Dwarf Galaxies Found Without Influence From Dark Matter

Dwarf galaxy in Fornax.
The dwarf galaxy NGC1427A flies through the Fornax galaxy cluster and undergoes disturbances which would not be possible if this galaxy were surrounded by a heavy and extended dark matter halo, as required by standard cosmology. Courtesy ESO.

Ask astronomers about dark matter and one of the things they talk about is that this invisible, mysterious “stuff” permeates the universe. In particular, it exists in halos surrounding most galaxies. The mass of the halo exerts a strong gravitational influence on the galaxy itself, as well as on others in the neighborhood. That’s pretty much the standard view of dark matter and its influence on galaxies. However, there are problems with the idea of those halos. Apparently, some oddly shaped dwarf galaxies exist that look like they have no halos. How could this be? Do they represent an observationally induced challenge to the prevailing ideas about dark matter halos?

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A Nearby Dwarf Galaxy has a Surprisingly Massive Black Hole in its Heart

Since the 1970s, scientists have known that within the cores of most massive galaxies in the Universe, there beats the heart of a Supermassive Black Hole (SMBH). The presence of these giant black holes causes these galaxies to be particularly energetic, to the point where their central regions outshine all the stars in their disks combined – aka. Active Galactic Nuclei (AGN). The Milky Way galaxy has its own SMBH, known as Sagitarrius A*, which has a mass of over 4 million Suns.

For decades, scientists have studied these objects in the hopes of learning more about their role in galactic formation and evolution. However, current research has shown that SMBHs may not be restricted to massive galaxies. In fact, a team of astronomers from the University of Texas at Austin’s McDonald Observatory recently discovered a massive black hole at the heart of a dwarf galaxy that orbits the Milky Way (Leo I). This finding could redefine our understanding of how black holes and galaxies evolve together.

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It Took 50 Nights of Observations to Capture New Data on the Magellanic Clouds

Part of the SMASH dataset showing an unprecedented wide-angle view of the Large Magellanic Cloud. Image Credit: CTIO/NOIRLab/NSF/AURA/SMASH/D. Nidever (Montana State University) Acknowledgment: Image processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani & Davide de Martin

The Magellanic Clouds are two of our closest neighbours, in galactic terms. The pair of irregular dwarf galaxies were drawn into the Milky Way’s orbit in the distant past, and we’ve been looking up at them since the dawn of humanity. Some of our ancestors even gathered pigments and created images of them in petroglyphs and cave paintings.

Following in the footsteps of those ancient artists, astronomers recently used the Dark Energy Camera (DECam) to capture an in-depth portrait of the pair of galaxies.

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7% of the Stars in the Milky Way’s Center Came From a Single Globular Cluster That Got Too Close and Was Broken Up

Central region of the Milky Way in infrared light. With this image, NASA's Spitzer Space Telescope has photographed the inner 890 x 640 light years of the Milky Way. The nuclear star cluster is located in a small area near the central massive black hole. The extended structures in the image are mostly clouds of gas and dust from the spiral arms of the Milky Way, which lie in the line of sight between Earth and the Galactic Centre. Image Credit: NASA/JPL-Caltech/S. Stolovy (Spitzer Science Center/Caltech)

The heart of the Milky Way can be a mysterious place. A gigantic black hole resides there, and it’s surrounded by a retinue of stars that astronomers call a Nuclear Star Cluster (NSC). The NSC is one of the densest populations of stars in the Universe. There are about 20 million stars in the innermost 26 light years of the galaxy.

New research shows that about 7% of the stars in the NSC came from a single source: a globular cluster of stars that fell into the Milky Way between 3 and 5 billion years ago.

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This Dwarf Galaxy is all by Itself

Unlike a spiral or elliptical galaxy, the galaxy KK 246 looks like glitter spilled across a black velvet sheet. KK 246, also known as ESO 461-036, is a dwarf irregular galaxy residing within the Local Void. This image is made up of observations from Hubble’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) in the infrared and optical parts of the spectrum. Four filters were used to sample various wavelengths. The color results from assigning different hues to each monochromatic image associated with an individual filter. Image credit: NASA/ESA/Hubble/E. Shaya et al.

In these days of social distancing, it appears this beautiful little galaxy is leading by example, sitting all by itself in the middle of a cosmic void.

KK 246, also known as ESO 461-036, is a dwarf irregular galaxy, and ESA aptly described this picture as looking like “glitter spilled across a black velvet sheet.”

But the serene view can be deceiving.

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Gravitational Waves Might be the Key to Finding Dark Matter

The OzGrav supercomputer, which was recently installed at the Swinburne University of Technology, will assist in the hunt for gravitational waves. Credit: OzGrav

Exotic dark matter theories. Gravitational waves. Observatories in space. Giant black holes. Colliding galaxies. Lasers. If you’re a fan of all the awesomest stuff in the universe, then this article is for you.

