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Charlie Conroy - One of the best experts on this subject based on the ideXlab platform.
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the Initial Mass function in the coma berenices dwarf galaxy from deep near infrared hst observations
arXiv: Astrophysics of Galaxies, 2018Co-Authors: Mario Gennaro, Charlie Conroy, Thomas M Brown, Marla Geha, Kirill Tchernyshyov, Roberto J Avila, Ricardo R Munoz, Joshua D Simon, Jason TumlinsonAbstract:We use deep $HST$ WFC3/IR imaging to study the Initial Mass Function (IMF) of the ultra faint dwarf galaxy Coma Berenices (Com Ber). Our observations reach the lowest stellar Mass ever probed in a resolved galaxy, with 50\% completeness at $\sim 0.17$ M$_{\odot}$. Unresolved background galaxies however limit our purity below $\sim 0.23$ M$_{\odot}$. If modeled with a single power law, we find that the IMF slope is $-1.45^{+0.29}_{-0.3}$ (68\% credible intervals), compared to a Milky Way value of $-2.3$. For a broken power law, we obtain a low-Mass slope of $-1.18_{-0.33}^{+0.49}$, a high-Mass slope of $-1.88_{-0.49}^{+0.43}$ and a break Mass of $0.57_{-0.08}^{+0.12}$ M$_{\odot}$, compared to $-1.3$, $-2.3$ and 0.5 M$_{\odot}$ for a Kroupa IMF. For a log-normal IMF model we obtain values of $0.33_{-0.16}^{+0.15}$ M$_{\odot}$ for the location parameter and of $0.68_{-0.12}^{+0.17}$ for $\sigma$ (0.22 M$_{\odot}$ and 0.57 for the Chabrier system IMF). All three parametrizations produce similar agreement with the data. Our results agree with previous analysis of shallower optical HST data. However analysis of similar optical data of other dwarfs finds IMFs significantly more bottom-light than in the Milky Way. These results suggest two, non mutually exclusive, possibilities: that the discrepancy of the dwarf galaxies IMF with respect to the Milky Way is, at least partly, an artifact of using a single power law model, and that there is real variance in the IMF at low Masses between the currently studied nearby dwarfs, with Com Ber being similar to the Milky Way, but other dwarfs differing significantly.
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Initial Mass function variability or not among low velocity dispersion compact stellar systems
arXiv: Astrophysics of Galaxies, 2017Co-Authors: Alexa Villaume, Jean P Brodie, Charlie Conroy, Aaron J Romanowsky, Pieter G Van DokkumAbstract:Analyses of strong gravitational lenses, galaxy-scale kinematics, and absorption line stellar population synthesis (SPS) have all concluded that the stellar Initial Mass function (IMF) varies within the Massive early-type galaxy (ETG) population. However, the physical mechanism that drives variation in the IMF is an outstanding question. Here we use new SPS models to consider a diverse set of compact, low-velocity dispersion stellar systems: globular clusters (GCs), an ultra-compact dwarf (UCD), and the compact elliptical (cE) galaxy M32. We compare our results to Massive ETGs and available dynamical measurements. We find that the GCs have stellar Mass-to-light ratios (M/L) that are either consistent with a Kroupa IMF or are slightly bottom-light while the UCD and cE have mildly elevated M/L. The separation in derived IMFs for systems with similar metallicities and abundance patterns indicates that our SPS models can distinguish abundance and IMF effects. Variation among the sample in this paper is only $\sim 50\%$ in normalized M/L compared to the $\sim 4\times$ among the ETG sample. This suggests that metallicity is not the sole driver of IMF variability and additional parameters need to be considered.
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the stellar Initial Mass function in early type galaxies from absorption line spectroscopy iii radial gradients
The Astrophysical Journal, 2017Co-Authors: Pieter G Van Dokkum, Alexa Villaume, Jean P Brodie, Charlie Conroy, Aaron J RomanowskyAbstract:There is good evidence that the centers of Massive early-type galaxies have a bottom-heavy stellar Initial Mass function (IMF) compared to the IMF of the Milky Way. Here we study the radial variation of the IMF within such galaxies, using a combination of high quality Keck spectroscopy and a new suite of stellar population synthesis models that cover a wide range in metallicity. As in the previous studies in this series, the models are fitted directly to the spectra and treat all elemental abundance ratios as free parameters. Using newly obtained spectroscopy for six galaxies, including deep data extending to ~1Re for the galaxies NGC1407, NGC1600, and NGC2695, we find that the IMF varies strongly with galactocentric radius. For all six galaxies the IMF is bottom-heavy in the central regions, with average Mass-to-light ratio "mismatch" parameter a~2.5 at R=0. The IMF rapidly becomes more bottom-light with increasing radius, flattening off near the Milky Way value (a~1.1) at R>0.4Re. A consequence is that the luminosity-weighted average IMF depends on the measurement aperture: within R=Re we find =1.3-1.5, consistent with recent lensing and dynamical results from SLACS and ATLAS-3D. Our results are also consistent with several earlier studies that were based on analyses of radial gradients of line indices. The observed IMF gradients support galaxy formation models in which the central regions of Massive galaxies had a different formation history than their outer parts. Finally, we make use of the high signal-to-noise central spectra of NGC1407 and NGC2695 to demonstrate how we can disentangle IMF effects and abundance effects.
