Monthly Notices of the Royal Astronomical Society, 2022
Stellar evolution calculations with variable abundance ratios were used to gauge the effects on t... more Stellar evolution calculations with variable abundance ratios were used to gauge the effects on temperatures, luminosities, and lifetimes in various phases. The individual elements C, N, O, Mg, Si, and Fe were included. Most of the effect relevant to integrated light models is contained in the temperature variable, as opposed to the time-scale or luminosity. We derive a recipe for including abundance-sensitive temperature effects that is applicable to existing isochrone grids. The resultant enhanced isochrones are incorporated into composite stellar population models and compared with galaxy data from the Sloan Digital Sky Survey. A severe oxygen–age degeneracy is apparent, 2–3 Gyr per 0.1 dex in [O/R], where R represents a heavy element such as Fe. Over the range of early-type galaxy velocity dispersion, the spans of all abundance ratios are reduced but the age range increases and becomes systematically older. Allowing Fe-peak elements the freedom to vary accentuates this increase ...
The strength of the Hbeta line for integrated globular cluster spectra has been computed from the... more The strength of the Hbeta line for integrated globular cluster spectra has been computed from the evolutionary population synthesis models. These models include, for the first time, the systematic variations of horizontal-branch (HB) morphology with metallicity and age. Our models suggest that the strength of the Hbeta line is significantly affected by the presence of blue HB stars. The Hbeta line strength does not monotonically decrease as metallicity increases, instead it reaches maximum when the distribution of HB stars is centered around 9500 K, the temperature where the Hbeta line is most prominent. Our models indicate that the strength of the Hbeta line increases as much as 0.7 { Angstroms} due to the presence of HB stars. We note that the relations between Hbeta line index and Mg line indices of the globular clusters in M87 and in NGC 1399 are systematically shifted to higher metallicity compared to the Galactic counterparts. Our models suggest that this systematic difference...
The strength of the Hβ index has been computed for the integrated spectra of model globular clust... more The strength of the Hβ index has been computed for the integrated spectra of model globular clusters from the evolutionary population synthesis. These models take into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that the Hβ index is significantly affected by the presence of blue HB stars. Because of the contribution from blue HB stars, the Hβ does not monotonically decrease as metallicity increases at a given age. Instead, it reaches a maximum strength when the distribution of HB stars is centered around 9500 K, the temperature where the Hβ index becomes strongest. Our models indicate that the strength of the Hβ index increases as much as 0.75Å due to the presence of blue HB stars. The comparison of the recent Keck observations of the globular cluster system in the Milky Way Galaxy with those in giant elliptical galaxies, NGC 1399 and M87, shows a systematic shift in the Hβ against metallicity plane. Our models suggest that this systematic difference is understood if the globular cluster systems in giant elliptical galaxies are several billion years older, in the mean, than the Galactic counterpart. Further observations of globular cluster systems in the external galaxies from the large ground-based telescopes and space UV facilities will enable to clarify whether this difference is indeed due to the age difference or other explanations are also possible.
A collection of Johnson/Cousins photometry for stars with known [Fe/H] is used to generate color-... more A collection of Johnson/Cousins photometry for stars with known [Fe/H] is used to generate color-color relations that include the abundance dependence. Literature temperature and bolometric correction dependences are attached to the color relations. The JHK colors are transformed to the Bessell & Brett (1988) homogenized system. The main result of this work is the tabulation of seven colors and the V-band bolometric correction as a function of T ef f , log g, and [Fe/H] for −1.06 < V − K < 10.2 and an accompanying interpolation program. Improvements to the present calibration would involve filling photometry gaps, obtaining more accurate and on-system photometry, knowing better log g and [Fe/H] values, improving the statistics for data-impoverished groups of stars such as K dwarfs, applying small tweaks in the processing pipeline, and obtaining better empirical temperature and bolometric correction relations, especially for supergiants and M stars. A way to estimate dust extinction from M dwarf colors is pointed out.
The tip of the red giant branch is one of the widely used distance measurement methods for the re... more The tip of the red giant branch is one of the widely used distance measurement methods for the relatively nearby galaxies where the bright individual stars are resolved. Most of the earlier works have used the Photometry in the I-band such as HST ACS/WFC F814W or F850LP for the RGB tip method. Here we look into the RGB tip magnitudes in J-band such as HST WFC3/IR F110W and JWST NIRCam/WF F115W for a wide range of age and metallicity. We find that the J-band (HST WFC3/IR F110W and JWST NIRCam/WF F115W) RGB tip magnitudes do not significantly change compared to that of the I-band (HST ACS/WFC F814W, F850LP) for a wide range of metallicity at given age. Moreover, HST WFC3/IR F110W RGB tip magnitudes stay constant within 0.005 magnitudes for stellar populations with old ages (age > 5 Gyr) at given metallicity.
