Record-breaking faint satellite galaxy of the Milky Way discovered

November 22, 2016, Subaru Telescope
Figure 1: The position of Virgo I in the constellation of Virgo (left). The right panel shows a density map of Virgo I's member stars in a 0.1 deg x 0.1 deg area, based on the stars located inside the green zone in the color-magnitude diagram of Virgo I shown in Figure 4. The color range from blue -> white -> yellow -> red indicates increasing density. Credit: Tohoku University/National Astronomical Observation of Japan

An international team led by researchers from Tohoku University has found an extremely faint dwarf satellite galaxy of the Milky Way. The team's discovery is part of the ongoing Subaru Strategic Survey using Hyper Suprime-Cam. The satellite, named Virgo I, lies in the direction of the constellation Virgo. At the absolute magnitude of -0.8 in the optical waveband, it may well be the faintest satellite galaxy yet found. Its discovery suggests the presence of a large number of yet-undetected dwarf satellites in the halo of the Milky Way and provides important insights into galaxy formation through hierarchical assembly of dark matter.

Currently, some 50 satellite galaxies to the Milky Way have been identified. About 40 of them are faint and diffuse and belong to the category of so-called "dwarf spheroidal galaxies" (Figure 2). Many recently discovered dwarf galaxies, especially those seen in systematic photometric surveys such as the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) are very faint with absolute luminosity in the optical waveband below -8 magnitude. These are so-called "ultra-faint dwarf galaxies". However, previous searches made use of telescopes with a diameter of 2.5 to 4 meters, so only satellites relatively close to the Sun or those with higher magnitudes were identified. Those that are more distant or faint ones in the halo of the Milky Way are yet to be detected (Figure 3).

The combination of the large aperture of 8.2-meter Subaru Telescope and the large field-of-view Hyper Suprime-Cam (HSC) instrument is very powerful in this study. It enables an efficient search for very faint dwarf satellites over large areas of the sky. The first step in searching out a new dwarf galaxy is to identify an over density of stars in the sky, using photometric data. Next is to assess that the over dense appearance is not due to line-of-sight or accidental juxtapositions of unrelated dense fields, but is really a stellar system. The standard method for doing this is to look for a characteristic distribution of stars in the color-magnitude diagram (comparable to the Hertzsprung-Russell diagram (middle and left panels of Figure 4)). Stars in a general field shows no particular patterns in this diagram (right panel of Figure 4).

Finding Virgo I

An animation showing locations of Milky Way Galaxy's satellite galaxies, featuring the newly discovered Virgo I. The computer graphics was created using Mitaka, a four-dimensional digital universe viewer. In the image from the Subaru Telescope, Green circles denote the member candidate stars that might belong to Virgo I. Credit: NAOJ

Daisuke Homma, a graduate student at Tohoku University, found Virgo I under the guidance of his advisor, Masashi Chiba, and their international collaborators. "We have carefully examined the early data of the Subaru Strategic Survey with HSC and found an apparent over density of stars in Virgo with very high statistical significance, showing a characteristic pattern of an ancient stellar system in the color-magnitude diagram," he said. "Surprisingly, this is one of the faintest satellites, with absolute magnitude of -0.8 in the optical waveband. This is indeed a galaxy, because it is spatially extended with a radius of 124 light years – systematically larger than a globular cluster with comparable luminosity."

The faintest dwarf satellites identified so far was Segue I, discovered by SDSS (-1.5 mag) and Cetus II in DES (0.0 mag). Cetus II is yet to be confirmed, as it is too compact as a galaxy. Virgo I may ultimately turn out to be the faintest one ever discovered. It lies at a distance of 280,000 light years from the Sun, and such a remote galaxy with faint brightness has not been identified in previous surveys. It is beyond the reach of SDSS, which has previously surveyed the same area in the direction of the constellation Virgo (Figure 5).

Figure 2: Satellite galaxies associated with the Milky Way Galaxy. Squares are Large and Small Magellanic Clouds and circles are dwarf spheroidal galaxies.

According to Chiba, the leader of this search project, the discovery has profound implications. "This discovery implies hundreds of faint dwarf satellites waiting to be discovered in the halo of the Milky Way," he said. "How many satellites are indeed there and what properties they have, will give us an important clue of understanding how the Milky Way formed and how contributed to it."

