Several studies have reported the presence of sodium excess objects having neutral atomic absorption lines at 5895 Å (Na D) and 8190 Å that are deeper than expected based on stellar population models that match the stellar continuum. The origin of these lines is therefore hotly debated. van Dokkum & Conroy proposed that low-mass stars ( 0.3 M ) are more prevalent in massive early-type galaxies, which may lead to a strong Na i 8190 line strength. It is necessary to test this prediction, however, against other prominent optical line indices such as Na D, Mg b, and Fe 5270, which can be measured with a significantly higher signal-to-noise ratio than Na i 8190. We identified a new sample of roughly 1000 Na D excess objects (NEOs; ∼8% of galaxies in the sample) based on Na D line strength in the redshift range 0.00 z 0.08 from the Sloan Digital Sky Survey (SDSS) DR7 through detailed analysis of galaxy spectra. We explore the properties of these new objects here. The novelty of this work is that the galaxies were carefully identified through direct visual inspection of SDSS images, and we systematically compared the properties of NEOs and those of a control sample of galaxies with normal Na D line strengths. We note that the majority of galaxies with high velocity dispersions (σ e > 250 km s −1 ) show Na D excesses. Most late-type NEOs have strong Hβ line strengths and significant emission lines, which are indicative of the presence of young stellar populations. This result implies that the presence of the interstellar medium and/or dust contributes to the increase in Na D line strengths observed for these galaxies, which is in good agreement with the earlier study of Chen et al. who used the Na D line index to study outflow activity in star-forming disk galaxies. In contrast, the majority of early-type NEOs are predominantly luminous and massive systems, which is in agreement with the findings of van Dokkum & Conroy. However, we find that models used to reproduce the Na i 8190 line strengths that adopt a bottom-heavy initial mass function are not able to reproduce the observed Na D line strengths. By comparing the observed Na D, Mg b, and Fe 5270 line strengths with those of the models, we identify a plausible range of parameters that reproduce the observed values. In these models, the majority of early-type NEOs are "α-enhanced" ([α/Fe] ∼ 0.3), "metal-rich" ([Z/H] ∼ 0.3), and, especially, "Na-enhanced" ([Na/Fe] ∼ 0.3). An enhanced Na abundance is a particularly compelling hypothesis for the increase in the strength of the Na D line index in our early-type NEOs that appear devoid of dust, both in their SDSS images and spectra.

We present a comprehensive photometric study of the old open cluster, NGC 6819 using 1 • ×1 • field V I MOSAIC CCD imaging taken with the WIYN 0.9m telescope. The resultant color-magnitude diagram (CMD) shows a well developed main sequence (MS) extending from V ∼14.5 mag down to our photometric limit of V ∼21 mag. Fitting theoretical isochrones with adopted values of the reddening and metallicity (E(B − V ) =0.14, [F e/H] = +0.09 dex) to the observed CMD yields a distance modulus of (m − M ) 0 =11.93±0.10 and an age of ∼ 2.6 Gyr for NGC 6819. Our wide-field imaging reveals that NGC 6819 is larger in areal extent (R =13 ′ ) than previously thought. The wide-field also benefits our estimate of the degree of field star contamination, and ultimately yields improved measurements of the structural parameters (r c =2.80 ′ , r t =38.2 ′ , and r h =7 ′ ) and tidal mass of the cluster (M tid =3542.4 M ⊙ ). The flattened luminosity and mass functions indicate that NGC 6819 has experienced mass segregation as a result of its dynamical evolution. Our variability study of the cluster blue straggler star (BSS) population using the Welch-Stetson variability index (I W S ) has revealed a number of variable BSS candidates.

