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We examined the double red clump (RC) observed in the Galactic bulge, interpreted as a difference in distance ("X-shaped bulge scenario") or in chemical composition ("multiple population scenario"). To verify chemical differences between the RC groups, we performed low-resolution spectroscopy for RC and red giant branch (RGB) stars using Gemini-South/GMOS in three fields of the bulge, and collected diverse data from literature. We divided our sample stars not only into bright and faint RC groups, but also into bluer ([Fe/H] < -0.1) and redder ([Fe/H] > -0.1) groups following the recent u-band photometric studies. For the metal-poor stars, no statistically significant difference in CN index was detected between the bright and faint RC groups for all observed fields. However, we found, from cross-matching with high-resolution spectroscopic data, a sign of Na enhancement in the "metal-poor and bright" RC group compared to the "metal-poor and faint" group at (l,b)=(-1 deg,-8.5 deg). When the contributions of the RGB stars on the RC regimes are taken into account, the Na abundance difference between genuine RCs would correspond to approximately 0.23 dex, similar to globular cluster (GCs) with multiple populations. In contrast, the metal-rich stars do not show chemical differences between the bright and faint RCs. It implies that the double RC observed in the metal-poor component of the bulge might be linked to the multiple populations originated from GC-like subsystem, whereas that of the metal-rich component would have produced by the X-shaped structure. Our results support the previous studies suggesting composite nature of the Milky Way bulge., Comment: Accepted for publication in ApJ

Context. With the increasing number of discoveries of globular clusters in the inner Milky Way, the need for spectroscopic confirmation and further investigation of their stellar populations and chemodynamical properties has become crucial. Aims. Gran 5 is a newly reported low-mass globular cluster located close to the Galactic center, and it is thought to be an accreted object associated with the Gaia-Enceladus structure. This study aims to investigate the stellar populations of Gran 5 and their detailed chemical properties. Methods. We performed high-resolution near-infrared spectroscopy on seven stars in the field of Gran 5 using IGRINS on the Gemini-South telescope. Results. We identified six stars as cluster members and reveal that they are divided into two stellar populations with different metallicities, with mean [Fe/H] values of -0.76 dex and -0.55 dex, respectively. In addition, the chemodynamical properties of Gran 5 agree with those of in situ globular clusters. Conclusions. Our findings represent the first detection of two stellar populations with different metallicities in a low-mass globular cluster. This suggests that the metallicity variation in Gran 5 may have arisen from processes different from those in other globular clusters with metallicity variation, or that it may have lost a substantial amount of its initial mass during its evolution., Comment: 12 pages, 9 figures, accepted for publication in A&A

Wide binaries, with separations between two stars from a few AU to more than several thousand AU, are valuable objects for various research topics in Galactic astronomy. As the number of newly reported wide binaries continues to increase, studying the chemical abundances of their component stars becomes more important. We conducted high-resolution near-infrared (NIR) spectroscopy for six pairs of wide binary candidates using the Immersion Grating Infrared Spectrometer (IGRINS) at the Gemini-South telescope. One pair was excluded from the wide binary samples due to a significant difference in radial velocity between its component stars, while the remaining five pairs exhibited homogeneous properties in 3D motion and chemical composition among the pair stars. The differences in [Fe/H] ranged from 0.00 to 0.07 dex for these wide binary pairs. The abundance differences between components are comparable to the previous results from optical spectroscopy for other samples. In addition, when combining our data with literature data, it appears that the variation of abundance differences increases in wide binaries with larger separations. However, the SVO2324 and SVO3206 showed minimal differences in most elements despite their large separation, supporting the concept of multiple formation mechanisms depending on each wide binary. This study is the first approach to the chemical properties of wide binaries based on NIR spectroscopy. Our results further highlight that NIR spectroscopy is an effective tool for stellar chemical studies based on equivalent measurements of chemical abundances from the two stars in each wide binary system., Comment: 16 pages, 9 figures, accepted for publication in AJ

It is well established that the Hubble residuals of type Ia supernovae (SNe Ia) show the luminosity step with respect to their host galaxy stellar masses. This `mass-step' is taken as an additional correction factor for the SN Ia luminosity standardization. Here we investigate the root cause of the mass-step and propose that the bimodal nature of the host $age$ distribution is responsible for the step. In particular, by using the empirical $nonlinear$ mass-to-age relation of local galaxies, we convert the mass function of SN Ia hosts to their age distribution. We find that the age distribution shows clear bimodality: a younger ($<$ 6 Gyr) group with lower mass ($\sim 10^{9.5}{\rm M}_{\rm sun}$) and an older ($>$ 6 Gyr) group with higher mass ($\sim 10^{10.5}{\rm M}_{\rm sun}$). On the Hubble residual versus host mass plane, the two groups create the mass-step at $\sim 10^{10}{\rm M}_{\rm sun}$. This leads us to conclude that the host galaxy mass-step can be attributed to the bimodal age distribution in relation to a nonlinear relation between galaxy mass and age. We suggest that the mass-step is another manifestation of the old `red sequence' and the young `blue cloud' observed in the galactic color--magnitude diagram., Comment: Accepted for publication in ApJ, 10 pages, 5 figures, 1 table

Globular clusters (GCs) play an important role in the formation and evolution of the Milky Way. New candidates are continuously found, particularly in the high-extinction low-latitude regions of the bulge, although their existence and properties have yet to be verified. In order to investigate the new GC candidates, we performed high-resolution NIR spectroscopy of stars toward the bulge using the IGRINS instrument at the Gemini-South telescope. We selected 15 and 10 stars near Camargo 1103 and 1106, respectively, which have recently been reported as metal-poor GC candidates in the bulge. In contrast to the classical approaches used in optical spectroscopy, we determined stellar parameters from a combination of line-depth ratios and the equivalent width of a CO line. The stellar parameters of the stars follow the common trends of nearby APOGEE stars in a similar magnitude range. We also determined the abundances of Fe, Na, Mg, Al, Si, S, K, Ca, Ti, Cr, Ni, and Ce through spectrum synthesis. There is no clear evidence of a grouping in RV-[Fe/H] space that would indicate the characterization of either object as metal-poor GCs. This result emphasizes the necessity of follow-up spectroscopy for new GC candidates toward the bulge, although we cannot completely rule out a low probability that we only observed nonmember stars. We also note discrepancies between the abundances of Al, Ca, and Ti when derived from the H- vs. the K-band spectra. Although the cause of this discrepancy is not clear, the effects of atmosphere parameters or NLTE are discussed. Our approach and results demonstrate that IGRINS spectroscopy is a useful tool for studying the chemical properties of stars toward the Galactic bulge with a statistical uncertainty in [Fe/H] of 0.03 dex, while the systematic error through uncertainties of atmospheric parameter is slightly larger than in measurements from optical spectroscopy., Comment: 16 pages, 9 figures, accepted for publication in A&A

