Understanding the evolution of galaxies using a novel definition of the Green Valley

Green Valley (GV) galaxies are essential to the understanding of galaxy formation and evolution, which, in turn, is necessary to understand our Universe. In this thesis we explore a novel way to define the GV, using the 4000Å break strength, which is more resilient to dust and less model dependent, thus carrying less inherent systematics compared with traditional methods. This method finds a cleaner stratification between each GV region; divided into lower (lGV), middle (mGV) and upper (uGV) green valley. We find comparable results between novel (4000Å break) and traditional (dust-corrected colour) methods of selecting GV galaxies - i.e., similar fractions of AGN galaxies and matching transition timescales. However, a more in-depth analysis finds nuanced differences, such as more homogeneous stellar population properties between different types of galaxies in the novel definition. Furthermore, we find results suggesting a more rapid transition from Blue Cloud (BC) to GV than from GV to Red Sequence (RS). This thesis also tests how well hydrodynamical simulations, EAGLE and IllustrisTNG (TNG100), are able to reproduce the observed trend on the 4000Å break vs stellar mass plane. We find both simulations match the location of the BC, however produce the RS and GV at a higher 4000Å break value than observations. Comparison of GV galaxies between observation and simulations find both simulations to overproduce quiescent fractions. As this is mostly seen for M⋆ ≳ 10^{10.5} M⊙, it is assumed mostly to be due to the subgrid physics of AGN feedback. Finally, we also compared “Twin” galaxies, having similar overall properties such as morphology, colour, stellar mass, etc, but one hosts an AGN and the other doesn’t. From this, we find AGN galaxies to be generally more evolved and metal rich than their non-AGN “twin”

Exploring a new definition of the green valley and its implications

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).Comment: 5 pages, 4 figures. Accepted for publication in MNRAS Letter

A detailed look at the stellar populations in green valley galaxies

\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.Comment: 20 pages, 13+3 figures. Accepted for publication in MNRA

Non-gaussianity of optical emission lines in SDSS star-forming galaxies and its implications on galactic outflows

The shape of emission lines in the optical spectra of star-forming galaxies reveals the kinematics of the diffuse gaseous component. We analyse the shape of prominent emission lines in a sample of ~53,000 star-forming galaxies from the Sloan Digital Sky Survey, focusing on departures from gaussianity. Departures from a single gaussian profile allow us to probe the motion of gas and to assess the role of outflows. The sample is divided into groups according to their stellar velocity dispersion and star formation rate. The spectra within each group are stacked to improve the signal-to-noise ratio of the emission lines, to remove individual signatures, and to enhance the effect of star formation rate on the shapes of the emission lines. The moments of the emission lines, including kurtosis and skewness, are determined. We find that most of the emission lines in strong star-forming systems unequivocally feature negative kurtosis. This signature is present in H$\beta$, H$\alpha$, [N II] and [S II] in massive galaxies with high star formation rates. We attribute it as evidence of radial outflows of ionised gas driven by the star formation of the galaxies. Also, most of the emission lines in low-mass systems with high star formation rates feature negative skewness, and we interpret it as evidence of dust obscuration in the galactic disk. These signatures are however absent in the [O III] line, which is believed to trace a different gas component. The observed trend is significantly stronger in face-on galaxies, indicating that star formation drives the outflows along the galactic rotation axis, presumably the path of least resistance. The data suggest that outflows driven by star formation exert accumulated impacts on the interstellar medium, and the outflow signature is more evident in older galaxies as they have experienced a longer total duration of star formation.Comment: 16 pages, 13 figures, Accepted for publication in PAS