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the stellar Initial Mass function in early type galaxies from absorption line spectroscopy ii results
The Astrophysical Journal, 2012Co-Authors: Charlie Conroy, Pieter G Van DokkumAbstract:The spectral absorption lines in early-type galaxies contain a wealth of information regarding the detailed abundance pattern, star formation history, and stellar Initial Mass function (IMF) of the underlying stellar population. Using our new population synthesis model that accounts for the effect of variable abundance ratios of 11 elements, we analyze very high quality absorption line spectra of 38 early-type galaxies and the nuclear bulge of M31. These data extend to 1 μm and they therefore include the IMF-sensitive spectral features Na I, Ca II, and FeH at 0.82 μm, 0.86 μm, and 0.99 μm, respectively. The models fit the data well, with typical rms residuals 1%. Strong constraints on the IMF and therefore the stellar Mass-to-light ratio, (M/L)stars, are derived for individual galaxies. We find that the IMF becomes increasingly bottom-heavy with increasing velocity dispersion and [Mg/Fe]. At the lowest dispersions and [Mg/Fe] values the derived IMF is consistent with the Milky Way (MW) IMF, while at the highest dispersions and [Mg/Fe] values the derived IMF contains more low-Mass stars (is more bottom-heavy) than even a Salpeter IMF. Our best-fit (M/L)stars values do not exceed dynamically based M/L values. We also apply our models to stacked spectra of four metal-rich globular clusters in M31 and find an (M/L)stars that implies fewer low-Mass stars than a MW IMF, again agreeing with dynamical constraints. We discuss other possible explanations for the observed trends and conclude that variation in the IMF is the simplest and most plausible.
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the stellar Initial Mass function in early type galaxies from absorption line spectroscopy i data and empirical trends
The Astrophysical Journal, 2012Co-Authors: Pieter G Van Dokkum, Charlie ConroyAbstract:The strength of gravity-sensitive absorption lines in the integrated light of old stellar populations is one of the few direct probes of the stellar Initial Mass function (IMF) outside of the Milky Way. Owing to the advent of fully depleted CCDs with little or no fringing it has recently become possible to obtain accurate measurements of these features. Here, we present spectra covering the wavelength ranges 0.35-0.55 ?m and 0.72-1.03 ?m for the bulge of M31 and 34 early-type galaxies from the SAURON sample, obtained with the Low Resolution Imaging Spectrometer on Keck. The signal-to-noise ratio is 200 ??1 out to 1 ?m, which is sufficient to measure gravity-sensitive features for individual galaxies and to determine how they depend on other properties of the galaxies. Combining the new data with previously obtained spectra for globular clusters in M31 and the most Massive elliptical galaxies in the Virgo cluster, we find that the dwarf-sensitive Na I ?8183, 8195 doublet and the FeH ?9916 Wing-Ford band increase systematically with velocity dispersion, while the giant-sensitive Ca II ?8498, 8542, 8662 triplet decreases with dispersion. These trends are consistent with a varying IMF, such that galaxies with deeper potential wells have more dwarf-enriched Mass functions. In a companion paper, we use a comprehensive stellar population synthesis model to demonstrate that IMF effects can be separated from age and abundance variations and quantify the IMF variation among early-type galaxies.
Pavel Kroupa - One of the best experts on this subject based on the ideXlab platform.
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the formation of ultra compact dwarf galaxies and Massive globular clusters quasar like objects to test for a variable stellar Initial Mass function
Astronomy and Astrophysics, 2017Co-Authors: Pavel Kroupa, Jorg Dabringhausen, T Jeřabkova, M Hilker, Kenji BekkiAbstract:The stellar Initial Mass function (IMF) has been described as being invariant, bottom-heavy, or top-heavy in extremely dense star-burst conditions. To provide usable observable diagnostics, we calculate redshift dependent spectral energy distributions of stellar populations in extreme star-burst clusters, which are likely to have been the precursors of present day Massive globular clusters (GCs) and of ultra compact dwarf galaxies (UCDs). The retention fraction of stellar remnants is taken into account to assess the Mass to light ratios of the ageing star-burst. Their redshift dependent photometric properties are calculated as predictions for James Webb Space Telescope (JWST) observations. While the present day GCs and UCDs are largely degenerate concerning bottom-heavy or top-heavy IMFs, a metallicity- and density-dependent top-heavy IMF implies the most Massive UCDs, at ages < 100 Myr, to appear as objects with quasar-like luminosities with a 0.1−10% variability on a monthly timescale due to core collapse supernovae.