Using HSC to Trace Galaxy Formation

Formation of galaxies like the Milky Way is thought to proceed through the hierarchical assembly of dark matter, forming dark halos, and through the subsequent infall of gas and star formation affected by gravity. Standard models of galaxy formation in the context of the so-called cold dark matter (CDM) theory predict the presence of hundreds of small dark halos orbiting in a Milky Way-sized dark halo and a comparable number of luminous satellite companions. However, only tens of satellites have ever been identified. This falls well short of a theoretical predicted number, which is part of the so-called "missing problem". Astronomers may need to consider other types of dark matter than CDM or to invoke baryonic physics suppressing to explain the shortfall in the number of satellites. Another possibility is that they have seen only a fraction of all the satellites associated with the Milky Way due to various observational biases. The issue remains unsolved.

One of the motivations for the Subaru Strategic Survey using HSC is to do increase observations in the search for Milky Way satellites. The early data from this survey is what led to the discovery of Virgo I. This program will continue to explore much wider areas of the sky and is expected to find more satellites like Virgo I. These tiny companions to be discovered in the near future may tell us much more about history of the Milky Way's formation.

Figure 3: False-color dwarf galaxy images taken with Subaru Telescope. Left: Leo II (V-band absolute magnitude MV = -11.9 mag). Middle: Boötes I (MV = -6.3 mag), where both images are taken with Suprime Cam. Right: HSC image of Virgo I (MV = -0.8 mag). Ultra-faint dwarf galaxies (Boötes I and Virgo I) are hard to detect from these images. Credit: Subaru Telescope
Figure 4: Stars in the color-magnitude diagram. Old stellar populations show a characteristic distribution along the curve seen in the diagram. From left to right: Boötes I, Segue I, Virgo I, and a general field outside Virgo I. The spatial distribution of the stars, which are located inside the green band for Virgo I, is shown in the right panel of Figure 1. Note that stars in a general field outside Virgo I (right panel) show no characteristic feature. Credit: Subaru Telescope
Figure 5: The relation between the distance from the Sun and absolute magnitude in optical waveband for Milky Way satellites discovered so far. Virgo I is extremely faint and distant from the Sun and is beyond the reach of SDSS. Except for Virgo I, DES mostly discovers those outside SDSS's limit. Credit: Subaru Telescope

Explore further: Reconciling dwarf galaxies with dark matter

More information: Daisuke Homma et al. A NEW MILKY WAY SATELLITE DISCOVERED IN THE SUBARU/HYPER SUPRIME-CAM SURVEY, The Astrophysical Journal (2016). DOI: 10.3847/0004-637X/832/1/21

Related Stories

Reconciling dwarf galaxies with dark matter

September 7, 2016

Dwarf galaxies are enigmas wrapped in riddles. Although they are the smallest galaxies, they represent some of the biggest mysteries about our universe. While many dwarf galaxies surround our own Milky Way, there seem to ...

How a star cluster ruled out MACHOs

August 10, 2016

Are massive black holes hiding in the halos of galaxies, making up the majority of the universe's mysterious dark matter? This possibility may have been ruled out by a star cluster in a small galaxy recently discovered orbiting ...

Dark Energy Survey finds more celestial neighbors

August 17, 2015

Scientists on the Dark Energy Survey, using one of the world's most powerful digital cameras, have discovered eight more faint celestial objects hovering near our Milky Way galaxy. Signs indicate that they, like the objects ...

Dark matter satellites trigger massive birth of stars

March 9, 2016

One of the main predictions of the current model of the creation of structures in the universe, known at the Lambda Cold Dark Matter model, is that galaxies are embedded in very extended and massive halos of dark matter that ...

A shy galactic neighbor

September 16, 2015

The Sculptor Dwarf Galaxy, pictured in this new image from the Wide Field Imager camera, installed on the 2.2-metre MPG/ESO telescope at ESO's La Silla Observatory, is a close neighbour of our galaxy, the Milky Way. Despite ...

Recommended for you

Encouraging prospects for moon hunters

November 20, 2018

Astrophysicists of the University of Zürich, ETH Zürich and the NCCR PlanetS show how the icy moons of Uranus were born. Their result suggests that such potentially habitable worlds are much more abundant in the Universe ...

