TIMESCALES FOR LOW-MASS STAR FORMATION IN EXTRAGALACTIC ENVIRONMENTS: IMPLICATIONS FOR THE STELLAR INITIAL MASS FUNCTION

Starburst galaxies are currently forming massive stars at prodigious rates. I discuss the star-formation histories and the shape of the initial mass function, with particular emphasis on the high- and on the low-mass end. The classical Salpeter IMF is consistent with constraints from observations of the most massive stars, irrespective of environmental properties. The situation at the low-mass end is less clear: direct star counts in nearby giant H II regions show stars down to ~1 M⊙, whereas dynamical arguments in some starburst galaxies suggest a deficit of such stars.

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The star formation timescale of elliptical galaxies

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936636 ◽

2019 ◽

Vol 632 ◽

pp. A110 ◽

Cited By ~ 2

Author(s):

Zhiqiang Yan ◽

Tereza Jerabkova ◽

Pavel Kroupa

Keyword(s):

Star Formation ◽

Stellar Population ◽

Mass Function ◽

Elliptical Galaxies ◽

Initial Mass Function ◽

Different Types ◽

The Galaxy ◽

Stellar Initial Mass Function ◽

Low Mass ◽

Natural Solution

The alpha element to iron peak element ratio, for example [Mg/Fe], is a commonly applied indicator of the galaxy star formation timescale (SFT) since the two groups of elements are mainly produced by different types of supernovae that explode over different timescales. However, it is insufficient to consider only [Mg/Fe] when estimating the SFT. The [Mg/Fe] yield of a stellar population depends on its metallicity. Therefore, it is possible for galaxies with different SFTs and at the same time different total metallicity to have the same [Mg/Fe]. This effect has not been properly taken into consideration in previous studies. In this study, we assume the galaxy-wide stellar initial mass function (gwIMF) to be canonical and invariant. We demonstrate that our computation code reproduces the SFT estimations of previous studies, where only the [Mg/Fe] observational constraint is applied. We then demonstrate that once both metallicity and [Mg/Fe] observations are considered, a more severe “downsizing relation” is required. This means that either low-mass ellipticals have longer SFTs (> 4 Gyr for galaxies with mass below 1010 M⊙) or massive ellipticals have shorter SFTs (≈200 Myr for galaxies more massive than 1011 M⊙) than previously thought. This modification increases the difficulty in reconciling such SFTs with other observational constraints. We show that applying different stellar yield modifications does not relieve this formation timescale problem. The quite unrealistically short SFT required by [Mg/Fe] and total metallicity would be prolonged if a variable stellar gwIMF were assumed. Since a systematically varying gwIMF has been suggested by various observations this could present a natural solution to this problem.

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Variation in the Stellar Initial Mass Function from the Chromospheric Activity of M Dwarfs in Early-type Galaxies

The Astrophysical Journal ◽

10.3847/1538-4357/ac2a30 ◽

2021 ◽

Vol 923 (1) ◽

pp. 43

Author(s):

Pieter van Dokkum ◽

Charlie Conroy

Keyword(s):

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Early Type ◽

M Dwarfs ◽

Low Mass Stars ◽

Chromospheric Activity ◽

Stellar Initial Mass Function ◽

Low Mass ◽

The Imf

Abstract Mass measurements and absorption-line studies indicate that the stellar initial mass function (IMF) is bottom-heavy in the central regions of many early-type galaxies, with an excess of low-mass stars compared to the IMF of the Milky Way. Here we test this hypothesis using a method that is independent of previous techniques. Low-mass stars have strong chromospheric activity characterized by nonthermal emission at short wavelengths. Approximately half of the UV flux of M dwarfs is contained in the λ1215.7 Lyα line, and we show that the total Lyα emission of an early-type galaxy is a sensitive probe of the IMF with a factor of ∼2 flux variation in response to plausible variations in the number of low-mass stars. We use the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure the Lyα line in the centers of the massive early-type galaxies NGC 1407 and NGC 2695. We detect Lyα emission in both galaxies and demonstrate that it originates in stars. We find that the Lyα to i-band flux ratio is a factor of 2.0 ± 0.4 higher in NGC 1407 than in NGC 2695, in agreement with the difference in their IMFs as previously determined from gravity-sensitive optical absorption lines. Although a larger sample of galaxies is required for definitive answers, these initial results support the hypothesis that the IMF is not universal but varies with environment.

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Transition of the initial mass function in the metal-poor environments

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab2497 ◽

2021 ◽

Author(s):

Sunmyon Chon ◽

Kazuyuki Omukai ◽

Raffaella Schneider

Keyword(s):

Star Formation ◽

Mass Distribution ◽

Massive Stars ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Low Mass Stars ◽

