Thread by @ChrisMihos

Time for a little research update, highlighting our recent discovery of a really diffuse object/galaxy/ubersmudge in the Leo I galaxy group.

The full story can be found in our paper, posted last week on ArXiv: arxiv.org/abs/1807.11544

First, thanks to collaborators @therealCCarr, Aaron Watkins,
@Valeandromeo and Paul Harding. Aaron took the optical imaging, Chris discovered the object and worked on photometry, Tom provided the HI data, and Paul made the Burrell Schmidt the amazing LSB imager it is today.

This is the Leo I galaxy group, at a distance of 11 Mpc. A few years back we imaged it using CWRUAstro’s Burrell Schmidt telescope, searching for diffuse intragroup light. Even though our imaging went really deep (muB,lim=30), we didn’t find much, just a few small tidal streams.

But last summer, as we were getting ready for a dwarf galaxy conference here at CWRU, we had CWRU undergrad Chris Carr take another look at the data, this time looking for small fuzzy things — dwarf galaxies — that might be lurking about in the imaging.

After only a day of searching for fuzzies he found this thing. I was pretty skeptical. But we binned up the data to improve signal and there was a 3sigma blip, in both the B and V imaging, which were taken a year apart. So…. maybe? But, eh, 3sigma. I still wasn’t convinced.

And then as I was leaving our meeting, this happened:

Carr: Oh, and I did an online search at that coordinate, and there is a radio source there, too.
Mihos: <stops dead> Say what?
Carr: Yes, at… um... 21-cm, it says.
Me: Say what?

So yes the smudge sits right on a clump of HI gas.

Now Leo I is the home of the well-known and giant HI Leo Ring, and it turns out the smudge is embedded in an HI spur connecting the Ring to the spiral M86. But Chris didn’t know about the HI when he found the source optically.

Boy, howdy! Time for some photometry.

Here’s the surface brightness profile showing a peak (PEAK!) surfb of muB=28.8. That’s pretty dang faint — in fact, it makes it the faintest extragalactic object found via integrated light. That @therealCCarr found it at all is amazing.

We enlisted the help of Tom Oosterloo, whose Westerbork HI data was critical. The object is pretty compact, with an HI mass of 4.5E7 Msun, and a very velocity width of only few km/s. And its 970 km/s velocity is a good match to the galaxies in Leo I.

What about the color? After much work, and handwringing about proper metrics, we settled on a B-V color of 0.12 +/- 0.09. That’s really blue. Uber blue. So maybe we’re seeing young stars?

So then Aaron says, well, is it detected in GALEX? It is, barely, and it has a really blue UV color as well. The optical/UV colors together suggest an age of a few hundred Myr for the stellar pops.

But you know what else can blue? Scattered light from interstellar dust. Now it’s not Galactic dust, since the HI velocity puts it way outside of the Milky Way. But maybe it could be scattering from dust embedded in the clump at Leo. So I did a dust scattering calculation.

LET ME REPEAT THAT: I DID A DUST SCATTERING CALCULATION.

Sheesh, it was like being back in grad school. But short answer: even in the most optimal conditions, dust scattering predicts a much fainter signal. So no, it looks to be starlight.

So what do we have? Beats me... One possibility is that it is a tidal dwarf, spawned by an interaction in the Leo I group. If so, it may not be long-lived: the HI tidal tails and the central hole in its surfb profile suggest it may be in the process of tidal disruption.

Another possibility is that it’s an incredibly diffuse galaxy that has been running around in the Leo group for a while. If so, maybe tidal interactions triggered a little starburst a few hundred Myr ago. But wow, if so, it would be the most extreme LSB galaxy known to date.

Either way, it’s pretty cool. And Chris Carr found this thing in his very first week of scientific research! I mean, dang, at that point in my career I was battling just to figure out how to mount 9-track tapes in the basement of Robinson...

Anyway, for details go to our paper, scheduled for the Astrophysical Journal Letters. And thanks again to all my collaborators, particularly @therealCCarr and his smudge-spotting eyes. (BTW, Chris is applying for grad schools this fall — grab him, astropeeps, he’s good….)