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Astronomy Cast Ep. 498: Dwarf Galaxy Update

The Milky Way has gobbled up dozens of dwarf galaxies and added them to its structure. Today we’re going to look at the ongoing hunt for the wreckage of past mergers. And what we’ve discovered about dwarf galaxies in general.

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Dwarf Dark Matter Galaxy Hides In Einstein Ring

The large blue light is a lensing galaxy in the foreground, called SDP81, and the red arcs are the distorted image of a more distant galaxy. By analyzing small distortions in the red, distant galaxy, astronomers have determined that a dwarf dark galaxy, represented by the white dot in the lower left, is companion to SDP81. The image is a composite from ALMA and the Hubble. Image: Y. Hezaveh, Stanford Univ./ALMA (NRAO/ESO/NAOJ)/NASA/ESA Hubble Space Telescope
The large blue light is a lensing galaxy in the foreground, called SDP81, about 4 billion light years away. The red arcs are the distorted image of a more distant galaxy, about 12 billion light years away. By analyzing small distortions in the red, distant galaxy, astronomers have determined that a dwarf dark galaxy, represented by the white dot in the lower left, is bound to SDP81. The image is a composite from ALMA and the Hubble. Image: Y. Hezaveh, Stanford Univ./ALMA (NRAO/ESO/NAOJ)/NASA/ESA Hubble Space Telescope

Everybody knows that galaxies are enormous collections of stars. A single galaxy can contain hundreds of billions of them. But there is a type of galaxy that has no stars. That’s right: zero stars.

These galaxies are called Dark Galaxies, or Dark Matter Galaxies. And rather than consisting of stars, they consist mostly of Dark Matter. Theory predicts that there should be many of these Dwarf Dark Galaxies in the halo around ‘regular’ galaxies, but finding them has been difficult.

Now, in a new paper to be published in the Astrophysical Journal, Yashar Hezaveh at Stanford University in California, and his team of colleagues, announce the discovery of one such object. The team used enhanced capabilities of the Atacamas Large Millimeter Array to examine an Einstein ring, so named because Einstein’s Theory of General Relativity predicted the phenomenon long before one was observed.

An Einstein Ring is when the massive gravity of a close object distorts the light from a much more distant object. They operate much like the lens in a telescope, or even a pair of eye-glasses. The mass of the glass in the lens directs incoming light in such a way that distant objects are enlarged.

Einstein Rings and gravitational lensing allow astronomers to study extremely distant objects, by looking at them through a lens of gravity. But they also allow astronomers to learn more about the galaxy that is acting as the lens, which is what happened in this case.

If a glass lens had tiny water spots on it, those spots would add a tiny amount of distortion to the image. That’s what happened in this case, except rather than microscopic water drops on a lens, the distortions were caused by tiny Dwarf Galaxies consisting of Dark Matter. “We can find these invisible objects in the same way that you can see rain droplets on a window. You know they are there because they distort the image of the background objects,” explained Hezaveh. The difference is that water distorts light by refraction, whereas matter distorts light by gravity.

As the ALMA facility increased its resolution, astronomers studied different astronomical objects to test its capabilities. One of these objects was SDP81, the gravitational lens in the above image. As they examined the more distant galaxy being lensed by SDP81, they discovered smaller distortions in the ring of the distant galaxy. Hezaveh and his team conclude that these distortions signal the presence of a Dwarf Dark Galaxy.

But why does this all matter? Because there is a problem in the Universe, or at least in our understanding of it; a problem of missing mass.

Our understanding of the formation of the structure of the Universe is pretty solid, at least in the larger scale. Predictions based on this model agree with observations of the Cosmic Microwave Background (CMB) and galaxy clustering. But our understanding breaks down somewhat when it comes to the smaller scale structure of the Universe.

One example of our lack of understanding in this area is what’s known as the Missing Satellite Problem. Theory predicts that there should be a large population of what are called sub-halo objects in the halo of dark matter surrounding galaxies. These objects can range from things as large as the Magellanic Clouds down to much smaller objects. In observations of the Local Group, there is a pronounced deficit of these objects, to the tune of a factor of 10, when compared to theoretical predictions.

Because we haven’t found them, one of two things needs to happen: either we get better at finding them, or we modify our theory. But it seems a little too soon to modify our theories of the structure of the Universe because we haven’t found something that, by its very nature, is hard to find. That’s why this announcement is so important.

The observation and identification of one of these Dwarf Dark Galaxies should open the door to more. Once more are found, we can start to build a model of their population and distribution. So if in the future more of these Dwarf Dark Galaxies are found, it will gradually confirm our over-arching understanding of the formation and structure of the Universe. And it’ll mean we’re on the right track when it comes to understanding Dark Matter’s role in the Universe. If we can’t find them, and the one bound to the halo of SDP81 turns out to be an anomaly, then it’s back to the drawing board, theoretically.

It took a lot of horsepower to detect the Dwarf Dark Galaxy bound to SDP81. Einstein Rings like SDP81 have to have enormous mass in order to exert a gravitational lensing effect, while Dwarf Dark Galaxies are tiny in comparison. It’s a classic ‘needle in a haystack’ problem, and Hezaveh and his team needed massive computing power to analyze the data from ALMA.