Filamentary Structure ◽

Low Mass ◽

Cloud Core

Abstract We study star cluster formation in a low-metallicity environment using three dimensional hydrodynamic simulations. Starting from a turbulent cloud core, we follow the formation and growth of protostellar systems with different metallicities ranging from 10−6 to 0.1 Z⊙. The cooling induced by dust grains promotes fragmentation at small scales and the formation of low-mass stars with M* ∼ 0.01–0.1 M⊙ While the number of low-mass stars increases with metallicity, when Z/Z⊙ ≳ 10−5. the stellar mass distribution is still top-heavy for Z/Z⊙ ≲ 10−2 compared to the Chabrier initial mass function (IMF). In these cases, star formation begins after the turbulent motion decays and a single massive cloud core monolithically collapses to form a central massive stellar system. The circumstellar disk preferentially feeds the mass to the central massive stars, making the mass distribution top-heavy. When Z/Z⊙ = 0.1, collisions of the turbulent flows promote the onset of the star formation and a highly filamentary structure develops owing to efficient fine-structure line cooling. In this case, the mass supply to the massive stars is limited by the local gas reservoir and the mass is shared among the stars, leading to a Chabrier-like IMF. We conclude that cooling at the scales of the turbulent motion promotes the development of the filamentary structure and works as an important factor leading to the present-day IMF.

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Stellar initial mass function variation in massive early-type galaxies: the potential role of the deuterium abundance

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2679 ◽

2020 ◽

Vol 498 (3) ◽

pp. 4051-4059 ◽

Cited By ~ 1

Author(s):

Timothy A Davis ◽

Freeke van de Voort

Keyword(s):

Mass Loss ◽

Cosmic Time ◽

Mass Function ◽

Stellar Mass ◽

Initial Mass Function ◽

Initial Mass ◽

Early Type ◽

Root Cause ◽

Stellar Initial Mass Function ◽

Low Mass

ABSTRACT The observed stellar initial mass function (IMF) appears to vary, becoming bottom-heavy in the centres of the most massive, metal-rich early-type galaxies. It is still unclear what physical processes might cause this IMF variation. In this paper, we demonstrate that the abundance of deuterium in the birth clouds of forming stars may be important in setting the IMF. We use models of disc accretion on to low-mass protostars to show that those forming from deuterium-poor gas are expected to have zero-age main-sequence masses significantly lower than those forming from primordial (high deuterium fraction) material. This deuterium abundance effect depends on stellar mass in our simple models, such that the resulting IMF would become bottom-heavy – as seen in observations. Stellar mass loss is entirely deuterium free and is important in fuelling star formation across cosmic time. Using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation we show that stellar mass-loss-induced deuterium variations are strongest in the same regions where IMF variations are observed: at the centres of the most massive, metal-rich, passive galaxies. While our analysis cannot prove that the deuterium abundance is the root cause of the observed IMF variation, it sets the stage for future theoretical and observational attempts to study this possibility.

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Impact of metallicity and star formation rate on the time-dependent, galaxy-wide stellar initial mass function

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833055 ◽

2018 ◽

Vol 620 ◽

pp. A39 ◽

Cited By ~ 32

Author(s):

T. Jeřábková ◽

A. Hasani Zonoozi ◽

P. Kroupa ◽

G. Beccari ◽

Z. Yan ◽

...

Keyword(s):

Star Formation ◽

Dwarf Galaxies ◽

Mass Function ◽

Initial Mass Function ◽

Initial Mass ◽

Probability Density Distribution ◽

Star Forming ◽

The Galaxy ◽

Stellar Initial Mass Function ◽

Stellar Masses

The stellar initial mass function (IMF) is commonly assumed to be an invariant probability density distribution function of initial stellar masses. These initial stellar masses are generally represented by the canonical IMF, which is defined as the result of one star formation event in an embedded cluster. As a consequence, the galaxy-wide IMF (gwIMF) should also be invariant and of the same form as the canonical IMF; gwIMF is defined as the sum of the IMFs of all star-forming regions in which embedded clusters form and spawn the galactic field population of the galaxy. Recent observational and theoretical results challenge the hypothesis that the gwIMF is invariant. In order to study the possible reasons for this variation, it is useful to relate the observed IMF to the gwIMF. Starting with the IMF determined in resolved star clusters, we apply the IGIMF-theory to calculate a comprehensive grid of gwIMF models for metallicities, [Fe/H] ∈ (−3, 1), and galaxy-wide star formation rates (SFRs), SFR ∈ (10−5, 105) M⊙ yr−1. For a galaxy with metallicity [Fe/H] < 0 and SFR > 1 M⊙ yr−1, which is a common condition in the early Universe, we find that the gwIMF is both bottom light (relatively fewer low-mass stars) and top heavy (more massive stars), when compared to the canonical IMF. For a SFR < 1 M⊙ yr−1 the gwIMF becomes top light regardless of the metallicity. For metallicities [Fe/H] > 0 the gwIMF can become bottom heavy regardless of the SFR. The IGIMF models predict that massive elliptical galaxies should have formed with a gwIMF that is top heavy within the first few hundred Myr of the life of the galaxy and that it evolves into a bottom heavy gwIMF in the metal-enriched galactic centre. Using the gwIMF grids, we study the SFR−Hα relation and its dependency on metallicity and the SFR. We also study the correction factors to the Kennicutt SFRK − Hα relation and provide new fitting functions. Late-type dwarf galaxies show significantly higher SFRs with respect to Kennicutt SFRs, while star-forming massive galaxies have significantly lower SFRs than hitherto thought. This has implications for gas-consumption timescales and for the main sequence of galaxies. We explicitly discuss Leo P and ultra-faint dwarf galaxies.

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