arXiv subjects

Kenji Bekki

Publications and source records attributed to Kenji Bekki.

At least 127 records · Page 7Linked to original sources

arXiv2014

Evolution of dust and molecular hydrogen in the Magellanic System

We investigate the evolution of the interstellar medium (ISM) in self-consistent, chemodynamical simulations of the Magellanic Clouds (MCs) during their recent (z<0.3) past. An explicit modelling of dust and molecular hydrogen lifecycles enables the comparison of our models against the observed properties of the ISM, including elemental depletion from the gas-phase. Combining this model with a tidal-dominated paradigm for the formation for the Magellanic Stream and Bridge, we reproduce the age-metallicity relations, long gas depletion timescales, and presently observed dust and molecular hydrogen masses of the MCs to within their respective uncertainties. We find that these models' enrichment depends sensitively on the processing of dust within the ISM and the dynamical influence of external tides/stellar bars. The ratio of characteristic dust destruction timescales in our SMC and LMC models, a governing parameter of our models' evolution, is consistent with estimates based on observed supernova (SN) rates. Our reference MC models tend to exhibit the disputed universal dust-to-metal ratio, which we argue stems from the adoption of high SNe II condensation efficiencies. Our models are the first to reproduce the one-tenth solar metallicity of the Stream/Leading Arm following tidal stripping of the SMC; the hypothesis that the LMC contributes a metal-rich filament to the Stream, as implied by recent kinematic and abundance analyses, is also appraised in this study.

astro-ph.GA↗

arXiv2014

The VMC Survey. XI. Radial Stellar Population Gradients in the Galactic Globular Cluster 47 Tucanae

We present a deep near-infrared color--magnitude diagram of the Galactic globular cluster 47 Tucanae, obtained with the Visible and Infrared Survey Telescope for Astronomy (VISTA) as part of the VISTA near-infrared Y, J, K_s survey of the Magellanic System (VMC). The cluster stars comprising both the subgiant and red-giant branches exhibit apparent, continuous variations in color--magnitude space as a function of radius. Subgiant-branch stars at larger radii are systematically brighter than their counterparts closer to the cluster core; similarly, red-giant-branch stars in the cluster's periphery are bluer than their more centrally located cousins. The observations can very well be described by adopting an age spread of ~0.5 Gyr as well as radial gradients in both the cluster's helium abundance (Y) and metallicity (Z), which change gradually from Y = 0.28, Z = 0.005 in the cluster core to Y = 0.25, Z = 0.003 in its periphery. We conclude that the cluster's inner regions host a significant fraction of second-generation stars, which decreases with increasing radius; the stellar population in the 47 Tuc periphery is well approximated by a simple stellar population.

astro-ph.SR↗

arXiv2014

Tidal-induced lopsidedness in Magellanic-type galaxies

We investigate the tidally-induced conversion of barred late-type spirals to Magellanic-type discs with numerical simulations, to establish how the lifetime of lopsidedness (asymmetry) varies with numerical parametrizations. Using a reference model based on observed properties of the Large Magellanic Cloud (LMC), we show that its off-centre bar, one-arm spiral and one-sided star formation can be induced by a brief tidal interaction. We thereafter perform a detailed parameter study, and find that at the dynamical mass of LMC-type discs ($\sim$10$^{\rm 10}$ \md{}), stellar lopsidedness (as quantified by the m$=$1 Fourier mode) and bar off-centredness can vary widely in amplitude, but are generally short-lived ($\sim$Gyr). Tidal interactions induce more persistent lopsidedness in lower mass galaxies (several Gyr), in particular those with large halo-to-disc mass ratios as implied by recent halo occupation models. We suggest that the tidal interactions play a larger role in the observed ubiquity of lopsidedness than the presently favoured gas accretion mechanism for lower mass galaxies. Other characteristics of Magellanic-type galaxies, such as one-sided star formation, are quantified and tend to arise more prominently in discs with later-type spiral structure (more halo dominated inner disc, weaker bars) following retrograde orbital encounters.