Supernova (SN) cosmology is based on the assumption that the width-luminosity relation (WLR) and the color-luminosity relation (CLR) in the type Ia SN luminosity standardization would not show {absolute magnitude differences} with progenitor age. Unlike this expectation, recent age datings of stellar populations in host galaxies have shown significant correlations between progenitor age and Hubble residual (HR). Here we show that this correlation originates from a strong progenitor age dependence of the zero-points of the WLR and the CLR, in the sense that SNe from younger progenitors are fainter each at given light-curve parameters $x_1$ and $c$. This $4.6\sigma$ result is reminiscent of Baade's discovery of the zero-point variation of the Cepheid period-luminosity relation with age, and, as such, causes a serious systematic bias with redshift in SN cosmology. Other host properties show substantially smaller and insignificant offsets in the WLR and CLR for the same dataset. We illustrate that the differences between the high-$z$ and low-$z$ SNe in the WLR and CLR, and in HR after the standardization, are fully comparable to those between the correspondingly young and old SNe at intermediate redshift, indicating that the observed dimming of SNe with redshift may well be an artifact of over-correction in the luminosity standardization. When this systematic bias with redshift is properly taken into account, there is little evidence left for an accelerating universe, in discordance with other probes, urging the follow-up investigations with larger samples at different redshift bins., Comment: 13 pages, 11 figures, 1 table, Accepted for publication in MNRAS

We performed low-resolution spectroscopy for the red giant branch stars in an intriguing globular cluster (GC) NGC 2808, which hosts subpopulations with extreme helium and light-element abundances. In order to trace N, C, and Ca abundance differences among subpopulations, we measured CN, CH, and Ca II H&K spectral indices, respectively. We identified four subpopulations (G1, G2, G3, and G4) from CN and CH strength, with CN-weak/CH-strong G1, CN-intermediate/CH-strong G2, CN-strong/CH-intermediate G3, and CN-strong/CH-weak G4. Compared to [Na/O] from high-resolution spectroscopy, we show that CN index can more clearly separate G1 and G2. Since CN traces N abundance in a GC, it implies that G1 and G2 would show a larger difference in [N/Fe] compared to [Na/Fe], as predicted by chemical evolution models. Later generation stars G3 and G4, however, are better separated with high-resolution spectroscopy. We also found that G4 shows a stronger Ca II H&K line strength compared to that of G1, but we suspect this to be a result of unusually strong He enhancement and/or Mg depletion in G4 of this GC. This work illustrates that combining low- and high-resolution spectroscopic studies can improve the separation of subpopulations in GCs., Comment: Accepted for publication in AJ

The double red clump (RC) observed in color-magnitude diagrams of the Milky Way bulge is at the heart of the current debate on the structure and formation origin of the bulge. This feature can be explained by the difference between the two RCs either in distance ("X-shaped scenario") or in chemical composition ("multiple-population scenario"). Here we report our high-resolution spectroscopy for the RC and red giant branch stars in a high-latitude field (b ~ -8.5$\deg$) of the bulge. We find a difference in [Fe/H] between the stars in the bright and faint RC regimes, in the sense that the bright stars are enhanced in [Fe/H] with respect to the faint stars by 0.149 $\pm$ 0.036 dex. The stars on the bright RC are also enhanced in [Na/Fe] but appear to be depleted in [Al/Fe] and [O/Fe], although more observations are required to confirm the significance of these differences. Interestingly, these chemical patterns are similar to those observed among multiple stellar populations in the metal-rich bulge globular cluster Terzan 5. In addition, we find a number of Na-rich stars, which would corroborate the presence of multiple populations in the bulge. Our results support an origin of the double RC from dissolved globular clusters that harbor multiple stellar populations. Thus, our study suggests that a substantial fraction of the outer bulge stars would have originated from the assembly of such stellar systems in the early phase of the Milky Way formation., Comment: 17 pages, 15 figures, accepted for publication in ApJ

We review the history of space mission in Korea focusing on the field of astronomy and astrophysics. For each mission, scientific motivation and achievement are reviewed together with some technical details of the program including mission schedule. This review includes the ongoing and currently approved missions as well as some planned ones. Within the admitted limitations of authors' perspectives, some comments on the future direction of space program for astronomy and astrophysics in Korea are made at the end of this review., Comment: 29 pages, 31 figures, and 4 tables. To be published in the special issue of Journal of Korean Physical Society (JKPS) to celebrate the 25th anniversary of Astrophysics Division of Korean Physical Society (KPS)

We present an evolutionary population synthesis model of the surface brightness fluctuation (SBF) for normal and He-enriched simple stellar populations (SSPs). While our SBF model for the normal-He population agrees with other existing models, the He-rich population, containing hotter horizontal-branch stars and brighter red-clump stars than the normal-He population, entails a substantial change in the SBF of SSPs. We show that the SBF magnitudes are affected by He-rich populations at least $\sim$0.3~mag even in $I$- and near-IR bands at given colors, from which the SBF-based distances are often derived. Due to uncertainties both in observations and models, however, the SBFs of Galactic globular clusters and early-type galaxies do not allow verifying the He-enriched model. We propose that when combined with independent metallicity and age indicators such as ${\rm Mg}_2$ and ${\rm H}\beta$, the UV and optical SBFs can readily detect underlying He-rich populations in unresolved stellar systems at a distance out to $\gtrsim 20$\,Mpc. A full set of the spectro-photometric and SBF data for SSPs from the Yonsei Evolutionary Population Synthesis (YEPS) model is available for download at http://cosmic.yonsei.ac.kr/YEPS.htm., Comment: 24 pages, 14 figures, 4 tables, Accepted for publication in ApJS

