A Darker View

‘Ridiculously’ Dim Bevy of Stars Found Beyond Milky Way

A team of American, Canadian and Chilean astronomers have stumbled onto a remarkably faint cluster of stars orbiting the Milky Way that puts out as much light as only 120 modest Sun-like stars. The tiny cluster, called Muñoz 1, was discovered near a dwarf galaxy in a survey of satellites around the Milky Way using the Canada-France-Hawaii Telescope (CFHT) and confirmed using the Keck II telescope, both of which are on Mauna Kea, Hawaii.

“What’s neat about this is it’s the dimmest globular cluster ever found,” said Ricardo Muñoz, an astronomer at the University of Chile and the discoverer of the cluster. A globular cluster is a spherical group of stars bound to each other by gravity so that they orbit around a galaxy as a unit.

“While I was working on the Ursa Minor dwarf galaxy I noticed there was this tiny little object close by,” Muñoz recalled. He made the discovery while he was a postdoctoral associate at Yale University. Most globular clusters have in the range of 100,000 stars. Muñoz 1 has something like 500 stars. “This is very surprising,” he said.

The Muñoz 1 globular cluster is seen to the right of the Ursa Minor dwarf galaxy in this image from the Canada-France-Hawaii Telescope MegaCam imager.

“It’s ridiculously dim,” agreed Yale astronomer Marla Geha. “There are individual stars that would far outshine this entire globular cluster.” That puts Muñoz 1 head-to-head with the Segue 3 globular cluster (also orbiting the Milky Way) as the dimmest troupe of old stars ever found.

Muñoz 1’s discovery was the result of a survey done with the CFHT MegaCam imager in 2009 and 2010. It was then confirmed by spectroscopic study using the Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope. The researchers will be publishing their results soon in The Astrophysical Journal Letters.

The Keck data was critical for the study, said Geha, because it sorted out whether or not Muñoz 1 and the Ursa Minor dwarf galaxy were moving together.

“Nearly every galaxy has an entourage of globular clusters,” said Geha, “so we first thought that Muñoz 1 might be associated with the nearby Ursa Minor dwarf galaxy.” By using spectroscopic data to measure the relative velocities of the cluster and the dwarf galaxy, they discovered quite the opposite was the case.

“The velocities turned out to be wildly different,” said Geha. So the fact that they are near each other is just a coincidence, she said. What has been seen is more like a single snapshot of two cars traveling near each other and apparently together, but they really have different destinations and are traveling at very different speeds. Analysis of the brightness and colors of the stars belonging to Muñoz 1 and Ursa Minor also suggests that the tiny cluster is actually located about 100,000 light years in front of the dwarf galaxy.

As for how Muñoz 1 came to be so dim, a likely scenario is that it has gradually lost stars over the eons, said Geha. It’s also possible it was stripped of stars by passing through the Milky Way. But the direction of the cluster’s movement is not yet known, so it’s not known whether it has passed through the Milky Way.

Perhaps the most intriguing aspect of the discovery is the possibility that Muñoz 1 may be hinting that there are many more such globular clusters in the Galactic halo. After all, the CFHT survey covered only 40 square degrees of sky out of 40,000 square degrees in the entire sky.

“Assuming that we’re not just lucky to have found something very rare, there could be many others out there,” said Geha.

“To truly understand its nature, we will need to measure its mass,” added Munoz. To do that, astronomers would need to measure the velocities of individual stars in the cluster and see how they move with relation to each other. That, in turn, reveals the overall mass of the cluster. A lot of mass would suggest there is a lot of dark matter holding the cluster together, and maybe even qualify the cluster as the smallest, darkest galaxy ever discovered. Right now the Segue 1 dwarf galaxy holds that record. Geha was also involved in measurements with the Keck DEIMOS instrument that confirmed the nature of Segue 1.

“The goal of this survey was to understand the difference between dwarf galaxies and globular clusters,” said Geha. Muñoz 1 suggests there may be plenty of borderline objects out there waiting to be found, which could help sort that matter out.

A pdf of the paper is available at http://www.cfht.hawaii.edu/en/news/Munoz1/munoz12.pdf.

Diving South of Kona

I have not done much diving south of Kona, usually diving the shores close to home, the reefs of North and South Kohala. I have done a couple dives at Two Step, at Hōnaunau. The terrain is different further south, there are different species to see. Last weekend I got a chance to do some more diving down south.