astro-ph.GA↗

arXiv2014

Choirs HI Galaxy Groups: The metallicity of dwarf galaxies

We present a recalibration of the luminosity-metallicity relation for gas-rich, star-forming dwarfs to magnitudes as faint as M$_R$ ~ -13. We use the Dopita et al. (2013) metallicity calibrations to calibrate the relation for all of the data in this analysis. In metallicity-luminosity space we find two sub-populations within a sample of high-confidence SDSS DR8 star-forming galaxies; 52% are metal-rich giants and 48% are metal-medium galaxies. Metal-rich dwarfs classified as tidal dwarf galaxy (TDG) candidates in the literature are typically of metallicity 12 + log(O/H) = 8.70 $\pm$ 0.05, while SDSS dwarfs fainter than M$_R$ = -16 have a mean metallicity of 12 + log(O/H) = 8.28 $\pm$ 0.10, regardless of their luminosity, indicating that there is an approximate floor to the metallicity of low luminosity galaxies. Our hydrodynamical simulations predict that TDGs should have metallicities elevated above the normal luminosity-metallicity relation. Metallicity can therefore be a useful diagnostic for identifying TDG candidate populations in the absence of tidal tails. At magnitudes brighter than M$_R$ ~ -16 our sample of 53 star-forming galaxies in 9 HI gas-rich groups is consistent with the normal relation defined by the SDSS sample. At fainter magnitudes there is an increase in dispersion in metallicity of our sample, suggestive of a wide range of HI content and environment. In our sample we identify three (16% of dwarfs) strong TDG candidates (12 + log(O/H) > 8.6), and four (21%) very metal poor dwarfs (12 + log(O/H) < 8.0), which are likely gas-rich dwarfs with recently ignited star formation.

astro-ph.GA↗

arXiv2013

Galactic star formation enhanced and quenched by ram pressure in groups and clusters

We investigate how ram pressure of intragroup and intracluster medium can influence the spatial and temporal variations of star formation (SF) of disk galaxies with halo masses (M_h) ranging from 10^10 M_sun to 10^12 M_sun (i.e., from dwarf irregular to Milky Way-type) in groups and clusters with 10^13 < M_h/M_sun < 10^15 by using numerical simulations with a new model for time-varying ram pressure. The long-term evolution of SF rates and H_alpha morphologies corresponding to the distributions of star-forming regions are particularly investigated for different model parameters. The principal results are as follows. Whether ram pressure can enhance or reduce SF depends on M_h of disk galaxies and inclination angles of gas disks with respect to their orbital directions for a given orbit and a given environment. For example, SF can be moderately enhanced in disk galaxies with M_h=10^12 M_sun at the pericenter passages in a cluster with M_h=10^14 M_sun whereas it can be completely shut down (`quenching') for low-mass disks with M_h=10^10 M_sun. Ram pressure can reduce the H_alpha-to-optical-disk-size ratios of disks and the revel of the reduction depends on M_h and orbits of disk galaxies for a given environment. Disk galaxies under strong ram pressure show characteristic H_alpha morphologies such as ring-like, one-sided, and crescent-like distributions.

astro-ph.CO↗

arXiv2013

Origin of a bottom-heavy stellar initial mass function in elliptical galaxies

We investigate the origin of a bottom-heavy stellar initial mass function (IMF) recently observed in elliptical galaxies by using chemical evolution models with a non-universal IMF. We adopt the Kroupa IMF with the three slopes (alpha_1, alpha_2, and alpha_3) dependent on metallicities ([Fe/H]) and densities (rho_g) of star-forming gas clouds and thereby search for the best IMF model that can reproduce (i) the observed steep IMF slope (alpha_2 ~ 3, i.e., bottom-heavy) for low stellar masses (m < 1 M_sun) and (ii) the correlation of alpha_2 with chemical properties of elliptical galaxies in a self-consistent manner. We find that if the IMF slope alpha_2 depends both on [Fe/H] and rho_g, then elliptical galaxies with higher [Mg/Fe] can have steeper alpha_2 (~3) in our models. We also find that the observed positive correlation of stellar mass-to-light ratios (M/L) with [Mg/Fe] in elliptical galaxies can be quantitatively reproduced in our models with alpha_2 = beta[Fe/H]}+gamma log rho_g, where beta ~ 0.5 and gamma ~ 2. We discuss whether the IMF slopes for low-mass (alpha_2) and high-mass stars (alpha_3) need to vary independently from each other to explain a number of IMF-related observational results self-consistently. We also briefly discuss why alpha_2 depends differently on [Fe/H] in dwarf and giant elliptical galaxies.