We present simple stellar population (SSP) models with scaled-solar and αelement enhanced abundan... more We present simple stellar population (SSP) models with scaled-solar and αelement enhanced abundances. The SSP models are based on the Dartmouth Stellar Evolution Database, our library of synthetic stellar spectra, and a detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. In order to test the relative importance of a variety of SSP model ingredients, we compare our SSP models with integrated spectra of 41 Milky Way Globular Clusters (MWGCs) from Schiavon et al. (2005). Using the Mg b and Ca4227 indices, we confirm that Mg and Ca are enhanced by about +0.4 and +0.2 dex, respectively, in agreement with results from high resolution spectra of individual stars in MWGCs. Balmer lines, particularly Hγ and Hδ, of MWGCs are reproduced by our α-enhanced SSP models not only because of the combination of isochrone and spectral effects but also because of our reasonable HB treatment. Moreover, it is shown that the Mg abundance significantly influences Balmer and iron line indices. Finally, the investigation of power-law initial mass function (IMF) variations suggests that an IMF much shallower than Salpeter is unrealistic because the Balmer lines are too strong on the metal-poor side to be compatible with observations.
Publications of the Astronomical Society of Australia, 2003
The primary present-day observables upon which theories of galaxy evolution are based are a syste... more The primary present-day observables upon which theories of galaxy evolution are based are a system’s morphology, dynamics, colour, and chemistry. Individually, each provides an important constraint to any given model; in concert, the four represent a fundamental (intractable) boundary condition for chemodynamical simulations. We review the current state-of-the-art semi-analytical and chemodynamical models for the Milky Way, emphasising the strengths and weaknesses of both approaches.
Publications of the Astronomical Society of Australia, 2004
Using stellar population synthesis techniques, we explore the photometric signatures of white dwa... more Using stellar population synthesis techniques, we explore the photometric signatures of white dwarf progenitor dominated galactic halos, in order to constrain the fraction of halo mass that may be locked up in white dwarf stellar remnants. We first construct a 109M⊙stellar halo using the canonical Salpeter initial stellar mass distribution, and then allow for an additional component of low- and intermediate-mass stars, which ultimately give rise to white dwarf remnants. Microlensing observations towards the Large Magellanic Cloud, coupled with several ground-based proper motion surveys, have led to claims that in excess of 20% of the dynamical mass of the halo (1012M⊙) might be found in white dwarfs. Our results indicate that (1) even if only 1% of the dynamical mass of the dark halo today could be attributed to white dwarfs, their main sequence progenitors at high redshift (z≈ 3) would have resulted in halos more than 100 times more luminous than those expected from conventional in...
We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance me... more We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance method for the F110W (J 110 ) and F160W (H 160 ) bandpasses of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the Hubble Space Telescope. The calibrations are based on data for 16 early-type galaxies in the Virgo and Fornax clusters observed with WFC3/IR and are provided as functions of both the optical (g 475 -z 850 ) and nearinfrared (J 110 -H 160 ) colors. The scatter about the linear calibration relations for the luminous red galaxies in the sample is approximately 0.10 mag, corresponding to a statistical error of 5% in distance. Our results imply that the distance to any suitably bright elliptical galaxy can be measured with this precision out to about 80 Mpc in a single-orbit observation with WFC3/IR, making this a remarkably powerful instrument for extragalactic distances. The calibration sample also includes much bluer and lower-luminosity galaxies than previously used for IR SBF studies, revealing interesting population differences that cause the calibration scatter to increase for dwarf galaxies. Comparisons with single-burst population models show that, as expected, the redder early-type galaxies contain old, metal-rich populations, while the bluer dwarf ellipticals contain a wider range of ages and lower metallicities than their more massive counterparts. Radial SBF gradients reveal that IR color gradients are largely an age effect; the bluer dwarfs typically have their youngest populations near their centers, while the redder giant ellipticals show only weak trends and in the opposite sense. Because of the population variations among bluer galaxies, distance measurements in the near-IR are best limited to red early-type galaxies. We conclude with some practical guidelines for using WFC3/IR to measure reliable SBF distances.
Monthly Notices of the Royal Astronomical Society, 2005
This work continues our efforts to calibrate model surface brightness fluctuation luminosities fo... more This work continues our efforts to calibrate model surface brightness fluctuation luminosities for the study of unresolved stellar populations, through the comparison with data of Magellanic Cloud star clusters. We present here the relation between absolute K s -band fluctuation magnitude and (V -I) integrated colour, using data from the 2MASS and DENIS surveys, and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by . We find that intermediate-age to old star clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the M Ks vs. (V -I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V -I c ) 0 < 1.25, ours holds in the interval -5 M Ks -9, 0.3 (V -I) 1.25. This implies, according to and Mouhcine & Lançon (2003) models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, a slight offset between the galaxies and the star clusters [the latter are ∼ 0.7 mag fainter than the former at a given value of (V -I)], caused by the difference in metallicity of roughly a factor of two, confirms that the M Ks vs. (V -I) plane may contribute to break the age-metallicity degeneracy in intermediate-age and old stellar populations. The confrontation between models and galaxy data also suggests that galaxies with K s fluctuation magnitudes that are brighter than predicted, given their (V -I) colour, might be explained in part by longer lifetimes of TP-AGB stars. A preliminary comparison between the H 2MASS data of the Magellanic star clusters and the sample of 47 early-type galaxies and spiral bulges observed by through the F 160W HST filter leads to the same basic conclusions: galaxies and star clusters lie along correlations with the same slope, and there is a slight offset between the star cluster sample and the galaxies, caused by their different metallicities. Magellanic star clusters are single populations, while galaxies are composite stellar systems; moreover, the objects analised live in different environments. Therefore, our findings mean that the relationship between fluctuation magnitudes in the near-IR, and (V -I) might be a fairly robust tool for the study of stellar population ages and metallicities, could provide additional constraints on star formation histories, and aid in the calibration of near-IR SBFs for cosmological distance measurements.
