Dwarf Problem Research Articles

The M dwarf problem: Fe and Ti abundances in a volume-limited sample of M dwarf stars

ABSTRACT We report iron and titanium abundance measurements from high-resolution spectra in a volume-limited sample of 106 M0 and M0.5 dwarf stars. The sample includes stars north of the celestial equator and closer than 29 parsecs. The results imply that there is an M dwarf problem similar to the previously known G dwarf problem, in that the fraction of low-metallicity M dwarfs is not large enough to fit simple closed-box models of Galactic chemical evolution. This volume-limited sample avoids many of the statistical uncertainties present in a previous study using a brightness-limited sample of M dwarf stars.

TESS as a Low-surface-brightness Observatory

The low surface brightness Universe holds clues to the first formation of galaxies. Specifically, the shape and morphology of local stellar haloes have encoded in them the early formation history of their parent galaxies. Early progenitor galaxies were absorbed by the dark halo and scattered their stars in a diffuse halo around the main galaxy. If the accretion event was relatively recent, it may show as a coherent stream of stars within the halo. in addition, the low-mass, low-surface brightness satellite galaxies, perhaps the ultradiffuse galaxies recently reported would help solve the Missing Dwarf Problem, the apparent over-prediction of $\Lambda$CDM models of the number of satellite galaxies around a Milky Way Halo. However low surface brightness is not what most telescopes are optimized for, most are best for resolving point sources and not sensitivity for large-scale low-light. To be sensitive to the low surface brightness Universe, a telescope needs a simple, unobstructed light path (disfavoring mirrors), fast optics (low f/D), and relatively coarse sampling (big pixels). Exceptions are the superb Dragonfly and Huntsman telescopes which are purposely designed to be sensitive to low surface brighnesses. Similarly designed, if not with low surface brightness in mind is the successfully launched TESS satellite. We show in this Research Note that the envisaged total exposure times and optical setup are near-ideal for low surface brightness work in the local Universe. With combined TESS imaging, one can model the stellar halo surrounding a galaxy. Technical challenges include the image quality, zodiacal and Galactic cirrus background light, PSF characterization and subtraction. Once accounted for with a processing pipeline, one can model the stellar halo for all nearby galaxies and to search for substructure in these haloes.

OPTICAL–NEAR-INFRARED PHOTOMETRIC CALIBRATION OF M DWARF METALLICITY AND ITS APPLICATION

Based on a carefully constructed sample of dwarf stars, a new optical-near infrared photometric calibration to estimate the metallicity of late-type K and early-to-mid-type M dwarfs is presented. The calibration sample has two parts; the first part includes 18 M dwarfs with metallicities determined by high-resolution spectroscopy and the second part contains 49 dwarfs with metallicities obtained through moderate-resolution spectra. By applying this calibration to a large sample of around 1.3 million M dwarfs from the Sloan Digital Sky Survey and the Two-Micron All Sky Survey, the metallicity distribution of this sample is determined and compared with those of previous studies. Using photometric parallaxes, the Galactic heights of M dwarfs in the large sample are also estimated. Our results show that stars farther from the Galactic plane, on average, have lower metallicity, which can be attributed to the age-metallicity relation. A scarcity of metal-poor dwarf stars in the metallicity distribution relative to the Simple Closed Box Model indicates the existence of the "M dwarf problem," similar to the previously known G and K dwarf problems. Several more complicated Galactic chemical evolution models which have been proposed to resolve the G and K dwarf problems are tested and it is shown that these models could, to some extent, mitigate the M dwarf problem as well.