astro-ph.CO↗

arXiv2013

Simulating galaxy evolution with a non-universal stellar initial mass function

We consider that the stellar initial mass function (IMF) depends on physical properties of star-forming molecular clouds in galaxies and thereby investigate how such a non-universal IMF (NUIMF) influences galaxy evolution. We incorporate a NUIMF model into galaxy-scale chemodynamical simulations in order to investigate the differences in chemical and dynamical evolution of disk galaxies between the NUIMF and universal IMF (UIMF) models. In the adopted NUIMF model, the three slopes of the Kroupa IMF depend independently on densities and metallicities ([Fe/H]) of molecular gas clouds, and production rates of metals and dust from massive and AGB stars can vary according to the time evolution of the three IMF slopes. The preliminary results of the simulations are as follows. Star formation rates in actively star-forming disk galaxies can be significantly lower in the NUIMF model than in the UIMF model, and the differences between the two models can be more remarkable in galaxies with higher SFRs. Chemical enrichment can proceed more rapidly in the NUIMF model and [Mg/Fe] for a given metallicity is higher in the NUIMF model. The evolution of H2 fraction (f_H2) and dust-to-gas ratio (D) is more rapid in the NUIMF model so that the final f_H2 and $D$ can be higher in the NUIMF model. Formation of massive stellar clumps in gas-rich disks is more strongly suppressed owing to the stronger SN feedback effect in the NUIMF model. The radial density profiles of new stars within the central 1kpc are shallower in the NUIMF model.

astro-ph.CO↗

arXiv2013

Stripping of nitrogen-rich AGB ejecta from interacting dwarf irregular galaxies

Dwarf irregular galaxies (dIrrs) including the Magellanic Clouds in the local Universe, in many cases, exhibit an unusually low N/O abundance ratio (log N/O ~ -1.5) in H II regions as compared with the solar value (~-0.9). This ratio is broadly equivalent to the average level of extremely metal-poor stars in the Galactic halo, suggesting that N released from asymptotic giant branch (AGB) stars is missing in the present-day interstellar matter of these dIrrs. We find evidence for past tidal interactions in the properties of individual dIrrs exhibiting low N/O ratios, while a clear signature of interactions is unseen for dIrrs with high N/O ratios. Accordingly, we propose that the ejecta of massive AGB stars that correspond to a major production site of N can be stripped from dIrrs that have undergone a strong interaction with a luminous galaxy. The physical process of its stripping is made up of two stages: (i) the ejecta of massive AGB stars in a dIrr are first merged with those of the bursting prompt SNe Ia and pushed up together to the galaxy halo, and (ii) subsequently through tidal interactions with a luminous galaxy, these ejecta are stripped from a dwarf galaxy's potential well. Our new chemical evolution models with stripping of AGB ejecta succeed in reproducing the observed low N/O ratio. Furthermore, we perform N-body + hydrodynamical simulations to trace the fate of AGB ejecta inside a dIrr orbiting the Milky Way, and confirm that a tidal interaction is responsible for the efficient stripping of AGB ejecta from dIrrs.

astro-ph.GA↗

arXiv2013

Choirs, HI Galaxy Groups: Catalogue and Detection of Star-forming Dwarf Group Members

Hα observations centred on galaxies selected from the HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001) typically show one and sometimes two star-forming galaxies within the approximately 15-arcminute beam of the Parkes 64-m HI detections. In our Survey of Ionization in Neutral Gas Galaxies (SINGG, Meurer et al. 2006) we found fifteen cases of HIPASS sources containing four or more emission line galaxies (ELGs). We name these fields Choir groups. In the most extreme case we found a field with at least nine ELGs. In this paper we present a catalogue of Choir group members in the context of the wider SINGG sample. The dwarf galaxies in the Choir groups would not be individually detectable in HIPASS at the observed distances if they were isolated, but are detected in SINGG narrow-band imaging due to their membership of groups with sufficiently large total HI mass. The ELGs in these groups are similar to the wider SINGG sample in terms of size, Hα equivalent width, and surface brightness. Eight of these groups have two large spiral galaxies with several dwarf galaxies and may be thought of as morphological analogues of the Local Group. However, on average our groups are not significantly HI-deficient, suggesting that they are at an early stage of assembly, and more like the M81 group. The Choir groups are very compact at typically only 190 kpc in projected distance between the two brightest members. They are very similar to SINGG fields in terms of star formation efficiency (the ratio of star formation rate to HI mass; SFE), showing an increasing trend in SFE with stellar mass.

