Kroupa Initial Mass Function Research Articles

Reconciling M/L Ratios Across Cosmic Time: a Concordance IMF for Massive Galaxies

The stellar initial mass function (IMF) is thought to be bottom heavy in the cores of the most massive galaxies, with an excess of low-mass stars compared to the Milky Way. However, studies of the kinematics of quiescent galaxies at 2 < z < 5 find M/L ratios that indicate lighter IMFs. Light IMFs have also been proposed for the unexpected populations of luminous galaxies that JWST has uncovered at z > 7 to reduce tensions with galaxy formation models. Here we explore “ski slope” IMFs that are simultaneously bottom heavy, with a steep slope at low stellar masses, and top heavy, with a shallow slope at high masses. We derive a form of the IMF for massive galaxies that is consistent with measurements in the local universe and yet produces relatively low M/L ratios at high redshift. This concordance IMF has slopes γ 1 = 2.40 ± 0.09, γ 2 = 2.00 ± 0.14, and γ 3 = 1.85 ± 0.11 in the regimes 0.08 M ⊙ – 0.5 M ⊙, 0.5 M⊙ – 1 M ⊙, and >1 M ⊙, respectively. The IMF parameter α, the mass excess compared to a Milky Way IMF, ranges from log(α)≈+0.3 for present-day galaxies to log(α)≈−0.1 for their star-forming progenitors. The concordance IMF applies only to the central regions of the most massive galaxies, with velocity dispersions σ ∼ 300 km s−1, and their progenitors. However, it can be generalized using a previously measured relation between α and σ. We arrive at the following modification to the Kroupa (2001) IMF for galaxies with σ ≳ 160 km s−1: γ1≈1.3+4.3logσ160 , γ2≈2.3−1.2logσ160 , and γ3≈2.3−1.7logσ160 , with σ 160 = σ/160 km s−1. If galaxies grow primarily inside out, so that velocity dispersions are relatively stable, these relations should also hold at high redshift.

The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). VI. The High-mass Stellar Initial Mass Function of M33

We measure the high-mass stellar initial mass function (IMF) from resolved stars in M33 young stellar clusters. Leveraging the Hubble Space Telescope’s high resolving power, we fully model the IMF probabilistically. We first model the optical color–magnitude diagram of each cluster to constrain its power-law slope Γ, marginalized over other cluster parameters in the fit (e.g., cluster age, mass, and radius). We then probabilistically model the distribution of mass function (MF) slopes for a highly strict cluster sample of nine clusters more massive than log(Mass/M ⊙) = 3.6; above this mass, all clusters have well-populated main sequences of massive stars and should have accurate recovery of their MF slopes, based on extensive tests with artificial clusters. We find that the ensemble IMF is best described by a mean high-mass slope of Γ¯=1.49±0.18 , with an intrinsic scatter of σΓ2=0.020.00+0.16 , consistent with a universal IMF. We find no dependence of the IMF on environmental impacts such as the local star formation rate (SFR) or galactocentric radius within M33, which serves as a proxy for metallicity. This Γ¯ measurement is consistent with similar measurements in M31, despite M33 having a much higher SFR intensity. While this measurement is formally consistent with the canonical Kroupa (Γ = 1.30) IMF, as well as the Salpeter (Γ = 1.35) value, it is the second Local Group cluster sample to show evidence for a somewhat steeper high-mass IMF slope. We explore the impacts a steeper IMF slope has on a number of astronomical subfields.