We have revisited the ACS Virgo Cluster Survey (ACSVCS), a Hubble Space Telescope program to obta... more We have revisited the ACS Virgo Cluster Survey (ACSVCS), a Hubble Space Telescope program to obtain ACS/WFC g and z bands imaging for a sample of 100 early-type galaxies in the Virgo Cluster. In this study, we examine 51 nucleated early-type galaxies in the ACSVCS in order to look into the relationship between the photometric and structural properties of stellar nuclei and their host galaxies. We morphologically dissect galaxies into five classes. We note that (1) the stellar nuclei of dwarf early-type galaxies (dS0, dE, and dE,N) are generally fainter and bluer with g> 18.95 and (g − z) < 1.40 compared to some brighter and redder counterparts of the ellipticals (E) and lenticular galaxies (S0), (2) the g-band half-light radii of stellar nuclei of all dwarf early-type galaxies (dS0, dE, and dE,N) are smaller than 20 pc and their average is about 4 pc, and (3) the colors of red stellar nuclei with (g − z)> 1.40 in bright ellipticals and lenticular galaxies are bluer than the...
ABSTRACT We have measured the near-IR SBF with HST WFC3/IR Channel (F110W and F160W) for 16 early... more ABSTRACT We have measured the near-IR SBF with HST WFC3/IR Channel (F110W and F160W) for 16 early-type galaxies in Virgo and Fornax clusters. The measurements are compared with theoretical integrated photometric models based upon Padova and Teramo isochrones, respectively. Two theoretical models are significantly different in the sense that the models incorporating the Padova isochrones are more than 1 magnitude fainter in F160W SBF than that employing the Teramo isochrones at the metal poor side, [Fe/H] &lt; -0.7. We investigate and present the major differences between the two isochrones that result in the big disparities in the integrated photometric models, particularly F160W SBF magnitudes.
The tip of the red giant branch is one of the widely used distance measurement methods for the re... more The tip of the red giant branch is one of the widely used distance measurement methods for the relatively nearby galaxies where the bright individual stars are resolved (e.g., Lee, Freedman, & Madore 1993). Most of the earlier works have used the photometry in the I-band such as HST ACS/WFC F814W or F850LP for the RGB tip method (e.g., McQuinn et al. 2017; Lee & Jang 2017). Here we look into the RGB tip magnitudes in J-band such as HST WFC3/IR F110W and JWST NIRCam/WF F115W for a wide range of age and metallicity. Figure 1 depicts that the J-band (HST WFC3/IR F110W and JWST NIRCam/WF F115W) RGB tip magnitudes in the lower panel do not significantly change compared to that of the I-band (HST ACS/WFC F814W, F850LP) in the upper panel for a wide range of metallicity at 13 Gyr. We have employed the MIST isochrones (Choi et al. 2016) in this work. We, however, find that the latest PARSEC isochrones (Marigo et al. 2017) show the similar trend. Moreover, HST WFC3/IR F110W RGB tip magnitudes stay constant within 0.005 magnitudes for stellar populations with old ages (age > 5 Gyr) at given metallicity (Figures 2 and 3). Once successfully calibrated, the J-band RGBtip magnitudes would serve a better way to measure the distance to the nearby galaxies compared to the traditional I-band RGBtip magnitudes that demonstrate some significant color-dependency (e.g., Figure 15 of Jang & Lee 2017). It may further ameliorate the disagreement of Hubble constant measurement between the CMB observations and the galactic distances (Freedman 2017). Moreover, the theoretical study of the color transformation and the constitutional physics on the tip of the red giant branch luminosity (e.g., Serenelli et al. 2017) will enhance the usefulness of the J-band RGBtip magnitudes method for the robust distance measurements of the galaxies.
