New paper: The remnants of the first stars

There is building evidence that the first stars are not as massive as previously thought and that they are merely typical massive stars on the order of tens of solar masses instead of behemoths up to 300 solar masses.  Furthermore, a non-negligible fraction of this population form in binary systems. These stellar systems can leave behind stellar-mass black holes, chemically enriched regions from their supernovae, and X-ray binaries if the companion star overflows onto the black hole during its giant phase. All of this depends on the initial mass function of the first stars, which is highly uncertain at the moment, but luckily it’s an active area of research!

To determine the evolution and impact of these remnants, we must know where they migrate after their progenitor star dies.  This week, Hao Xu, Michael Norman at UCSD and I submitted a paper that focuses on exactly this point. About two years ago, we started thinking about running a massive galaxy formation simulation that can follow thousands of galaxies’ star formation histories and had enough mass resolution to include the first stars that can form in dark matter halos as small as 300,000 solar masses. This is a “zoom-in” calculation in which we re-simulate an overdense region of the universe with much higher resolution.

Left: projected gas density, showing values between a scale of log(Density) = -27.5 and -24.5 in units of g/cm^3. Right: projected gas temperature, showing values between a scale of log(Temperature) = 3 and 4.5 in units of K. The field of view of both panels is 50 proper kpc.

This simulation has 1.3 billion computational cells, follows 25 data fields in each cell, and 475 million dark matter particles.  In addition, the UV radiation field is calculated from 15,000 radiation sources at any given time, using adaptive ray tracing. So far, it has progressed to redshift 15 (300 million years after the Big Bang) at which point we analyzed the data to determine where the remnants of the first stars are located. Above, I show a snapshot of the simulation at redshift 16.5 of the projected density and gas temperature in a field of view of (50 kpc)2, which is encompasses only 2% of the high-resolution region in the simulation!

In our analysis of this overdense region, we have found that most halos do not form stars until they reach a halo mass of 107 solar masses because their molecular hydrogen formation, which is the primary coolant in metal-free gas, is suppressed by high UV radiation from nearby galaxies.  However, this doesn’t prevent metal-free stars from forms before galaxy formation commences, as we find that they continue to form at a rate of 10-4 solar masses per year per comoving Mpc3.  We also find that the most massive starless halo has a mass of 7 x 107 solar masses, which could be a potential site for the gas to directly collapse into a massive black hole with M~105 solar masses.  The most massive halos (M ~ 109 solar masses) in our simulation at redshift 15 have approximately 50 first star remnants on average.  If these remnants are still X-ray binaries, they could affect the thermal structure of these early galaxies and contribute to cosmic reionization. Furthermore, the black holes could be the progenitors of central black holes found in most galaxies today or possibly those elusive intermediate mass black holes! The multiplicity of black holes in the smallest galaxies at early times should also be important in determining the black hole merger rate as these stellar-mass black holes spiral inwards toward the centers of their host galaxies.

This paper is just the first of many from this large radiation hydrodynamics simulation of galaxy formation, which contains a wealth of information about the building blocks of present-day galaxies and the conditions during the first billion years in the universe.

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