Improving the open cluster census

Context. The census of open clusters has exploded in size thanks to data from the Gaia satellite. However, it is likely that many of these reported clusters are not gravitationally bound, making the open cluster census impractical for many scientific applications. Aims. We aim to test different physically motivated methods for distinguishing between bound and unbound clusters, using them to create a cleaned star cluster catalogue. Methods. We derived completeness-corrected photometric masses for 6956 clusters from our earlier work. Then, we used these masses to compute the size of the Roche surface of these clusters (their Jacobi radius) and distinguish between bound and unbound clusters. Results. We find that only 5647 (79%) of the clusters from our previous catalogue are compatible with bound open clusters, dropping to just 11% of clusters within 250 pc. Our catalogue contains 3530 open clusters in a more strongly cut high-quality sample of objects. The moving groups in our sample show different trends in their size as a function of age and mass, suggesting that they are unbound and undergoing different dynamical processes. Our cluster mass measurements constitute the largest catalogue of Milky Way cluster masses to date, which we also use for further science. Firstly, we inferred the mass-dependent completeness limit of the open cluster census, showing that the census is complete within 1.8 kpc only for objects heavier than 230 M⊙. Next, we derived a completeness-corrected age and mass function for our open cluster catalogue, including estimating that the Milky Way contains a total of 1.3 × 105 open clusters, only ∼4% of which are currently known. Finally, we show that most open clusters have mass functions compatible with the Kroupa initial mass function. Conclusions. We demonstrate Jacobi radii for distinguishing between bound and unbound star clusters, and publish an updated star cluster catalogue with masses and improved cluster classifications.

The Present-day Mass Function of Star Clusters in the Solar Neighborhood

This work analyzes the present-day mass function (PDMF) of 93 star clusters utilizing Gaia Data Release 3 data, with membership determined by the StarGo machine-learning algorithm. The impact of unresolved binary systems on mass estimation is rigorously assessed, adopting three mass ratio profiles for correction. The PDMF is characterized by the power-law index, α, derived through a robust maximum likelihood method that avoids biases associated with data binning. The value of α for stars between the completeness limited mass of Gaia (with a mean 0.3 M ⊙ for our cluster samples) and 2 M ⊙ exhibits stability for clusters younger than 200 Myr, decreasing for older clusters, particularly when considering stars within the half-mass radius. The PDMF of these star clusters is consistent with a dynamically evolved Kroupa initial mass function via the loss of low-mass stars. Cluster morphology shows a correlation with α, as α values exhibit a decreasing trend from filamentary to tidal-tail clusters, mirroring the sequence of increasing cluster age. The dependence of α on the total cluster mass is weak, with a subtle increase for higher-mass clusters, especially outside the half-mass radius. We do not observe a correlation between α and the mean metallicity of the clusters. Younger clusters have lower metallicity compared to their older counterparts, which indicates that the older clusters might have migrated to the solar neighborhood from the inner disk. A comparison with numerical models incorporating a black hole population suggests the need for observations of distant, older, massive open clusters to determine whether or not they contain black holes.

Dynamical Stellar Mass-to-light Ratio Gradients: Evidence for Very Centrally Concentrated IMF Variations in ETGs?

Evidence from different probes of the stellar initial mass function (IMF) of massive early-type galaxies (ETGs) has repeatedly converged on IMFs more bottom heavy than in the Milky Way (MW). This consensus has come under scrutiny due to often contradictory results from different methods on the level of individual galaxies. In particular, a number of strong lensing probes are ostensibly incompatible with a non-MW IMF. Radial gradients of the IMF—related to gradients of the stellar mass-to-light ratio ϒ—can potentially resolve this issue. We construct Schwarzschild models allowing for ϒ-gradients in seven massive ETGs with MUSE and SINFONI observations. We find dynamical evidence that ϒ increases toward the center for all ETGs. The gradients are confined to subkiloparsec scales. Our results suggest that constant-ϒ models may overestimate the stellar mass of galaxies by up to a factor of 1.5. For all except one galaxy, we find a radius where the total dynamical mass has a minimum. This minimum places the strongest constraints on the IMF outside the center and appears at roughly 1 kpc. We consider the IMF at this radius characteristic for the main body of each ETG. In terms of the IMF mass-normalization α relative to a Kroupa IMF, we find on average an MW-like IMF 〈α main〉 = 1.03 ± 0.19. In the centers, we find concentrated regions with increased mass normalizations that are less extreme than previous studies suggested, but still point to a Salpeter-like IMF, 〈α cen〉 = 1.54 ± 0.15.