The H_beta and some metal line indices, such as Mg2, Fe52, Fe53 of single-age and single-metallic... more The H_beta and some metal line indices, such as Mg2, Fe52, Fe53 of single-age and single-metallicity populations are computed based on the method of evolutionary population synthesis, with careful consideration of the variation of the horizontal-branch morphology with metallicity and age. We find (a) that while metal lines are little affected, the H_beta index is severely enhanced (up to 30%) by the presence of the blue horizontal-branch stars, frustrating the current age-estimations from this index without careful consideration of these stars, and (b) that there is a systematic trend in the sense that the globular clusters in giant elliptical galaxies appear to be older than those in our Galaxy by several billion years. We also calculate these indices for the stellar populations with a metallicity spread, by adopting metallicity distribution functions predicted by chemical evolution models. The comparison of the models with the observed indices of the central regions of the early-t...
Publications of the Astronomical Society of the Pacific, 2001
Theoretical integrated spectrophotometric quantities, such as spectral line indices in the Lick s... more Theoretical integrated spectrophotometric quantities, such as spectral line indices in the Lick system (Hβ, Mg 2 , Mg b, Fe5270, Fe5335), broadband color indices (in the UBV RIHK and Washington CMT 1 bandpasses), and surface brightness fluctuation (SBF) magnitudes and colors, ranging from far-UV to near-IR have been consistently computed for simple stellar populations from our evolutionary population synthesis code. The age range of −
Publications of the Astronomical Society of Australia, 2004
Using stellar population synthesis techniques, we explore the photometric signatures of white dwa... more Using stellar population synthesis techniques, we explore the photometric signatures of white dwarf progenitor dominated galactic halos, in order to constrain the fraction of halo mass that may be locked-up in white dwarf stellar remnants. We first construct a 10 9 M⊙ stellar halo using the canonical Salpeter initial stellar mass distribution, and then allow for an additional component of low-and intermediate-mass stars, which ultimately give rise to white dwarf remnants. Microlensing observations towards the Large Magellanic Cloud, coupled with several ground-based proper motion surveys, have led to claims that in excess of 20% of the dynamical mass of the halo (10 12 M⊙) might be found in white dwarfs. Our results indicate that (1) even if only 1% of the dynamical mass of the dark halo today could be attributed to white dwarfs, their main sequence progenitors at high redshift (z≈3) would have resulted in halos more than 100 times more luminous than those expected from conventional initial mass functions alone, and (2) any putative halo white dwarf progenitor dominated initial mass function component, regardless of its dynamical importance, would be virtually impossible to detect at the present-day, due to its extremely faint surface brightness.
Proceedings of The International Astronomical Union, 2002
We present our recent revision of model constructions for the horizontal-branch (HB) morphology o... more We present our recent revision of model constructions for the horizontal-branch (HB) morphology of globular clusters, which suggests the HB morphology is more sensitive to age compared to our earlier models. We also present our high precision CCD photometry for the classic second parameter pair M3 and M13. The relative age dating based on this photometry indicates that M13 is
Monthly Notices of the Royal Astronomical Society, 2004
The rotation curves of low surface brightness (LSB) galaxies suggest that they possess significan... more The rotation curves of low surface brightness (LSB) galaxies suggest that they possess significantly higher mass-to-light (M/L) ratios than their high surface brightness counterparts, indicating that LSB galaxies may be dark matter dominated. This interpretation is hampered by the difficulty of disentangling the disc and dark halo contributions from the disc dynamics of LSB galaxies. Recently, Fuchs has attempted such a disentanglement using spiral arm density wave and swing amplification theory, allowing an independent measurement of the disc mass; this work suggests that LSB discs are significantly more massive than previously believed. This would considerably reduce the amount of matter required in the dark haloes in fitting the rotation curves. Interestingly, the high mass-to-light ratios derived for the discs appear inconsistent with standard stellar population synthesis models.
We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance me... more We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance method for the F110W (J 110 ) and F160W (H 160 ) bandpasses of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the Hubble Space Telescope. The calibrations are based on data for 16 early-type galaxies in the Virgo and Fornax clusters observed with WFC3/IR and are provided as functions of both the optical (g 475 −z 850 ) and nearinfrared (J 110 −H 160 ) colors. The scatter about the linear calibration relations for the luminous red galaxies in the sample is approximately 0.10 mag, corresponding to a statistical error of 5% in distance. Our results imply that the distance to any suitably bright elliptical galaxy can be measured with this precision out to about 80 Mpc in a single-orbit observation with WFC3/IR, making this a remarkably powerful instrument for extragalactic distances. The calibration sample also includes much bluer and lower-luminosity galaxies than previously used for IR SBF studies, revealing interesting population differences that cause the calibration scatter to increase for dwarf galaxies. Comparisons with single-burst population models show that, as expected, the redder early-type galaxies contain old, metal-rich populations, while the bluer dwarf ellipticals contain a wider range of ages and lower metallicities than their more massive counterparts. Radial SBF gradients reveal that IR color gradients are largely an age effect; the bluer dwarfs typically have their youngest populations near their centers, while the redder giant ellipticals show only weak trends and in the opposite sense. Because of the population variations among bluer galaxies, distance measurements in the near-IR are best limited to red early-type galaxies. We conclude with some practical guidelines for using WFC3/IR to measure reliable SBF distances.
