Title: Massive star formation in overdense regions of the early universe
Author: John A. Regan
Institution of the first author: CASM, Maynooth University, Ireland
Status: submitted to the Open Journal of Astrophysics [open access]available on arXiv
In most large galaxies we observe – like the Milky Way – there appears to be a supermassive black hole (SMBH) at the center. These black holes are incredibly massive (well an explicit name in astronomy, here are some examples of how not to do it), starting at a million times the mass of the Sun all the way up to billions of solar masses. Here’s the problem: how did supermassive black holes get supermassive? A lot can happen once a lot of time has passed, and the universe has been around for quite a while. Yet astronomers don’t know how these black holes grew to such staggering masses. Moreover, already very early in the universe – even before its billionth birthday – there were SMBHs with more than a billion solar masses around. Currently, the general idea is that they are the product of mergers; smaller black holes merge into larger ones until they reach SMBH masses. The main problem is that it’s really hard to explain why this merger went so fast and therefore why we see them.
How did we come here?
One of the most annoying things about black holes is that they tend to be difficult to observe (unsurprisingly). Now the black hole zoo is diverse. The ones we’ve seen before are stellar-mass black holes – the remnant dying stars (there are plenty of them) – and supermassive black holes, usually found in galactic centers – they’re very bright, or at least their surroundings l ‘is. There is also the suspicion that the so-called intermediate-mass black holes exist, with masses between a thousand and a million solar masses. These massive black holes (MBH) are neither bright nor abundant: they are not observed. Although sometimes considered a red flag in astronomy, these objects are still believed to hold the key to understanding how SMBHs form.
There is a wide range of explanations for the formation of MBHs, with two main categories (see also Figure 1):
- Light Seeds: An MBH grows from many population III standard stars or their remnants or can simply grow to enormous masses by consuming a very dense local gas cloud.
- Heavy Seeds: Much heavier structures (on the order of at least a thousand solar masses) form and fuse into MBH.
The main problem with the light seed scenario is that there is a maximum rate at which black holes consume matter: when matter falls into the black hole, the disk of matter (= the accretion disk in scientific jargon) around it warms up and emits more and more light. At some point, the disk will become so bright that the amount of light is so strong that it effectively prevents more matter from falling. When we reach a maximum amount of stuff falling into the black hole, nothing more will just be stopped by the radiation from the disc. This is called the Eddington limit. It is this limit that limits (pun intended) the rapid formation of MBHs in the light seed scenario: it simply cannot happen fast enough.
On the heavy seed side, it is more difficult to explain how these much more massive objects formed in the first place. But not impossible!
Superstars
To get the heavy seeds that MBHs need, the author of today’s article turns to supermassive stars (SMS). As a very special case of population III stars, they should be formed with hundreds of thousands of solar masses, a bit mind-boggling considering that the stars we have today only reach a few hundred solar masses. Now, there are two problems with Population III stars: they only happened a very long time ago (so we have to look at huge distances) and they would have died out very quickly. So, unfortunately, we haven’t seen them yet.
These stars will, however, theoretically form in halos of dark matter in the early universe. Fortunately, cosmological simulations are very good at this. For our SMS, we need particularly dense dark matter halos to allow them to form. Cosmological simulations are not easy, but step-by-step zooming in on structures of interest can reduce the effort. This is shown in Figure 2.
There, the author identifies an interesting overdense region. In the overdense region of Figure 2, the author indicates three clusters (aptly named C1, C2 and C3) that could contain the dark matter halos where SMS could have a chance to form. It’s even easier said than done though, because there seem to be a bunch of conditions we need to form these crazy stars:
- We basically need almost zero “metal” in our halos. This works best if no other stars have formed that have spewed harmful metals (in the astronomical sense) that facilitate low mass star formation and don’t give our SMSes a chance.
- We need a lot of material movement in the halo; the more local the motion, the better it prevents clouds from collapsing into smaller stars early.
- The halo should form quickly. The heat formed by the rapid formation of the halo during its contraction must be greater than the radiative cooling of the material in the halo.
So basically we want to prevent the formation of lower mass stars in order to get the monstrosity of a star that we need to get an MBH. Analysis of these three clusters shows that at least C2 and C3 have dark matter halos that can form SMS. A rough estimate of star formation in these promising halos shows that some stars containing hundreds of thousands of solar masses could form – in the mass range of what we need for MBHs. These stellar behemoths would have been quite rare due to their very finicky training requirements, but they could have been there.
As these SMS live out their lives (very short, a few million years at most), they will eventually collapse into fairly heavy black holes. Black holes that are exactly in the mass range we want for MBHs, so SMSs serve well as heavy seeds for MBH formation. In any case, the possibility that these supermassive stars could have existed brings us one step closer to the massive black hole and the eventual formation of a supermassive black hole.
Astrobite edited by Katya Gozman
Featured image credit: Figure 15 c) from James et al. 2015
About Roel Lefever
Roel is a first-year PhD student at the University of Heidelberg, studying astrophysics. He works on massive stars and simulates their atmospheres/flows. In his spare time he enjoys hiking/biking in nature, playing (many) video games, playing/listening to music (movie soundtracks!) and reading (currently The Wheel of Time, but any fantasy really).
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