The Peak of the Fallback Rate from Tidal Disruption Events: Dependence on Stellar Type

A star completely destroyed in a tidal disruption event (TDE) ignites a luminous flare that is powered by the fallback of tidally stripped debris to a supermassive black hole (SMBH) of mass M •. We analyze two estimates for the peak fallback rate in a TDE, one being the “frozen-in” model, which predicts a strong dependence of the time to peak fallback rate, t peak, on both stellar mass and age, with 15 days ≲ t peak ≲ 10 yr for main sequence stars with masses 0.2 ≤ M ⋆/M ⊙ ≤ 5 and M • = 106 M ⊙. The second estimate, which postulates that the star is completely destroyed when tides dominate the maximum stellar self-gravity, predicts that t peak is very weakly dependent on stellar type, with tpeak=23.2±4.0daysM•/106M⊙1/2 for 0.2 ≤ M ⋆/M ⊙ ≤ 5, while tpeak=29.8±3.6daysM•/106M⊙1/2 for a Kroupa initial mass function truncated at 1.5M ⊙. This second estimate also agrees closely with hydrodynamical simulations, while the frozen-in model is discrepant by orders of magnitude. We conclude that (1) the time to peak luminosity in complete TDEs is almost exclusively determined by SMBH mass, and (2) massive-star TDEs power the largest accretion luminosities. Consequently, (a) decades-long extra-galactic outbursts cannot be powered by complete TDEs, including massive-star disruptions, and (b) the most highly super-Eddington TDEs are powered by the complete disruption of massive stars, which—if responsible for producing jetted TDEs—would explain the rarity of jetted TDEs and their preference for young and star-forming host galaxies.

Massive Star Formation in the Tarantula Nebula

In this work, we present 299 candidate young stellar objects (YSOs) in 30 Doradus discovered using Spitzer and Herschel point-source catalogs, 276 of which are new. We study the parental giant molecular clouds in which these YSO candidates form using recently published Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 7 observations of 12CO and 13CO. The threshold for star formation in 30 Doradus inferred by the LTE-based mass surface density is 178 M ⊙ pc−2, 40% higher than the threshold for star formation in the Milky Way. This increase in star formation threshold in comparison to the Milky Way and increase in line width seen in clumps 11 pc away in comparison to clumps 45 pc away from the R136 super star cluster could be due to injected turbulent energy, increase in interstellar medium pressure, and/or local magnetic field strength. Of the 299 YSO candidates in this work, 62% are not associated with 12CO molecular gas. This large fraction can be explained by the fact that 75%–97% of the H2 gas is not traced by CO. We fit a Kroupa initial mass function to the YSO candidates and find that the total integrated stellar mass is 18,000 M ⊙ and that the region has a star formation rate (SFR) of 0.18 M ⊙ yr−1. The initial mass function determined here applies to the four 150″ × 150″ (37.5 pc × 37.5 pc) subfields and one 150″ × 75″ (37.5 pc × 18.8 pc) subfield observed with ALMA. The SFR in 30 Doradus has increased in the past few million years.

Did the Milky Way just light up? The recent star formation history of the Galactic disc