Supernova (SN) cosmology is based on the assumption that the corrected luminosity of SN Ia would not evolve with redshift. Recently, our age dating of stellar populations in early-type host galaxies (ETGs) from high-quality spectra has shown that this key assumption is most likely in error. It has been argued though that the age-Hubble residual (HR) correlation from ETGs is not confirmed from two independent age datasets measured from multi-band optical photometry of host galaxies of all morphological types. Here we show, however, that one of them is based on highly uncertain and inappropriate luminosity-weighted ages derived, in many cases, under serious template mismatch. The other dataset employs more reliable mass-weighted ages, but the statistical analysis involved is affected by regression dilution bias, severely underestimating both the slope and significance of the age-HR correlation. Remarkably, when we apply regression analysis with a standard posterior sampling method to this dataset comprising a large sample ($N=102$) of host galaxies, very significant ($> 99.99 \%$) correlation is obtained between the global population age and HR with the slope ($-0.047 \pm 0.011$~mag/Gyr) highly consistent with our previous spectroscopic result from ETGs. For the local age of the environment around the site of SN, a similarly significant ($> 99.96 \%$) correlation is obtained with a steeper slope ($-0.057 \pm 0.016$ mag/Gyr). Therefore, the SN luminosity evolution is strongly supported by the age dating based on multi-band optical photometry and can be a serious systematic bias in SN cosmology., Comment: 7 pages, 3 figures, Accepted for publication in ApJ

A few dozen dwarf satellite galaxies of the Milky Way have been discovered, which are often viewed as the remaining building blocks of our Galaxy. The follow-up spectroscopy showed that dwarf galaxies have a sizeable spread in their metallicities. Several scenarios were suggested to explain the metallicity spread, which can be tested by the structural patterns of stellar subpopulations with distinct metallicities. However, such chemical plus structural examination, to which we refer to as "chemostructural study" is hindered by the lack of stars with spectroscopic metallicity. Here we propose the Ca$-$$by$ photometry as an alternative way to secure metallicities for a 2$-$3 orders of magnitude larger stellar sample than the spectroscopic sample and thus enable us to perform a chemostructural study on dwarf galaxies. In particular, we use the $hk$ index [$\equiv($Ca$-b)-(b-y)$], whose validity as a photometric metallicity indicator (and crass insensitivity to age) for red-giant-branch stars was upheld via Galactic globular clusters, and observe three dwarf spheroidal galaxies$-$Draco, Sextans, and Canes Venatici I (CVnI)$-$with Subaru/Suprime-Cam. We find that in all the galaxies the metal-rich stellar populations are more centrally concentrated than the metal-poor counterparts, suggesting that the central regions of the galaxies underwent extended star formation. Such a negative radial metallicity gradient for Sextans and CVnI opposes to the traditional spectroscopic results. We also find that their metallicity distribution functions (MDFs) can be characterized by a unimodal, skewed Gaussian shape with a metal-rich peak and a metal-poor tail. We discuss their features in the chemostructure and MDF in terms of dwarf galaxy formation theories., Comment: Accepted for publication in ApJS, data will be available at http://vo.kasi.re.kr/Phot_dSphs/

The most direct and strongest evidence for the presence of dark energy is provided by the measurement of galaxy distances using SNe Ia. This result is based on the assumption that the corrected brightness of SN Ia through the empirical standardization would not evolve with look-back time. Recent studies have shown, however, that the standardized brightness of SN Ia is correlated with host morphology, host mass, and local star formation rate (SFR), suggesting a possible correlation with stellar population property. To understand the origin of these correlations, we have continued our spectroscopic observations to cover most of the reported nearby early-type host galaxies. From high-quality (signal-to-noise ratio ~175) spectra, we obtained the most direct and reliable estimates of population age and metallicity for these host galaxies. We find a significant correlation between SN luminosity (after the standardization) and stellar population age at a 99.5 % confidence level. As such, this is the most direct and stringent test ever made for the luminosity evolution of SN Ia. Based on this result, we further show that the previously reported correlations with host morphology, host mass, and local SFR are most likely originated from the difference in population age. This indicates that the light-curve fitters used by the SNe Ia community are not quite capable of correcting for the population age effect, which would inevitably cause a serious systematic bias with look-back time. Notably, taken at face values, most of the Hubble residual used in the discovery of the dark energy appears to be affected by the luminosity evolution., Comment: To be published in 20 January 2020 issue of ApJ; see Figure 16 for the luminosity evolution mimicking dark energy

Recent stellar evolution models for globular clusters (GCs) in multiple population paradigm suggest that horizontal-branch (HB) morphology and mean period of type ab RR Lyrae variables are mostly determined by He & CNO abundances and relative ages for subpopulations. These parameters are also provided by chemical evolution models constructed to reproduce the Na-O anti-correlation. Therefore, a consistency check is possible between the synthetic HB and chemical evolution models. Furthermore, by combining them, a better constraint might be attained for star formation history and chemical abundances of subpopulations in GCs. We find, from such efforts made for four GCs, M4, M5, M15, and M80, that consistent results can be obtained from these two independent studies. In our unified model, He and Na abundances gradually increase over the generation, and therefore, the various extensions observed in both HB morphology and Na-O chemical pattern depend on the presence of later generation stars after the second generation. It is schematically shown that this observed diversity, however, would not be naturally explained by the models requiring dilution. Further spectroscopic observations are required, for metal-poor GCs in particular, to obtain a more detailed constraint from this approach., Comment: Accepted for publication in ApJ

Recent interpretation of the color$-$magnitude diagrams of the Milky Way (MW) bulge has suggested that the observed double red-clump feature can be a natural consequence of He-enhanced stellar populations in the MW bulge. This implies that globular clusters (GCs), where the He-enhanced second-generation (SG) stars can be efficiently created, are the most likely candidate contributors of He-rich stars to the MW bulge. We extend this idea to the Galactic inner halo and investigate the fraction of the SG stars as a function of the Galactocentric distance. We use bluer blue-horizontal branch (bBHB) stars, which are assumed to be originated from He-rich SG populations, as proxies of SG stars, and find that the fraction of bBHB stars increases with decreasing Galactocentric distance. Simulations of the GC evolution in the MW tidal field qualitatively support the observed trend of bBHB enhancement in the inner halo. In these simulations, the increasing tidal force with decreasing Galactocentric distance leads to stripping of stars not only from the outskirts but also from the central regions of GCs, where SG stars are more abundant. We discuss the implication and prospect of our findings concerning the formation history of the bulge and inner halo of the MW., Comment: 11 pages, 4 figures, accepted for publication in the ApJL