This was made possible through a friend and fellow Keck engineer. Dennis owns a boat. Not just any boat… The Aqua Safari… A big boat, a fast boat, a dive boat, equipped for scuba, actually a pretty nice boat. I was invited to join the guys for a dive outing. I didn’t even ask where we were going, it really didn’t matter, I just wanted to dive.

Dropoff — Mark Devenot exploring the reef drop-off at Tanks

We headed south of Kona to a dive site I had never explored before, a place called Amphitheater. Just around the corner from the famous Kealakekua Bay, the site features some large sea caves carved into the cliffs by the winter waves. An excellent site, featuring good fish, numerous lava tubes, and good coral cover. Visibility was great, allowing good photographic conditions.

A find of the dive was a Bearded Cusk Eel hiding in a crevice at the base of a rock wall. I managed one half-decent photograph before it disappeared further into the crevice, out of sight. I guess it did not like the strobe, cusk eels are notoriously shy.

Mark, Patti and I shared the meal preparations. They had brought sandwich makings, I brought tuna mac, drinks and chips. Deb even sent a long a batch of home-made chocolate chip cookies. We ate well indeed.

The second dive site was Tanks, a site just north of the old Kona Airport and south of Honokohau. A fair amount of surge made mooring quite a challenge, we actually gave up on one mooring buoy, too close to the rocks where we were getting bounced around. There was quite a bit of surge underneath as well, and visibility was poor. At least poor by Kona standards, fairly good for most anywhere else. The surge and vis did not preclude a decent dive, we descended to the edge of the dropoff, where the slope plunges into the deep blue abyss. A few good photos, including a Undulated Moray. We spent the last part of the dive exploring a number of small caves just under the shoreline, where we found several white tip reef sharks, including one of the largest I had ever seen. This shark was a bit more than six feet, and quite rotund. He lived up to the scientific name for the species Triaenodon obesus.

A great day and a couple good dives. That was the goal, and that was what we achieved. Thanks Dennis!

Postcard from Hawai’i – View From Above

Big Island from ISS — The Big Island of Hawai'i photographed from the International Space Station, Dec 29, 2011, image credit: ISS expedition 30, NASA

Postcard from the Reef – Snaggletooth

The books says these guys grow to 3½ feet in length. Uh, right. This guy was at least six feet long, with a mouth big enough to slide my whole hand into. His scarred head and ragged fangs spoke of a lifetime of life and death struggle on the reef. The book also says that undulated morays are one of the meanest morays, cautioning not to feed or play with these eels. I had not read the book before the encounter, but I needed no warnings to give this fellow a good deal of respect.

Postcard from the Universe – Rising Milky Way

MIlky Way over A'a — The rising Milky Way silhouetting and old a'a lava flow on Mauna Kea, Canon 60D @ ISO12800, 17mm f/4, eight 10 second frames aligned and averaged

Jupiter Exits the Evening Sky

This week sees Jupiter disappear into the sunset. The planet will pass through superior conjunction on May 13th, to reappear in the dawn during the last days of May.

Guard Crab

The Moon and Venus

A pretty crescent Moon will join Venus in the evening sky tonight. The pair will be a little less than 6° apart with a 13% illuminated Moon. It should make a lovely pairing high in the dusk.

Adhara

The morning routine was predictable… Moments after the first notes from the alarm clock there would be a familiar thump at the foot of the bed. That would be followed by whiskers in the face. Time to get up and start the day, which included feeding the cat.

Not this morning, nor tomorrow, nor anytime soon…

Today I buried a cat that had been part of my life for 16 years.

No more expectant eyes standing on the corner of the bed, demanding a skritch while I was attempting to dress for the day. Inevitably sitting atop of whatever clean shirt I had put there moments before. Gone is the purr from beside me while I attempt to get to sleep. I remember an escape artist that defeated my ever more elaborate barriers to keep a litter of kittens safely in a linoleum floored kitchen. I remember hot days of Tucson sun, lying in the shade of the old tangerine tree. I remember hours of yowling as we drove to San Diego, the first leg of the move to Hawai’i.

I picked her from the litter, the little grey furball. Named for the star Epsilon Canis Majoris, Adhara, sometimes spelled Adara. This star was once the brightest star in the Earth’s sky, though it has faded as it has drifted away from Earth over the eons.

Now, all we have in an empty house and freshly turned earth beneath an adenium in the front yard.

The Moon and Jupiter

This evening will see a razor thin Moon just above the planet Jupiter, low in the sunset. The planet will be about 5° below a 3% illuminated Moon. The Moon itself will be only 19° from the Sun at sunset, about 18:42HST. It should be a dramatic scene with the pair immersed in the colors of sunset.