We map the stellar age distribution (≲1 Gyr) across a 6 kpc × 6 kpc area of the Galactic disc in order to constrain our Galaxy’s recent star formation history. Our modelling draws on a sample that encompasses all presumed disc O-, B-, and A-type stars (∼500 000 sources) with G &lt; 16. To be less sensitive to reddening, we did not forward-model the detailed CMD distribution of these stars; instead, we forward-modelled the K-band absolute magnitude distribution, n(MK), among stars with MK &lt; 0 and Teff &gt; 7000 K at certain positions x in the disc as a step function with five age bins, b(τ | x, α), logarithmically spaced in age from τ = 5 Myr to τ ∼ 1 Gyr. Given a set of isochrones and a Kroupa initial mass function, we sampled b(τ | x, α) to maximise the likelihood of the data n(MK | x, α), accounting for the selection function. This results in a set of mono-age stellar density maps across a sizeable portion of the Galactic disc. These maps show that some, but not all, spiral arms are reflected in overdensities of stars younger than 500 Myr. The maps of the youngest stars (&lt; 10 Myr) trace major star-forming regions. The maps at all ages exhibit an outward density gradient and distinct spiral-like spatial structure, which is qualitatively similar on large scales among the five age bins. When summing over the maps’ areas and extrapolating to the whole disc, we find an effective star formation rate over the last 10 Myr of ≈ 3.3 M⊙ yr−1, higher than previously published estimates that had not accounted for unresolved binaries. Remarkably, our stellar age distribution implies that the star formation rate has been three times lower throughout most of the last Gyr, having risen distinctly only in the very recent past. Finally, we used TNG50 simulations to explore how justified the common identification of local age distribution with global star formation history is: we find that the global star formation rate at a given radius in Milky Way-like galaxies is approximated within a factor of ∼1.5 by the young age distribution within a 6 kpc × 6 kpc area near R⊙.

Stellar population analysis of MaNGA early-type galaxies: IMF dependence and systematic effects

ABSTRACT We study systematics associated with estimating simple stellar population (SSP) parameters – age, metallicity [M/H], α-enhancement [α/Fe], and initial mass function (IMF) shape – and associated M*/L gradients, of elliptical slow rotators (E-SRs), fast rotators (E-FRs), and S0s from stacked spectra of galaxies in the MaNGA survey. These systematics arise from (i) how one normalizes the spectra when stacking; (ii) having to subtract emission before estimating absorption line strengths; (iii) the decision to fit the whole spectrum or just a few absorption lines; (iv) SSP model differences (e.g. isochrones, enrichment, IMF). The MILES+Padova SSP models, fit to the Hβ, 〈Fe〉, TiO2SDSS, and [MgFe] Lick indices in the stacks, indicate that out to the half-light radius Re: (a) ages are younger and [α/Fe] values are lower in the central regions but the opposite is true of [M/H]; (b) the IMF is more bottom-heavy in the center, but is close to Kroupa beyond about Re/2; (c) this makes M*/L about 2 × larger in the central regions than beyond Re/2. While the models of Conroy et al. return similar [M/H] and [α/Fe] profiles, the age and (hence) M*/L profiles can differ significantly even for solar abundances and a Kroupa IMF; different responses to non-solar abundances and IMF parametrization further compound these differences. There are clear (model independent) differences between E-SRs, E-FRs, and S0s: younger ages and less enhanced [α/Fe] values suggest that E-FRs and S0s are not SSPs, but relaxing this assumption is unlikely to change their inferred M*/L gradients significantly.

The MASSIVE Survey. XVI. The Stellar Initial Mass Function in the Center of MASSIVE Early-type Galaxies

The stellar initial mass function (IMF) is a fundamental property in the measurement of stellar masses and galaxy star formation histories. In this work, we focus on the most massive galaxies in the nearby universe . We obtain high-quality Magellan/LDSS-3 long-slit spectroscopy with a wide wavelength coverage of 0.4–1.01 μm for 41 early-type galaxies (ETGs) in the MASSIVE survey and derive high signal-to-noise spectra within an aperture of R e/8. Using detailed stellar synthesis models, we constrain the elemental abundances and stellar IMF of each galaxy through full spectral modeling. All the ETGs in our sample have an IMF that is steeper than a Milky Way (Kroupa) IMF. The best-fit IMF mismatch parameter, α IMF = (M/L)/(M/L)MW, ranges from 1.1 to 3.1, with an average of 〈α IMF〉 = 1.84, suggesting that on average, the IMF is more bottom heavy than Salpeter. Comparing the estimated stellar masses with the dynamical masses, we find that most galaxies have stellar masses that are smaller than their dynamical masses within the 1σ uncertainty. We complement our sample with lower-mass galaxies from the literature and confirm that is positively correlated with , , and . From the combined sample, we show that the IMF in the centers of more massive ETGs is more bottom heavy. In addition, we find that is positively correlated with both [Mg/Fe] and the estimated total metallicity [Z/H]. We find suggestive evidence that the effective stellar surface density ΣKroupa might be responsible for the variation of α IMF. We conclude that σ, [Mg/Fe], and [Z/H] are the primary drivers of the global stellar IMF variation.

