 2010 ) , which are preferentially found in galaxies with large bulges ( Bundy et al. 2010; Masters et al. 2010 ) . These ﬁ ndings support the “ morphological quenching ” mechanism proposed by Martig et al. ( 2009 ) , although studies of cold gas in massive galaxies indicate that a reduction in gas content is also required ( e.g., Fabello et al. 2011 ) . For central galaxies in massive dark matter halos above a critical mass of ~ 10 12 M , “ radio-mode ” active
Do the theorised different formation mechanisms of fast and slow rotators produce an observable difference in their star formation histories? To study this we identify quenching slow rotators in the MaNGA sample by selecting those which lie below the star forming sequence and identify a sample of quenching fast rotators which were matched in stellar mass. This results in a total sample of 194 kinematically classified galaxies, which is agnostic to visual morphology. We use u − r and N U V − u colours from SDSS and GALEX and an existing inference package, starpy , to conduct a first look at the onset time and exponentially declining rate of quenching of these galaxies. An Anderson-Darling test on the distribution of the inferred quenching rates across the two kinematic populations reveals they are statistically distinguishable (3.2σ). We find that fast rotators quench at a much wider range of rates than slow rotators, consistent with a wide variety of physical processes such as secular evolution, minor mergers, gas accretion and environmentally driven mechanisms. Quenching is more likely to occur at rapid rates (τ ∼ < 1 Gyr) for slow rotators, in agreement with theories suggesting slow rotators are formed in dynamically fast processes, such as major mergers. Interestingly, we also find that a subset of the fast rotators quench at these same rapid rates as the bulk of the slow rotator sample. We therefore discuss how the total gas mass of a merger, rather than the merger mass ratio, may decide a galaxy’s ultimate kinematic fate.
As with 7443-12701, this galaxy is also likely to be an S0 galaxy, but the stellar populations show a very different trend. Due to the small differences in the Hβ line strengths in older stellar popu- lations, it can be assumed that the bulge and disc contain stellar populations of approximately similar ages. The decomposed bulge spectrum however shows a higher average metallicity than the disc spectrum. The older ages suggest that it is likely that this galaxy un- derwent the transformation into an S0 longer ago than 7443-12701, however the small differences in the H β line strengths at such ages make it difficult to determine reliable age gradients across this galaxy. The metallicity gradient on the other hand suggests that the gas that fuelled the most recent star formation within the inner re- gions of the galaxy was more metal enriched than the gas fuelling the latest star formation in the outer regions. Since bars are thought to drive gas through the disc into the inner regions of a galaxy (Kormendy & Kennicutt 2004), it is possible that this metallicity gradient was produced by a bar that existed before the star forma- tion ceased. Flat age gradients between the bulge and disc regions of S0s have also been found within the CALIFA survey Gonz´alez Delgado et al. (2015), while Rawle et al. (2008) have also detected flat age and negative metallicity gradients across early type galaxies in Abell ∼ 3389.
Integral Field Spectroscopic (IFS) surveys provide spatially resolved properties of galaxies enabling detailed identification of external influence. Previous work has focussed on the gas content in early-type galaxies (ETGs) and the fraction of which that are significantly kinematically misaligned (i.e. global position angle between rotational directions of gas and stars is greater than 30 ◦ ) indicating external influence. Davis et al. (2011) identify that approximately 36% of fast-rotating ETGs are kinematically misaligned within the volume-limited sample of ATLAS 3D (Cappellari et al. 2011a), setting a lower limit on the importance of externally acquired gas. This can also be used to constrain the time-scales of misalignment. Davis & Bureau (2016) utilise a toy model to propose that misaligned gas could relax gradually over time-scales of 1-5 Gyr. A faster time-scale of relaxation would require merger rates of ≈ 5 Gyr -1 and hence is disfavoured. The interplay between the strength and persistence of the gas in-flow and the re-aligning torque of the stellar component dictates the exact time-scale of misalignment for an individual galaxy. The strength of a galaxy’s stellar torque scales as a function of radius, with the central component of a galaxy re-aligning on a quicker time-scale than the outer regions.