The M dwarf problem in the Galaxy

We present evidence that there is an M dwarf problem similar to the previously identified G dwarf and K dwarf problems: the number of low-metallicity M dwarfs is not sufficient to match simple closed-box models of local Galactic chemical evolution. We estimated the metallicity of 4141 M dwarf stars with spectra from the Sloan Digital Sky Survey (SDSS) using a molecular band strength versus metallicity calibration developed using high resolution spectra of nearby M dwarfs. Using a sample of M dwarfs with measured magnitudes, parallaxes, and metallicities, we derived a relation that describes the absolute magnitude variation as a function of metallicity. When we examined the metallicity distribution of SDSS stars, after correcting for the different volumes sampled by the magnitude-limited survey, we found that there is an M dwarf problem, with the number of M dwarfs at [Fe/H] ~ -0.5 less than 1% the number at [Fe/H] = 0, where a simple model of Galactic chemical evolution predicts a more gradual drop in star numbers with decreasing metallicity.

Placing Chandra’s work in historical context

Placing Chandra’s work in historical context

Abundance distribution functions for nearby late-type dwarfs

AbstractA major role to the understanding of the chemical evolution of the Galaxy is played by studies of stellar chemical abundance distribution. In the last years, there has been an increase in the number of spectroscopic surveys of late-type stars. Classical problems on the chemical evolution of the Galaxy, such as the G dwarf problem and the age-metallicity relation, can be reinvestigated with better accuracy. We present a chemical abundance survey of 325 solar neighborhood G dwarfs stars situated within 25 pc from the Sun. We reinvestigate classical observational constraints, namely the metallicity distribution, using a number of chemical elements (Na, Si, Ca, Ni, Fe and Ba) as metallicity indicators. The abundance probability density function for each of the surveyed element was derived using a Gaussian kernel estimator. We have found mean values of −0.11, −0.14, −0.07, −0.05, −0.16 and −0.12 dex for the [Fe/H], [Na/H], [Si/H], [Ca/H], [Ni/H] and [Ba/H] pdfs, respectively. We also show that abundance distributions having higher mean values have smaller dispersion, in contradiction to the predictions of the Simple Model with Delayed Production. We discuss this result in the context and present an alternate explanation for this pattern.

Environmental Effect on the Subhalo Abundance—a Solution to the Missing Dwarf Problem

Abstract Recent high-resolution simulations of the formation of dark-matter halos have shown that the distribution of subhalos is scale-free, in the sense that if scaled by the velocity dispersion of the parent halo, the subhalo velocity distribution functions of galaxy-sized and cluster-sized halos are identical. For cluster-sized halos, the simulation results agreed well with the observations. The simulations, however, predicted far too many subhalos for galaxy-sized halos. Our galaxy has several tens of known dwarf galaxies. On the other hand, simulated dark-matter halos contain thousands of subhalos. We performed a simulation of a single large volume, and measured the abundance of subhalos in all massive halos. We found that the variation of the subhalo abundance is very large, and those with the largest number of subhalos correspond to the simulated halos in previous studies. The subhalo abundance depends strongly on the local density of the background. Halos in high-density regions contain a large number of subhalos. Our Galaxy is in the low-density region. For our simulated halos in low-density regions, the number of subhalos is within a factor of four to that of our Galaxy. We argue that the “missing dwarf problem” is not a real problem, but is caused by biased selections of the initial conditions in previous studies, which were not appropriate for field galaxies.

Missing Dwarf Problem in Galaxy Clusters

Abstract We investigated the formation and evolution of CDM subhalos in galaxy-sized and cluster-sized halos by means of $N$-body simulations. Our aim was to make clear what the “dwarf galaxy problem” is. It has been argued that the number of subhalos in simulated galaxy-sized halos is too large compared with the observed number of dwarfs in the local group, while that in cluster-sized halos is consistent with the observed number of galaxies in clusters, such as the Virgo cluster. We simulated nine halos with several different mass resolutions and physical scales. We found that the dependence of the cumulative number of subhalos, $N_c$, on their maximum circular velocity, $V_c$, is given by $N_c\propto V_c^{-3}$, down to the reliability limit, independent of the mass of the main halo. This implies that simulations for cluster-sized halos give too many halos with $V_c \sim$140 km s$^{-1}$ or less. Previous comparisons of cluster-sized halos gave a much smaller number of subhalos in this regime, simply because of their limited resolution. Our result implies that any theory that attempts to resolve the missing dwarf problem should also explain the discrepancy of the simulation and observation in cluster-sized halos.

