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Register a new userThe importance of magnetic fields for the initial mass function of the first stars
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Magnetic fields play an important role for the formation of stars in both local and high-redshift galaxies. Recent studies of dynamo amplification in the first dark matter haloes suggest that significant magnetic fields were likely present during the formation of the first stars in the Universe at redshifts of 15 and above. In this work, we study how these magnetic fields potentially impact the initial mass function (IMF) of the first stars. We perform 200 high-resolution, three-dimensional (3D), magneto-hydrodynamic (MHD) simulations of the collapse of primordial clouds with different initial turbulent magnetic field strengths as predicted from turbulent dynamo theory in the early Universe, forming more than 1100 first stars in total. We detect a strong statistical signature of suppressed fragmentation in the presence of strong magnetic fields, leading to a dramatic reduction in the number of first stars with masses low enough that they might be expected to survive to the present day. Additionally, strong fields shift the transition point where stars go from being mostly single to mostly multiple to higher masses. However, irrespective of the field strength, individual simulations are highly chaotic, show different levels of fragmentation and clustering, and the outcome depends on the exact realisation of the turbulence in the primordial clouds. While these are still idealised simulations that do not start from cosmological initial conditions, our work shows that magnetic fields play a key role for the primordial IMF, potentially even more so than for the present-day IMF.
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The characteristic mass that sets the peak of the stellar initial mass function (IMF) is closely linked to the thermodynamic behaviour of interstellar gas, which controls how gas fragments as it collapses under gravity. As the Universe has grown in metal abundance over cosmic time, this thermodynamic behaviour has evolved from a primordial regime dominated by the competition between compressional heating and molecular hydrogen cooling to a modern regime where the dominant process in dense gas is protostellar radiation feedback, transmitted to the gas by dust-gas collisions. In this paper we map out the primordial-to-modern transition by constructing a model for the thermodynamics of collapsing, dusty gas clouds at a wide range of metallicities. We show the transition from the primordial regime to the modern regime begins at metallicity $Zsim 10^{-4} rm{Z_odot}$, passes through an intermediate stage where metal line cooling is dominant at $Z sim 10^{-3},rm{Z_{odot}}$, and then transitions to the modern dust- and feedback-dominated regime at $Zsim 10^{-2} rm{Z_odot}$. In low pressure environments like the Milky Way, this transition is accompanied by a dramatic change in the characteristic stellar mass, from $sim 50,rm{M_odot}$ at $Z sim 10^{-6},rm{Z_{odot}}$ to $sim 0.3,rm{M_odot}$ once radiation feedback begins to dominate, which marks the appearance of the modern bottom-heavy Milky Way IMF. In the high pressure environments typical of massive elliptical galaxies, the characteristic mass for the modern, dust-dominated regime falls to $sim 0.1,rm{M_{odot}}$, thus providing an explanation for the brown dwarf rich population observed in these galaxies. We conclude that metallicity is a key driver of variations in the characteristic stellar mass, and by extension, the IMF.
Many results in modern astrophysics rest on the notion that the Initial Mass Function (IMF) is universal. Our observations of HI selected galaxies in the light of H-alpha and the far-ultraviolet (FUV) challenge this notion. The flux ratio H-alpha/FUV from these two star formation tracers shows strong correlations with the surface-brightness in H-alpha and the R band: Low Surface Brightness (LSB) galaxies have lower ratios compared to High Surface Brightness galaxies and to expectations from equilibrium star formation models using commonly favored IMF parameters. Weaker but significant correlations of H-alpha/FUV with luminosity, rotational velocity and dynamical mass are found as well as a systematic trend with morphology. The correlated variations of H-alpha/FUV with other global parameters are thus part of the larger family of galaxy scaling relations. The H-alpha/FUV correlations can not be due to dust correction errors, while systematic variations in the star formation history can not explain the trends with both H-alpha and R surface brightness. LSB galaxies are unlikely to have a higher escape fraction of ionizing photons considering their high gas fraction, and color-magnitude diagrams. The most plausible explanation for the correlations are systematic variations of the upper mass limit and/or slope of the IMF at the upper end. We outline a scenario of pressure driving the correlations by setting the efficiency of the formation of the dense star clusters where the highest mass stars form. Our results imply that the star formation rate measured in a galaxy is highly sensitive to the tracer used in the measurement. A non-universal IMF also calls into question the interpretation of metal abundance patterns in dwarf galaxies and star formation histories derived from color magnitude diagrams. Abridged.