There is growing evidence for the dependence of Type Ia supernova (SN Ia) luminosities on their environments. While the impact of this trend on estimating cosmological parameters is widely acknowledged, the origin of this correlation is still under debate. In order to explore this problem, we first construct the YONSEI (YOnsei Nearby Supernova Evolution Investigation) SN catalog. The catalog consists of 1231 spectroscopically confirmed SNe Ia over a wide redshift range (0.01 < z < 1.37) from various SN surveys and includes the light-curve fit data from two independent light-curve fitters of SALT2 and MLCS2k2. For a sample of 674 host galaxies, we use the stellar mass and the star formation rate data in Kim et al. (2018). We find that SNe Ia in low-mass and star-forming host galaxies are $0.062\pm0.009$ mag and $0.057\pm0.010$ mag fainter than those in high-mass and passive hosts, after light-curve corrections with SALT2 and MLCS2k2, respectively. When only local environments of SNe Ia (e.g., locally star-forming and locally passive) are considered, this luminosity difference increases to $0.081\pm0.018$ mag for SALT2 and $0.072\pm0.018$ mag for MLCS2k2. Considering the significant difference in the mean stellar population age between the two environments, this result suggests that the origin of environmental dependence is most likely the luminosity evolution of SNe Ia with redshift., Comment: 25 pages, 27 figures,and 10 tables. Accepted for publication in JKAS (Journal of the Korean Astronomical Society)

Recent investigations of the double red clump in the color-magnitude diagram of the Milky Way bulge cast serious doubts on the structure and formation origin of the outer bulge. Unlike previous interpretation based on an X-shaped bulge, stellar evolution models and CN-band observations have suggested that this feature is another manifestation of the multiple stellar population phenomenon observed in globular clusters (GCs). This new scenario requires a significant fraction of the outer bulge stars with chemical patterns uniquely observed in GCs. Here we show from homogeneous high-quality spectroscopic data that the red giant branch stars in the outer bulge (> 5.5 degrees from the Galactic center) are clearly divided into two groups according to Na abundance in the [Na/Fe] - [Fe/H] plane. The Na-rich stars are also enhanced in Al, while the differences in O and Mg are not observed between the two Na groups. The population ratio and the Na and Al differences between the two groups are also comparable with those observed in metal-rich GCs. The only plausible explanation for these chemical patterns and characteristics appears to be that the outer bulge was mostly assembled from disrupted proto-GCs in the early history of the Milky Way., Comment: Accepted for publication in ApJL

In order to investigate the origin of multiple stellar populations found in globular clusters (GCs) in the halo and bulge of the Milky Way, we have constructed chemical evolution models for their putative low-mass progenitors. In light of recent theoretical developments, we assume that supernova blast waves undergo blowout without expelling the pre-enriched ambient gas, while relatively slow winds of massive stars, together with the winds and ejecta from low to high mass asymptotic-giant-branch stars, are all locally retained in these less massive systems. Interestingly, we find that the observed Na-O anti-correlations in metal-poor GCs can be reproduced when multiple episodes of starburst and enrichment are allowed to continue in these subsystems. A specific form of star formation history with decreasing time intervals between the successive stellar generations, however, is required to obtain this result, which is in good agreement with the parameters obtained from synthetic horizontal-branch models. The "mass budget problem" is also much alleviated by our models without ad-hoc assumptions on star formation efficiency, initial mass function, and the preferential loss of first-generation stars. We also applied these models to investigate the origin of super-He-rich red clump stars in the metal-rich bulge suggested by Lee et al. (2015). We find that chemical enrichment by the winds of massive stars can naturally reproduce the required strong He enhancement in metal-rich subsystems. Our results further underscore that gas expulsion or retention is a key factor in understanding the multiple populations in GCs., Comment: Accepted for publication in ApJ

The two red clumps (RCs) observed in the color-magnitude diagram of the Milky Way bulge is widely accepted as evidence for an X-shaped structure originated from the bar instability. A drastically different interpretation has been suggested, however, based on the He-enhanced multiple stellar population phenomenon as is observed in globular clusters (GCs). Because these two scenarios imply very different pictures on the formation of the bulge and elliptical galaxies, understanding the origin of the double RC is of crucial importance. Here we report our discovery that the stars in the two RCs show a significant (> 5.3 {\sigma}) difference in CN-band strength, in stark contrast to that expected in the X-shaped bulge scenario. The difference in CN abundance and the population ratio between the two RCs are comparable to those observed in GCs between the first- and later generation stars. Since CN-strong stars trace a population with enhanced N, Na, and He abundances originated in GCs, this is direct evidence that the double RC is due to the multiple population phenomenon, and that a significant population of stars in the Milky Way bulge were assembled from disrupted proto-GCs. Our result also calls for the major revision of the 3D structure of the Milky Way bulge given that the current view is based on the previous interpretation of the double RC phenomenon., Comment: Accepted for publication in ApJL

A number of recent studies have claimed that the double red clump (RC) observed in the Milky Way bulge is a consequence of a giant X-shaped structure. In particular, Ness & Lang (2016) reported a direct detection of a faint X-shaped structure from the residual map of the Wide-Field Infrared Survey Explorer (WISE) bulge image. Here we show, however, that their result is affected substantially by whether the conventional dust extinction correction is applied or not and partly by a bulge model subtracted from the original image. We find that the residuals obtained by subtracting either ellipsoidal or boxy bulge models from the dereddened images show no obvious X-shaped structure. We further show that, even if it is real, the stellar density in the claimed X-shaped structure is way too low to be observed as a strong double RC at l=0., Comment: Submitted to AJ

It is now established that there is a dependence of the luminosity of type Ia supernovae (SNe Ia) on environment: SNe Ia in young, star-forming, metal-poor stellar populations appear fainter after light-curve shape corrections than those in older, passive, metal-rich environments. This is accounted for in cosmological studies using a global property of the SN host galaxy, typically the host galaxy stellar mass. However, recent low-redshift studies suggest that this effect manifests itself most strongly when using the local star-formation rate (SFR) at the SN location, rather than the global SFR or stellar mass of the host galaxy. At high-redshift, such local SFRs are difficult to determine; here, we show that an equivalent 'local' correction can be made by restricting the SN Ia sample in globally star-forming host galaxies to a low-mass host galaxy subset ($\le10^{10} M_{\odot}$). Comparing this sample of SNe Ia (in locally star-forming environments) to those in locally passive host galaxies, we find that SNe Ia in locally star-forming environments are $0.081\pm0.018$ mag fainter ($4.5\sigma$), consistent with the result reported by Rigault et al. (2015), but our conclusion is based on a sample ~5 times larger over a wider redshift range. This is a larger difference than when splitting the SN Ia sample based on global host galaxy SFR or host galaxy stellar mass. This method can be used in ongoing and future high-redshift SN surveys, where local SN Ia environments are difficult to determine., Comment: 16 pages, 3 figures, and 5 tables, accepted for publication in ApJ