Non-Parametric Confidence Bands in Deconvolution Density Estimation

SummaryUniform confidence bands for densities f via non-parametric kernel estimates were first constructed by Bickel and Rosenblatt. In this paper this is extended to confidence bands in the deconvolution problem g=f*ψ for an ordinary smooth error density ψ. Under certain regularity conditions, we obtain asymptotic uniform confidence bands based on the asymptotic distribution of the maximal deviation (L∞-distance) between a deconvolution kernel estimator f^ and f. Further consistency of the simple non-parametric bootstrap is proved. For our theoretical developments the bias is simply corrected by choosing an undersmoothing bandwidth. For practical purposes we propose a new data-driven bandwidth selector that is based on heuristic arguments, which aims at minimizing the L∞-distance between f^ and f. Although not constructed explicitly to undersmooth the estimator, a simulation study reveals that the bandwidth selector suggested performs well in finite samples, in terms of both area and coverage probability of the resulting confidence bands. Finally the methodology is applied to measurements of the metallicity of local F and G dwarf stars. Our results confirm the ‘G dwarf problem’, i.e. the lack of metal poor G dwarfs relative to predictions from ‘closed box models’ of stellar formation.

Complexity on Small Scales. II. Metallicities and Ages in the Leo II Dwarf Spheroidal Galaxy

We present metallicities and ages for 52 red giants in the remote Galactic dwarf spheroidal (dSph) galaxy Leo II. These stars cover the entire surface area of Leo II and are radial velocity members. We obtained medium-resolution multifiber spectroscopy with FLAMES as part of a Large Program with the Very Large Telescope at the European Southern Observatory, Chile. The metallicities were determined based on the well-established near-infrared Ca II triplet technique. This allowed us to achieve a mean random error of 0.16 dex on the metallicities, while other systematic effects, such as unknown variations in the dSph's [Ca/Fe] ratio, may introduce a further source of uncertainty of the order of 0.1 dex. The resulting metallicity distribution is asymmetric and peaks at [Fe/H] = -1.74 dex on the Carretta & Gratton scale. The full range in metallicities extends from -2.4 to -1.08 dex. As in other dSph galaxies, no extremely metal-poor red giants were found. We compare Leo II's observed metallicity distribution with model predictions for several other Galactic dSphs from the literature. Leo II clearly exhibits a lack of more metal-poor stars, analogous to the classical G dwarf problem, which may indicate a comparable K giant problem. Moreover, its evolution appears to have been affected by galactic winds. We use our inferred metallicities as an input parameter for isochrone fits to Sloan Digital Sky Survey photometry of our target stars and derive approximate ages. The resulting age-metallicity distribution covers the full age range from 2 to about 15 Gyr on our adopted isochrone scale. During the first ~7 Gyr relative to the oldest stars, the metallicity of Leo II appears to have remained almost constant, centering on the mean metallicity of this galaxy. The almost constant metallicity at higher ages and a slight drop by about 0.3 dex thereafter may be indicative of rejuvenation by low-metallicity gas. Overall, the age-metallicity relation appears to support the formation of Leo II from preenriched gas. Evidence for enrichment is seen during the recent 2-4 Gyr. Our findings support earlier derived photometric findings of Leo II as a galaxy with a prominent old population and dominant intermediate-age populations. We do not see a significant indication of a radial metallicity gradient or age gradient in our current data.