Monthly Notices of the Royal Astronomical Society, 2022
Stellar evolution calculations with variable abundance ratios were used to gauge the effects on t... more Stellar evolution calculations with variable abundance ratios were used to gauge the effects on temperatures, luminosities, and lifetimes in various phases. The individual elements C, N, O, Mg, Si, and Fe were included. Most of the effect relevant to integrated light models is contained in the temperature variable, as opposed to the time-scale or luminosity. We derive a recipe for including abundance-sensitive temperature effects that is applicable to existing isochrone grids. The resultant enhanced isochrones are incorporated into composite stellar population models and compared with galaxy data from the Sloan Digital Sky Survey. A severe oxygen–age degeneracy is apparent, 2–3 Gyr per 0.1 dex in [O/R], where R represents a heavy element such as Fe. Over the range of early-type galaxy velocity dispersion, the spans of all abundance ratios are reduced but the age range increases and becomes systematically older. Allowing Fe-peak elements the freedom to vary accentuates this increase ...
The strength of the Hbeta line for integrated globular cluster spectra has been computed from the... more The strength of the Hbeta line for integrated globular cluster spectra has been computed from the evolutionary population synthesis models. These models include, for the first time, the systematic variations of horizontal-branch (HB) morphology with metallicity and age. Our models suggest that the strength of the Hbeta line is significantly affected by the presence of blue HB stars. The Hbeta line strength does not monotonically decrease as metallicity increases, instead it reaches maximum when the distribution of HB stars is centered around 9500 K, the temperature where the Hbeta line is most prominent. Our models indicate that the strength of the Hbeta line increases as much as 0.7 { Angstroms} due to the presence of HB stars. We note that the relations between Hbeta line index and Mg line indices of the globular clusters in M87 and in NGC 1399 are systematically shifted to higher metallicity compared to the Galactic counterparts. Our models suggest that this systematic difference...
The strength of the Hβ index has been computed for the integrated spectra of model globular clust... more The strength of the Hβ index has been computed for the integrated spectra of model globular clusters from the evolutionary population synthesis. These models take into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that the Hβ index is significantly affected by the presence of blue HB stars. Because of the contribution from blue HB stars, the Hβ does not monotonically decrease as metallicity increases at a given age. Instead, it reaches a maximum strength when the distribution of HB stars is centered around 9500 K, the temperature where the Hβ index becomes strongest. Our models indicate that the strength of the Hβ index increases as much as 0.75Å due to the presence of blue HB stars. The comparison of the recent Keck observations of the globular cluster system in the Milky Way Galaxy with those in giant elliptical galaxies, NGC 1399 and M87, shows a systematic shift in the Hβ against metallicity plane. Our models suggest that this systematic difference is understood if the globular cluster systems in giant elliptical galaxies are several billion years older, in the mean, than the Galactic counterpart. Further observations of globular cluster systems in the external galaxies from the large ground-based telescopes and space UV facilities will enable to clarify whether this difference is indeed due to the age difference or other explanations are also possible.
A collection of Johnson/Cousins photometry for stars with known [Fe/H] is used to generate color-... more A collection of Johnson/Cousins photometry for stars with known [Fe/H] is used to generate color-color relations that include the abundance dependence. Literature temperature and bolometric correction dependences are attached to the color relations. The JHK colors are transformed to the Bessell & Brett (1988) homogenized system. The main result of this work is the tabulation of seven colors and the V-band bolometric correction as a function of T ef f , log g, and [Fe/H] for −1.06 < V − K < 10.2 and an accompanying interpolation program. Improvements to the present calibration would involve filling photometry gaps, obtaining more accurate and on-system photometry, knowing better log g and [Fe/H] values, improving the statistics for data-impoverished groups of stars such as K dwarfs, applying small tweaks in the processing pipeline, and obtaining better empirical temperature and bolometric correction relations, especially for supergiants and M stars. A way to estimate dust extinction from M dwarf colors is pointed out.
The tip of the red giant branch is one of the widely used distance measurement methods for the re... more The tip of the red giant branch is one of the widely used distance measurement methods for the relatively nearby galaxies where the bright individual stars are resolved. Most of the earlier works have used the Photometry in the I-band such as HST ACS/WFC F814W or F850LP for the RGB tip method. Here we look into the RGB tip magnitudes in J-band such as HST WFC3/IR F110W and JWST NIRCam/WF F115W for a wide range of age and metallicity. We find that the J-band (HST WFC3/IR F110W and JWST NIRCam/WF F115W) RGB tip magnitudes do not significantly change compared to that of the I-band (HST ACS/WFC F814W, F850LP) for a wide range of metallicity at given age. Moreover, HST WFC3/IR F110W RGB tip magnitudes stay constant within 0.005 magnitudes for stellar populations with old ages (age > 5 Gyr) at given metallicity.