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the chemical evolution of galaxies with a variable integrated galactic Initial Mass function
Monthly Notices of the Royal Astronomical Society, 2015Co-Authors: S Recchi, Pavel KroupaAbstract:ABSTRACT Standard analytical chemical evolution modelling of galaxies has been assuming thestellar Initial Mass function (IMF) to be invariant and fully sampled allowing fractionsof Massive stars to contribute even in dwarf galaxies with very low star formationrates (SFRs). Recent observations show the integrated galactic Initial Mass function(IGIMF) of stars, i.e. the galaxy-wide IMF, to become systematically top-heavy withincreasing SFR. This has been predicted by the IGIMF theory, which is here usedto develop the analytical theory of the chemical evolution of galaxies. This theoryis non-linear and requires the iterative solution of implicit integral equations due tothe dependence of the IGIMF on the metallicity and on the SFR. It is shown thatthe Mass–metallicity relation of galaxies emerges naturally, although at low Masses thetheoreticalpredictionsoverestimatethe observationsby 0.3–0.4dex. A good agreementwith the observation can be obtained only if gas flows are taken into account. Inparticular, we are able to reproduce the Mass–metallicity relation observed by Lee etal. (2006) with modest amounts of infall and with an outflow rate which decreasesas a function of the galactic Mass. The outflow rates required to fit the data areconsiderably smaller than required in models with invariant IMFs.Key words: Stars: abundances – stars: luminosity function, Mass function – su-pernovae: general – Galaxies: evolution – Galaxies: dwarf – Galaxies: star clusters:general
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evidence for top heavy stellar Initial Mass functions with increasing density and decreasing metallicity
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: Pavel Kroupa, Michael S Marks, Jorg Dabringhausen, Marcel S PawlowskiAbstract:ABSTRACT Residual-gas expulsion after cluster formation has recently been shown to leave animprint in the low-Mass present-day stellar Mass function (PDMF) which allowedthe estimation of birth conditions of some Galactic globular clusters (GCs) such asMass, radius and star formation efficiency. We show that in order to explain theircharacteristics (Masses, radii, metallicity, PDMF) their stellar Initial Mass function(IMF) must have been top-heavy. It is found that the IMF is required to becomemore top-heavy the lower the cluster metallicity and the larger the pre-GC cloud-core density are. The deduced trends are in qualitative agreement with theoreticalexpectation. The results are consistent with estimates of the shape of the high-Massend of the IMF in the Arches cluster, Westerlund 1, R136 and NGC 3603, as wellas with the IMF independently constrained for ultra-compact dwarf galaxies (UCDs).The latter suggests that GCs and UCDs might have formed along the same channel orthat UCDs formed via mergers of GCs. A fundamental plane is found which describesthe variation of the IMF with density and metallicity of the pre-GC cloud-cores. Theimplications for the evolution of galaxies and chemical enrichment over cosmologicaltimes are expected to be major.Keywords: stars: formation – stars: Mass-function – stars: early-type – stars: late-type – globular clusters: general
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evidence for top heavy stellar Initial Mass functions with increasing density and decreasing metallicity
arXiv: Astrophysics of Galaxies, 2012Co-Authors: Pavel Kroupa, Michael S Marks, Jorg Dabringhausen, Marcel S PawlowskiAbstract:Residual-gas expulsion after cluster formation has recently been shown to leave an imprint in the low-Mass present-day stellar Mass function (PDMF) which allowed the estimation of birth conditions of some Galactic globular clusters (GCs) such as Mass, radius and star formation efficiency. We show that in order to explain their characteristics (Masses, radii, metallicity, PDMF) their stellar Initial Mass function (IMF) must have been top-heavy. It is found that the IMF is required to become more top-heavy the lower the cluster metallicity and the larger the pre-GC cloud-core density are. The deduced trends are in qualitative agreement with theoretical expectation. The results are consistent with estimates of the shape of the high-Mass end of the IMF in the Arches cluster, Westerlund 1, R136 and NGC 3603, as well as with the IMF independently constrained for ultra-compact dwarf galaxies (UCDs). The latter suggests that GCs and UCDs might have formed along the same channel or that UCDs formed via mergers of GCs. A fundamental plane is found which describes the variation of the IMF with density and metallicity of the pre-GC cloud-cores simultaneously. The implications for the evolution of galaxies and chemical enrichment over cosmological times are expected to be major.