Type Ia Supernovae in a Hierarchical Galaxy Formation Model: The Milky Way

We investigate chemical evolution in Milky Way-like galaxies based on the cold dark matter model in which cosmic structures form via hierarchical merging. We introduce chemical enrichment due to Type Ia supernovae (SNe Ia) into the Mitaka semianalytic galaxy formation model developed by Nagashima & Yoshii. For the first time we derive distributions of stellar metallicities and their ratios in Milky Way-like galaxies treating chemical enrichment due to SNe Ia in a hierarchical galaxy formation model self-consistently. As a first attempt, we assume all SNe Ia to have the same lifetime and assume instantaneous recycling for Type II supernovae (SNe II). We find that our model reproduces well the metal abundance ratio [O/Fe] against [Fe/H] and the iron metallicity distribution function in the solar neighborhood. This means that the so-called G dwarf problem is resolved by the hierarchical formation of galaxies, and a gas infall term introduced in traditional monolithic collapse models to solve this problem is well explained by the mixture of some physical processes such as hierarchical merging of dark halos, gas cooling, energy feedback, and injection of gas and metals into hot gas due to supernovae. We also discuss how physical processes affect the metal abundance ratio by varying the lifetime of SNe Ia, the star formation timescale, and the strength of supernova feedback. We find that the supernova feedback plays a key role among them, and therefore there is no one-to-one correspondence between star formation histories and stellar metallicity ratio distributions.

Structural Parameters of Thin and Thick Disks in Edge-on Disk Galaxies

We analyze the global structure of 34 late-type, edge-on, undisturbed, disk galaxies spanning a wide range of mass. We measure structural parameters for the galaxies using two-dimensional least-squares fitting to our R-band photometry. The fits require both a thick and a thin disk to adequately fit the data. The thick disks have larger scale heights and longer scale lengths than the embedded thin disks by factors of ~2 and ~1.25, respectively. The observed structural parameters agree well with the properties of thick and thin disks derived from star counts in the Milky Way and from resolved stellar populations in nearby galaxies. We find that massive galaxies' luminosities are dominated by the thin disk. However, in low-mass galaxies (Vc 120 km s-1) thick disk stars contribute nearly half the luminosity and dominate the stellar mass. Thus, although low-mass dwarf galaxies appear blue, the majority of their stars are probably quite old. Our data are most easily explained by a formation scenario in which the thick disk is assembled through direct accretion of stellar material from merging satellites while the thin disk is formed from accreted gas. The baryonic fraction in the thin disk therefore constrains the gas richness of the merging pregalactic fragments. If we include the mass in H I as part of the thin disk, the thick disk contains 10% of the baryons in high-mass galaxies and ~25%–30% of the baryons in low-mass galaxies. Our data, therefore, indicate that the fragments were quite gas rich at the time of merging (fgas = 75%–90%). However, because low-mass galaxies have a smaller fraction of baryons in their thin disks, the pregalactic fragments from which they assembled must have been systematically more gas poor. We believe this trend results from increased outflow due to supernova-driven winds in the lower mass pregalactic fragments. We estimate that ~60% of the total baryonic mass in these systems was lost due to outflows. Pushing the episode of significant winds to early times allows the mass-metallicity relationship for disks to be established early, before the main disk is assembled, and obviates the difficulty in driving winds from diffuse disks with low star formation efficiencies. We discuss other implications of this scenario for solving the G dwarf problem, for predicting abundance trends in thick disks, and for removing discrepancies between semianalytic galaxy formation models and the observed colors of low-mass galaxies.

Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy

The Carina dwarf spheroidal galaxy is the only galaxy of this type that shows clearly episodic star formation separated by long pauses. Here we present metallicities for 437 radial velocity members of this Galactic satellite. The metallicities and radial velocities were measured as part of a Large Programme with the Very Large Telescope at the European Southern Observatory, Chile. We obtained medium-resolution spectroscopy with the multiobject spectrograph FLAMES. Our target red giants cover the entire projected surface area of Carina. Our spectra are centered at the near-infrared Ca II triplet, which is a well-established metallicity indicator for old and intermediate-age red giants. The resulting data sample provides the largest collection of spectroscopically derived metallicities for a Local Group dwarf spheroidal galaxy to date. Four of our likely radial velocity members of Carina lie outside this galaxy's nominal tidal radius, supporting earlier claims of the possible existence of such stars beyond the main body of Carina. We find a mean metallicity of [Fe/H] ~ -1.7 dex on the 1997 metallicity scale of Carretta and Gratton for Carina. The formal FWHM of the metallicity distribution function is 0.92 dex, while the full range of metallicities is found to span approximately -3.0 dex < [Fe/H] < 0.0 dex. The metallicity distribution function might be indicative of several subpopulations distinct in metallicity. There appears to be a mild radial gradient such that more metal-rich populations are more centrally concentrated, matching a similar trend for an increasing fraction of intermediate-age stars (see the 2001 work of Harbeck and coworkers). This, as well as the photometric colors of the more metal-rich red giants, suggests that Carina exhibits an age-metallicity relation. Indeed, the age-metallicity degeneracy seems to conspire to form a narrow red giant branch despite the considerable spread in metallicity and wide range of ages. The metallicity distribution function is not well matched by a simple closed-box model of chemical evolution. Qualitatively better matches are obtained by chemical models that also take into account infall and outflows. A G dwarf problem remains for all these models.

Abundances of Baade’s Window Giants from Keck HIRES Spectra. I. Stellar Parameters and [Fe/H] Values

We present the first results of a new abundance survey of the Milky Way bulge based on Keck HIRES spectra of 27 K giants in the Baade's Window (l = 1°, b = -4°) field. The spectral data used in this study are of much higher resolution and signal-to-noise ratio than previous optical studies of Galactic bulge stars. The [Fe/H] values of our stars, which range between -1.29 and +0.51, were used to recalibrate large low-resolution surveys of bulge stars. Our best value for the mean [Fe/H] of the bulge is -0.10 ± 0.04. This mean value is similar to the mean metallicity of the local disk and indicates that there cannot be a strong metallicity gradient inside the solar circle. The metallicity distribution of stars confirms that the bulge does not suffer from the so-called G dwarf problem. This paper also details the new abundance techniques necessary to analyze very metal-rich K giants, including a new Fe line list and regions of low blanketing for continuum identification.

Spectroscopy of Red Giants in the Large Magellanic Cloud Bar: Abundances, Kinematics, and the Age-Metallicity Relation

We report metallicities and radial velocities derived from spectra at the near-infrared calcium triplet for 373 red giants in a 200 arcmin2 area at the optical center of the LMC bar. These are the first spectroscopic abundance measurements of intermediate-age and old field stars in the high surface brightness heart of the LMC. The metallicity distribution is sharply peaked at the median value [Fe/H] = -0.40, with a small tail of stars extending down to [Fe/H] ≤ -2.1; 10% of the red giants are observed to have [Fe/H] ≤ -0.7. The relative lack of metal-poor stars indicates that the LMC has a G dwarf problem, similar to the Milky Way. The abundance distribution can be closely approximated by two Gaussians containing 89% and 11% of the stars, respectively: the first component is centered at [Fe/H] = -0.37 with σ = 0.15, and the second at [Fe/H] = -1.08 with σ = 0.46. The dominant population has a metallicity distribution similar to that of the LMC's intermediate-age star clusters. The mean heliocentric radial velocity of the sample is 257 km s-1, corresponding to the same center-of-mass velocity as the disk (measured at larger radii). Because of the central location of our field, kinematic constraints are not strong, but there is no evidence that the bar deviates from the general motion of the LMC disk. The velocity dispersion of the whole sample is σv = 24.7 ± 0.4 km s-1. When cut by metallicity, the most metal-poor 5% of stars ([Fe/H] < -1.15) show σv = 40.8 ± 1.7 km s-1, more than twice the value for the most metal-rich 5%; this suggests that an old, thicker disk or halo population is present. The age-metallicity relation (AMR) is almost flat during the period from 5 to 10 Gyr ago, with an apparent scatter of ±0.15 dex about the mean metallicity for a given age. Comparing this to chemical evolution models from the literature, we find that a burst of star formation 3 Gyr ago does not reproduce the observed AMR more closely than a steadily declining star formation rate. The AMR suggests that the epoch of enhanced star formation, if any, must have commenced earlier, ≈6 Gyr ago—the exact time is model dependent. We compare the properties of the LMC and the Milky Way, and discuss our results in the context of models that attempt to use tidal interactions with the Milky Way and Small Magellanic Cloud to explain the star and cluster formation histories of the LMC.