We present simple stellar population (SSP) models with scaled-solar and αelement enhanced abundan... more We present simple stellar population (SSP) models with scaled-solar and αelement enhanced abundances. The SSP models are based on the Dartmouth Stellar Evolution Database, our library of synthetic stellar spectra, and a detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. In order to test the relative importance of a variety of SSP model ingredients, we compare our SSP models with integrated spectra of 41 Milky Way Globular Clusters (MWGCs) from Schiavon et al. (2005). Using the Mg b and Ca4227 indices, we confirm that Mg and Ca are enhanced by about +0.4 and +0.2 dex, respectively, in agreement with results from high resolution spectra of individual stars in MWGCs. Balmer lines, particularly Hγ and Hδ, of MWGCs are reproduced by our α-enhanced SSP models not only because of the combination of isochrone and spectral effects but also because of our reasonable HB treatment. Moreover, it is shown that the Mg abundance significantly influences Balmer and iron line indices. Finally, the investigation of power-law initial mass function (IMF) variations suggests that an IMF much shallower than Salpeter is unrealistic because the Balmer lines are too strong on the metal-poor side to be compatible with observations.
Publications of the Astronomical Society of Australia, 2003
The primary present-day observables upon which theories of galaxy evolution are based are a syste... more The primary present-day observables upon which theories of galaxy evolution are based are a system’s morphology, dynamics, colour, and chemistry. Individually, each provides an important constraint to any given model; in concert, the four represent a fundamental (intractable) boundary condition for chemodynamical simulations. We review the current state-of-the-art semi-analytical and chemodynamical models for the Milky Way, emphasising the strengths and weaknesses of both approaches.
Publications of the Astronomical Society of Australia, 2004
Using stellar population synthesis techniques, we explore the photometric signatures of white dwa... more Using stellar population synthesis techniques, we explore the photometric signatures of white dwarf progenitor dominated galactic halos, in order to constrain the fraction of halo mass that may be locked up in white dwarf stellar remnants. We first construct a 109M⊙stellar halo using the canonical Salpeter initial stellar mass distribution, and then allow for an additional component of low- and intermediate-mass stars, which ultimately give rise to white dwarf remnants. Microlensing observations towards the Large Magellanic Cloud, coupled with several ground-based proper motion surveys, have led to claims that in excess of 20% of the dynamical mass of the halo (1012M⊙) might be found in white dwarfs. Our results indicate that (1) even if only 1% of the dynamical mass of the dark halo today could be attributed to white dwarfs, their main sequence progenitors at high redshift (z≈ 3) would have resulted in halos more than 100 times more luminous than those expected from conventional in...
We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance me... more We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance method for the F110W (J 110 ) and F160W (H 160 ) bandpasses of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the Hubble Space Telescope. The calibrations are based on data for 16 early-type galaxies in the Virgo and Fornax clusters observed with WFC3/IR and are provided as functions of both the optical (g 475 -z 850 ) and nearinfrared (J 110 -H 160 ) colors. The scatter about the linear calibration relations for the luminous red galaxies in the sample is approximately 0.10 mag, corresponding to a statistical error of 5% in distance. Our results imply that the distance to any suitably bright elliptical galaxy can be measured with this precision out to about 80 Mpc in a single-orbit observation with WFC3/IR, making this a remarkably powerful instrument for extragalactic distances. The calibration sample also includes much bluer and lower-luminosity galaxies than previously used for IR SBF studies, revealing interesting population differences that cause the calibration scatter to increase for dwarf galaxies. Comparisons with single-burst population models show that, as expected, the redder early-type galaxies contain old, metal-rich populations, while the bluer dwarf ellipticals contain a wider range of ages and lower metallicities than their more massive counterparts. Radial SBF gradients reveal that IR color gradients are largely an age effect; the bluer dwarfs typically have their youngest populations near their centers, while the redder giant ellipticals show only weak trends and in the opposite sense. Because of the population variations among bluer galaxies, distance measurements in the near-IR are best limited to red early-type galaxies. We conclude with some practical guidelines for using WFC3/IR to measure reliable SBF distances.
Monthly Notices of the Royal Astronomical Society, 2005
This work continues our efforts to calibrate model surface brightness fluctuation luminosities fo... more This work continues our efforts to calibrate model surface brightness fluctuation luminosities for the study of unresolved stellar populations, through the comparison with data of Magellanic Cloud star clusters. We present here the relation between absolute K s -band fluctuation magnitude and (V -I) integrated colour, using data from the 2MASS and DENIS surveys, and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by . We find that intermediate-age to old star clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the M Ks vs. (V -I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V -I c ) 0 < 1.25, ours holds in the interval -5 M Ks -9, 0.3 (V -I) 1.25. This implies, according to and Mouhcine & Lançon (2003) models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, a slight offset between the galaxies and the star clusters [the latter are ∼ 0.7 mag fainter than the former at a given value of (V -I)], caused by the difference in metallicity of roughly a factor of two, confirms that the M Ks vs. (V -I) plane may contribute to break the age-metallicity degeneracy in intermediate-age and old stellar populations. The confrontation between models and galaxy data also suggests that galaxies with K s fluctuation magnitudes that are brighter than predicted, given their (V -I) colour, might be explained in part by longer lifetimes of TP-AGB stars. A preliminary comparison between the H 2MASS data of the Magellanic star clusters and the sample of 47 early-type galaxies and spiral bulges observed by through the F 160W HST filter leads to the same basic conclusions: galaxies and star clusters lie along correlations with the same slope, and there is a slight offset between the star cluster sample and the galaxies, caused by their different metallicities. Magellanic star clusters are single populations, while galaxies are composite stellar systems; moreover, the objects analised live in different environments. Therefore, our findings mean that the relationship between fluctuation magnitudes in the near-IR, and (V -I) might be a fairly robust tool for the study of stellar population ages and metallicities, could provide additional constraints on star formation histories, and aid in the calibration of near-IR SBFs for cosmological distance measurements.
