Hello Space Fans and welcome to another edition of Space Fan News.
Well if the activity in my inbox is any indication, you guys want to talk about black holes, and
not just any black holes but the really big ones, Ultra Massive Black Holes.
Earlier this month astronomers revised some mass estimates of a black hole at the center
of the elliptical galaxy Holmberg 15A, an enormous galaxy 700 million light years away
that is home to the second largest black hole we’ve ever seen.
There is no doubt that black holes are fascinating objects.
They are enigmatic points in space that contain boundaries that defy our observations.
There are horizons close to these objects where we cannot hope to observe, the confines
of physics prevent us from ever seeing inside.
These horizons are defined by a radius of the gravitational well, which directly related
to the mass, and it is by mass that we catalog black holes.
They come in many sizes, from relatively tiny stellar-sized black holes, remnants of the
deaths of massive stars, to objects billions of times the mass of our Sun.
Black holes get bigger by merging with other black holes and astronomers believe that almost
every galaxy in our universe has one at their centers.
The roles black holes play in galaxy formation and evolution is not completely understood,
but it is generally believed that as galaxies interact with one another through collisions
and mergers, the central black holes also get bigger.
In some cases much, much bigger.
Which brings me to the galaxy Holmberg 15A, a member of the class of galaxies known as
elliptical galaxies and if you’ve ever seen my video on IC 1101, you know that these galaxies
are not only the largest, but they are also the oldest.
They are named because of their shape which has been sculpted by billions of years of
interactions with other galaxies, each gravitational encounter smearing out any defined morphology
it may once have had, leaving a blobby roughly spherical shape.
The color of these galaxies comes from the stars inside.
They are almost all old stars, the hot, young blue stars in these galaxies have long since
died leaving only older, redder stars with lifetimes in the tens of billions of years.
They contain very little gas for new stars to form as well meaning that star formation
has virtually stopped.
About sixty percent of all galaxies in the universe are ellipticals.
So Holmberg 15A is huge, old and dying.
And earlier this month, astronomers studying this galaxy have for the first time, directly
imaged the stars going around the black hole, Holmberg 15A star using high resolution, wide-field
spectral observations on the MUSE instrument at the Very Large Telescope in Chile and based
on the motions of those stars calculated that Holm 15A* is forty billion solar masses.
By comparison, Sagittarius A*, the black hole at the center of the Milky Way is estimated
to be only four million solar masses, so this one is a doozy.
The Schwartzchild radius, also called the event horizon of this black hole, the point
beyond which nothing can escape, is a whopping 790 astronomical units.
That’s way bigger than our solar system.
By comparison, Pluto on on average 39.5 AU away and our heliopause which is an ill-defined
area where the solar wind no longer pushes against interstellar space is around 123 AU.
So having a forty billion solar mass black hole with an event horizon reaching 790 AU
out is pretty impressive and this is probably among the first of this class of black hole
we’ll measure, they are calling the Ultra Massive Black holes of course cuz that’s
how astronomers roll: Super, Ultra… then Mega no doubt, followed by UltraMega and SuperUltraMega…
the list goes on.
Still, this isn’t the biggest black hole we know about.
The biggest is a black hole may belong to TON 618, a distant, extremely luminous quasar
that has a black hole 66 billion times the mass of the Sun.
The caveat here is this estimate comes from indirect observing methods of the galaxy itself,
which aren’t as accurate as measuring the individual stars in orbit around the black
hole, so this may be revised.
Quasars are galaxies with really active black holes with accretion disks of very hot gas.
These black holes shed radiation as material falls in that is the brightest in the universe.
They are also really far away, making measurements like this all but impossible, TON 618 shine
with the luminosity of 140 trillion Suns and gets its name from having been observed first
at Tonantzintia Observatory in Mexico.
So right now Holm 15A* is the most massive black hole measured using direct observations
of stars in a galaxy 700 million light years, and while I think all of that is amazing to
think about, I mean come on, a 40 billion solar mass black hole with a radius 790 AU
out in a galaxy so old that stars have long ago stopped forming?
You gotta love it, but my takeaway from this comes from my years working with telescopes.
The fact that we can now directly measure the motions of individual stars in a crowded,
somewhat dim galaxy 700 million light years away is stupendous.
It’s mind-boggling that we now have this kind of resolution.
And it wasn’t done in space but on the ground, using equipment located under the Earth’s
blanket of air.
This technique involved measuring the motions of the stars going around Holm 14A* then applying
orbit-based, axisymmetric Schwarzschild models to those paths to get the result they obtained
in this paper.
The golden age folks, what can I say.
Well that’s it for this episode Space Fans thank you Deep Astronomy Patreon Patrons for
your continued support and thanks to OPT telescopes, a world leader in telescopes for amateur and
professional astronomers.
Thanks to all of you for watching and as always, keep looking up!