Galaxy Formation: Cold Dark Matter, Feedback, and the Hubble Sequence

TreeSPH simulations of galaxy formation in a standard Λ cold dark matter cosmology, including star formation and the effects of energetic stellar feedback processes and of a metagalactic UV field, have been performed, resulting in a mix of realistic disk, lenticular, and elliptical galaxies at redshift z = 0. The disk galaxies are deficient in angular momentum by only about a factor of 2 compared with observed disk galaxies for simulations with fairly strong starbursts in early, protogalactic clouds, leading to "blow-away" of the remaining gas in the clouds. In this respect the present scenario is hence doing almost as well as the warm dark matter (WDM) scenarios discussed by Sommer-Larsen & Dolgov. The surface density profiles of the stellar disks are approximately exponential, and those of the bulges range from exponential to r1/4, as observed. The bulge-to-disk ratios of the disk galaxies are consistent with observations, as are their integrated B-V colors, which have been calculated using stellar population synthesis techniques. Furthermore, the observed I-band Tully-Fisher relation can be matched, provided that the stellar mass-to-light ratio of disk galaxies is M/LI ~ 0.8, similar to what was found by Sommer-Larsen & Dolgov from their WDM simulations and in fair agreement with several recent observational determinations of M/LI for disk galaxies. The elliptical and lenticular galaxies have approximately r1/4 stellar surface density profiles, are dominated by nondisklike kinematics, and are flattened as a result of nonisotropic stellar velocity distributions, again consistent with observations. Hot halo gas is predicted to cool out and be accreted onto the Galactic disk at a rate of 0.5-1 M☉ yr-1 at z = 0, consistent with upper limits deduced from Far Ultraviolet Spectroscopic Explorer observations of O VI. We have analyzed in detail the formation history of two disk galaxies with circular speeds comparable to that of the Milky Way and find gas accretion rates, and hence bolometric X-ray luminosities of the halos, 6-7 times larger at z ~ 1 than at z = 0 for these disk galaxies. More generally, it is found that gas infall rates onto these disks are nearly exponentially declining with time, both for the total disk and for the "solar cylinder." This theoretical result hence supports the exponentially declining gas infall approximation often used in chemical evolution models. The infall timescales deduced are ~5-6 Gyr, comparable to what is adopted in current chemical evolution models to solve the "G dwarf problem." The disk of one of the two galaxies forms "inside-out," the other "outside-in," but in both cases the mean ages of the stars in the outskirts of the disks are ≳6-8 Gyr, fairly consistent with the findings of Ferguson & Johnson for the disk of M31. The amount of hot gas in disk galaxy halos is consistent with observational upper limits. The globular cluster M53 and the LMC are "inserted" in the halos of the two Milky Way-like disk galaxies, and dispersion measures to these objects are calculated. The results are consistent with upper limits from observed dispersion measures to pulsars in these systems. Finally, the results of the simulations indicate that the observed peak in the cosmic star formation rate at redshift z ~ 2 can be reproduced. Depending on the star formation and feedback scenarios, one predicts either a cosmic star formation rate that decreases monotonically with redshift beyond these redshifts or a second peak at z ~ 6-8, corresponding to the putative Population III and interestingly similar to recent estimates of the redshift at which the universe was reionized. These various scenarios should hence be observationally constrainable with upcoming instruments such as the James Webb Space Telescope and the Atacama Large Millimeter Array.