A study of 1000 galaxies with unusually young and massive stars in the SDSS: a search for hidden black holes

ABSTRACT We select 1076 galaxies with extinction-corrected H α equivalent widths too large to be explained with a Kroupa initial mass function, and compare these with a control sample of galaxies that is matched in stellar mass, redshift, and 4000 Å break strength, but with normal H α equivalent widths. Our goal is to study how processes such as black hole growth and energetic feedback processes from massive stars differ between galaxies with extreme central H α emission and galaxies with normal young central stellar populations. The stellar mass distribution of H α excess galaxies is peaked at $3 \times 10^{10}\, {\rm M}_{\odot }$ and almost all fall well within the star-forming locus in the [O iii]/H β versus [N ii]/H α Baldwin, Philipps &amp; Terlevich diagram. H α excess galaxies are twice as likely to exhibit H α line asymmetries and 1.55 times more likely to be detected at 1 GHz in the VLA FIRST survey compared to control sample galaxies. The radio luminosity per unit stellar mass decreases with the stellar age of the system. Using stacked spectra, we demonstrate that [Ne v] emission is not present in the very youngest of the radio-quiet H α excess galaxies with detectable Wolf–Rayet features, suggesting that black hole growth has not yet commenced in such systems. [Ne v] emission is detected in H α excess galaxies with radio detections and the strength of the line correlates with the radio luminosity. This is the clearest indication for a population of black holes that may be forming in a subset of the H α excess population.

Birth cluster simulations of planetary systems with multiple super-Earths: initial conditions for white dwarf pollution drivers

ABSTRACT Previous investigations have revealed that eccentric super-Earths represent a class of planets that are particularly effective at transporting minor bodies towards white dwarfs and subsequently polluting their atmospheres with observable chemical signatures. However, the lack of discoveries of these planets beyond a few astronomical units from their host stars prompts a better understanding of their orbital architectures from their nascent birth cluster. Here, we perform stellar cluster simulations of three-planet and seven-planet systems containing super-Earths on initially circular, coplanar orbits. We adopt the typical stellar masses of main-sequence progenitors of white dwarfs ($1.5\, \mathrm{M}_{\odot }$–$2.5\, \mathrm{M}_{\odot }$) as host stars and include 8000 main-sequence stars following a Kroupa initial mass function in our clusters. Our results reveal that about 30 per cent of the simulated planets generate eccentricities of at least 0.1 by the time of cluster dissolution, which would aid white dwarf pollution. We provide our output parameters to the community for potential use as initial conditions for subsequent evolution simulations.

A Parametric Galactic Model toward the Galactic Bulge Based on Gaia and Microlensing Data