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galactic field Initial Mass functions of Massive stars
The Astrophysical Journal, 2003Co-Authors: Pavel Kroupa, Carsten WeidnerAbstract:Over the past years observations of young and populous star clusters have shown that the stellar Initial Mass function (IMF) appears to be an invariant featureless Salpeter power law with an exponent ? = 2.35 for stars more Massive than a few M?. A consensus has also emerged that most, if not all, stars form in stellar groups and star clusters and that the Mass function of young star clusters in the solar neighborhood and in interacting galaxies can be described, over the Mass range of a few 10 to 107 M?, as a power law with an exponent ? ? 2. These two results imply that galactic-field IMFs for early-type stars cannot, under any circumstances, be a Salpeter power law, but that they must have a steeper exponent, ?field 2.8. This has important consequences for the distribution of stellar remnants and for the chemodynamical and photometric evolution of galaxies.
Tommaso Treu - One of the best experts on this subject based on the ideXlab platform.
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stellar velocity dispersion and Initial Mass function gradients in dissipationless galaxy mergers
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Carlo Nipoti, Carlo Cannarozzo, F Calura, A Sonnenfeld, Tommaso TreuAbstract:The stellar Initial Mass function (IMF) is believed to be non-universal among early-type galaxies (ETGs). Parameterizing the IMF with the so-called IMF mismatch parameter αIMF, which is a measure of the stellar Mass-to-light ratio of an ensemble of stars and thus of the ‘heaviness’ of its IMF, one finds that for ETGs αe (i.e. αIMF integrated within the effective radius Re) increases with σe (the line-of-sight velocity dispersion σlos integrated within Re) and that, within the same ETG, αIMF tends to decrease outwards. We study the effect of dissipationless (dry) mergers on the distribution of the IMF mismatch parameter αIMF in ETGs using the results of binary major and minor merging simulations. We find that dry mergers tend to make the αIMF profiles of ETGs shallower, but do not alter significantly the shape of the distributions in the spatially resolved σlosαIMF space. Individual galaxies undergoing dry mergers tend to decrease their αe, due to erosion of αIMF gradients and mixing with stellar populations with lighter IMF. Their σe can either decrease or increase, depending on the merging orbital parameters and Mass ratio, but tends to decrease for cosmologically motivated merging histories. The αe-σe relation can vary with redshift as a consequence of the evolution of individual ETGs: based on a simple dry-merging model, ETGs of given σe are expected to have higher αe at higher redshift, unless the accreted satellites are so diffuse that they contribute negligibly to the inner stellar distribution of the merger remnant.
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the swells survey vi hierarchical inference of the Initial Mass functions of bulges and discs
Monthly Notices of the Royal Astronomical Society, 2014Co-Authors: Brendon J Brewer, Aaron A Dutton, Tommaso Treu, M W Auger, P J Marshall, Matteo BarnabeAbstract:The long-standing assumption that the stellar Initial Mass function (IMF) is universal has recently been challenged by a number of observations. Several studies have shown that a \heavy" IMF (e.g., with a Salpeter-like abundance of low Mass stars and thus normalisation) is preferred for Massive early-type galaxies, while this IMF is inconsistent with the properties of less Massive, later-type galaxies. These discoveries motivate the hypothesis that the IMF may vary (possibly very slightly) across galaxies and across components of individual galaxies (e.g. bulges vs discs). In this paper we use a sample of 19 late-type strong gravitational lenses from the SWELLS survey to investigate the IMFs of the bulges and discs in late-type galaxies. We perform a joint analysis of the galaxies’ total Masses (constrained by strong gravitational lensing) and stellar Masses (constrained by optical and near-infrared colors in the context of a stellar population synthesis [SPS] model, up to an IMF normalisation parameter). Using minimal assumptions apart from the physical constraint that the total stellar Mass m within any aperture must be less than the total Mass mtot within the aperture, we nd that the bulges of the galaxies cannot have IMFs heavier (i.e. implying high Mass per unit luminosity) than Salpeter, while the disc IMFs are not well constrained by this data set. We also discuss the necessity for hierarchical modelling when combining incomplete information about multiple astronomical objects. This modelling approach allows us to place upper limits on the size of any departures from universality. More data, including spatially resolved kinematics (as in paper V) and stellar population diagnostics over a range of bulge and disc Masses, are needed to robustly quantify how the IMF varies within galaxies.