Pollution of Stellar Atmospheres by Interstellar Dust Material: A Possible Solution to the G Dwarf Problem

Accretion of interstellar dust material onto stellar atmospheres could be responsible for the lack of low-metallicity G dwarfs observed in the disk of our Galaxy, known as the G dwarf problem. This interpretation of the G dwarf problem would be consistent with a peaked distribution of G dwarf metallicities and reconcile the observations of both a well-mixed interstellar gas and a large scatter in the age-metallicity relation. The absence of G dwarf problem in the Galactic halo could also be easily explained by the lower density of dust grains and the higher velocity of the stars in the halo. In order to test this idea of atmospheric pollution by interstellar dust material, the Sun accretion rate of interstellar dust (ISD) iron is estimated, making use of in situ measurements at AU distances from the Sun and taking into account the solar gravitational and radiation pressure forces to describe the grains dynamics closer to the Sun. The dust grains do not actually reach the Sun surface, but their iron content does after their sublimation at a few solar radii. The flux of ISD measured by Ulysses and Galileo results in an iron accretion rate of the order of 10-20 M☉ yr-1; the flux of very large ISD recently measured with AMOR gives an accretion rate several orders of magnitude higher, 10-17 M☉ yr-1 being a probable value. For a G dwarf of real metallicity [Fe/H]r, the metallicity of the stellar atmosphere (the observed one) will therefore differ from the metallicity [Fe/H]r of the stellar interior if the dilution of the accreted iron into the interior of the star happens on a timescale longer than 1-100 × 10 yr. Further studies of the particle transport between the atmosphere and convection zone will be needed to conclude about the plausibility of these values.

The Distribution of Heavy Elements in Spiral and Elliptical Galaxies

ABSTRACTIn large disk and spheroidal galaxies, spatially resolved abundance information can be extracted by analysis of either emission lines, absorption lines, or both, depending on the situation. This review recaps significant results as they apply to nondwarf galaxies, including the Milky Way, spiral disks and bulges, and elliptical and lenticular galaxies. Methods for determining abundances are explained in appendices.Conclusions that span the galaxy types treated here are as follows. All galaxies, on average, have heavy‐element abundances (metallicities) that systematically decrease outward from their galactic centers while their global metallicities increase with galaxy mass. Abundance gradients are steepest in normal spirals and are seen to be progressively flatter going in order from barred spirals to lenticulars to ellipticals. The distribution of abundances N(Z) versus Z is strongly peaked compared with simple closed‐box model predictions of chemical enrichment in all galaxy types. That is, a “G dwarf problem,” commonly known in the solar cylinder, exists for all large galaxies.For spiral galaxies, local metallicity appears to be correlated with total (disk+bulge) surface density. Examination of N/O versus O/H in spiral disks indicates that production of N is dominated by primary processes at low metallicity and secondary processes at high metallicity. Carbon production increases with increasing metallicity. Abundance ratios Ne/O, S/O, and Ar/O appear to be universally constant and independent of metallicity, which argues either that the initial mass function (IMF) is universally constant or that these ratios are not sensitive to IMF variations. In the Milky Way, there is a rough age‐metallicity trend with much scatter, in the sense that older stars are more metal poor.In elliptical galaxies, nuclear abundances are in the range [Z/H] = 0.0–0.4, but the element mixture is not scaled‐solar. In large elliptical galaxies [Mg/Fe] is in the range 0.3–0.5, decreasing to ≈0 in smaller elliptical galaxies. Other light elements track the Mg enhancement, but the heavier Ca tracks Fe. Velocity dispersion appears to be a key parameter in the modulation of [Mg/Fe], but the cause of the connection is unclear.