We present [Fe/H] and [Ca/Fe] of $\sim600$ red giant branch (RGB) members of the globular cluster $\omega$ Centauri. We collect medium-resolution ($R\sim2000$) spectra using the Blanco 4 m telescope at the Cerro Tololo Inter-American Observatory equipped with Hydra, the fiber-fed multi-object spectrograph. We demonstrate that blending of stellar light in optical fibers severely limits the accuracy of spectroscopic parameters in the crowded central region of the cluster. When photometric temperatures are taken in the spectroscopic analysis, our kinematically selected cluster members, excluding those that are strongly affected by flux from neighboring stars, include relatively fewer stars at intermediate metallicity ([Fe/H]$\sim-1.5$) than seen in the previous high-resolution survey for brighter giants in Johnson & Pilachowski. As opposed to the trend of increasing [Ca/Fe] with [Fe/H] found by those authors, our [Ca/Fe] estimates, based on Ca II H & K measurements, show essentially the same mean [Ca/Fe] for most of the metal-poor and metal-intermediate populations in this cluster, suggesting that mass- or metallicity-dependent SN II yields may not be necessary in their proposed chemical evolution scenario. Metal-rich cluster members in our sample show a large spread in [Ca/Fe], and do not exhibit a clear bimodal distribution in [Ca/Fe]. We also do not find convincing evidence for a radial metallicity gradient among RGB stars in $\omega$ Centauri., Comment: 10 pages, 10 figures. Astronomical Journal, in press

In order to investigate the origin of multiple stellar populations in the halo and bulge of the Milky Way, we have constructed chemical evolution models for the low-mass proto-Galactic subsystems such as globular clusters (GCs). Unlike previous studies, we assume that supernova blast waves undergo blowout without expelling the pre-enriched gas, while relatively slow winds of massive stars (WMS), together with the winds and ejecta from low and intermediate mass asymptotic-giant-branch stars (AGBs), are all locally retained in these less massive systems. We find that the observed Na-O anti-correlation in metal-poor GCs can be reproduced, when multiple episodes of starbursts are allowed to continue in these subsystems. Specific star formation history (SFH) with decreasing time intervals between the stellar generations, however, is required to obtain this result, which is in good agreement with the parameters obtained from our stellar evolution models for the horizontal-branch. The "mass budget problem" is also much alleviated by our models without ad-hoc assumptions on star formation efficiency (SFE) and initial mass function (IMF). We also applied these models to investigate the origin of super-helium-rich red clump stars in the metal-rich bulge as recently suggested by Lee et al. (2015). We find that chemical enrichments by the WMS can naturally reproduce the required helium enhancement (dY/dZ = 6) for the second generation stars. Disruption of proto-GCs in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field., Comment: To appear in the proceedings of the IAU Symposium 334 "Rediscovering our Galaxy", Potsdam, 10-14 July 2017

We performed low-resolution spectroscopy for the red giant stars in the Galactic globular cluster (GC) NGC 5286, which is known to show intrinsic heavy element abundance variations. We found that the observed stars in this GC are clearly divided into three subpopulations by CN index (CN-weak, CN-intermediate, and CN-strong). The CN-strong stars are also enhanced in the calcium HK' (7.4{\sigma}) and CH (5.1{\sigma}) indices, while the CN-intermediate stars show no significant difference in the strength of HK' index with CN-weak stars. From the comparison with high-resolution spectroscopic data, we found that the CN- and HK'-strong stars are also enhanced in the abundances of Fe and s-process elements. It appears, therefore, that these stars are later generation stars affected by some supernovae enrichment in addition to the asymptotic giant branch ejecta. In addition, unlike normal GCs, sample stars in NGC 5286 show the CN-CH positive correlation, strengthening our previous suggestion that this positive correlation is only discovered in GCs with heavy element abundance variations such as M22 and NGC 6273., Comment: 8 pages, 6 figures, 1 table, accepted for publication in ApJ

The discovery of multiple stellar populations in Milky Way globular clusters (GCs) has stimulated various follow-up studies on helium-enhanced stellar populations. Here we present the evolutionary population synthesis models for the spectro-photometric evolution of simple stellar populations (SSPs) with varying initial helium abundance ($Y_{\rm ini}$). We show that $Y_{\rm ini}$ brings about {dramatic} changes in spectro-photometric properties of SSPs. Like the normal-helium SSPs, the integrated spectro-photometric evolution of helium-enhanced SSPs is also dependent on metallicity and age for a given $Y_{\rm ini}$. {We discuss the implications and prospects for the helium-enhanced populations in relation to the second-generation populations found in the Milky Way GCs.} All of the models are available at \url{http://web.yonsei.ac.kr/cosmic/data/YEPS.htm}., Comment: Accepted for publication in ApJ, 21 pages, 12 figures, 3 tables

A recent survey toward the Milky Way bulge has discovered two sequences of RR Lyrae stars on the period-amplitude diagram with a maximum period-shift of {\Delta}log P = 0.015 between the two populations. Here we show, from our synthetic horizontal-branch models, that this period-shift is most likely due to the small difference in helium abundance ({\Delta}Y = 0.012) between the first and second-generation stars (G1 and G2), as is the case in our models for the inner halo globular clusters with similar metallicity ([Fe/H] = -1.1). We further show that the observed double red clump (RC) in the bulge is naturally reproduced when these models are extended to solar metallicity following {\Delta}Y/{\Delta}Z = 6 for G2, as would be expected from the chemical evolution models. Therefore, the two populations of RR Lyrae stars and the double RC observed in the bulge appear to be different manifestations of the same multiple population phenomenon in the metal-poor and metal-rich regimes respectively., Comment: Accepted for publication in ApJ

Most globular clusters (GCs) are now known to host multiple stellar populations with different light element abundances. Here we use narrow-band photometry and low-resolution spectroscopy for NGC 362 and NGC 6723 to investigate their chemical properties and radial distributions of subpopulations. We confirm that NGC 362 and NGC 6723 are among the GCs with multiple populations showing bimodal CN distribution and CN-CH anti-correlation without a significant spread in calcium abundance. These two GCs show more centrally concentrated CN-weak earlier generation stars compared to the later generation CN-strong stars. These trends are reversed with respect to those found in previous studies for many other GCs. Our findings, therefore, seem contradictory to the current scenario for the formation of multiple stellar populations, but mass segregation acting on the two subpopulations might be a possible solution to explain this reversed radial trend., Comment: 12 pages, 8 figures, 2 tables, accepted for publication in ApJ

