Can stars form in the stellar halo?

Question

The wiki page for Galactic Halo states:

Star formation in the stellar halo of the Milky Way ceased long ago.

In addition, the wiki page for Stellar Halo states:

Astrophysical simulations of galaxies have predicted that stellar halos should have two components; one inner region dominated by stars which formed within the galaxy, and an outer region primarily composed of stars accreted through merger events.

This implies that newly formed stars in the galactic halo of the Milky Way, if any in the first place, do not originate from the halo itself. My question is why? Would it not be possible in theory for stars to form from the galactic corona (before it cools down and fall back into the galactic disc) for example?

Side note: If star formation in the stellar halo of the Milky Way has ceased, how did they even form long ago in the first place?

Related: How do disk-shaped galaxies then form at the sites of spherical galactic halos? Do the halo stars start orbiting or just sit in place?, Gas halo of our Milky Way Galaxy

Answer 1

Stars form from cold gas, not hot. Hot gas consists of fast-moving particles which cannot condense. So stars are born in the densest regions in molecular clouds where the temperature is 10–100 K, and particles move around with¹ ~1 km/s. In contrast, gas in the galactic halo has temperatures of millions of degrees², and hence particle velocities of hundreds of km/s. Indeed, this is the reason the gas forms a halo rather than condensing to the galactic disk.

Galaxies are formed through mergers of smaller clumps in the past. Gas clouds collided, dissipating their energy through various cooling processes (which I describe here), and settled down into a disk. But the stars that had already formed were (in practise) collisionless and hence did not form a part of the disk, instead keeping their original random motions in all directions and hence forming a halo. Eventually the most massive stars died so that, today, only the low-mass, long-lived (and red) ones are left.

Gas at such high temperatures does not easily cool, but stays hot for billions of years, and is moreover replenished by gas that has been heated by stellar feedback.

That's why the halo is a halo, and that's why no new stars form here.

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_{¹ The formula to calculate this is $v = \sqrt{2k_\mathrm{B}T/m}$, where $k_\mathrm{B}$ is Boltzmann's constant, $T$ is the temperature, and $m$ is the mass of the particle.}

_{² The characteristic temperature in a cloud that collapses under gravity but is unable to cool is the "virial temperatur", $T_\mathrm{vir} \simeq \frac{m}{2 k_\mathrm{B}} \frac{G M_\mathrm{vir}}{R_\mathrm{vir}}$, where $G$ is the gravitational constant, and $M_\mathrm{vir}$ and $R_\mathrm{vir}$ are the mass and radius of the collapsed cloud. For the Milky Way this evaluates to $\sim10^7\,\mathrm{K}$.}