The necessity of feedback physics in setting the peak of the initial mass function

Guszejnov, Dávid; Krumholz, Mark R.; Hopkins, Philip F.

doi:10.1093/mnras/stw315

Cited by 63 publications

(66 citation statements)

References 66 publications

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“…Hennebelle & Chabrier (2008), Hopkins (2012), andChabrier et al (2014) argue that the IMF emerges from the combined effects of gravity and lognormal density fluctuations in a turbulent medium (with the important assumption that the ratio between core and stellar mass is constant). Recently, Guszejnov et al (2016) combined these ideas by considering a model that includes both gravito-turbulent fragmentation and stellar radiation feedback.…”

Section: Discussionmentioning

confidence: 99%

The Stellar Initial Mass Function in Early-type Galaxies from Absorption Line Spectroscopy. IV. A Super-Salpeter IMF in the Center of NGC 1407 from Non-parametric Models

Conroy

Dokkum

Villaume

2017

ApJ

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It is now well-established that the stellar initial mass function (IMF) can be determined from the absorption line spectra of old stellar systems, and this has been used to measure the IMF and its variation across the early-type galaxy population. Previous work focused on measuring the slope of the IMF over one or more stellar mass intervals, implicitly assuming that this is a good description of the IMF and that the IMF has a universal low-mass cutoff. In this work we consider more flexible IMFs, including two-component power-laws with a variable low-mass cutoff and a general non-parametric model. We demonstrate with mock spectra that the detailed shape of the IMF can be accurately recovered as long as the data quality are high (S/N 300Å −1 ) and cover a wide wavelength range (0.4µm − 1.0µm). We apply these flexible IMF models to a high S/N spectrum of the center of the massive elliptical galaxy NGC 1407. Fitting the spectrum with non-parametric IMFs, we find that the IMF in the center shows a continuous rise extending toward the hydrogen-burning limit, with a behavior that is well-approximated by a power-law with an index of −2.7. These results provide strong evidence for the existence of extreme (super-Salpeter) IMFs in the cores of massive galaxies.

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Section: Discussionmentioning

confidence: 99%

The Stellar Initial Mass Function in Early-type Galaxies from Absorption Line Spectroscopy. IV. A Super-Salpeter IMF in the Center of NGC 1407 from Non-parametric Models

Conroy

Dokkum

Villaume

2017

ApJ

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“…On the other hand, increasing density makes it easier for gas to fragment by lowering the Bonnor-Ebert mass. Krumholz (2011) and Guszejnov, Krumholz & Hopkins (2016) find that these two effects nearly cancel, yielding a characteristic mass that is close to independent of density. However, this proposition remains untested by simulations.…”

Section: Caveats and Cautionsmentioning

confidence: 96%

“…However, in these simulations the gas temperature and thus the effective equation of state was not calculated self-consistently, and the calculations did not include the effects of radiation from the stars themselves. That this radiation is in fact the dominant mechanism in determining the gas temperature structure was first pointed out analytically by Krumholz (2006) and numerically by Bate (2009), and both analytic models and simulations locating the origin of the IMF peak in stellar radiative feedback have been published by a number of authors (e.g., Offner et al 2009;Bate 2009Bate , 2012Bate , 2014Krumholz 2011;; Guszejnov, Krumholz & Hopkins 2016).…”

Section: Introductionmentioning

confidence: 97%

What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations

Krumholz

Myers

Klein

et al. 2016

Mon. Not. R. Astron. Soc.

