arXiv subjects

S. K. Yi

Publications and source records attributed to S. K. Yi.

41 records · Page 3Linked to original sources

arXiv2004

GALEX Ultraviolet Photometry of Globular Clusters in M31

We present ultraviolet photometry for globular clusters (GCs) in M31 from 15 square deg of imaging using the Galaxy Evolution Explorer (GALEX). We detect 200 and 94 GCs with certainty in the near-ultraviolet (NUV; 1750 - 2750 Angstroms) and far-ultraviolet (FUV; 1350 - 1750 Angstroms) bandpasses, respectively. Our rate of detection is about 50% in the NUV and 23% in the FUV, to an approximate limiting V magnitude of 19. Out of six clusters with [Fe/H]>-1 seen in the NUV, none is detected in the FUV bandpass. Furthermore, we find no candidate metal-rich clusters with significant FUV flux, because of the contribution of blue horizontal-branch (HB) stars, such as NGC 6388 and NGC 6441, which are metal-rich Galactic GCs with hot HB stars. We show that our GALEX photometry follows the general color trends established in previous UV studies of GCs in M31 and the Galaxy. Comparing our data with Galactic GCs in the UV and with population synthesis models, we suggest that the age range of M31 and Galactic halo GCs are similar.

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arXiv2004

The Look-back Time Evolution of Far-UV Flux from Elliptical Galaxies: The Fornax Cluster and Abell 2670

In order to investigate the origin of the far-UV (FUV) flux from the early-type galaxies, Galaxy Evolution Explorer (GALEX) is collecting the UV data for the elliptical-rich clusters at moderate redshifts (z < 0.2) where the dominant FUV source is predicted to be hot horizontal-branch (HB) stars and their post-HB progeny. Here we present our first result for the early-type galaxies in Abell 2670 at z = 0.076. Compared to NGC 1399, a nearby giant elliptical galaxy in the Fornax cluster, it appears that the rest-frame FUV - V color of the giant ellipticals gets redder by ~ 0.7 mag at the distance of Abell 2670 (z = 0.076; look-back time ~ 1.0 Gyr). Although a detailed comparison with the models is postponed until more cluster data are accumulated, it is interesting to note that this value is consistent with the variation predicted by the population synthesis models where the mean temperature of HB stars declines rapidly with increasing look-back time.

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arXiv2004

Systematics of the Ultraviolet Rising flux in a GALEX/SDSS sample of Early-type Galaxies

We present ultraviolet photometry for a sample of morphologically early-type galaxies selected by matching the Sloan Digital Sky Survey Data Release 1 with the GALEX Medium and All-sky Imaging Surveys. We obtain a working sample of 1032 early-type galaxies with GALEX FUV detections, SDSS spectroscopy, and $z<0.2$. Using the SDSS spectra to identify galaxies with even weak star formation or evidence of AGN, and further removing galaxies with any evidence of non early-type morphology, we derive a final sample of 172 red quiescent early-type galaxies. We find that the $FUV-r$ color has a full range of 5 mag. Plotting against the $FUV-r$ color the metallicity sensitive Lick $\rm Mg_2$ and D4000 indices, and the stellar velocity dispersion, we find no correlation between our measurement of UV rising flux, and any parameter sensitive to metallicity.

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arXiv2004

Spectral dating of high-redshift stellar populations

(Abridged) Age derivation techniques for unresolved stellar populations at high redshifts are explored using the NUV spectrum of LBDS~53W091 and LBDS~53W069. The photometry and morphology of these galaxies suggest they are early-type systems,a feature that makes them ideal test beds for the analysis of their ages and metallicities. In the analysis that is based on simple stellar population models,we find a significant degeneracy between the derived ages and metallicities both in optical+NIR photometric and NUV spectroscopic analyses. This degeneracy is not so strong for LBDS~53W069. However even in this case the stellar age cannot be constrained better than to a range roughly encompassing one third of the age of the Universe at the observed redshift. We have explored several independent population synthesis models and consistently found similar results. Broadband photometry straddling the rest-frame 4000A break is also subject to a strong age-metallicity degeneracy. The use of realistic chemical enrichment assumptions significantly helps in disentangling the degeneracy. Based on this method, we derive the average stellar age for both galaxies around 3.6-3.8 Gyr with better constraints on the youngest possible ages. From the observational point of view, the most efficient (and feasible) way to set limits on unresolved stellar populations comprises a combination of Balmer absorption lines along with either low SNR rest frame NUV spectroscopy or accurate optical and NIR photometry.

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arXiv2004

Globular Clusters as Probes of Galaxy Evolution: NGC 5128

We present the results of our analysis of the photometric data of globular clusters in the elliptical galaxy NGC 5128 (Cen A). We show that the integrated colour U-B can be an effective metallicity indicator for simple stellar populations. This is because it is sensitive to metallicity via the opacity effect but relatively insensitive to the effective main sequence turn-off temperature of the population (and thus to age) when Teff ~ 7000-12000K, that is, when the Balmer Jump is strong. This flat U-B vs Teff relation is a result of the fact that the blueing effect of the optical continuum with increasing temperature is temporarily stopped by the Balmer Jump which becomes greater with temperature in this range. In this study we use U-B and B-V as metallicity and age indicators, respectively. We first show that the use of the U-B vs B-V two-colour diagram roughly yields the metallicities and ages of the Milky Way globular clusters independently determined, and then apply the technique to the clusters in NGC 5128. There is a large range in U-B, which corresponds to [Fe/H] of -2.0 through over +0.3. The large uncertainties from the data and the models currently prevent us from pinning down their ages and metallicities. Although a constant age for all these clusters cannot be ruled out, there is a hint of the metal-rich clusters being younger. Significance of these results and caveats of the analysis are discussed.

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