Abstract We developed a parametric Galactic model toward the Galactic bulge by fitting to spatial distributions of the Gaia DR2 disk velocity, VVV proper motion, BRAVA radial velocity, OGLE-III red clump star count, and OGLE-IV star count and microlens rate, optimized for use in microlensing studies. We include the asymmetric drift of Galactic disk stars and the dependence of velocity dispersion on Galactic location in the kinematic model, which has been ignored in most previous models used for microlensing studies. We show that our model predicts a microlensing parameter distribution significantly different from those typically used in previous studies. We estimate various fundamental model parameters for our Galaxy through our modeling, including the initial mass function (IMF) in the inner Galaxy. Combined constraints from star counts and the microlensing event timescale distribution from the OGLE-IV survey, in addition to a prior on the bulge stellar mass, enable us to successfully measure IMF slopes using a broken power-law form over a broad mass range, α bd = 0.22 − 0.55 + 0.20 for M &lt; 0.08 M ⊙, α ms = 1.16 − 0.15 + 0.08 for 0.08 M ⊙ ≤ M &lt; M br, and α hm = 2.32 − 0.10 + 0.14 for M ≥ M br, as well as a break mass at M br = 0.90 − 0.14 + 0.05 M ⊙ . This is significantly different from the Kroupa IMF for local stars, but similar to the Zoccali IMF measured from a bulge luminosity function. We also estimate the dark matter mass fraction in the bulge region of 28% ± 7% which could be larger than a previous estimate. Because our model is purely parametric, it can be universally applied using the parameters provided in this paper. (A tool for microlensing simulation using our Galactic model has been published, Koshimoto &amp; Ranc 2021, and can be downloaded at https://github.com/nkoshimoto/genulens.)

A study of outer disc stellar populations of face-on star-forming galaxies in SDSS-IV MaNGA: causes of H α deficiency

ABSTRACT Integral field unit spectra of face-on star-forming galaxies from the MaNGA survey are stacked in radial bins so as to reach an S/N high enough to measure emission lines and Lick indices out to 2.5–3Re. Two-thirds of galaxies have stellar populations in the outer discs that are older, more metal poor, and less dusty than in the inner discs. Recent bursts of star formation have occurred more frequently in the outer disc, but extinction-corrected H α equivalent widths are significantly lower at fixed Dn(4000) in these regions. I examine the properties of a subset of galaxies with the most H α-deficient outer discs. These regions contain young stellar populations that must have formed within the last 0.5 Gyr, but have extinction-corrected H α values well below the values predicted for a standard Kroupa initial mass function. The H α-deficient galaxies have flat Dn(4000) and H δA profiles with little radial fluctuation, indicating that star formation has occurred extremely uniformly across the entire disc. The H α line profiles indicate that the ionized gas kinematics is also very regular across the disc. The main clue to the origin of the H α deficiency is that it sets in at the same radius where the dust extinction abruptly decreases, suggesting a mode of star formation deficient in massive stars in quiescent, H i-dominated gas. Finally, I have carried out a search for galaxies with signatures of unusual H α kinematics and find that 15 per cent of the sample exhibits evidence for significant ionized gas that is displaced from the systemic velocity of the disc.

Dynamical masses of brightest cluster galaxies – II. Constraints on the stellar IMF

ABSTRACT We use stellar and dynamical mass profiles, combined with a stellar population analysis, of 32 brightest cluster galaxies (BCGs) at redshifts of 0.05 ≤$z$ ≤ 0.30, to place constraints on their stellar initial mass function (IMF). We measure the spatially resolved stellar population properties of the BCGs, and use it to derive their stellar mass-to-light ratios ($\Upsilon _{\star \rm POP}$). We find young stellar populations (&amp;lt;200 Myr) in the centres of 22 per cent of the sample, and constant $\Upsilon _{\star \rm POP}$ within 15 kpc for 60 per cent of the sample. We further use the stellar mass-to-light ratio from the dynamical mass profiles of the BCGs ($\Upsilon _{\star \rm DYN}$), modelled using a multi-Gaussian expansion and Jeans Anisotropic Method, with the dark matter contribution explicitly constrained from weak gravitational lensing measurements. We directly compare the stellar mass-to-light ratios derived from the two independent methods, $\Upsilon _{\star \rm POP}$ (assuming some IMF) to $\Upsilon _{\star \rm DYN}$ for the subsample of BCGs with no young stellar populations and constant $\Upsilon _{\star \rm POP}$. We find that for the majority of these BCGs, a Salpeter (or even more bottom-heavy) IMF is needed to reconcile the stellar population and dynamical modelling results although for a small number of BCGs, a Kroupa (or even lighter) IMF is preferred. For those BCGs better fit with a Salpeter IMF, we find that the mass-excess factor against velocity dispersion falls on an extrapolation (towards higher masses) of known literature correlations. We conclude that there is substantial scatter in the IMF amongst the highest mass galaxies.

