We investigate two-dimensional image decomposition of nearby, morphologicallyselected early-type galaxies (ETGs). We are motivated by recent observationalevidence of significant size growth of quiescent galaxies and theoreticaldevelopment advocating a two-phase formation scenario for ETGs. We find that asignificant fraction of nearby ETGs show changes in isophotal shape thatrequire multi-component models. The characteristic sizes of the inner and outercomponent are $\sim 3$ and $\sim 15$ kpc. The inner component lies on themass-size relation of ETGs at $z \sim 0.25-0.75$, while the outer componenttends to be more elliptical and hints at a stochastic buildup process. We findreal physical differences between the single- and double-component ETGs, withthe double-component galaxies being younger and more metal-rich. The fractionof double component ETGs increases with increasing $\sigma$ and decreases indenser environments. We hypothesize that double-component systems were able toaccrete gas and small galaxies until later times, boosting their centraldensities, building up their outer parts, and lowering their typical centralages. In contrast, the oldest galaxies, perhaps due to residing in richerenvironments, have no remaining hints of their last accretion episode.

For the first time, we present the size evolution of a mass-complete(log(M*/Msol)>10) sample of star-forming galaxies over redshifts z=1-7,selected from the FourStar Galaxy Evolution Survey (ZFOURGE). Observed H-bandsizes are measured from the Cosmic Assembly Near-Infrared Deep ExtragalacticLegacy Survey (CANDELS) Hubble Space Telescope (HST)/F160W imaging.Distributions of individual galaxy masses and sizes illustrate that a clearmass-size relation exists up to z~7. At z~7, we find that the average galaxysize from the mass-size relation is more compact at a fixed mass oflog(M*/Msol)=10.1, with r_1/2,maj=1.02+/-0.29 kpc, than at lower redshifts.This is consistent with our results from stacking the same CANDELS HST/F160Wimaging, when we correct for galaxy position angle alignment. We find that thesize evolution of star-forming galaxies is well fit by a power law of the formr_e = 7.07(1 + z)^-0.89 kpc, which is consistent with previous works for normalstar-formers at 1<z<4. In order to compare our slope with those derived Lymanbreak galaxy studies, we correct for different IMFs and methodology and find aslope of -0.97+/-0.02, which is shallower than that reported for the evolutionof Lyman break galaxies at z>4 (r_e\propto(1 +z)^-1.2+/-0.06). Therefore, weconclude the Lyman break galaxies likely represent a subset of highlystar-forming galaxies that exhibit rapid size growth at z>4.

The stellar initial mass function (IMF), which is often assumed to beuniversal across unresolved stellar populations, has recently been suggested tobe "bottom-heavy" for massive ellipticals. In these galaxies, the prevalence ofgravity-sensitive absorption lines (e.g. Na I and Ca II) in their near-IRspectra implies an excess of low-mass ($m <= 0.5$ $M_\odot$) stars over thatexpected from a canonical IMF observed in low-mass ellipticals. A directextrapolation of such a bottom-heavy IMF to high stellar masses ($m >= 8$$M_\odot$) would lead to a corresponding deficit of neutron stars and blackholes, and therefore of low-mass X-ray binaries (LMXBs), per unit near-IRluminosity in these galaxies. Peacock et al. (2014) searched for evidence ofthis trend and found that the observed number of LMXBs per unit $K$-bandluminosity ($N/L_K$) was nearly constant. We extend this work using new andarchival Chandra X-ray Observatory (Chandra) and Hubble Space Telescope (HST)observations of seven low-mass ellipticals where $N/L_K$ is expected to be thelargest and compare these data with a variety of IMF models to test which areconsistent with the observed $N/L_K$. We reproduce the result of Peacock et al.(2014), strengthening the constraint that the slope of the IMF at $m >= 8$$M_\odot$ must be consistent with a Kroupa-like IMF. We construct an IMF modelthat is a linear combination of a Milky Way-like IMF and a broken power-lawIMF, with a steep slope ($\alpha_1=$ $3.84$) for stars < 0.5 $M_\odot$ (assuggested by near-IR indices), and that flattens out ($\alpha_2=$ $2.14$) forstars > 0.5 $M_\odot$, and discuss its wider ramifications and limitations.

We show that the stellar surface-brightness profiles in discgalaxies---observed to be approximately exponential---can be explained ifradial migration efficiently scrambles the individual stars' angular momentawhile conserving the circularity of the orbits and the total mass and angularmomentum. In this case the disc's distribution of specific angular momenta $j$should be near a maximum-entropy state and therefore approximately exponential,$dN\propto\exp(-j/\langle j\rangle)dj$. This distribution translates to asurface-density profile that is generally not an exponential function ofradius: $\Sigma(R)\propto\exp[-R/R_e(R)]/(RR_e(R))(1+d\log v_c(R)/d\log R)$,for a rotation curve $v_c(R)$ and $R_e(R)\equiv\langle j\rangle/v_c(R)$. Weshow that such a profile matches the observed surface-brightness profiles ofdisc-dominated galaxies as well as the empirical exponential profile. Discgalaxies that exhibit population gradients cannot have fully reached amaximum-entropy state but appear to be close enough that theirsurface-brightness profiles are well-fit by this idealized model.