astro-ph.CO↗

arXiv2013

Coevolution of dust, gas, and stars in galaxies - I. Spatial distributions and scaling-relations of dust and molecular hydrogen

We investigate the time evolution of dust properties, molecular hydrogen (H_2) contents, and star formation histories in galaxies by using our original chemodynamical simulations. The simulations include the formation of dust in the stellar winds of supernovae (SNe) and asymptotic giant branch (AGB) stars, the growth and destruction processes of dust in the interstellar medium (ISM), the formation of polycyclic aromatic hydrocarbon (PAH) dust in carbon-rich AGB stars, the H_2 formation on dust grains, and the H_2 photo-dissociation due to far ultra-violet (FUV) light in a self-consistent manner. We focus mainly on disk galaxies with different total masses in this preliminary study. The principle results are as follows: The star formation histories of disk galaxies can be regulated by the time evolution of interstellar dust, mainly because the formation rates of H_2 can be controlled by dust properties. The observed correlation between dust-to-gas-ratios (D) and gas-phase oxygen abundances (A_O = 12+log(O/H)) can be reproduced reasonably well in the present models. The disks show negative radial gradients (i.e., larger in inner regions) of H_2 fraction (f_H2), PAH-to-dust mass ratio (f_PAH), D, and A_O and these gradients evolve with time. The surface-mass densities of dust (Sigma_dust) are correlated more strongly with the total surface gas densities than with those of H_2. Local gaseous regions with higher D are more likely to have higher f_H2 in individual disks and total H_2 masses correlate well with total dust masses. More massive disk galaxies are more likely to have higher D, f_PAH, and f_H2 and smaller dust-to-stellar mass ratios. We also compare between galactic star formation histories in the metallicity-dependent and dust-dependent star formation models and find no major differences.

astro-ph.CO↗

arXiv2013

Evolution of the high-mass end of the stellar initial mass functions in starburst galaxies

We investigate the time evolution and spatial variation of the stellar initial mass function (IMF) in star-forming disk galaxies by using chemodynamical simulations with an IMF model depending both on local densities and metallicities ([Fe/H]) of the interstellar medium (ISM). We find that the slope (alpha) of a power-law IMF (N(m) ~ m^-alpha) for stellar masses larger than 1M_sun evolves from the canonical Salpeter IMF (alpha ~ 2.35) to be moderately top-heavy one (alpha ~ 1.9) in the simulated disk galaxies with starbursts triggered by galaxy interaction. We also find that alpha in star-forming regions correlates with star formation rate densities (Sigma_SFR in units of M_sun yr^{-1} kpc^{-2}). Feedback effects of Type Ia and II supernovae are found to prevent IMFs from being too top-heavy (alpha < 1.5). The simulation predicts alpha ~ 0.23 log Sigma_SFR + 1.7 for log Sigma_SFR > -2 (i.e., more top-heavy in higher Sigma_SFR), which is reasonably consistent well with corresponding recent observational results. The present study also predicts that inner regions of starburst disk galaxies have smaller alpha thus are more top-heavy (d alpha/d R ~ 0.07 kpc^{-1} for R < 5 kpc). The predicted radial alpha gradient can be tested against future observational studies of the alpha variation in star-forming galaxies.

astro-ph.CO↗

arXiv2012

Chemical evolution of the Large Magellanic Cloud

We adopt a new chemical evolution model for the Large Magellanic Cloud (LMC) and thereby investigate its past star formation and chemical enrichment histories. The delay time distribution of type Ia supernovae recently revealed by type Ia supernova surveys is incorporated self-consistently into the new model. The principle results are summarized as follows. The present gas mass fraction and stellar metallicity as well as the higher [Ba/Fe] in metal-poor stars at [Fe/H]<-1.5 can be more self-consistently explained by models with steeper initial mass functions. The observed higher [Mg/Fe] (> 0.3) at [Fe/H] ~ -0.6 and higher [Ba/Fe] (>0.5) at [Fe/H] ~ -0.3 can be due to significantly enhanced star formation about 2 Gyr ago. The observed overall [Ca/Fe]-[Fe/H] relation and remarkably low [Ca/Fe] (<-0.2) at [Fe/H]>-0.6 are consistent with models with short-delay supernova Ia and with the more efficient loss of Ca possibly caused by an explosion mechanism of type II supernovae. Although the metallicity distribution functions do not show double peaks in the models with a starburst about 2 Gyr ago, they show characteristic double peaks in the models with double starbursts at ~200 Myr and ~2 Gyr ago. The observed apparent dip of [Fe/H] around ~1.5 Gyr ago in the age--metallicity relation can be reproduced by models in which a large amount (~10^9 M_{sun}) of metal-poor ([Fe/H]<-1) gas can be accreted onto the LMC.

