April 12, 2020 – See-Through Galaxies

Not kidding, there are galaxies that are transparent!  Our Milky Way is not one of them, we are quite obscure thank you.

In 1984 Allan Sandage and Bruno Binggeli discovered the first see-through galaxy, in the Virgo supercluster of galaxies.   This discovery, like many, changed how astronomers thought about galaxy formation.  It also led to the term ultra-diffuse galaxy (UDG).  Problem is, they are extremely difficult to detect, and this proved to be a huge challenge.  Guess Sandage and Binggeli got lucky.

Fast forward to 2013.  Two astronomers were wondering how to detect these virtually invisible galaxies. One came up with the idea of using new fast telephoto lenses from Canon (400mm f2.8) with special sub-wavelength coatings, in an array.  An array would produce the equivalent of a larger lens, with a focal ratio even faster than f2.8. The idea was to capture very faint objects in wide field images, like the one Sandage and Binggeli found.  Their instrument’s lens coatings were optimal for imaging them.

Starting in 2013 with an eight lens array the astronomers imaged M101 in Ursa Major and discovered an anomaly in its halo.  With this encouraging result they eventually settled on two arrays of 48 lenses each.  So, each array has the light gathering capability of a one-meter telescope with an f ratio of 0.4, extremely fast!  I have written about these lens arrays previously, so this part is a repeat.  The arrays are called Dragonfly (DF).

Armed with their Dragonfly arrays, the team searches for UDGs and not surprisingly have found a few, DF2, DF4, and DF44.  Here’s where it gets interesting.

These tenuous galaxies appear to either have little to no dark matter, or be mostly dark matter!  When conflicts like this occur in science, it usually means something is amiss.  In the case of UDGs, there might be issues with understanding how they form.   Or they are weird.

We do know they have few stars but can be as large as the Milky Way.

How do UDGs form?  One possibility is they are the result of galactic mergers.  During the messy merger all kinds of stuff gets thrown around so they could be what are termed tidal dwarf galaxies. They might be the detritus collections of gas, dust, and stars tossed out from merging galaxies.  Another idea is they are caused by a nearby quasar’s ion winds causing gas clouds to form.  Or, in another scenario, they could form from gas clouds between close galaxies in a group.  Gas flows along gravitational “streams” but could be disrupted by one of the galaxy’s supermassive black hole’s relativistic jet of particles.  It might get kicked away as a cloud big enough to form a UDG.  All these hypotheses have problems fitting known models.

Let’s just say, UDGs are a work in progress.  And weird.

What’s in the Sky?

April 14-16; dawn; southeast:  The Moon, Jupiter, Saturn, and Mars line up above the horizon