We have revisited the ACS Virgo Cluster Survey (ACSVCS), a Hubble Space Telescope program to obta... more We have revisited the ACS Virgo Cluster Survey (ACSVCS), a Hubble Space Telescope program to obtain ACS/WFC g and z bands imaging for a sample of 100 early-type galaxies in the Virgo Cluster. In this study, we examine 51 nucleated early-type galaxies in the ACSVCS in order to look into the relationship between the photometric and structural properties of stellar nuclei and their host galaxies. We morphologically dissect galaxies into five classes. We note that (1) the stellar nuclei of dwarf early-type galaxies (dS0, dE, and dE,N) are generally fainter and bluer with g> 18.95 and (g − z) < 1.40 compared to some brighter and redder counterparts of the ellipticals (E) and lenticular galaxies (S0), (2) the g-band half-light radii of stellar nuclei of all dwarf early-type galaxies (dS0, dE, and dE,N) are smaller than 20 pc and their average is about 4 pc, and (3) the colors of red stellar nuclei with (g − z)> 1.40 in bright ellipticals and lenticular galaxies are bluer than the...
ABSTRACT We have measured the near-IR SBF with HST WFC3/IR Channel (F110W and F160W) for 16 early... more ABSTRACT We have measured the near-IR SBF with HST WFC3/IR Channel (F110W and F160W) for 16 early-type galaxies in Virgo and Fornax clusters. The measurements are compared with theoretical integrated photometric models based upon Padova and Teramo isochrones, respectively. Two theoretical models are significantly different in the sense that the models incorporating the Padova isochrones are more than 1 magnitude fainter in F160W SBF than that employing the Teramo isochrones at the metal poor side, [Fe/H] &lt; -0.7. We investigate and present the major differences between the two isochrones that result in the big disparities in the integrated photometric models, particularly F160W SBF magnitudes.
The tip of the red giant branch is one of the widely used distance measurement methods for the re... more The tip of the red giant branch is one of the widely used distance measurement methods for the relatively nearby galaxies where the bright individual stars are resolved (e.g., Lee, Freedman, & Madore 1993). Most of the earlier works have used the photometry in the I-band such as HST ACS/WFC F814W or F850LP for the RGB tip method (e.g., McQuinn et al. 2017; Lee & Jang 2017). Here we look into the RGB tip magnitudes in J-band such as HST WFC3/IR F110W and JWST NIRCam/WF F115W for a wide range of age and metallicity. Figure 1 depicts that the J-band (HST WFC3/IR F110W and JWST NIRCam/WF F115W) RGB tip magnitudes in the lower panel do not significantly change compared to that of the I-band (HST ACS/WFC F814W, F850LP) in the upper panel for a wide range of metallicity at 13 Gyr. We have employed the MIST isochrones (Choi et al. 2016) in this work. We, however, find that the latest PARSEC isochrones (Marigo et al. 2017) show the similar trend. Moreover, HST WFC3/IR F110W RGB tip magnitudes stay constant within 0.005 magnitudes for stellar populations with old ages (age > 5 Gyr) at given metallicity (Figures 2 and 3). Once successfully calibrated, the J-band RGBtip magnitudes would serve a better way to measure the distance to the nearby galaxies compared to the traditional I-band RGBtip magnitudes that demonstrate some significant color-dependency (e.g., Figure 15 of Jang & Lee 2017). It may further ameliorate the disagreement of Hubble constant measurement between the CMB observations and the galactic distances (Freedman 2017). Moreover, the theoretical study of the color transformation and the constitutional physics on the tip of the red giant branch luminosity (e.g., Serenelli et al. 2017) will enhance the usefulness of the J-band RGBtip magnitudes method for the robust distance measurements of the galaxies.
The H_beta and some metal line indices, such as Mg2, Fe52, Fe53 of single-age and single-metallic... more The H_beta and some metal line indices, such as Mg2, Fe52, Fe53 of single-age and single-metallicity populations are computed based on the method of evolutionary population synthesis, with careful consideration of the variation of the horizontal-branch morphology with metallicity and age. We find (a) that while metal lines are little affected, the H_beta index is severely enhanced (up to 30%) by the presence of the blue horizontal-branch stars, frustrating the current age-estimations from this index without careful consideration of these stars, and (b) that there is a systematic trend in the sense that the globular clusters in giant elliptical galaxies appear to be older than those in our Galaxy by several billion years. We also calculate these indices for the stellar populations with a metallicity spread, by adopting metallicity distribution functions predicted by chemical evolution models. The comparison of the models with the observed indices of the central regions of the early-t...
