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Register a new userThe effect of the environment-dependent IMF on the formation and metallicities of stars over the cosmic history
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Added by
Martyna Chru\\'sli\\'nska
Publication date
2020
fields
Physics
and research's language is
English
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Recent observational and theoretical studies indicate that the stellar initial mass function (IMF) varies systematically with the environment (star formation rate - SFR, metallicity). Although the exact dependence of the IMF on those properties is likely to change with improving observational constraints, the reported trend in the shape of the IMF appears robust. We present the first study aiming to evaluate the effect of the IMF variations on the measured cosmic SFR density (SFRD) as a function of metallicity and redshift, $f_{rm SFR}$(Z,z). We also study the expected number and metallicity of white dwarf, neutron star and black hole progenitors under different IMF assumptions. Applying the empirically driven IMF variations described by the integrated galactic IMF (IGIMF) theory, we correct $f_{rm SFR}$(Z,z) obtained by Chruslinska & Nelemans (2019) and find lower SFRD at high redshifts as well as a higher fraction of metal-poor stars being formed. In the local Universe, our calculation applying the IGIMF theory suggests more white dwarf and neutron star progenitors in comparison with the universal IMF scenario, while the number of black hole progenitors remains unaffected.
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We combine constraints on galaxy formation histories with planet formation models, yielding the Earth-like and giant planet formation histories of the Milky Way and the Universe as a whole. In the Hubble Volume (10^13 Mpc^3), we expect there to be ~10^20 Earth-like and ~10^20 giant planets; our own galaxy is expected to host ~10^9 and ~10^10 Earth-like and giant planets, respectively. Proposed metallicity thresholds for planet formation do not significantly affect these numbers. However, the metallicity dependence for giant planets results in later typical formation times and larger host galaxies than for Earth-like planets. The Solar System formed at the median age for existing giant planets in the Milky Way, and consistent with past estimates, formed after 80% of Earth-like planets. However, if existing gas within virialised dark matter haloes continues to collapse and form stars and planets, the Universe will form over 10 times more planets than currently exist. We show that this would imply at least a 92% chance that we are not the only civilisation the Universe will ever have, independent of arguments involving the Drake Equation.
The question how much star formation is occurring at low metallicity throughout the cosmic history appears crucial for the discussion of the origin of various energetic transients, and possibly - double black hole mergers. We revisit the observation-based distribution of birth metallicities of stars (f$_{rm SFR}$(Z,z)), focusing on several factors that strongly affect its low metallicity part: (i) the method used to describe the metallicity distribution of galaxies (redshift-dependent mass metallicity relation - MZR, or redshift-invariant fundamental metallicity relation - FMR), (ii) the contribution of starburst galaxies and (iii) the slope of the MZR. We empirically construct the FMR based on the low-redshift scaling relations, which allows us to capture the systematic differences in the relation caused by the choice of metallicity and star formation rate (SFR) determination techniques and discuss the related f$_{rm SFR}$(Z,z) uncertainty. We indicate factors that dominate the f$_{rm SFR}$(Z,z) uncertainty in different metallicity and redshift regimes. The low metallicity part of the distribution is poorly constrained even at low redshifts (even a factor of $sim$200 difference between the model variations) The non-evolving FMR implies a much shallower metallicity evolution than the extrapolated MZR, however, its effect on the low metallicity part of the f$_{rm SFR}$(Z,z) is counterbalanced by the contribution of starbursts (assuming that they follow the FMR). A non-negligible fraction of starbursts in our model may be necessary to satisfy the recent high-redshift SFR density constraints.
We use 12000 stars from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic data to show that the metallicities of Kepler field stars as given in the Kepler Input Catalog (KIC) systematically underestimate both the true metallicity and the dynamic range of the Kepler sample. Specifically, to the first order approximation, we find [Fe/H]_KIC = -0.20 + 0.43 [Fe/H]_LAMOST, with a scatter of ~0.25 dex, due almost entirely to errors in KIC. This relation is most secure for -0.3<[Fe/H]_LAMOST<+0.4 where we have >200 comparison stars per 0.1 dex bin and good consistency is shown between metallicities determined by LAMOST and high-resolution spectra. It remains approximately valid in a slightly broader range. When the relation is inverted, the error in true metallicity as derived from KIC is (0.25 dex)/0.43~0.6 dex. We thereby quantitatively confirm the cautionary note by Brown et al. (2011) that KIC estimates of [Fe/H] should not be used by anyone with a particular interest in stellar metallicities. Fortunately, many more LAMOST spectroscopic metallicities will be available in the near future.
This article is based on an invited talk given by V. P. Kulkarni at the 8th Cosmic Dust meeting. Dust has a profound effect on the physics and chemistry of the interstellar gas in galaxies and on the appearance of galaxies. Understanding the cosmic evolution of dust with time is therefore crucial for understanding the evolution of galaxies. Despite the importance of interstellar dust, very little is known about its nature and composition in distant galaxies. We summarize the results of our ongoing programs using observations of distant quasars to obtain better constraints on dust grains in foreground galaxies that happen to lie along the quasar sightlines. These observations consist of a combination of mid-infrared data obtained with the Spitzer Space Telescope and optical/UV data obtained with ground-based telescopes and/or the Hubble Space Telescope. The mid-IR data target the 10 $mu$m and 18 $mu$m silicate absorption features, while the optical/UV data allow determinations of element depletions, extinction curves, 2175 {AA} bumps, etc. Measurements of such properties in absorption-selected galaxies with redshifts ranging from $zsim0$ to $z>2$ provide constraints on the evolution of interstellar dust over the past $> 10$ Gyr. The optical depth of the 10 $mu$m silicate absorption feature ($tau_{10}$) in these galaxies is correlated with the amount of reddening along the sightline. But there are indications [e.g., based on the $tau_{10}$ /$E(B-V)$ ratio and possible grain crystallinity] that the dust in these distant galaxies differs in structure and composition from the dust in the Milky Way and the Magellanic Clouds. We briefly discuss the implications of these results for the evolution of galaxies and their star formation history.
Metallicity is one of the crucial factors that determine stellar evolution. To characterize the properties of stellar populations one needs to know the fraction of stars forming at different metallicities. Knowing how this fraction evolves over time is necessary e.g. to estimate the rates of occurrence of any stellar evolution related phenomena (e.g. double compact object mergers, gamma ray bursts). Such theoretical estimates can be confronted with observational limits to validate the assumptions about the evolution of the progenitor system leading to a certain transient. However, to perform the comparison correctly one needs to know the uncertainties related to the assumed star formation history and chemical evolution of the Universe. We combine the empirical scaling relations and other observational properties of the star forming galaxies to construct the distribution of the cosmic star formation rate density at different metallicities and redshifts. We address the question of uncertainty of this distribution due to currently unresolved questions, such as the absolute metallicity scale, the flattening in the star formation--mass relation or the low mass end of the galaxy mass function. We find that the fraction of stellar mass formed at metallicities <10% solar (>solar) since z=3 varies by ~18% (~26%) between the extreme cases considered in our study. This uncertainty stems primarily from the differences in the mass metallicity relations obtained with different methods. We confront our results with the local core-collapse supernovae observations. Our model is publicly available.
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