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Register a new userSimulating Star Clusters Across Cosmic Time: I. Initial Mass Function, Star Formation Rates and Efficiencies
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We present radiation-magneto-hydrodynamic simulations of star formation in self-gravitating, turbulent molecular clouds, modeling the formation of individual massive stars, including their UV radiation feedback. The set of simulations have cloud masses between $m_{rm gas}=10^3$~M$_odot$ to $3 times 10^5$~M$_odot$ and gas densities typical of clouds in the local universe ($overline n_{rm gas} sim 1.8times 10^2$~cm$^{-3}$) and 10$times$ and 100$times$ denser, expected to exist in high-redshift galaxies. The main results are: {it i}) The observed Salpeter power-law slope and normalisation of the stellar initial mass function at the high-mass end can be reproduced if we assume that each star-forming gas clump (sink particle) fragments into stars producing on average a maximum stellar mass about $40%$ of the mass of the sink particle, while the remaining $60%$ is distributed into smaller mass stars. Assuming that the sinks fragment according to a power-law mass function flatter than Salpeter, with log-slope $0.8$, satisfy this empirical prescription. {it ii}) The star formation law that best describes our set of simulation is $drho_*/dt propto rho_{gas}^{1.5}$ if $overline n_{gas}<n_{cri}approx 10^3$~cm$^{-3}$, and $drho_*/dt propto rho_{rm gas}^{2.5}$ otherwise. The duration of the star formation episode is roughly $6$ clouds sound crossing times (with $c_s=10$~km/s). {it iii}) The total star formation efficiency in the cloud is $f_*=2% (m_{rm gas}/10^4~M_odot)^{0.4}(1+overline n_{rm gas}/n_{rm cri})^{0.91}$, for gas at solar metallicity, while for metallicity $Z<0.1$~Z$_odot$, based on our limited sample, $f_*$ is reduced by a factor of $sim 5$. {it iv)} The most compact and massive clouds appear to form globular cluster progenitors, in the sense that star clusters remain gravitationally bound after the gas has been expelled.
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We investigate the balance of power between stars and AGN across cosmic history, based on the comparison between the infrared (IR) galaxy luminosity function (LF) and the IR AGN LF. The former corresponds to emission from dust heated by stars and AGN, whereas the latter includes emission from AGN-heated dust only. We find that at all redshifts (at least up to z~2.5), the high luminosity tails of the two LFs converge, indicating that the most infrared-luminous galaxies are AGN-powered. Our results shed light to the decades-old conundrum regarding the flatter high-luminosity slope seen in the IR galaxy LF compared to that in the UV and optical. We attribute this difference to the increasing fraction of AGN-dominated galaxies with increasing total infrared luminosity (L_IR). We partition the L_IR-z parameter space into a star-formation and an AGN-dominated region, finding that the most luminous galaxies at all epochs lie in the AGN-dominated region. This sets a potential `limit to attainable star formation rates, casting doubt on the abundance of `extreme starbursts: if AGN did not exist, L_IR>10^13 Lsun galaxies would be significantly rarer than they currently are in our observable Universe. We also find that AGN affect the average dust temperatures (T_dust) of galaxies and hence the shape of the well-known L_IR-T_dust relation. We propose that the reason why local ULIRGs are hotter than their high redshift counterparts is because of a higher fraction of AGN-dominated galaxies amongst the former group.
The inner few hundred parsecs of the Milky Way harbours gas densities, pressures, velocity dispersions, an interstellar radiation field and a cosmic ray ionisation rate orders of magnitude higher than the disc; akin to the environment found in star-forming galaxies at high-redshift. Previous studies have shown that this region is forming stars at a rate per unit mass of dense gas which is at least an order of magnitude lower than in the disc, potentially violating theoretical predictions. We show that all observational star formation rate diagnostics - both direct counting of young stellar objects and integrated light measurements - are in agreement within a factor two, hence the low star formation rate is not the result of the systematic uncertainties that affect any one method. As these methods trace the star formation over different timescales, from $0.1 - 5$ Myr, we conclude that the star formation rate has been constant to within a factor of a few within this time period. We investigate the progression of star formation within gravitationally bound clouds on $sim$ parsec scales and find $1 - 4$ per cent of the cloud masses are converted into stars per free-fall time, consistent with a subset of the considered volumetric star formation models. However, discriminating between these models is obstructed by the current uncertainties on the input observables and, most importantly and urgently, by their dependence on ill-constrained free parameters. The lack of empirical constraints on these parameters therefore represents a key challenge in the further verification or falsification of current star formation theories.
We calculate the hydrogen and helium-ionizing radiation escaping star-forming molecular clouds, as a function of the star cluster mass and compactness, using a set of high-resolution radiation-magneto-hydrodynamic simulations of star formation in self-gravitating, turbulent molecular clouds. In these simulations, presented in He, Ricotti and Geen (2019), the formation of individual massive stars are well resolved, and their UV radiation feedback and lifetime on the main sequence are modeled self-consistently. We find that the escape fraction of ionizing radiation from molecular clouds, $langle f_{rm esc}^{scriptscriptstyle rm MC}rangle$, decreases with increasing mass of the star cluster and with decreasing compactness. Molecular clouds with densities typically found in the local Universe have negligible $langle f_{rm esc}^{scriptscriptstyle rm MC}rangle$, ranging between $0.5%$ to $5%$. Ten times denser molecular clouds have $langle f_{rm esc}^{scriptscriptstyle rm MC}rangle approx 10%-20%$, while $100times$ denser clouds, which produce globular cluster progenitors, have $langle f_{rm esc}^{scriptscriptstyle rm MC}rangle approx 20%-60%$. We find that $langle f_{rm esc}^{scriptscriptstyle rm MC}rangle$ increases with decreasing gas metallicity, even when ignoring dust extinction, due to stronger radiation feedback. However, the total number of escaping ionizing photons decreases with decreasing metallicity because the star formation efficiency is reduced. We conclude that the sources of reionization at $z>6$ must have been very compact star clusters forming in molecular clouds about $100times$ denser than in todays Universe, which leads to a significant production of old globular clusters progenitors.
The stellar initial mass function (IMF) is a fundamental property of star formation, offering key insight into the physics driving the process as well as informing our understanding of stellar populations, their by-products, and their impact on the surrounding medium. While the IMF appears to be fairly uniform in the Milky Way disk, it is not yet known how the IMF might behave across a wide range of environments, such as those with extreme gas temperatures and densities, high pressures, and low metallicities. We discuss new opportunities for measuring the IMF in such environments in the coming decade with JWST, WFIRST, and thirty-meter class telescopes. For the first time, we will be able to measure the high-mass slope and peak of the IMF via direct star counts for massive star clusters across the Milky Way and Local Group, providing stringent constraints for star formation theory and laying the groundwork for understanding distant and unresolved stellar systems.
We analyse a hydrodynamical simulation of star formation. Sink particles in the simulations which represent stars show episodic growth, which is presumably accretion from a core that can be regularly replenished in response to the fluctuating conditions in the local environment. The accretion rates follow $dot{m} propto m^{2/3}$, as expected from accretion in a gas-dominated potential, but with substantial variations over-laid on this. The growth times follow an exponential distribution which is tapered at long times due to the finite length of the simulation. The initial collapse masses have an approximately lognormal distribution with already an onset of a power-law at large masses. The sink particle mass function can be reproduced with a non-linear stochastic process, with fluctuating accretion rates $propto m^{2/3}$, a distribution of seed masses and a distribution of growth times. All three factors contribute equally to the form of the final sink mass function. We find that the upper power law tail of the IMF is unrelated to Bondi-Hoyle accretion.
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