Mysterious molecular mysteries

Ultra Diffuse Galaxies are big flooffy things without many stars. But they do have some. So at some point they must have had some level of star formation activity. Maybe they made them all in one go, or maybe they made them very slowly, but they must have done something, at some point, somehow.

What's especially weird is that some of them have quite a lot of atomic gas - comparable to Milky Way-sized galaxies, which in general form stars at a quite respectable rate. But very little is known about the molecular gas, and while recently the mood is swinging away from "molecular gas is the be-all and end-all of star formation", it's still undeniably an important part of the process. So these authors use the IRAM 30m dish to look for CO emission (everyone's favourite tracer of H2) in six lucky UDGs, all rich in HI gas. Which, you may remember, is a project I've done myself, but not got around to publishing yet.

This is just a letter, so they concentrate heavily on their main result : a very tentative detection in one galaxy of their sample. It's difficult to tell exactly how tentative it is, since the part of the spectrum they show is so small that there's almost no baseline visible for comparison. They say it's 4 sigma, but it would be a lot more interesting with more of the spectrum visible.

To be fair, they don't ever claim this as more than a tentative possibility. They note that the HI doesn't overlap the velocity range of this possible CO emission, but it would have been easy to actually show this directly (I'd have made the figure myself but they don't plot the absolute velocities on the x-axis, so it's above the effort/reward threshold for me). It's true that this doesn't rule out the CO emission, but one would think you could say something more interesting about the kinematics than simply commenting on the validity of the detection. If these galaxies have low star formation thanks to a very extended, low-density gas disc, then why should the kinematics of the CO be different from the HI ?

The other five galaxies have no CO detected at all, though they don't give any tables and figures. They say from the upper limits that this means they have low H2/HI ratios, so "the inefficient star formation in such galaxies is mainly due to the low efficiency of forming molecules from atomic gas". Well, yes, but what's keeping that molecular star formation low ? There's just as much atomic gas as in galaxies which are perfectly happy to form stars, so why don't these ones ? For now, the middling molecular mass measurement makes mysterious mysteries more mysterious. Maybe.

CO observations toward HI-rich Ultra Diffuse Galaxies

We present CO observations toward a sample of six HI-rich Ultra-diffuse galaxies (UDGs) as well as one UDG (VLSB-A) in the Virgo Cluster with the IRAM 30-m telescope. CO 1-0 is marginally detected at 4sigma level in AGC122966, as the first detection of CO emission in UDGs.

A Bridge Quite Far

Specifically, a bridge between two galaxy clusters that spans a whopping 2 Mpc. They'd seen some earlier hints of this in previous data, but now have higher sensitivity from multiple telescopes to try and confirm it. And it's rock solid at 144 MHz and (in my opinion - they're more cautious) 53 MHz, but tentative at best at 383 MHz and not visible at all at 1.5 GHz.

Such bridges are rare in the extreme - this is one of only two such features known, making them hard to understand. You can't, for example, say what types of system most commonly host them, so it's difficult to comment on their origins. Even the emission mechanism is uncertain, since it's only clearly detected at one frequency. They say :

According to Brunetti & Vazza (2020), radio bridges may originate from second-order Fermi acceleration of electrons interacting with turbulent motions triggered by the complex dynamics in the overdense region between premerging clusters.

Basically it's from very low energy, low density, ionised gas. Interestingly there's a correlation with the much hotter X-ray emission, pointing to a connection between thermal and non-thermal sources, but that's about all they can say from this.

The larger question of whether this gas is primordial, i.e. we're seeing the cosmic web itself, or non-primordial gas from interactions, isn't addressed directly. And fair enough really - when you've discovered a bridge of gas 2 Mpc in length, you're absolutely entitled to write a short letter than says little more besides, "we found a bridge of gas 2 Mpc in length". Interestingly though, both cases of giant radio bridges (or in one slightly amusing typo, radio brides) are found in massive, pre-merging clusters that themselves consist of interacting sub-structures. So I guess both options are open, though presumably the sheer length of the thing would more naturally suggest a primordial structure. But until more of these are found, there's little else to be said apart from, "hey, that's neat !".

A giant radio bridge connecting two clusters in Abell 1758

Collisions between galaxy clusters dissipate enormous amounts of energy in the intra-cluster medium (ICM) through turbulence and shocks. In the process, Mpc-scale diffuse synchrotron emission in form of radio halos and relics can form. However, little is known about the very early phase of the collision.