astro-ph.CO↗

arXiv2012

Feedback effects of aspherical supernovae explosions on galaxies

We investigate how explosions of aspherical supernovae (A-SNe) can influence star formation histories and chemical evolution of dwarf galaxies by using a new chemodynamical model. We mainly present the numerical results of two comparative models so that the A-SN feedback effects on galaxies can be more clearly seen. SNe originating from stars with masses larger than 30M_sun are A-SNe in the "ASN" model whereas all SNe are spherical ones (S-SNe) in the "SSN" model. Each S-SN and A-SN are assumed to release feedback energy of 10^{51} erg and 10^{52} erg, respectively, and chemical yields and feedback energy of A-SN ejecta depend on angles between the axis of symmetry and the ejection directions. We find that star formation can become at least by a factor of ~3 lower in the ASN model in comparison with the SSN one owing to the more energetic feedback of A-SNe. As a result of this, chemical evolution can proceed very slowly in the ASN model. A-SN feedback effects can play a significant role in the formation of giant gaseous holes and energetic gaseous outflow and unique chemical abundances (e.g., high [Mg/Ca]). Based on these results, we provide a number of implications of the A-SN feedback effects on galaxy formation and evolution.

astro-ph.CO↗

arXiv2012

First Evidence of Globular Cluster Formation from the Ejecta of Prompt Type Ia Supernovae

Recent spectroscopic observations of globular clusters (GCs) in the Large Magellanic Cloud (LMC) have discovered that one of the intermediate-age GC, NGC 1718 with [Fe/H]=-0.7 has an extremely low [Mg/Fe] ratio of ~-0.9. We propose that NGC 1718 was formed from the ejecta of type Ia supernovae (SNe Ia) mixed with very metal-poor ([Fe/H] <-1.3) gas about ~ 2 Gyr ago. The proposed scenario is shown to be consistent with the observed abundances of Fe-group elements such as Cr, Mn, and Ni. In addition, compelling evidence for asymptotic giant branch stars playing a role in chemical enrichment during this GC formation is found. We suggest that the origin of the metal-poor gas is closely associated with the efficient gas-transfer from the outer gas disk of the Small Magellanic Cloud to the LMC disk. We anticipate that the outer part of the LMC disk contains field stars exhibiting significantly low [Mg/Fe] ratios, formed through the same process as NGC 1718.

astro-ph.GA↗

arXiv2012

The Tidal Origin of the Magellanic Stream and the Possibility of a Stellar Counterpart

We present an N-body model that reproduces the morphology and kinematics of the Magellanic Stream (MS), a vast neutral hydrogen (HI) structure that trails behind the Large and Small Magellanic Clouds (LMC and SMC, respectively) in their orbit about the Milky Way. After investigating 8 million possible orbits consistent with the latest proper motions, we adopt an orbital history in which the LMC and SMC have only recently become a strongly interacting binary pair. We find that their first close encounter 2 Gyr ago provides the necessary tidal forces to disrupt the disk of the SMC and thereby create the MS. The model also reproduces the on-sky bifurcation of the two filaments of the MS, and we suggest that a bound association with the Milky Way is required to reproduce the bifurcation. Additional HI structures are created during the tidal evolution of the SMC disk, including the Magellanic Bridge, the "Counter-Bridge", and two branches of leading material. Insights into the chemical evolution of the LMC are also provided, as a substantial fraction of the material stripped away from the SMC is engulfed by the LMC. Lastly we compare three different N-body realizations of the stellar component of the SMC, which we model as a pressure-supported spheroid motivated by recent kinematical observations. We find that an extended spheroid is better able to explain the stellar periphery of the SMC, and the tidal evolution of the spheroid may imply the existence of a stellar stream akin to the gaseous MS.

astro-ph.GA↗

arXiv2012

The influences of the Magellanic Clouds on the Galaxy: Pole shift, warp, and star formation history