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the swells survey v a salpeter stellar Initial Mass function in the bulges of Massive spiral galaxies
Monthly Notices of the Royal Astronomical Society, 2013Co-Authors: Brendon J Brewer, Aaron A Dutton, Tommaso Treu, Philip J Marshall, Matteo Barnabe, M W Auger, Adam S BoltonAbstract:Recent work has suggested that the stellar Initial Mass function (IMF) is not universal, but rather is correlated with galaxy stellar Mass, stellar velocity dispersion or morphological type. In this paper, we investigate variations of the IMF within individual galaxies. For this purpose, we use strong lensing and gas kinematics to measure independently the normalization of the IMF of the bulge and disc components of a sample of five Massive spiral galaxies with substantial bulge components taken from the Sloan WFC Edge-on Late-type Lens Survey (SWELLS). We find that the stellar Masses of the bulges are tightly constrained by the lensing and kinematic data. A comparison with Masses based on stellar population synthesis models fitted to optical and near-infrared photometry favours a Salpeter-like normalization of the IMF. Conversely, the disc Masses are less well constrained due to degeneracies with the dark matter halo, but are consistent with Milky Way-type IMFs in agreement with previous studies. The discs are submaximal at 2.2 disc scale lengths, but due to the contribution of the bulges, the galaxies are baryon dominated at 2.2 disc scale lengths. Globally, our inferred IMF normalization is consistent with that found for early-type galaxies of comparable stellar Mass (>10(11) M-circle dot). Our results suggest a non-universal IMF within the different components of spiral galaxies, adding to the well-known differences in stellar populations between discs and bulges.
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evidence for dark matter contraction and a salpeter Initial Mass function in a Massive early type galaxy
The Astrophysical Journal, 2012Co-Authors: Alessandro Sonnenfeld, Tommaso Treu, Philip J Marshall, L V E Koopmans, R Gavazzi, M W Auger, S H Suyu, A BoltonAbstract:Stars and dark matter account for most of the Mass of early-type galaxies, but uncertainties in the stellar population and the dark matter profile make it challenging to distinguish between the two components. Nevertheless, precise observations of stellar and dark matter are extremely valuable for testing the many models of structure formation and evolution. We present a measurement of the stellar Mass and inner slope of the dark matter halo of a Massive early-type galaxy at z = 0.222. The galaxy is the foreground deflector of the double Einstein ring gravitational lens system SDSSJ0946+1006, also known as the Jackpot. By combining the tools of lensing and dynamics we first constrain the mean slope of the total Mass density profile () within the radius of the outer ring to be γ' = 1.98 ± 0.02 ± 0.01. Then we obtain a bulge-halo decomposition, assuming a power-law form for the dark matter halo. Our analysis yields γDM = 1.7 ± 0.2 for the inner slope of the dark matter profile, in agreement with theoretical findings on the distribution of dark matter in ellipticals, and a stellar Mass from lensing and dynamics M LD * = 5.5–1.3 +0.4 × 1011 M ☉. By comparing this measurement with stellar Masses inferred from stellar population synthesis fitting we find that a Salpeter Initial Mass function (IMF) provides a good description of the stellar population of the lens while the probability of the IMF being heavier than Chabrier is 95%. Our data suggest that growth by accretion of small systems from a compact red nugget is a plausible formation scenario for this object.
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the swells survey iii disfavouring heavy Initial Mass functions for spiral lens galaxies
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: Brendon J Brewer, Aaron A Dutton, Tommaso Treu, Matthew W Auger, Philip J Marshall, Matteo BarnabeAbstract:We present gravitational lens models for 20 strong gravitational lens systems observed as part of the Sloan WFC Edge-on Late-type Lens Survey (SWELLS) project. 15 of the lenses are taken from Paper I, while five are newly discovered systems. The systems are galaxy-galaxy lenses where the foreground deflector has an inclined disc, with a wide range of morphological types, from late-type spiral to lenticular. For each system, we compare the total Mass inside the critical curve inferred from gravitational lens modelling to the stellar Mass inferred from stellar population synthesis (SPS) models, computing the stellar Mass fraction f*≡MSPS/Mlens. We find that, for the lower Mass SWELLS systems, adoption of a Salpeter stellar Initial Mass function (IMF) leads to estimates of f* that exceed 1. This is unphysical and provides strong evidence against the Salpeter IMF being valid for these systems. Taking the lower Mass end of the SWELLS sample (σSIE <230 km s-1), we find that the IMF is lighter (in terms of stellar Mass-to-light ratio) than Salpeter with 98 per cent probability, and consistent with the Chabrier IMF and IMFs between the two. This result is consistent with previous studies of spiral galaxies based on independent techniques. In combination with recent studies of Massive early-type galaxies that have favoured a heavier Salpeter-like IMF, this result strengthens the evidence against a universal stellar IMF.
Volker Bromm - One of the best experts on this subject based on the ideXlab platform.