Gravitationally lensed quasars

November 19, 2018

The path of light is bent by mass, an effect predicted by Einstein's theory of gravity, and when a massive galaxy or cluster lies along our line-of-sight to a more distant galaxy its matter will act as a lens to image the ...

9 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

chauffeurkp
1 / 5 (2) Nov 22, 2016
"These are so-called "ultra-faint dwarf galaxies". However, previous searches made use of telescopes with a diameter of 2.5 to 4 meters, so only satellites relatively close to the Sun or those with higher magnitudes were identified."

Ultra-faint dwarf galaxies close to the sun? For luminosity? Does the author understand the scale?
TheGhostofOtto1923
3.7 / 5 (3) Nov 22, 2016
Do you understand that they were obviously talking about the solar neighborhood? Duh.
chauffeurkp
1 / 5 (1) Nov 22, 2016
Solar Neighborhood? HA! "It lies at a distance of 280,000 light years from the Sun, and such a remote galaxy with faint brightness has not been identified in previous surveys."

fyi - the earth is about 9 light MINUTES from the sun - and yeah, earth is in the "solar neighborhood".
barakn
5 / 5 (2) Nov 22, 2016
"These are so-called "ultra-faint dwarf galaxies". However, previous searches made use of telescopes with a diameter of 2.5 to 4 meters, so only satellites relatively close to the Sun or those with higher magnitudes were identified."

Ultra-faint dwarf galaxies close to the sun? For luminosity? Does the author understand the scale?
Yes, judging from the use of the word 'relatively.'
chauffeurkp
1 / 5 (2) Nov 22, 2016
Well clearly we speak a different language. Look at the 3D graph above showing the various dwarf galaxies in proximity to the Milky Way. (Granted, a 2D image of a 3D graph is perplexing.) The gray oval in the middle is the Milky Way. It is apparent that gauging a distance, "relatively close to the sun" which is buried inside this small gray oval is confusing at best and shows a lack of understanding of the scale.
Protoplasmix
5 / 5 (2) Nov 22, 2016
It is apparent that gauging a distance, "relatively close to the sun" which is buried inside this small gray oval is confusing at best and shows a lack of understanding of the scale
As you mentioned, we're less than 10 light-minutes from the sun and the context is galaxies. It was pointed out in the paper ( preprint: https://arxiv.org...09.04346 ) that an object as faint as Virgo I couldn't be detected in the Sloan Digital Sky Survey unless it was within about 28 kpc (this is a heliocentric distance, obviously, since most all observations are made from somewhere along a circle centered around the sun with radius of just 1 AU). The limit for seeing such faint objects with Subaru/Hyper Suprime-Cam is about 89 kpc from the sun, so Virgo I was almost "too far from the sun" to be observed. As you've demonstrated, it's when you can see a 3-dimensional plot of the galactic neighborhood that distances other than heliocentric ones become noticeable.
chauffeurkp
3 / 5 (2) Nov 23, 2016
Since our sun is about 8kpc from the center of the Milky Way, it's confusing to relate distances to a dwarf galaxy that is over 10 times that distance away. It's relationship is to our galaxy, not our sun. However, I see your point that accuracy may be better served with this confusing explanation.
FredJose
1 / 5 (5) Nov 23, 2016
provides important insights into galaxy formation through hierarchical assembly of dark matter.

Once more the totally meaningless statement of insights that simply lead further up the creek. This observation reveals absolutely nothing about galaxy formation whatsoever. It might tell us about the current structures but in no ways does it reveal anything about how the galaxy[ies] came to be.
Once again - the meaningless phrase simply indicates that the researcher(s) are still groping in the dark as fervently as before.
TheGhostofOtto1923
3.7 / 5 (3) Nov 23, 2016
It is apparent that gauging a distance, "relatively close to the sun" which is buried inside this small gray oval is confusing at best and shows a lack of understanding of the scale
So you think that researchers and science writers who clearly know more than you about the subject, were confused about where these things were located?

That's just silly.
This observation reveals absolutely nothing about galaxy formation whatsoever
This observation was made by pros who know a great deal more about galaxy formation than can be included in one article or paper or even the entire book of Genesis. This is implicit yes?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.