The initial mass function in the extended ultraviolet disc of M83

ABSTRACT Using Hubble Space Telescope ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations of four fields within the extended ultraviolet disc (XUV disc) of M83. These observations show a clumpy distribution of main-sequence stars and a mostly smooth distribution of red giant branch stars. We constrain the upper end of the initial mass function (IMF) in the outer disc using the detected population of main-sequence stars and an assumed constant star formation rate (SFR) over the last 300 Myr. By comparing the observed main-sequence luminosity function to simulations, we determine the best-fitting IMF to have a power-law slope α = −2.35 ± 0.3 and an upper mass limit $M_{\rm u}=25_{-3}^{+17} \, \mathrm{M}_\odot$. This IMF is consistent with the observed H $\rm \alpha$ emission, which we use to provide additional constraints on the IMF. We explore the influence of deviations from the constant SFR assumption, finding that our IMF conclusions are robust against all but strong recent variations in SFR, but these are excluded by causality arguments. These results, along with our similar studies of other nearby galaxies, indicate that some XUV discs are deficient in high-mass stars compared to a Kroupa IMF. There are over one hundred galaxies within 5 Mpc, many already observed with HST, thus allowing a more comprehensive investigation of the IMF, and how it varies, using the techniques developed here.

The total stellar halo mass of the Milky Way

ABSTRACT We measure the total stellar halo luminosity using red giant branch (RGB) stars selected from Gaia data release 2. Using slices in magnitude, colour, and location on the sky, we decompose RGB stars belonging to the disc and halo by fitting two-dimensional Gaussians to the Galactic proper motion distributions. The number counts of RGB stars are converted to total stellar halo luminosity using a suite of isochrones weighted by age and metallicity, and by applying a volume correction based on the stellar halo density profile. Our method is tested and calibrated using Galaxia and N-body models. We find a total luminosity (out to 100 kpc) of $L_{\rm halo} = 7.9 \pm 2.0 \times 10^8\, \mathrm{L}_\odot$ excluding Sgr, and $L_{\rm halo} = 9.4 \pm 2.4 \times 10^8\, \mathrm{L}_\odot$ including Sgr. These values are appropriate for our adopted stellar halo density profile and metallicity distribution, but additional systematics related to these assumptions are quantified and discussed. Assuming a stellar mass-to-light ratio appropriate for a Kroupa initial mass function (M⋆/L = 1.5), we estimate a stellar halo mass of $M^\star _{\rm halo} = 1.4 \pm 0.4\times 10^9 \, \mathrm{M}_\odot$. This mass is larger than previous estimates in the literature, but is in good agreement with the emerging picture that the (inner) stellar halo is dominated by one massive dwarf progenitor. Finally, we argue that the combination of a ${\sim}10^9\, \mathrm{M}_\odot$ mass and an average metallicity of 〈[Fe/H]〉 ∼ −1.5 for the Galactic halo points to an ancient (∼10 Gyr) merger event.