ALMA will consist of 66 individual antennae like these when it is complete. The facility is located in the Atacama Desert in Chile, at 5,000 meters above sea level. Credit: ALMA (ESO / NAOJ / NRAO)
ALMA will consist of 66 individual antennae like these when it is complete. The facility is located in the Atacama Desert in Chile, at 5,000 meters above sea level. Credit: ALMA (ESO / NAOJ / NRAO)

ALMA, and the methodology developed by Hezaveh and team will hopefully shed more light on Dwarf Dark Galaxies in the future. The team thinks that ALMA has great potential to discover more of these halo objects, which should in turn improve our understanding of the structure of the Universe. As they say in the conclusion of their paper, “… ALMA observations have the potential to significantly advance our understanding of the abundance of dark matter substructure.”

The Milky Way Galaxy’s Dark Halo Of Star Formation

Dark matter is invisible. Based on the effect of gravitational lensing, a ring of dark matter has been inferred in this image of a galaxy cluster (CL0024+17) and has been represented in blue. Image: NASA/ESA.
Dark matter is invisible. Based on the effect of gravitational lensing, a ring of dark matter has been inferred in this image of a galaxy cluster (CL0024+17) and has been represented in blue. Image: NASA/ESA.

Dark Matter is rightly called one of the greatest mysteries in the Universe. In fact, so mysterious is it, that we here in the opulent sky-scraper offices of Universe Today often joke that it should be called “Dark Mystery.” But that sounds like a cheesy History Channel show, and here at Universe Today we don’t like cheesy, so Dark Matter it remains.

Though we still don’t know what exactly Dark Matter is, we keep learning more about how it interacts with the rest of the Universe, and nibbling around at the edges of what it might be. But before we get into the latest news about Dark Matter, it’s worth stepping back a bit to remind ourselves of what is known about Dark Matter.

Evidence from cosmology shows that about 25% of the mass of the Universe is Dark Matter, also known as non-baryonic matter. Baryonic matter is ‘normal’ matter, which we are all familiar with. It’s made up of protons and neutrons, and it’s the matter that we interact with every day.

Cosmologists can’t see the 25% of matter that is Dark Matter, because it doesn’t interact with light. But they can see the effect it has on the large scale structure of the Universe, on the cosmic microwave background, and in the phenomenon of gravitational lensing. So they know it’s there.

Large galaxies like our own Milky Way are surrounded by what is called a halo of Dark Matter. These huge haloes are in turn surrounded by smaller sub-haloes of Dark Matter. These sub-haloes have enough gravitational force to form dwarf galaxies, like the Milky Way’s own Sagittarius and Canis Major dwarf galaxies. Then, these dwarf galaxies themselves have their own Dark Matter haloes, which at this scale are now much too small to contain gas or stars. Called dark satellites, these smaller haloes are of course invisible to telescopes, but theory states they should be there.

But proving that these dark satellites are even there requires some evidence of the effect they have on their host galaxies.

Now, thanks to Laura Sales, who is an assistant professor at the University of California, Riverside’s, Department of Physics and Astronomy, and her collaborators at the Kapteyn Astronomical Institute in the Netherlands, Tjitske Starkenberg and Amina Helmi, there is more evidence that these dark satellites are indeed there.

In their paper “Dark influences II: gas and star formation in minor mergers of dwarf galaxies with dark satellites,” from November 2015, they provide an analysis of theory-based computer simulations of the interaction between a dwarf galaxy and a dark satellite.

Their paper shows that when a dark satellite is at its closest point to a dwarf galaxy, the satellite’s gravitational influence compresses the gas in the dwarf. This causes a sustained period of star formation, called a starburst, that can last for billions of years.

NGC 5253 is one of the nearest of the known Blue Compact Dwarf (BCD) galaxies, and is located at a distance of about 12 million light-years from Earth in the southern constellation of Centaurus. It is experiencing a starburst of hot, young stars, which could be caused by dark satellites. Image: NASA/ESA/Hubble.
NGC 5253 is one of the nearest of the known Blue Compact Dwarf (BCD) galaxies, and is located at a distance of about 12 million light-years from Earth in the southern constellation of Centaurus. It is experiencing a starburst of hot, young stars, which could be caused by dark satellites. Image: NASA/ESA/Hubble.

Their modelling suggests that dwarf galaxies should be exhibiting a higher rate of star formation than would otherwise be expected. And observation of dwarf galaxies reveals that that is indeed the case. Their modelling also suggests that when a dark satellite and a dwarf galaxy interact, the shape of the dwarf galaxy should change. And again, this is born out by the observation of isolated spheroidal dwarf galaxies, whose origin has so far been a mystery.

The exact nature of Dark Matter is still a mystery, and will probably remain a mystery for quite some time. But studies like this keep shining more light on Dark Matter, and I encourage readers who want more detail to read it.