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Register a new userExploring a new definition of the green valley and its implications
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The distribution of galaxies on a colour-magnitude diagram reveals a bimodality, featuring a passively evolving red sequence and a star-forming blue cloud. The region between these two, the Green Valley (GV), represents a fundamental transition where quenching processes operate. We exploit an alternative definition of the GV using the 4,000 Angstrom break strength, an indicator that is more resilient than colour to dust attenuation. We compare and contrast our GV definition with the traditional one, based on dust-corrected colour, making use of data from the Sloan Digital Sky Survey. Our GV selection - that does not need a dust correction and thus does not carry the inherent systematics - reveals very similar trends regarding nebular activity (star formation, AGN, quiescence) to the standard dust-corrected $^{0.1}(g-r)$. By use of high SNR stacked spectra of the quiescent GV subsample, we derive the simple stellar population (SSP) age difference across the GV, a rough proxy of the quenching timescale ($Delta$t). We obtain an increasing trend with velocity dispersion ($sigma$), from $Delta$t$sim$1.5Gyr at $sigma$=100km/s, up to 3.5Gyr at $sigma$=200km/s, followed by a rapid decrease in the most massive GV galaxies ($Delta$t$sim$1Gyr at $sigma$=250km/s), suggesting two different modes of quenching, or the presence of an additional channel (rejuvenation).
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The galaxies found in optical surveys fall in two distinct regions of a diagram of optical colour versus absolute magnitude: the red sequence and the blue cloud with the green valley in between. We show that the galaxies found in a submillimetre survey have almost the opposite distribution in this diagram, forming a `green mountain. We show that these distinctive distributions follow naturally from a single, continuous, curved Galaxy Sequence in a diagram of specific star-formation rate versus stellar mass without there being the need for a separate star-forming galaxy Main Sequence and region of passive galaxies. The cause of the red sequence and the blue cloud is the geometric mapping between stellar mass/specific star-formation rate and absolute magnitude/colour, which distorts a continuous Galaxy Sequence in the diagram of intrinsic properties into a bimodal distribution in the diagram of observed properties. The cause of the green mountain is Malmquist bias in the submillimetre waveband, with submillimetre surveys tending to select galaxies on the curve of the Galaxy Sequence, which have the highest ratios of submillimetre-to-optical luminosity. This effect, working in reverse, causes galaxies on the curve of the Galaxy Sequence to be underrepresented in optical samples, deepening the green valley. The green valley is therefore not evidence (1) for there being two distinct populations of galaxies, (2) for galaxies in this region evolving more quickly than galaxies in the blue cloud and the red sequence, (c) for rapid quenching processes in the galaxy population.
$require{mediawiki-texvc}$The green valley (GV) represents an important transitional state from actively star-forming galaxies to passively evolving systems. Its traditional definition, based on colour, rests on a number of assumptions that can be subject to non-trivial systematics. In Angthopo et al. (2019), we proposed a new definition of the GV based on the 4000$AA$ break strength. In this paper, we explore in detail the properties of the underlying stellar populations by use of ~230 thousand high-quality spectra from the Sloan Digital Sky Survey (SDSS), contrasting our results with a traditional approach via dust-corrected colours. We explore high quality stacked SDSS spectra, and find a population trend that suggests a substantial difference between low- and high-mass galaxies, with the former featuring younger populations with star formation quenching, and the latter showing older (post-quenching) populations that include rejuvenation events. Subtle but measurable differences are found between a colour-based approach and our definition, especially as our selection of GV galaxies produces a cleaner stratification of the GV, with more homogeneous population properties within sections of the GV. Our definition based on 4000$AA$ break strength gives a clean representation of the transition to quiescence, easily measurable in the upcoming and future spectroscopic surveys.
We test cosmological hydrodynamical simulations of galaxy formation regarding the properties of the Blue Cloud (BC), Green Valley (GV) and Red Sequence (RS), as measured on the 4000$small{ mathring {mathrm A}}$ break strength vs stellar mass plane at $z=0.1$. We analyse the RefL0100N1504 run of EAGLE and the TNG100 run of IllustrisTNG project, by comparing them with the Sloan Digital Sky Survey, while taking into account selection bias. Our analysis focuses on the GV, within stellar mass $log,mathrm{M_star/M_{odot}} simeq 10-11$, selected from the bimodal distribution of galaxies on the D$_n$(4000) vs stellar mass plane, following Angthopo et al. methodology. Both simulations match the fraction of AGN in the green-valley. However, they over-produce quiescent GV galaxies with respect to observations, with IllustrisTNG yielding a higher fraction of quiescent GV galaxies than EAGLE. In both, GV galaxies have older luminosity-weighted ages with respect to the SDSS, while a better match is found for mass-weighted ages. We find EAGLE GV galaxies quench their star formation early, but undergo later episodes of star formation, matching observations. In contrast, IllustrisTNG GV galaxies have a more extended SFH, and quench more effectively at later cosmic times, producing the excess of quenched galaxies in GV compared with SDSS, based on the 4000$small{ mathring {mathrm A}}$ break strength. These results suggest the AGN feedback subgrid physics, more specifically, the threshold halo mass for black hole input and the black hole seed mass, could be the primary cause of the over-production of quiescent galaxies found with respect to the observational constraints.
We explore constraints on the joint photometric and morphological evolution of typical low redshift galaxies as they move from the blue cloud through the green valley and onto the red sequence. We select GAMA survey galaxies with $10.25<{rm log}(M_*/M_odot)<10.75$ and $z<0.2$ classified according to their intrinsic $u^*-r^*$ colour. From single component Sersic fits, we find that the stellar mass-sensitive $K-$band profiles of red and green galaxy populations are very similar, while $g-$band profiles indicate more disk-like morphologies for the green galaxies: apparent (optical) morphological differences arise primarily from radial mass-to-light ratio variations. Two-component fits show that most green galaxies have significant bulge and disk components and that the blue to red evolution is driven by colour change in the disk. Together, these strongly suggest that galaxies evolve from blue to red through secular disk fading and that a strong bulge is present prior to any decline in star formation. The relative abundance of the green population implies a typical timescale for traversing the green valley $sim 1-2$~Gyr and is independent of environment, unlike that of the red and blue populations. While environment likely plays a r^ole in triggering the passage across the green valley, it appears to have little effect on time taken. These results are consistent with a green valley population dominated by (early type) disk galaxies that are insufficiently supplied with gas to maintain previous levels of disk star formation, eventually attaining passive colours. No single event is needed quench their star formation.
We propose a method to identify quasars radiating closest to the Eddington limit, defining primary and secondary selection criteria in the optical, UV and X-ray spectral range based on the 4D eigenvector 1 formalism. We then show that it is possible to derive a redshift-independent estimate of luminosity for extreme Eddington ratio sources. Using preliminary samples of these sources in three redshift intervals (as well as two mock samples), we test a range of cosmological models. Results are consistent with concordance cosmology but the data are insufficient for deriving strong constraints. Mock samples indicate that application of the method proposed in this paper using dedicated observations would allow to set stringent limits on Omega_M and significant constraints on Omega_Lambda.
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