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As star-forming clouds collapse, the gas within them fragments to ever-smaller masses. Naively one might expect this process to continue down to the smallest mass that is able to radiate away its binding energy on a dynamical timescale, the opacity limit for fragmentation, at ∼ 0.01 M . However, the observed peak of the initial mass function (IMF) lies a factor of 20 − 30 higher in mass, suggesting that some other mechanism halts fragmentation before the opacity limit is reached. In this paper we analyse radiation-magnetohydrodynamic simulations of star cluster formation in typical Milky Way environments in order to determine what physical process limits fragmentation in them. We examine the regions in the vicinity of stars that form in the simulations to determine the amounts of mass that are prevented from fragmenting by thermal and magnetic pressure. We show that, on small scales, thermal pressure enhanced by stellar radiation heating is the dominant mechanism limiting the ability of the gas to further fragment. In the brown dwarf mass regime, ∼ 0.01 M , the typical object that forms in the simulations is surrounded by gas whose mass is several times its own that is unable to escape or fragment, and instead is likely to accrete. This mechanism explains why ∼ 0.01 M objects are rare: unless an outside agent intervenes (e.g., a shock strips away the gas around them), they will grow by accreting the warmed gas around them. In contrast, by the time stars grow to masses of ∼ 0.2 M , the mass of heated gas is only tens of percent of the central star mass, too small to alter its final mass by a large factor. This naturally explains why the IMF peak is at ∼ 0.2 M .

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“…While IMF slopes this steep have yet to be robustly observed, an IMF slope this extreme could have more of an immediate implication for analytical IMF variation theories. For example, the functional form of the CMF could be mapped to a unimodal IMF slope to determine how high of a Mach number would be required to produce these extreme * M L ratios in theories that predict that high-Mach-number environments promote a bottom-heavy IMF (Hopkins 2013;Chabrier et al 2014;Guszejnov et al 2016).…”

Section: Implications For Imf Theorymentioning

confidence: 99%

“…Some models find that the larger density fluctuations associated with higher Mach numbers in star-forming disks cause the low-mass turnover of the pre-stellar core mass function (CMF) to shift to lower masses, implying a more bottom-heavy IMF (e.g., Hopkins 2013;Chabrier et al 2014;Guszejnov et al 2016;but see Bertelli Motta et al 2016). These models are able to reconcile the universal IMF found across a range of Milky Way stellar populations with a bottomheavy IMF in more extreme star formation environments such as starbursts.…”

Section: Introductionmentioning

confidence: 99%

Implications of Galaxy Buildup for Putative IMF Variations in Massive Galaxies

Blancato

Genel

Bryan

2017

ApJ

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Recent observational evidence for initial mass function (IMF) variations in massive quiescent galaxies at z=0 challenges the long-established paradigm of a universal IMF. While a few theoretical models relate the IMF to birth cloud conditions, the physical driver underlying these putative IMF variations is still largely unclear. Here we use post-processing analysis of the Illustris cosmological hydrodynamical simulation to investigate possible physical origins of IMF variability with galactic properties. We do so by tagging stellar particles in the simulation (each representing a stellar population of »  M 10 6) with individual IMFs that depend on various physical conditions, such as velocity dispersion, metallicity, or star formation rate, at the time and place in which the stars are formed. We then follow the assembly of these populations throughout cosmic time and reconstruct the overall IMF of each z=0 galaxy from the many distinct IMFs it is composed of. Our main result is that applying the observed relations between IMF and galactic properties to the conditions at the star formation sites does not result in strong enough IMF variations between z=0 galaxies. Steeper physical IMF relations are required for reproducing the observed IMF trends, and some stellar populations must form with more extreme IMFs than those observed. The origin of this result is the hierarchical nature of massive galaxy assembly, and it has implications for the reliability of the strong observed trends, for the ability of cosmological simulations to capture certain physical conditions in galaxies, and for theories of star formation aiming to explain the physical origin of a variable IMF.

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The necessity of feedback physics in setting the peak of the initial mass function

Cited by 63 publications

References 66 publications

The Stellar Initial Mass Function in Early-type Galaxies from Absorption Line Spectroscopy. IV. A Super-Salpeter IMF in the Center of NGC 1407 from Non-parametric Models

The Stellar Initial Mass Function in Early-type Galaxies from Absorption Line Spectroscopy. IV. A Super-Salpeter IMF in the Center of NGC 1407 from Non-parametric Models

What physics determines the peak of the IMF? Insights from the structure of cores in radiation-magnetohydrodynamic simulations

Implications of Galaxy Buildup for Putative IMF Variations in Massive Galaxies

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