It has been suggested that merging plays an important role in the formation and the evolution of elliptical galaxies. While gas dissipation by star formation is believed to steepen metallicity and color gradients of the merger products, mixing of stars through dissipation-less merging (dry merging) is believed to flatten them. In order to understand the past merging history of elliptical galaxies, we studied the optical-near infrared (NIR) color gradients of 204 elliptical galaxies. These galaxies are selected from the overlap region of the Sloan Digital Sky Survey (SDSS) Stripe 82 and the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). The use of the optical and the NIR data (g, r, and K) provides large wavelength baselines, and breaks the age-metallicity degeneracy, allowing us to derive age and metallicity gradients. The use of the deep SDSS Stripe 82 images make it possible for us to examine how the color/age/metallicity gradients are related to merging features. We find that the optical-NIR color and the age/metallicity gradients of elliptical galaxies with tidal features are consistent with those of relaxed ellipticals suggesting that the two populations underwent a similar merging history on average and that mixing of stars was more or less completed before the tidal features disappear. Elliptical galaxies with dust features have steeper color gradients than the other two types, even after masking out dust features during the analysis, which can be due to a process involving wet merging. More importantly, we find that the scatter in the color/age/metallicity gradients of the relaxed and merging feature types, decreases as their luminosities (or masses) increase at M > 10 11.4 M ⊙ but stays to be large at lower luminosities. Mean metallicity gradients appear nearly constant over the explored mass range, but a possible flattening is observed at the massive end. According to our toy model that predicts how the distribution of metallicity gradients changes as a result of major dry merging, the mean metallicity gradient should flatten by 40% and its scatter become smaller by 80% per a mass doubling scale if ellipticals evolve only through major dry merger. Our result, although limited by a number statistics at the massive end, is consistent with the picture that major dry merging is an important mechanism for the evolution for ellipticals at M > 10 11.4 M ⊙ , but less important at the lower mass range.

We analyze the intrinsic flux ratios of various visible-near-infrared filters with respect to 3.5 µm for simple and composite stellar populations (CSPs), and their dependence on age, metallicity, and star formation history (SFH). UV/optical light from stars is reddened and attenuated by dust, where different sightlines across a galaxy suffer varying amounts of extinction. Tamura et al. (2009) developed an approximate method to correct for dust extinction on a pixel-by-pixel basis, dubbed the "β V " method, by comparing the observed flux ratio to an empirical estimate of the intrinsic ratio of visible and ∼3.5 µm data. Through extensive modeling, we aim to validate the "β V " method for various filters spanning the visible through near-infrared wavelength range, for a wide variety of simple and CSPs. Combining Starburst99 and BC03 models, we built spectral energy distributions (SEDs) of simple (SSP) and composite (CSP) stellar populations for various realistic SFHs, while taking metallicity evolution into account. We convolve various 0.44-1.65 µm filter throughput curves with each model SED to obtain intrinsic flux ratios β λ,0 . When unconstrained in redshift, the total allowed range of β V,0 is 0.6-4.7, or almost a factor of eight. At known redshifts, and in particular at low redshifts (z 0.01), β V,0 is predicted to span a narrow range of 0.6-1.9, especially for early-type galaxies (0.6-0.7), and is consistent with observed β V values. The β λ method can therefore serve as a first-order dust-correction method for large galaxy surveys that combine JWST (rest-frame 3.5µm) and HST (rest-frame visible-near-IR) data.

We present the result of the Infrared Medium-deep Survey (IMS) z ∼ 6 quasar survey, using the combination of the IMS near-infrared images and the Canada–France–Hawaii Telescope Legacy Survey optical images. The traditional color selection method results in 25 quasar candidates over 86 deg2. We introduce the corrected Akaike information criterion (AICc) with the high-redshift quasar and late-type star models to prioritize the candidates efficiently. Among the color-selected candidates, seven plausible candidates finally passed the AICc selection, of which three are known quasars at z ∼ 6. The follow-up spectroscopic observations for the remaining four candidates were carried out, and we confirmed that two out of four are z ∼ 6 quasars. With this complete sample, we revisited the quasar space density at z ∼ 6 down to M 1450 ∼ −23.5 mag. Our result supports the low quasar space density at the luminosity where the quasar’s ultraviolet ionizing emissivity peaks, favoring a minor contribution of quasars to the cosmic reionization.

We present and analyze spatially-resolved maps for the observed V -and g-band to 3.6 µm flux ratios and the inferred dust extinction values, A V , for a sample of 257 nearby NGC and IC galaxies. Flux ratio maps are constructed using PSF-matched mosaics of SDSS gand r-band images and Spitzer/IRAC 3.6 µm mosaics, with all pixels contaminated by foreground stars or background objects masked out. By applying the β V method (Tamura et al. 2009(Tamura et al. , 2010, which was recently calibrated as a function of redshift and morphological type by Kim, Jansen, & Windhorst (2017), dust extinction maps were created for each galaxy. The typical 1-σ scatter in β V around the average, both within a galaxy and in each morphological type bin, is ∼20%. Combined, these result in a ∼0.4 mag scatter in A V . β V becomes insensitive to small-scale variations in stellar populations once resolution elements subtend an angle larger than that of a typical giant molecular cloud (∼200 pc). We find noticeably redder V −3.6 µm colors in the center of star-forming galaxies and galaxies with a weak AGN. The derived intrinsic V −3.6 µm colors for each Hubble type are generally consistent with the model predictions of Kim et al. (2017). Finally, we discuss the applicability of the β V dust-correction method to more distant galaxies, for which well-matched HST rest-frame visible and JWST rest-frame ∼3.5µm images will become available in the near-future.