The double red clump (RC) observed in the Milky Way bulge is widely interpreted as evidence for an X-shaped structure. We have recently suggested, however, an alternative interpretation based on the multiple population phenomenon, where the bright RC is from helium enhanced second-generation stars (G2), while the faint RC is representing first-generation stars (G1) with normal helium abundance. Here our RC models are constructed in a large parameter space to see the effects of metallicity, age, and helium abundance on the double RC feature. Our models show that the luminosity of RC stars is mainly affected by helium abundance, while the RC color is primarily affected by metallicity. The effect of age is relatively small, unless it is older than 12 Gyr or much younger than 6 Gyr. The observed double RC feature can therefore be reproduced in a relatively large parameter space, once {\Delta}Y between G2 and G1 is assumed to be greater than $\sim$0.10. We further show that the longitude dependence of the double RC feature at $b \approx -8.5 \deg$, which was pointed out by Gonzalez et al. (2015) as a potential problem of our model, is well explained in our scenario by a classical bulge embedded in a tilted bar., Comment: Accepted for publication in ApJ, 7 pages, 13 figures

Recently, both stellar mass-segregation and binary-fractions were uniformly measured on relatively large samples of Galactic Globular Clusters (GCs). Simulations show that both sizeable binary-star populations and Intermediate-Mass Black Holes (IMBHs) quench mass-segregation in relaxed GCs. Thus mass-segregation in GCs with a reliable binary-fraction measurement is a valuable probe to constrain IMBHs. In this paper we combine mass-segregation and binary-fraction measurements from the literature to build a sample of 33 GCs (with measured core-binary fractions), and a sample of 43 GCs (with a binary fraction measurement in the area between the core radius and the half-mass radius). Within both samples we try to identify IMBH-host candidates. These should have relatively low mass-segregation, a low binary fraction (< 5%), and short (< 1 Gyr) relaxation time. Considering the core binary fraction sample, no suitable candidates emerge. If the binary fraction between the core and the half-mass radius is considered, two candidates are found, but this is likely due to statistical fluctuations. We also consider a larger sample of 54 GCs where we obtained an estimate of the core binary fraction using a predictive relation based on metallicity and integrated absolute magnitude. Also in this case no suitable candidates are found. Finally, we consider the GC core- to half-mass radius ratio, that is expected to be larger for GCs containing either an IMBH or binaries. We find that GCs with large core- to half-mass radius ratios are less mass-segregated (and show a larger binary fraction), confirming the theoretical expectation that the energy sources responsible for the large core are also quenching mass-segregation, Comment: 5 pages, 7 figures, ApJ accepted

We present new HST optical and near-infrared (NIR) photometry of the rich globular cluster (GC) system of NGC 4874, the cD galaxy in the core of the Coma cluster (Abell 1656). NGC 4874 was observed with the HST Advanced Camera for Surveys in the F475W (g) and F814W (I) passbands and the Wide Field Camera 3 IR Channel in F160W (H). The GCs in this field exhibit a bimodal optical color distribution with more than half of the GCs falling on the red side at g-I > 1. Bimodality is also present, though less conspicuously, in the optical-NIR I-H color. Consistent with past work, we find evidence for nonlinearity in the g-I versus I-H color-color relation. Our results thus underscore the need for understanding the detailed form of the color-metallicity relations in interpreting observational data on GC bimodality. We also find a very strong color-magnitude trend, or "blue tilt," for the blue component of the optical color distribution of the NGC 4874 GC system. A similarly strong trend is present for the overall mean I-H color as a function of magnitude; for M_814 < -10 mag, these trends imply a steep mass-metallicity scaling with $Z\propto M_{\rm GC}^{1.4\pm0.4}$, but the scaling is not a simple power law and becomes much weaker at lower masses. As in other similar systems, the spatial distribution of the blue GCs is more extended than that of the red GCs, partly because of blue GCs associated with surrounding cluster galaxies. In addition, the center of the GC system is displaced by 4+/-1 kpc towards the southwest from the luminosity center of NGC 4874, in the direction of NGC 4872. Finally, we remark on a dwarf elliptical galaxy with a noticeably asymmetrical GC distribution. Interestingly, this dwarf has a velocity of nearly -3000 km/s with respect to NGC 4874; we suggest it is on its first infall into the cluster core and is undergoing stripping of its GC system by the cluster potential., Comment: 24 pages, 20 figures, accepted for publication in ApJ

Type Ia supernova (SN Ia) cosmology provides the most direct evidence for the presence of dark energy. This result is based on the assumption that the look-back time evolution of SN Ia luminosity, after light-curve corrections, would be negligible. Recent studies show, however, that the Hubble residual (HR) of SN Ia is correlated with the mass and morphology of host galaxies, implying the possible dependence of SN Ia luminosity on host galaxy properties. In order to investigate this more directly, we have initiated spectroscopic survey for the early-type host galaxies, for which population age and metallicity can be more reliably determined from the absorption lines. As the first paper of the series, here we present the results from high signal-to-noise ratio (>100 per pixel) spectra for 27 nearby host galaxies in the southern hemisphere. For the first time in host galaxy studies, we find a significant (~3.9sigma) correlation between host galaxy mass (velocity dispersion) and population age, which is consistent with the "downsizing" trend among non-host early-type galaxies. This result is rather insensitive to the choice of population synthesis models. Since we find no correlation with metallicity, our result suggests that stellar population age is mainly responsible for the relation between host mass and HR. If confirmed, this would imply that the luminosity evolution plays a major role in the systematic uncertainties of SN Ia cosmology., Comment: Accepted for publication in ApJS

The metallicity distribution function of globular clusters (GCs) in galaxies is a key to understanding galactic formation and evolution. The calcium II triplet (CaT) index has recently become a popular metal abundance indicator thanks to its sensitivity to GC metallicity. Here we revisit and assess the reliability of CaT as a metallicity indicator using our new stellar population synthesis simulations based on empirical, high-resolution fluxes. The model shows that the CaT strength of old ($>$ 10 Gyr) GCs is proportional to ${\rm [Fe/H]}$ below $-0.5$. In the modest metal-rich regime, however, CaT does not increase anymore with ${\rm [Fe/H]}$ due to the little contribution from coolest red giant stars to the CaT absorption. The nonlinear nature of the color--$CaT$ relation is confirmed by the observations of GCs in nearby early-type galaxies. This indicates that the CaT should be used carefully when deriving metallicities of metal-rich stellar populations. Our results offer an explanation for the observed sharp difference between the color and $CaT$ distributions of GCs in the same galaxies. We take this as an analogy to the view that metallicity--color and metallicity--Lick index nonlinearity of GCs is primarily responsible for their observed "bimodal" distributions of colors and absorption indices., Comment: 27 pages, 10 figures, 2 tables, accepted for publication in ApJ

