The Photometric Structure And Assembly History Of Nearby Elliptical Galaxies

OTIVATED by recent developments, the assembly history of massive galaxies has gradually gained more and more attention, and has become an unique topic under the big picture of galaxy formation and evolution. More and more observations have revealed that massive galaxies in the nearby universe must have gone through an different phases during their evolution, and have experienced significant structural transformation. To further confirm the picture for their structural evolution, and pro-vide better constraints to the formation scenario of massive galaxies, I have explored the detailed photometric structure of massive early-type galaxies in both nearby uni-verse and at moderate redshift using a carefully developed pipeline of two-dimensional image decomposition method.First, I have conducted detailed image decomposition analysis to a representative sample of94bright, nearby elliptical galaxies, using high-quality optical images from the Carnegie-Irvine Galaxy Survey. The sample spans a range of environments and stellar masses, from M*=1010.2to1012.0M(?). I exploit the unique capabilities of two-dimensional image decomposition to explore the possibility that local elliptical galaxies may contain photometrically distinct substructure that can shed light on their evolu-tionary history. Compared with the traditional one-dimensional approach, these two-dimensional models are capable of consistently recovering the surface brightness dis-tribution and the systematic radial variation of geometric information at the same time. Contrary to conventional perception, I find that the global light distribution of the ma-jority ((?)75%) of elliptical galaxies is not well described by a single Sersic function. In-stead, I propose that local elliptical galaxies generically contain three subcomponents: a compact (Re(?)1kpc) inner component with luminosity fraction f≈0.1-0.15; an intermediate-scale (Re≈2.5kpc) middle component wit f≈0.2-0.25; and a dominant (f=0.6), extended (Re≈10kpc) outer envelope. All subcomponents have average Sersic indices n-1-2, significantly lower than the values typically ob-tained from single-component fits. The individual subcomponents follow well-defined photometric scaling relations and the stellar mass-size relation. I discuss the physical nature of the substructures and their implications for the formation of massive elliptical galaxies.To further understand the physical implications of the substructures I found under the context of the structural evolution of massive galaxies during the last10Gyrs, I at-tempt to relate these substructures with the properties of early-type galaxies observed at higher redshifts. I find that a hypothetical structure formed from combining the inner plus the middle components of local ellipticals follows a strikingly tight stellar mass-size relation, one that resembles the distribution of early-type galaxies at z≈1. Out-side of the central kpc, the stellar mass surface density profiles of this composite struc-ture are also qualitatively similar to those of compact galaxies at z≥1.5, although in roughly half of the sample the profiles appear to have undergone additional size growth at lower redshifts. I propose that the central substructures in nearby ellipticals are the evolutionary descendants of the "red nuggets" formed under highly dissipative ("wet") conditions at high redshifts, as envisioned in the initial stages of the two-phase forma-tion scenario recently advocated for massive galaxies. Subsequent accretion, plausibly through non-dissipation ("dry") minor mergers, builds the outer regions of the galaxy identified as the outer envelope in our decomposition. The large scatter exhibited by this component on the stellar mass-size plane testifies to the stochastic nature of the accretion events.Although the so-called "two-phase" formation scenario has gradually gained in-creasingly more supports from different lines of observational evidence, including my results from photometric decomposition of nearby elliptical galaxies, it is still a long way to goal before we could confindently rely on this scenario. Within this model, one physical quantity:the mass distribution of the accreted quiescent systems during the second evolutionary phase; or more straightforward:the average mass ratio for the se-ries of dry mergers, is particularly important, since it decides the degree of size growth and the transformation of the surface density profile at larger radii. Notwithstanding its importance, the observational constraint for such parameter is extremely hard to archive. For this purpose, I propose a novel method to estimate this parameter using the multi-color, multi-component photometric models of nearby elliptical galaxies. I have successfully extended the models I obtained for V-band images to B-band and R-band based on very simple and reasonable constraint. Based on these groups of multi-color models, I can relate the difference in optical color between the composite inner component and the extended outer component with the average merger mass ra-tio of the second phase in the "two-phase" formation scenario using a few assumptions. It is previously known that early-type galaxies follow a well-defined stellar mass-color relation. And, if we assume that once the star-formation in the progenitors of nearby elliptical galaxies has been quenched, they only evolve along this relation toward the massive end through dry merger events, then the color difference between the "in-situ" and "accreted" components happens to provide an interesting constraint of the average merger mass ratio. Although this approach is still subjected to large observational un-certainties, the rough estimation I get,5-15:1, is actually quite consistent with the results from recent simulations of massive galaxy evolution.Furthermore, I propose that, to provide an even more straightforward picture for the structural evolution of massive galaxies, it should be very benificial if we could make use of the images of early type galaxies at higher redshift obtained by HST obser-vations and a consistent method of two-dimensional image decomposition method. If the "two-phase" formation scenario is indeed reliable, then we should naturally expect to witness the transformation of the inner structure and the gradual accumulation of the outer envelope. However, it becomes much more challenging to apply the image decomposition method to high-redshift galaxies even with the help of HST images. As the first step, I take advantage of a small sample of7early-type galaxies at z=0.6-1.5who are luckily covered by the unique HUDF field. Based on the comparison of the photometric decomposition results from both extremely deep, rest-frame near-infrared images from HUDF12project and the regular exposure image for the same galaxy in the same filter from the CANDELS project, it becomes clear that:(1). A large fraction of early-type galaxies at moderate redshift have already shown concrete evidence for photometric substructures;(2). Although the current HST images are still not perfect, the quality is enough for detailed structural analysis of massive early-type galaxies at z(?)1.5. This result is only based on a very small and incomplete sample, but it always gives me the faith to further pusrsue this direction using larger sample from HST sur-veys.