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building up the population iii Initial Mass function from cosmological Initial conditions
Monthly Notices of the Royal Astronomical Society, 2016Co-Authors: Athena Stacy, Volker Bromm, A T LeeAbstract:We simulate the growth of a Population III stellar system, starting from cosmological Initial conditions at z=100. We follow the formation of a minihalo and the subsequent collapse of its central gas to high densities, resolving scales as small as ~ 1 AU. Using sink particles to represent the growing protostars, we model the growth of the photodissociating and ionizing region around the first sink, continuing the simulation for ~ 5000 yr after Initial protostar formation. Along with the first-forming sink, several tens of secondary sinks form before an ionization front develops around the most Massive star. The resulting cluster has high rates of sink formation, ejections from the stellar disc, and sink mergers during the first ~ 2000 yr, before the onset of radiative feedback. By this time a warm ~ 5000 K phase of neutral gas has expanded to roughly the disc radius of 2000 AU, slowing Mass flow onto the disc and sinks. By 5000 yr the most Massive star grows to 20 M_sol, while the total stellar Mass approaches 75 M_sol. Out of the ~ 40 sinks, approximately 30 are low-Mass (M_* < 1 M_sol), and if the simulation had resolved smaller scales an even greater number of sinks might have formed. Thus, protostellar radiative feedback is insufficient to prevent rapid disc fragmentation and the formation of a high-member Pop III cluster before an ionization front emerges. Throughout the simulation, the majority of stellar Mass is contained within the most Massive stars, further implying that the Pop III Initial Mass function is top-heavy.
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gravitational fragmentation in turbulent primordial gas and the Initial Mass function of population iii stars
The Astrophysical Journal, 2011Co-Authors: Paul C Clark, Simon C O Glover, Ralf S Klessen, Volker BrommAbstract:We report results from numerical simulations of star formation in the early universe that focus on the dynamical behavior of metal-free gas under different Initial and environmental conditions. In particular we investigate the role of turbulence, which is thought to ubiquitously accompany the collapse of high-redshift halos. We distinguish between two main cases: the birth of Population III.1 stars—those which form in the pristine halos unaffected by prior star formation—and the formation of Population III.2 stars—those forming in halos where the gas has an increased ionization fraction. We find that turbulent primordial gas is highly susceptible to fragmentation in both cases, even for turbulence in the subsonic regime, i.e., for rms velocity dispersions as low as 20% of the sound speed. Fragmentation is more vigorous and more widespread in pristine halos compared to pre-ionized ones. If such levels of turbulent motions were indeed present in star-forming minihalos, Population III.1 stars would be on average of somewhat lower Mass, and form in larger groups, than Population III.2 stars. We find that fragment Masses cover over two orders of magnitude, suggesting that the Population III Initial Mass function may have been much broader than previously thought. This prompts the need for a large, high-resolution study of the formation of dark matter minihalos that is capable of resolving the turbulent flows in the gas at the moment when the baryons become self-gravitating. This would help to determine the applicability of our results to primordial star formation.
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gravitational fragmentation in turbulent primordial gas and the Initial Mass function of population iii stars
arXiv: Astrophysics of Galaxies, 2010Co-Authors: Paul C Clark, Simon C O Glover, Ralf S Klessen, Volker BrommAbstract:We report results from numerical simulations of star formation in the early universe that focus on the dynamical behavior of metal-free gas under different Initial and environmental conditions. In particular we investigate the role of turbulence, which is thought to ubiquitously accompany the collapse of high-redshift halos. We distinguish between two main cases: the birth of Population III.1 stars - those which form in the pristine halos unaffected by prior star formation - and the formation of Population III.2 stars - those forming in halos where the gas is still metal free but has an increased ionization fraction. This latter case can arise either from exposure to the intense UV radiation of stellar sources in neighboring halos, or from the high virial temperatures associated with the formation of Massive halos, that is, those with Masses greater than 1e8 solar Masses. We find that turbulent primordial gas is highly susceptible to fragmentation in both cases, even for turbulence in the subsonic regime, i.e. for rms velocity dispersions as low as 20 % of the sound speed. Contrary to our original expectations, fragmentation is more vigorous and more widespread in pristine halos compared to pre-ionized ones. We therefore predict Pop III.1 stars to be on average of somewhat lower Mass, and form in larger groups, than Pop III.2 stars. We find that fragment Masses cover over two orders of magnitude, indicating that the resulting Population III Initial Mass function was significantly extended in Mass as well. This prompts the need for a large, high-resolution study of the formation of dark matter minihalos that is capable of resolving the turbulent flows in the gas at the moment when the baryons become self-gravitating. This would help determine which, if any, of the Initial conditions presented in our study are realized in nature.