In our recent investigation (Lim et al. 2015), we have shown that narrow-band photometry can be combined with low-resolution spectroscopy to effectively search for globular clusters (GCs) with supernovae (SNe) enrichments. Here we apply this technique to the metal-poor bulge GC NGC 6273, and find that the red giant branch stars in this GC are clearly divided into two distinct subpopulations having different calcium abun- dances. The Ca rich subpopulation in this GC is also enhanced in CN and CH, showing a positive correlation between them. This trend is identical to the result we found in M22, suggesting that this might be a ubiquitous nature of GCs more strongly affected by SNe in their chemical evolution. Our results suggest that NGC 6273 was massive enough to retain SNe ejecta which would place this cluster in the growing group of GCs with Galactic building block characteristics, such as {\omega} Centauri and Terzan 5., Comment: 5 pages, 5 figures, accepted for the publication in ApJL

The formation of the Milky Way stellar halo is thought to be the result of merging and accretion of building blocks such as dwarf galaxies and massive globular clusters. Recently, Deason et al. (2015) suggested that the Milky Way outer halo formed mostly from big building blocks, such as dwarf spheroidal galaxies, based on the similar number ratio of blue straggler (BS) stars to blue horizontal-branch (BHB) stars. Here we demonstrate, however, that this result is seriously biased by not taking into detailed consideration on the formation mechanism of BHB stars from helium enhanced second-generation population. In particular, the high BS-to-BHB ratio observed in the outer halo fields is most likely due to a small number of BHB stars provided by GCs rather than to a large number of BS stars. This is supported by our dynamical evolution model of GCs which shows preferential removal of first generation stars in GCs. Moreover, there are a sufficient number of outer halo GCs which show very high BS-to-BHB ratio. Therefore, the BS-to-BHB number ratio is not a good indicator to use in arguing that more massive dwarf galaxies are the main building blocks of the Milky Way outer halo. Several lines of evidence still suggest that GCs can contribute a significant fraction of the outer halo stars., Comment: 5 pages, 3 figures, 1 table, accepted for publication in MNRAS

The presence of two red clumps (RCs) in high latitude fields of the Milky Way bulge is interpreted as evidence for an X-shaped structure originated from the bar instability. Here we show, however, that this double RC phenomenon is more likely to be another manifestation of multiple populations observed in globular clusters (GCs) in the metal-rich regime. As in the bulge GC Terzan 5, the helium enhanced second generation stars (G2) in the classical bulge component of the Milky Way are placed on the bright RC, which is about 0.5 mag brighter than the normal RC originated from the first generation stars (G1), producing the observed double RC. In a composite bulge, where a classical bulge can coexist with a boxy pseudo bulge, our models can also reproduce key observations, such as the dependence of the double RC feature on metallicity and Galactic latitude and longitude. If confirmed by Gaia trigonometric parallax distances, this would indicate that the Milky Way bar is not sufficiently buckled to form the X-shaped structure in the bulge, and suggest that the early-type galaxies would be similarly prevailed by super-helium-rich subpopulation., Comment: Accepted for publication in MNRAS

In our recent investigation of the Oosterhoff dichotomy in the multiple population paradigm (Jang et al. 2014), we have suggested that the RR Lyrae variables in the Oosterhoff groups I, II, and III globular clusters (GCs) are produced mostly by the first, second, and third generation stars (G1, G2, and G3), respectively. Here we show, for the first time, that the observed dichotomies in the inner and outer halo GCs can be naturally reproduced when these models are extended to all metallicity regimes, while maintaining reasonable agreements in the horizontal-branch type versus [Fe/H] correlations. In order to achieve this, however, specific star formation histories are required for the inner and outer halos. In the inner halo GCs, the star formation commenced and ceased earlier with relatively short formation timescale between the subpopulations (~0.5 Gyr), while in the outer halo, the formation of G1 was delayed by ~0.8 Gyr with more extended timescale between G1 and G2 (~1.4 Gyr). This is consistent with the dual origin of the Milky Way halo. Despite the difference in detail, our models show that the Oosterhoff period groups observed in both outer and inner halo GCs are all manifestations of the "population-shift" effect within the instability strip, for which the origin can be traced back to the two or three discrete episodes of star formation in GCs., Comment: Accepted for publication in ApJS

There is increasing evidence for the presence of multiple red giant branches (RGBs) in the color-magnitude diagrams of massive globular clusters (GCs). In order to investigate the origin of this split on the RGB, we have performed new narrow-band Ca photometry and low-resolution spectroscopy for M22, NGC 1851, and NGC 288. We find significant differences (more than 4 sigma) in calcium abundance from the spectroscopic HK' index for M22 and NGC 1851. We also find more than 8 sigma differences in CN band strength between the Ca-strong and Ca-weak subpopulations for these GCs. For NGC 288, however, a large difference is detected only in the CN strength. The calcium abundances of RGB stars in this GC are identical to within the errors. This is consistent with the conclusion from our new Ca photometry, where the RGB splits are confirmed in M22 and NGC 1851, but not in NGC 288. We also find interesting differences in CN-CH correlations among these GCs. While CN and CH are anti-correlated in NGC 288, they show positive correlation in M22. NGC 1851, however, shows no difference in CH between the two groups of stars with different CN strengths. We suggest that all of these systematic differences would be best explained by how strongly type II supernovae enrichment has contributed to the chemical evolution of these GCs., Comment: 11 pages, 14 figures. Accepted for publication in ApJS

The globular cluster NGC 288 was previously reported to exhibit two distinct red giant branches (RGBs) in the narrow-band Calcium (HK) and Str\"omgren b and y band passes. In order to investigate this phenomenon further, we obtained moderate resolution (R$\sim$18,000) spectra of 27 RGB stars in NGC 288 with the Hydra multifiber spectrograph on the Blanco 4m telescope at Cerro Tololo Inter-American Observatory. From these data we derive iron ($\langle$[Fe/H]$\rangle$=-1.19; $\sigma$=0.12), oxygen ($\langle$[O/Fe]$\rangle$=$+$0.25; $\sigma$=0.13), and sodium ($\langle$[Na/Fe]$\rangle$=$+$0.15; $\sigma$=0.26) abundances using standard equivalent width and spectrum synthesis techniques. Combining these data with those available in the literature indicates that the two giant branches have distinctly different light element chemistry but do not exhibit a significant spread in [Fe/H]. A new transmission tracing for the CTIO Ca filter, obtained for this project, shows that CN contamination is the primary spectral feature driving the split RGB. Interestingly, the CN leak in the current CTIO Ca filter may be used as an efficient means to search for CN-weak and CN-strong stars in systems with otherwise small Ca abundance variations.

