This week in astronomy: weak winds, early worlds and wandering black holes
Some weeks astronomy delivers one enormous headline and everything else gets swept aside. This week was quieter, and rather more interesting for it. Four results landed, and each removes an assumption that a lot of other work has been resting on: how the first massive stars lost their mass, what the Solar System's first bodies were built from, where a galaxy's black holes end up, and whether a spiral has to spin all in one direction.
Metal-poor stars have surprisingly feeble winds, and that matters for Webb's strangest galaxies
Massive stars do not simply sit there and shine. They blow, hard, driving winds off their surfaces that carry away a serious fraction of their mass in a few million years. How hard they blow sets almost everything downstream: how bright the star stays, how much ionising ultraviolet light escapes, how much material is returned to the galaxy, and what sort of corpse gets left behind. Astronomers have long assumed those winds scale smoothly with the iron and other heavy elements in the star, since it is those elements that catch the outgoing light and get pushed.
Grace Telford at the University of Utah has now tested that assumption where it matters most, and found it breaks. Her team's survey, cheerfully acronymed TEMPOS (Treasury of Extremely Metal-Poor O Stars), used the Cosmic Origins Spectrograph on the Hubble Space Telescope to take ultraviolet spectra of 29 massive stars across six nearby dwarf galaxies. These are patient observations: some individual stars needed as much as 35 hours of Hubble time. Below roughly a tenth of the Sun's metal content, the expected smooth trend stops behaving. Wind speeds fall away sharply rather than tapering gently. The team also found that iron abundance varies considerably from star to star even within the very metal-poor regime, which complicates the tidy picture of one number describing a galaxy's stellar population.
This reaches well beyond a handful of dwarf galaxies, because those stars are the best local stand-ins we have for the first generations of massive stars, the ones JWST keeps finding in galaxies that look far too bright and far too odd for their age. Every model of those early galaxies contains a wind prescription, extrapolated from metal-rich stars nearby. If the extrapolation fails at low metallicity, a good deal of modelling needs revisiting. Telford is careful about the leap and so are we: the link to Webb's galaxies is an inference rather than a measurement. It is a well-motivated one, and it arrives with 29 new data points where there were previously very few.
Reference: Galaxies in the early Universe are weird. These stars may explain why (University of Utah), published in The Astrophysical Journal Supplement Series.
The young Solar System chose fire over ice
Ask where the outer Solar System's first solid bodies came from and the intuitive answer is ice: cold, volatile-rich dust from beyond the snow line, clumping together far from the Sun. Damanveer Grewal and colleagues at Yale have made a strong case that the opposite happened, and that it happened remarkably early.
Their evidence comes from iron meteorites, the shattered cores of planetesimals that melted completely when radioactive aluminium-26 heated them from within. Melting destroys texture, which is why these objects are awkward to read. Grewal's team got around that using two tracers that survive the melt: sulphur concentration, which was concentrated in the fine-grained matrix, and the oxidation state of the iron, which records how much water ice and oxidised dust went into the mix. Both point the same way. The earliest outer Solar System planetesimals were built from roughly 83 to 92 per cent chondrules, the little beads of once-molten rock, with only 8 to 17 per cent of the icy matrix that dominates later bodies.
The timing is the part that stings. This sorting was already in place within the first million years, whereas previous evidence for chondrule-rich assembly came from two to four million years after the Solar System's birth. As Grewal puts it, the process "was remarkably selective from the very beginning". What the team does not yet offer is a mechanism: something concentrated chondrules and excluded fine icy dust very efficiently and very early, and identifying it is the obvious next question. Given how often exoplanet formation models borrow their starting conditions from our own system, the answer will travel.
Reference: From the start, the Solar System chose fire over ice to build its first bodies (Yale News), published in Nature Astronomy.
Black holes that wandered off may be the most informative ones
When two galaxies merge, their central black holes are supposed to sink towards the middle of the combined galaxy and eventually pair up. Emma Jane Weller, a graduate student at Yale working with Priyamvada Natarajan, has been following the ones that do not. Using ASTRID, a cosmological simulation that tracks dark matter, gas, stars and black holes together across 12.6 billion years, the team followed black holes through galaxies spanning stellar masses from ten million to a trillion times the Sun's.
Plenty never make it home. Knocked off course by mergers, they spend billions of years roaming their hosts instead of settling at the centre, and they turn up disproportionately in smaller galaxies and in galaxies still forming stars. That last point is the useful one. Small galaxies have quiet histories and shallow gravitational wells, so they hold on to traces of the original black hole seed population in a way that large galaxies, having been through repeated mergers, do not. Natarajan's phrase for it is nicely put: black holes are "remarkable cosmic archivists".
This is a simulation rather than a set of observations, so it is best read as a prediction with a to-do list attached. The encouraging thing is that the list is answerable. Wandering black holes in nearby dwarf galaxies are exactly the sort of faint, off-centre, variable source that wide surveys are now built to catch.
Reference: Roaming black holes may tell the hidden history of galaxies (Yale News), published in The Astrophysical Journal Letters.
A galaxy that spins out of step with itself
Finally, something for anyone who has ever pointed a telescope at Virgo. The ESA/Hubble Picture of the Month for September is NGC 4698, a spiral roughly 55 million light years away and a member of the Virgo Cluster. It looks conventional at first glance: bright bulge, winding arms, dust lanes threading through knots of blue and red star formation. What it is doing is not conventional at all. The stars and gas closest to the centre rotate perpendicular to the rest of the disc, and the bulge sticks out at right angles to the disc plane, which is genuinely rare among spirals.
The likeliest explanation is that the inner regions did not grow up with the outer ones. Gas funnelled in from outside, or a small companion was swallowed, and the material that ended up in the middle arrived carrying the wrong angular momentum and never sorted itself out. The new data come from a Hubble programme led by David Thilker, part of the MAUVE-HST survey working through Virgo Cluster galaxies to study their star clusters and nebulae and to understand how life in a crowded cluster changes a galaxy.
Virgo is happy hunting ground for anyone imaging from a dark site, and NGC 4698 is within reach of the sort of remote setups our own users work with. Worth knowing, next time it drifts through a field, that you are looking at a galaxy assembled from two mismatched pieces.
Reference: A galaxy spinning out of sync (ESA/Hubble Picture of the Month, potm2609a).
Until next week
Four results, four assumptions loosened. Clear skies, and we will see you in seven days.