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generic spectrum and ionization efficiency of a heavy Initial Mass function for the first stars
The Astrophysical Journal, 2001Co-Authors: Volker Bromm, R P Kudritzki, Abraham LoebAbstract:We calculate the generic spectral signature of an early population of Massive stars at high redshifts. For metal-free stars with Mass above 300 M☉, we find that the combined spectral luminosity per unit stellar Mass is almost independent of the Mass distribution of these stars. To zeroth order, the generic spectrum resembles a blackbody with an effective temperature of ~105 K, making these stars highly efficient at ionizing hydrogen and helium. The production rate of ionizing radiation per stellar Mass by stars more Massive than ~300 M☉ is larger by ~1 order of magnitude for hydrogen and He I and by ~2 orders of magnitude for He II than the emission from a standard Initial Mass function. This would result in unusually strong hydrogen and helium recombination lines from the surrounding interstellar medium. It could also alleviate the current difficulty of ionizing the intergalactic medium at z 6 with the cosmic star formation rate inferred at somewhat lower redshifts.
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generic spectrum and ionization efficiency of a heavy Initial Mass function for the first stars
arXiv: Astrophysics, 2000Co-Authors: Volker Bromm, R P Kudritzki, Abraham LoebAbstract:We calculate the generic spectral signature of an early population of Massive stars at high redshifts. For metal-free stars in the Mass range of 100-1000 solar Masses, we find that the combined spectral luminosity per unit stellar Mass is almost independent of the Mass distribution of these stars. To zeroth order, the generic spectrum resembles a black-body with an effective temperature of ~10^5 K, making these stars highly efficient at ionizing hydrogen and helium. The production rate of ionizing radiation per stellar Mass by stars more Massive than 100 solar Masses is larger by about an order of magnitude for hydrogen and He I, and by about two orders of magnitude for He II, than the emission from a standard Initial Mass function. This would result in unusually strong hydrogen and helium recombination lines from the surrounding interstellar medium. It could also alleviate the current difficulty of ionizing the intergalactic medium at z>6 with the cosmic star formation rate inferred at somewhat lower redshifts.
Aaron A Dutton - One of the best experts on this subject based on the ideXlab platform.
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sdss iv manga variation of the stellar Initial Mass function in spiral and early type galaxies
The Astrophysical Journal, 2017Co-Authors: Hongyu Li, Aaron A Dutton, Michele Cappellari, Junqiang Ge, R J Long, Ran Li, Eric Emsellem, Cheng LiAbstract:We perform Jeans anisotropic modeling (JAM) on elliptical and spiral galaxies from the MaNGA DR13 sample. By comparing the stellar Mass-to-light ratios estimated from stellar population synthesis (SPS) and from JAM, we find a similar systematic variation of the Initial Mass function (IMF) as in the earlier $\rm ATLAS^{3D}$ results. Early type galaxies (elliptical and lenticular) with lower velocity dispersions within one effective radius are consistent with a Chabrier-like IMF while galaxies with higher velocity dispersions are consistent with a more bottom heavy IMF such as the Salpeter IMF. Spiral galaxies have similar systematic IMF variations, but with slightly different slopes and larger scatters, due to the uncertainties caused by higher gas fractions and extinctions for these galaxies. Furthermore, we examine the effects of stellar Mass-to-light ratio gradients on our JAM modeling, and find that the trends from our results becomes stronger after considering the gradients.
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the swells survey vi hierarchical inference of the Initial Mass functions of bulges and discs
Monthly Notices of the Royal Astronomical Society, 2014Co-Authors: Brendon J Brewer, Aaron A Dutton, Tommaso Treu, M W Auger, P J Marshall, Matteo BarnabeAbstract:The long-standing assumption that the stellar Initial Mass function (IMF) is universal has recently been challenged by a number of observations. Several studies have shown that a \heavy" IMF (e.g., with a Salpeter-like abundance of low Mass stars and thus normalisation) is preferred for Massive early-type galaxies, while this IMF is inconsistent with the properties of less Massive, later-type galaxies. These discoveries motivate the hypothesis that the IMF may vary (possibly very slightly) across galaxies and across components of individual galaxies (e.g. bulges vs discs). In this paper we use a sample of 19 late-type strong gravitational lenses from the SWELLS survey to investigate the IMFs of the bulges and discs in late-type galaxies. We perform a joint analysis of the galaxies’ total Masses (constrained by strong gravitational lensing) and stellar Masses (constrained by optical and near-infrared colors in the context of a stellar population synthesis [SPS] model, up to an IMF normalisation parameter). Using minimal assumptions apart from the physical constraint that the total stellar Mass m within any aperture must be less than the total Mass mtot within the aperture, we nd that the bulges of the galaxies cannot have IMFs heavier (i.e. implying high Mass per unit luminosity) than Salpeter, while the disc IMFs are not well constrained by this data set. We also discuss the necessity for hierarchical modelling when combining incomplete information about multiple astronomical objects. This modelling approach allows us to place upper limits on the size of any departures from universality. More data, including spatially resolved kinematics (as in paper V) and stellar population diagnostics over a range of bulge and disc Masses, are needed to robustly quantify how the IMF varies within galaxies.