We investigate how the Large Magellanic Cloud (LMC) influences the evolution of the Galaxy after the LMC enters into the virial radius of the dark matter halo of the Galaxy for the first time. Both the Galaxy and the LMC are modeled as N-body particles in our models so that the dynamical influences of the LMC on the Galaxy can be investigated in a fully self-consistent manner. Furthermore, the orbital parameters for the LMC are carefully chosen such that the present location of the LMC in the Galaxy can be rather precisely reproduced in our simulations. We particularly investigate the influences of the LMC on the precession rate, the outer stellar and gaseous structures, and the star formation history of the Galaxy. Our principals results are summarized as follows. The LMC-Galaxy dynamical interaction can cause "pole shift" (or irregular precession/nutation) of the Galaxy and the typical rate of pole shift (dot θ_{d}) is ~2 degree Gyr^{-1} corresponding to ~ 7 muas yr^{-1}. The LMC-Galaxy interaction induces the formation of the outer warp structures of the Galaxy, which thus confirms the results of previous numerical simulations on the formation of the Galactic warp. The LMC-Galaxy interaction also induces the formation of outer (R>20 kpc) spiral arms and increases the vertical velocity dispersion of the outer disk significantly. The mean star formation rate of the Galaxy for the last several Gyrs can be hardly influenced by the LMC's tidal force. The age and metallicity distribution of stars in the solar-neighborhood (7 kpc < R < 10 kpc) for the last several Gyr can be only slightly changed by the past LMC-Galaxy interaction. Based on these results, we discuss how the possible ongoing Galactic pole shift with 10 muas yr^{-1} can be detected by future observational studies by GAIA.

astro-ph.GA↗

arXiv2012

Two-Component Galactic Bulge Probed with Renewed Galactic Chemical Evolution Model

Results of recent observations of the Galactic bulge demand that we discard a simple picture of its formation, suggesting the presence of two stellar populations represented by two peaks of stellar metallicity distribution (MDF) in the bulge. To assess this issue, we construct Galactic chemical evolution models that have been updated in two respects: First, the delay time distribution (DTD) of type Ia supernovae (SNe Ia) recently revealed by extensive SN Ia surveys is incorporated into the models. Second, the nucleosynthesis clock, the s-processing in asymptotic giant branch (AGB) stars, is carefully considered in this study. This novel model first shows that the Galaxy feature tagged by the key elements, Mg, Fe, Ba for the bulge as well as thin and thick disks is compatible with a short-delay SN Ia. We present a successful modeling of a two-component bulge including the MDF and the evolutions of [Mg/Fe] and [Ba/Mg], and reveal its origin as follows. A metal-poor component (<[Fe/H]>~-0.5) is formed with a relatively short timescale of ~1 Gyr. These properties are identical to the thick disk's characteristics in the solar vicinity. Subsequently from its remaining gas mixed with a gas flow from the disk outside the bulge, a metal-rich component (<[Fe/H]>~+0.3) is formed with a longer timescale (~4 Gyr) together with a top-heavy initial mass function that might be identified with the thin disk component within the bulge.

astro-ph.GA↗

arXiv2011

A possible physical connection between helium-rich stellar populations of massive globular clusters and the UV upturn of galactic spheroids

We discuss a possible physical connection between helium-rich (Y > 0.35) stellar populations of massive globular clusters (GCs) and the ultraviolet (UV) upturn of galactic spheroids by using analytical and numerical models. In our model, all stars are initially formed as bound or unbound star clusters (SCs) formed from giant molecular clouds (GMCs) and the SCs can finally become GCs, open clusters, and field stars depending on physical properties of their host GMCs. An essential ingredient of the model is that helium-rich stars are formed almost purely from gas ejected from massive asymptotic giant branch (AGB) stars. The helium-rich star formation is assumed to occur within massive SCs if the masses of the progenitor GMCs are larger than a threshold mass (M_thres). These massive SCs can finally become either massive GCs or helium-rich field stars depending on whether they are disintegrated or not. Using this model, we show that if the initial mass functions (IMFs) in galactic spheroids are mildly top-heavy, then the mass fractions of helium-rich main-sequence stars (F_He) can be as large as ~ 0.1 for M_thres=10^7 M_sun. F_He is found to depend on IMFs and M_thres such that it can be larger for shallower IMFs and smaller M_thres. The inner regions of galactic spheroids show larger F_He in almost all models. Based on these results, we suggest that if the UV upturn of elliptical galaxies is due to the larger fractions of helium-rich stars, then the origin can be closely associated with top-heavy IMFs in the galaxies.

astro-ph.CO↗