Publications of the Astronomical Society of the Pacific, 2001
Theoretical integrated spectrophotometric quantities, such as spectral line indices in the Lick s... more Theoretical integrated spectrophotometric quantities, such as spectral line indices in the Lick system (Hβ, Mg 2 , Mg b, Fe5270, Fe5335), broadband color indices (in the UBV RIHK and Washington CMT 1 bandpasses), and surface brightness fluctuation (SBF) magnitudes and colors, ranging from far-UV to near-IR have been consistently computed for simple stellar populations from our evolutionary population synthesis code. The age range of −
Publications of the Astronomical Society of Australia, 2004
Using stellar population synthesis techniques, we explore the photometric signatures of white dwa... more Using stellar population synthesis techniques, we explore the photometric signatures of white dwarf progenitor dominated galactic halos, in order to constrain the fraction of halo mass that may be locked-up in white dwarf stellar remnants. We first construct a 10 9 M⊙ stellar halo using the canonical Salpeter initial stellar mass distribution, and then allow for an additional component of low-and intermediate-mass stars, which ultimately give rise to white dwarf remnants. Microlensing observations towards the Large Magellanic Cloud, coupled with several ground-based proper motion surveys, have led to claims that in excess of 20% of the dynamical mass of the halo (10 12 M⊙) might be found in white dwarfs. Our results indicate that (1) even if only 1% of the dynamical mass of the dark halo today could be attributed to white dwarfs, their main sequence progenitors at high redshift (z≈3) would have resulted in halos more than 100 times more luminous than those expected from conventional initial mass functions alone, and (2) any putative halo white dwarf progenitor dominated initial mass function component, regardless of its dynamical importance, would be virtually impossible to detect at the present-day, due to its extremely faint surface brightness.
Proceedings of The International Astronomical Union, 2002
We present our recent revision of model constructions for the horizontal-branch (HB) morphology o... more We present our recent revision of model constructions for the horizontal-branch (HB) morphology of globular clusters, which suggests the HB morphology is more sensitive to age compared to our earlier models. We also present our high precision CCD photometry for the classic second parameter pair M3 and M13. The relative age dating based on this photometry indicates that M13 is
Monthly Notices of the Royal Astronomical Society, 2004
The rotation curves of low surface brightness (LSB) galaxies suggest that they possess significan... more The rotation curves of low surface brightness (LSB) galaxies suggest that they possess significantly higher mass-to-light (M/L) ratios than their high surface brightness counterparts, indicating that LSB galaxies may be dark matter dominated. This interpretation is hampered by the difficulty of disentangling the disc and dark halo contributions from the disc dynamics of LSB galaxies. Recently, Fuchs has attempted such a disentanglement using spiral arm density wave and swing amplification theory, allowing an independent measurement of the disc mass; this work suggests that LSB discs are significantly more massive than previously believed. This would considerably reduce the amount of matter required in the dark haloes in fitting the rotation curves. Interestingly, the high mass-to-light ratios derived for the discs appear inconsistent with standard stellar population synthesis models.
We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance me... more We present new calibrations of the near-infrared surface brightness fluctuation (SBF) distance method for the F110W (J 110 ) and F160W (H 160 ) bandpasses of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the Hubble Space Telescope. The calibrations are based on data for 16 early-type galaxies in the Virgo and Fornax clusters observed with WFC3/IR and are provided as functions of both the optical (g 475 −z 850 ) and nearinfrared (J 110 −H 160 ) colors. The scatter about the linear calibration relations for the luminous red galaxies in the sample is approximately 0.10 mag, corresponding to a statistical error of 5% in distance. Our results imply that the distance to any suitably bright elliptical galaxy can be measured with this precision out to about 80 Mpc in a single-orbit observation with WFC3/IR, making this a remarkably powerful instrument for extragalactic distances. The calibration sample also includes much bluer and lower-luminosity galaxies than previously used for IR SBF studies, revealing interesting population differences that cause the calibration scatter to increase for dwarf galaxies. Comparisons with single-burst population models show that, as expected, the redder early-type galaxies contain old, metal-rich populations, while the bluer dwarf ellipticals contain a wider range of ages and lower metallicities than their more massive counterparts. Radial SBF gradients reveal that IR color gradients are largely an age effect; the bluer dwarfs typically have their youngest populations near their centers, while the redder giant ellipticals show only weak trends and in the opposite sense. Because of the population variations among bluer galaxies, distance measurements in the near-IR are best limited to red early-type galaxies. We conclude with some practical guidelines for using WFC3/IR to measure reliable SBF distances.
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Papers by Hyun-chul Lee