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the swells survey v a salpeter stellar Initial Mass function in the bulges of Massive spiral galaxies
Monthly Notices of the Royal Astronomical Society, 2013Co-Authors: Brendon J Brewer, Aaron A Dutton, Tommaso Treu, Philip J Marshall, Matteo Barnabe, M W Auger, Adam S BoltonAbstract:Recent work has suggested that the stellar Initial Mass function (IMF) is not universal, but rather is correlated with galaxy stellar Mass, stellar velocity dispersion or morphological type. In this paper, we investigate variations of the IMF within individual galaxies. For this purpose, we use strong lensing and gas kinematics to measure independently the normalization of the IMF of the bulge and disc components of a sample of five Massive spiral galaxies with substantial bulge components taken from the Sloan WFC Edge-on Late-type Lens Survey (SWELLS). We find that the stellar Masses of the bulges are tightly constrained by the lensing and kinematic data. A comparison with Masses based on stellar population synthesis models fitted to optical and near-infrared photometry favours a Salpeter-like normalization of the IMF. Conversely, the disc Masses are less well constrained due to degeneracies with the dark matter halo, but are consistent with Milky Way-type IMFs in agreement with previous studies. The discs are submaximal at 2.2 disc scale lengths, but due to the contribution of the bulges, the galaxies are baryon dominated at 2.2 disc scale lengths. Globally, our inferred IMF normalization is consistent with that found for early-type galaxies of comparable stellar Mass (>10(11) M-circle dot). Our results suggest a non-universal IMF within the different components of spiral galaxies, adding to the well-known differences in stellar populations between discs and bulges.
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the swells survey iii disfavouring heavy Initial Mass functions for spiral lens galaxies
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: Brendon J Brewer, Aaron A Dutton, Tommaso Treu, Matthew W Auger, Philip J Marshall, Matteo BarnabeAbstract:We present gravitational lens models for 20 strong gravitational lens systems observed as part of the Sloan WFC Edge-on Late-type Lens Survey (SWELLS) project. 15 of the lenses are taken from Paper I, while five are newly discovered systems. The systems are galaxy-galaxy lenses where the foreground deflector has an inclined disc, with a wide range of morphological types, from late-type spiral to lenticular. For each system, we compare the total Mass inside the critical curve inferred from gravitational lens modelling to the stellar Mass inferred from stellar population synthesis (SPS) models, computing the stellar Mass fraction f*≡MSPS/Mlens. We find that, for the lower Mass SWELLS systems, adoption of a Salpeter stellar Initial Mass function (IMF) leads to estimates of f* that exceed 1. This is unphysical and provides strong evidence against the Salpeter IMF being valid for these systems. Taking the lower Mass end of the SWELLS sample (σSIE <230 km s-1), we find that the IMF is lighter (in terms of stellar Mass-to-light ratio) than Salpeter with 98 per cent probability, and consistent with the Chabrier IMF and IMFs between the two. This result is consistent with previous studies of spiral galaxies based on independent techniques. In combination with recent studies of Massive early-type galaxies that have favoured a heavier Salpeter-like IMF, this result strengthens the evidence against a universal stellar IMF.
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dark halo response and the stellar Initial Mass function in early type and late type galaxies
Monthly Notices of the Royal Astronomical Society, 2011Co-Authors: Aaron A Dutton, Charlie Conroy, Frank C Van Den Bosch, Luc Simard, Trevor J Mendel, Stephane Courteau, Avishai Dekel, Surhud More, Francisco PradaAbstract:We investigate the origin of the relations between stellar Mass and optical circular velocity for early-type galaxies (ETGs) and late-type galaxies (LTGs) – the Faber–Jackson (FJ) and Tully–Fisher (TF) relations. We combine measurements of dark halo Masses (from satellite kinematics and weak lensing), and the distribution of baryons in galaxies (from a new compilation of galaxy scaling relations), with constraints on dark halo structure from cosmological simulations. The principal unknowns are the halo response to galaxy formation and the stellar Initial Mass function (IMF). The slopes of the TF and FJ relations are naturally reproduced for a wide range of halo response and IMFs. However, models with a universal IMF and universal halo response cannot simultaneously reproduce the zero-points of both the TF and FJ relations. For a model with a universal Chabrier IMF, LTGs require halo expansion, while ETGs require halo contraction. A Salpeter IMF is permitted for high-Mass (σ≳ 180 km s−1) ETGs, but is inconsistent for intermediate Masses, unless Vcirc(Re)/σe≳ 1.6. If the IMF is universal and close to Chabrier, we speculate that the presence of a major merger may be responsible for the contraction in ETGs while clumpy accreting streams and/or feedback leads to expansion in LTGs. Alternatively, a recently proposed variation in the IMF disfavours halo contraction in both types of galaxies. Finally we show that our models naturally reproduce flat and featureless circular velocity profiles within the optical regions of galaxies without fine-tuning.