We study how an observationally-motivated, metallicity-dependent initial mass function (IMF) affects the feedback budget and observables of an ultra-faint dwarf galaxy. We model the evolution of a low-mass ($approx 8 , times , 10^{8} , rm M_{odot}$) dark matter halo with cosmological, zoomed hydrodynamical simulations capable of resolving individual supernovae explosions. We complement the EDGE galaxy formation model from Agertz et al. (2020) with a new prescription for IMF variations according to Geha et al. (2013). At the low metallicities typical of faint dwarf galaxies, the IMF becomes top-heavy, increasing the efficiency of supernova and photo-ionization feedback in regulating star formation. This results in a 100-fold reduction of the final stellar mass of the dwarf compared to a canonical IMF, at fixed dynamical mass. The increase in the feedback budget is nonetheless met by increased metal production from more numerous massive stars, leading to nearly constant iron content at $z=0$. A metallicity-dependent IMF therefore provides a mechanism to produce low-mass ($rm M_{star}sim 10^3 rm M_{odot}$), yet enriched ($rm [Fe/H]approx -2$) field dwarf galaxies, thus opening a self-consistent avenue to populate the plateau in $rm [Fe/H]$ at the faintest end of the mass-metallicity relation.
We simulate the formation of a metal-poor (10^-2 Zsun) stellar cluster in one of the first galaxies to form in the early Universe, specifically a high-redshift atomic cooling halo (z~14). This is the first calculation that resolves the formation of individual metal-enriched stars in simulations starting from realistic cosmological initial conditions. We follow the evolution of a single dense clump among several in the parent halo. The clump forms a cluster of ~40 stars and sub-stellar objects within 7000 years and could continue forming stars ~5 times longer. Protostellar dust heating has a negligible effect on the star formation efficiency, at least during the early evolutionary stages, but it moderately suppresses gaseous fragmentation and brown dwarf formation. We observe fragmentation in thin gaseous filaments and sustained accretion in larger, rotating structures as well as ejections by binary interactions. The stellar initial mass function above 0.1 Msun, evaluated after ~10^4 years of fragmentation and accretion, seems in agreement with the recent measurement in ultra-faint dwarf spheroidal Galactic satellites of Geha et al. (2013).
Using the Oxford Short Wavelength Integral Field specTrograph (SWIFT), we trace radial variations of initial mass function (IMF) sensitive absorption features of three galaxies in the Coma cluster. We obtain resolved spectroscopy of the central 5kpc for the two central brightest-cluster galaxies (BCGs) NGC4889, NGC4874, and the BCG in the south-west group NGC4839, as well as unresolved data for NGC4873 as a low-$sigma_*$ control. We present radial measurements of the IMF-sensitive features sodium NaI$_{rm{SDSS}}$, calcium triplet CaT and iron-hydride FeH0.99, along with the magnesium MgI0.88 and titanium oxide TiO0.89 features. We employ two separate methods for both telluric correction and sky-subtraction around the faint FeH feature to verify our analysis. Within NGC4889 we find strong gradients of NaI$_{rm{SDSS}}$ and CaT but a flat FeH profile, which from comparing to stellar population synthesis models, suggests an old, $alpha$-enhanced population with a Chabrier, or even bottom-light IMF. The age and abundance is in line with previous studies but the normal IMF is in contrast to recent results suggesting an increased IMF slope with increased velocity dispersion. We measure flat NaI$_{rm{SDSS}}$ and FeH profiles within NGC4874 and determine an old, possibly slightly $alpha$-enhanced and Chabrier IMF population. We find an $alpha$-enhanced, Chabrier IMF population in NGC4873. Within NGC4839 we measure both strong NaI$_{rm{SDSS}}$ and strong FeH, although with a large systematic uncertainty, suggesting a possible heavier IMF. The IMFs we infer for these galaxies are supported by published dynamical modelling. We stress that IMF constraints should be corroborated by further spectral coverage and independent methods on a galaxy-by-galaxy basis.
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