The presence of multiple populations is now well-established in most globular clusters in the Milky Way. In light of this progress, here we suggest a new model explaining the origin of the Sandage period-shift and the difference in mean period of type ab RR Lyrae variables between the two Oosterhoff groups. In our models, the instability strip in the metal-poor group II clusters, such as M15, is populated by second generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the relatively metal rich group I clusters like M3 are mostly produced by first generation stars (G1) without these enhancements. This population shift within the instability strip with metallicity can create the observed period-shift between the two groups, since both helium and CNO abundances play a role in increasing the period of RR Lyrae variables. The presence of more metal-rich clusters having Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most metal-rich clusters having RR Lyraes with longest periods (group III) can also be reproduced, as more helium-rich third and later generations of stars (G3) penetrate into the instability strip with further increase in metallicity. Therefore, for the most general cases, our models predict that the RR Lyraes are produced mostly by G1, G2, and G3, respectively, for the Oosterhoff groups I, II, and III., Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letters

Recent spectroscopic observations of the outer halo globular cluster (GC) NGC 2419 show that it is unique among GCs, in terms of chemical abundance patterns, and some suggest that it was originated in the nucleus of a dwarf galaxy. Here we show, from the Subaru narrow-band photometry employing a calcium filter, that the red giant-branch (RGB) of this GC is split into two distinct subpopulations. Comparison with spectroscopy has confirmed that the redder RGB stars in the $hk$[=(Ca$-b)-(b-y)$] index are enhanced in [Ca/H] by $\sim$0.2 dex compared to the bluer RGB stars. Our population model further indicates that the calcium-rich second generation stars are also enhanced in helium abundance by a large amount ($\Delta$Y = 0.19). Our photometry, together with the results for other massive GCs (e.g., $\omega$ Cen, M22, and NGC 1851), suggests that the discrete distribution of RGB stars in the $hk$ index might be a universal characteristic of this growing group of peculiar GCs. The planned narrow-band calcium photometry for the Local Group dwarf galaxies would help to establish an empirical connection between these GCs and the primordial building blocks in the hierarchical merging paradigm of galaxy formation., Comment: 4 pages, 4 figures, 1 table, accepted for the publication in ApJL

Context. Stellar collision rates in globular clusters (GCs) do not appear to correlate with horizontal branch (HB) morphology, sug- gesting that dynamics does not play a role in the second-parameter problem. However, core densities and collision rates derived from surface-brightness may be significantly underestimated as the surface-brightness profile of GCs is not necessarily a good indicator of the dynamical state of GC cores. Core-collapse may go unnoticed if high central densities of dark remnants are present. Aims. We test whether GC HB morphology data supports a dynamical contribution to the so-called second-parameter effect. Methods. To remove first-parameter dependence we fitted the maximum effective temperature along the HB as a function of metal- licity in a sample of 54 Milky Way GCs. We plotted the residuals to the fit as a function of second-parameter candidates, namely dynamical age and total luminosity. We considered dynamical age (i.e. the ratio between age and half-light relaxation time) among possible second-parameters. We used a set of direct N-body simulations, including ones with dark remnants to illustrate how core density peaks, due to core collapse, in a dynamical-age range similar to that in which blue HBs are overabundant with respect to the metallicity expectation, especially for low-concentration initial conditions. Results. GC total luminosity shows nonlinear behavior compatible with the self-enrichment picture. However, the data are amenable to a different interpretation based on a dynamical origin of the second-parameter effect. Enhanced mass-stripping in the late red-giant- branch phase due to stellar interactions in collapsing cores is a viable candidate mechanism. In this picture, GCs with HBs bluer than expected based on metallicity are those undergoing core-collapse., Comment: 7 pages, 7 figures, A&A accepted

The mean color of globular clusters (GCs) in early-type galaxies is in general bluer than the integrated color of halo field stars in host galaxies. Metal-rich GCs often appear more associated with field stars than metal-poor GCs, yet show bluer colors than their host galaxy light. Motivated by the discovery of multiple stellar populations in Milky Way GCs, we present a new scenario in which the presence of second-generation (SG) stars in GCs is responsible for the color discrepancy between metal-rich GCs and field stars. The model assumes that the SG populations have enhanced helium abundance as evidenced by observations, and it gives a good explanation of the bluer optical colors of metal-rich GCs than field stars as well as strong Balmer lines and blue UV colors of metal-rich GCs. Ours may be complementary to the recent scenario suggesting the difference in stellar mass functions (MFs) as an origin for the GC-to-star color offset. A quantitative comparison is given between the SG and MF models., Comment: 12 pages, 3 figures, 1 table, accepted for publication in the ApJL

Recent spectroscopy on the globular cluster (GC) system of M31 with unprecedented precision witnessed a clear bimodality in absorption-line index distributions of old GCs. Such division of extragalactic GCs, so far asserted mainly by photometric color bimodality, has been viewed as the presence of merely two distinct metallicity subgroups within individual galaxies and forms a critical backbone of various galaxy formation theories. Given that spectroscopy is a more detailed probe into stellar population than photometry, the discovery of index bimodality may point to the very existence of dual GC populations. However, here we show that the observed spectroscopic dichotomy of M31 GCs emerges due to the nonlinear nature of metallicity-to-index conversion and thus one does not necessarily have to invoke two separate GC subsystems. We take this as a close analogy to the recent view that metallicity-color nonlinearity is primarily responsible for observed GC color bimodality. We also demonstrate that the metallicity-sensitive magnesium line displays non-negligible metallicity-index nonlinearity and Balmer lines show rather strong nonlinearity. This gives rise to bimodal index distributions, which are routinely interpreted as bimodal metallicity distributions, not considering metallicity-index nonlinearity. Our findings give a new insight into the constitution of M31's GC system, which could change much of the current thought on the formation of GC systems and their host galaxies., Comment: Accepted for publication in ApJ, 34 pages, 6 figures, 8 tables