Mass Function Gradients and the Need for Dark Matter

There is both theoretical and empirical evidence that the initial mass function (IMF) may be a function of the local star formation conditions. In particular, the IMF is predicted to flatten with increasing local luminosity density ρl, with the formation of massive stars being preferentially enhanced in brighter regions. In R136, the bright stellar cluster in 30 Doradus, the IMF gradient is ∂Γ/∂log ρl=0.28±0.06, where Γ is the slope of the IMF. If such IMF gradients are indeed general features of galaxies, this implies that several previous astrophysical measurements, such as the surface mass densities of spirals (obtained assuming constant mass-to-light ratios), were plagued by substantial systematic errors. In this Letter, calculations that account for possible IMF gradients are presented for surface densities of spiral galaxies. Compared with previous estimates, the mass surface densities corrected for IMF gradients are higher in the outer regions of the disks. For a model based on the Milky Way but with an IMF scaled according to R136, the rotation curve without the traditional dark halo component falls with Galactocentric radius, although slower than it would without IMF gradients. For a second model of the Milky Way in which the IMF gradient is increased to 0.42, the rotation curve is approximately flat in the outer disk, with a rotational velocity below ≃220 km s−1 only before the traditional dark halo component is added. For a third model in which substantial arm/interarm density contrasts are additionally assumed, the solar vicinity mass density drops to 0.10 M☉ pc−3, which is consistent with observations. These results, if generalizable to other galaxies, not only call into question the assertion that dark matter halos are compatible with the flat rotation curves of spiral galaxies but also may clarify our understanding of a wide variety of other astrophysical phenomena such as the G dwarf problem, metallicity gradients, and the Tully-Fisher relation.

The G Dwarf Problem Exists in Other Galaxies

view Abstract Citations (93) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The G Dwarf Problem Exists in Other Galaxies Worthey, Guy ; Dorman, Ben ; Jones, Lewis A. Abstract Stellar population models with abundance distributions determined from the analytic Simple model of chemical evolution fail to match observations of the nuclei of bulge-dominated galaxies in three respects. First, the spectral energy distribution in the mid-ultraviolet range 200O < λ < 2400 A exceeds observation by ~0.6 mag. Most of that excess is due to metal-poor main sequence stars. Second, the models do not reproduce metal-sensitive optical absorption features that arise mainly from red giant stars. Third, the strength of a Ca II index sensitive to hot stars does not jibe with the predicted number of A-type horizontal branch stars. The number of metal-poor stars in galaxies is at least a factor of 2 less than predicted by the Simple model, exactly similar to the "G dwarf problem" in the solar cylinder. Observations at larger radii in local group galaxies indicate that the paucity of metal- poor stars applies globally, rather than only in the nuclei. Because of the dominance of metal-rich stars, primordial galaxies will have a plentiful dust supply early in their star formation history, and thus will probably have weak Lyα emission, as is apparently observed. We confirm that early-type galaxies cannot have been formed exclusively from mergers of small all-stellar subsystems, a result already established by dynamical simulations. The constraint of peaked abundance distributions will limit future chemical evolution models. It will also make age estimates for the stellar populations in early-type galaxies and bulges more secure. Publication: The Astronomical Journal Pub Date: September 1996 DOI: 10.1086/118068 arXiv: arXiv:astro-ph/9606017 Bibcode: 1996AJ....112..948W Keywords: GALAXIES: STELLAR CONTENT; GALAXIES: ISM; STARS: ABUNDANCES; Astrophysics E-Print: 14 pages, LaTeX includes 3 postscript figures. Uses AAS LaTeX v 4.0 and times.sty. Accepted for publication in the Astronomical Journal. Postscript available at http://shemesh.gsfc.nasa.gov/~dorman/Ben.html full text sources arXiv | ADS | data products SIMBAD (4) NED (4)