We present the KASI-Yonsei Deep Imaging Survey of Clusters (KYDISC) targeting 14 clusters at 0.015 z 0.144 using the Inamori Magellan Areal Camera and Spectrograph on the 6.5-meter Magellan Baade telescope and the MegaCam on the 3.6-meter Canada-France-Hawaii Telescope. We provide a catalog of cluster galaxies that lists magnitudes, redshifts, morphologies, bulge-to-total ratios, and local density. Based on the 1409 spectroscopically-confirmed cluster galaxies brighter than -19.8 in the r-band, we study galaxy morphology, color, and visual features generated by galaxy mergers. We see a clear trend between morphological content and cluster velocity dispersion, which was not presented by previous studies using local clusters. Passive spirals are preferentially found in a highly dense region (i.e., cluster center), indicating that they have gone through the environmental quenching. In deep images (µ r ∼ 27 mag arcsec −2 ), 20% of our sample show signatures of recent mergers, which is not expected from theoretical predictions and a low frequency of ongoing mergers in our sample (∼ 4%). Such a high fraction of recent mergers in the cluster environment supports a scenario that the merger events that made the features have preceded the galaxy accretion into the cluster environment. We conclude that mergers affect a cluster population mainly through the pre-processing on recently accreted galaxies.

We report the detection of RR Lyrae variable stars in Crater II, a recently discovered large and diffuse satellite dwarf galaxy of the Milky Way (MW). Based on B, V time-series photometry obtained with the Korea Microlensing Telescope Network (KMTNet) 1.6 -m telescope at CTIO, we identified 83 ab -type and 13 c -type pulsators by fitting template light curves. The detected RR Lyrae stars are centrally concentrated, which ensures that most of them are members of Crater II. In terms of the distribution of RRab stars in the period-amplitude diagram, Crater II is clearly different from ultrafaint dwarf (UFD) galaxies, but very similar to the two classical MW dwarf spheroidal (dSph) galaxies Draco and Carina with Oosterhoff-intermediate (Oo-int) properties. Combined with the mean period of ab -type variables ( P ab = 0.631±0.004 d) and the c -type fraction (∼0.14) in Crater II, this suggests an Oo-int classification for Crater II and implies that its nature is more like a dSph rather than a UFD. We also estimated the mean metallicity, reddening, and distance of Crater II, from the photometric and pulsation properties of the RR Lyrae stars. The stellar population model we have constructed indicates that Crater II is dominated by an old population, but is relatively younger than the oldest globular clusters in the MW. With a lack of high-amplitude short-period RRab stars, Crater II, like most of the other less massive dSphs, is probably not a surviving counterpart of the major building blocks of the MW halo.

As a case study to understand the coevolution of Brightest Cluster Galaxies (BCGs) and their host clusters, we investigate the BCGs in dynamically young and old clusters, Abell 1139 (A1139) and Abell 2589 (A2589). We analyze the pixel color-magnitude diagrams (pCMDs) using deep g-and r-band images, obtained from the Canada-France-Hawaii Telescope observations. After masking foreground/background objects and smoothing pixels in consideration of the observational seeing size, detailed pCMD features are compared between the two BCGs. (1) While the overall shapes of the pCMDs are similar to those of typical early-type galaxies, the A2589-BCG tends to have redder mean pixel color and smaller pixel color deviation at given surface brightness than the A1139-BCG. (2) The mean pixel color distribution as a function of pixel surface brightness (pCMD backbone) indicates that the A2589-BCG formed a larger central body (∼ 2.0 kpc in radius) by major dry mergers at an early epoch than the A1139-BCG (a central body ∼ 1.3 kpc in radius), while they have grown commonly by subsequent minor mergers. (3) The spatial distributions of the pCMD outliers reveal that the A1139-BCG experienced considerable tidal events more recently than the A2589-BCG, whereas the A2589-BCG has an asymmetric compact core possibly resulting from major dry merger at an early epoch. (4) The A2589-BCG shows a very large faint-to-bright pixel number ratio compared to early-type non-BCGs, whereas the ratio for the A1139-BCG is not distinctively large. These results are consistent with the idea that the BCG in the dynamically older cluster (A2589) formed earlier and is relaxed better.