The young stellar population of the metal-poor galaxy NGC 6822

ABSTRACT We present a comprehensive study of massive young stellar objects (YSOs) in the metal-poor galaxy NGC 6822 using IRAC and MIPS data obtained from the Spitzer Space Telescope. We find over 500 new YSO candidates in seven massive star formation regions; these sources were selected using six colour–magnitude cuts. Via spectral energy distribution fitting to the data with YSO radiative transfer models we refine this list, identifying 105 high-confidence and 88 medium-confidence YSO candidates. For these sources, we constrain their evolutionary state and estimate their physical properties. The majority of our YSO candidates are massive protostars with an accreting envelope in the initial stages of formation. We fit the mass distribution of the Stage I YSOs with a Kroupa initial mass function and determine a global star formation rate of 0.039 $\mathrm{M}_{\odot } \, \mathrm{yr}^{-1}$. This is higher than star formation rate estimates based on integrated UV fluxes. The new YSO candidates are preferentially located in clusters which correspond to seven active high-mass star-formation regions which are strongly correlated with the 8 and 24 μm emission from PAHs and warm dust. This analysis reveals an embedded high-mass star formation region, Spitzer I, which hosts the highest number of massive YSO candidates in NGC 6822. The properties of Spitzer I suggest it is younger and more active than the other prominent H ii and star-formation regions in the galaxy.

Galaxy properties as revealed by MaNGA – II. Differences in stellar populations of slow and fast rotator ellipticals and dependence on environment

ABSTRACT We present estimates of stellar population (SP) gradients from stacked spectra of slow rotator (SR) and fast rotator (SR) elliptical galaxies from the MaNGA-DR15 survey. We find that (1) FRs are ∼5 Gyr younger, more metal rich, less α-enhanced and smaller than SRs of the same luminosity Lr and central velocity dispersion σ0. This explains why when one combines SRs and FRs, objects which are small for their Lr and σ0 tend to be younger. Their SP gradients are also different. (2) Ignoring the FR/SR dichotomy leads one to conclude that compact galaxies are older than their larger counterparts of the same mass, even though almost the opposite is true for FRs and SRs individually. (3) SRs with σ0 ≤ 250 km s−1 are remarkably homogeneous within ∼Re: they are old, α-enhanced, and only slightly supersolar in metallicity. These SRs show no gradients in age and M*/Lr, negative gradients in metallicity, and slightly positive gradients in [α/Fe] (the latter are model dependent). SRs with σ0 ≥ 250 km s−1 are slightly younger and more metal rich, contradicting previous work suggesting that age increases with σ0. They also show larger M*/Lr gradients. (4) Self-consistently accounting for M*/L gradients yields Mdyn ≈ M* because gradients reduce Mdyn by ∼0.2 dex while only slightly increasing the M* inferred using a Kroupa (not Salpeter) initial mass function. (5) The SR population starts to dominate the counts above $M_*\ge 3\times 10^{11}\, \mathrm{M}_\odot$; this is the same scale at which the size–mass correlation and other scaling relations change. Our results support the finding that this is an important mass scale that correlates with the environment and above which mergers matter.

Unraveling the Spectral Energy Distributions of Clustered YSOs

Abstract Despite significant evidence suggesting that intermediate- and high-mass stars form in clustered environments, how stars form when the available resources are shared is still not well understood. A related question is whether the initial mass function (IMF) is in fact universal across galactic environments, or whether it is an average of IMFs that differ, for example, in massive versus low-mass molecular clouds. One of the long-standing problems in resolving these questions and in the study of young clusters is observational: how to accurately combine multiwavelength data sets obtained using telescopes with different spatial resolutions. The resulting confusion hinders our ability to fully characterize clustered star formation. Here we present a new method that uses Bayesian inference to fit the blended spectral energy distributions and images of individual young stellar objects (YSOs) in confused clusters. We apply this method to the infrared photometry of a sample comprising 70 Spitzer-selected, low-mass (M cl &lt; 100 M ⊙) young clusters in the galactic plane, and we use the derived physical parameters to investigate how the distribution of YSO masses within each cluster relates to the total mass of the cluster. We find that for low-mass clusters this distribution is indistinguishable from a randomly sampled Kroupa IMF for this range of cluster masses. Therefore, any effects of self-regulated star formation that affect the IMF sampling are likely to play a role only at larger cluster masses. Our results are also compatible with smoothed particle hydrodynamics models that predict a dynamical termination of the accretion in protostars, with massive stars undergoing this stopping at later times in their evolution.