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Astronomers see through the dust of the Milky Way to trace radiation to the center of the galaxy



The center of our galaxy may be one of the most mysterious places in the universe. Astronomers have to probe dense dust to see what’s going on there. All that dust makes life difficult for astronomers who are trying to understand all the radiation in the center of the Milky Way and what exactly is its source.

A new study based on 20 years of data – and a hydrogen bubble where there shouldn’t be – is helping astronomers understand all that energy.

It is an astronomical peculiarity that we somehow know more about other galaxies than we do about ours. Scientists examined energy from the center of thousands of other spiral galaxies in visible light. But for our Milky Way, that knowledge is blocked by thick clouds of gas and dust.

A team of researchers examined decades of data from the Wisconsin H-Alpha Mapper (WHAM) telescope for clues to the energy of the Milky Way. Their results are in a document entitled “Discovery of optical emission lines diffused by the internal galaxy: evidence of gas similar to ER LI (N)”

;. The main author is Dhanesh Krishnarao of the University of Wisconsin. The article is published in the journal Science Advances.

There is a huge amount of hydrogen near the center of the Milky Way. That hydrogen is ionized by energy from the galactic center. As ionized gas, its electrons have been removed. The WHAM telescope is designed to see ionized hydrogen, which appears red when viewed with the scope.

The WHAM telescope is at the Inter-American Cerro-Tololo Observatory in Chile. It is a fully robotic structure, controlled remotely. Image Credit: WHAM / University of Wisconsin.
The WHAM telescope is at the Inter-American Cerro-Tololo Observatory in Chile. It is a fully robotic structure, controlled remotely. Image Credit: WHAM / University of Wisconsin.

It’s not just that hydrogen is ionized. After ionizing a gas, the ions normally recombine to neutrality in a short period of time. The fact that all this hydrogen is continuously ionized by an energy source is the link between WHAM data and the energy at the center of the Milky Way. Astronomers thought that the star formation is the energy source for this ionization, but this is not conclusive.

WHAM is tailor-made to study ionized gas. The Milky Way contains a thick layer, called Warm Ionized Medium (WIM), which is a distinct and important component of the galactic interstellar medium. WIM is the main target of WHAM.

“Without a continuous source of energy, free electrons usually find and recombine to return to a neutral state in a relatively short period of time,” explained co-author L. Matthew Haffner of the Embry-Riddle Air Force University, in a press release. “Being able to view ionized gas in new ways should help us discover the types of sources that could be responsible for keeping all that gas energized.”

It all started when co-author Bob Benjamin, UW – Whitewater astronomy professor, was reviewing WHAM data a couple of years ago. The data came from observations of ionized hydrogen across the Milky Way. Benjamin found what he called a “red flag”. Protruding from the dusty center of the Milky Way was an ionized hydrogen bubble with a strange shape.

And that bubble was moving in the direction of the Earth.

The researchers used the Wisconsin H-alpha Mapper telescope, or WHAM, the telescope to measure the emission of visible light from hydrogen in an inclined disk-shaped region under the plane of the Milky Way, highlighted in red. DHANESH KRISHNARAO / AXEL MELLINGER SIDE IMAGE
The researchers used the Wisconsin H-alpha Mapper telescope, or WHAM, the telescope to measure the emission of visible light from hydrogen in an inclined disk-shaped region under the plane of the Milky Way, highlighted in red. DHANESH KRISHNARAO / AXEL MELLINGER SIDE IMAGE

Astronomers call this function “Inclined disk”. Its strange shape cannot be explained by physical causes such as rotation of the galaxy. There is something else behind it.

The researchers realized that this was a rare opportunity: the disc protruded from its usual thick dust cover. Now they could study it in optical light, thanks to WHAM. Normally, the tilted disc is visible only in infrared or radio, due to the dusty veil. This allowed researchers to compare the center of the Milky Way with observations of visible light from other spiral galaxies.

“For the first time, we were able to directly compare measurements from our galaxy with that large population.”

L. Matthew Haffner, Co-author of the study, Embry-Riddle Aeronautical University

“The ability to perform these measurements in optical light has allowed us to compare the core of the Milky Way with other galaxies much more easily,” said Haffner. “Many past studies have measured the quantity and quality of ionized gas from the centers of thousands of spiral galaxies across the universe. For the first time, we were able to directly compare measurements from our Galaxy with that vast population. “

There are existing scientific models of the ionized gas that makes up the WIM. In this new research, lead author Dhanesh Krishnarao used one to predict how much ionized gas should be in the red flag region identified by Benjamin. He perfected those predictions with WHAM raw data and produced an accurate three-dimensional image of the bubble structure. Using spectroscopy, the researchers identified the amount of nitrogen and oxygen present, giving them more clues about the overall composition of the structure.

The results show that 48% of the gas in the Tilted Disk function is ionized by an unknown energy source. As lead author Krishnarao states, “The Milky Way can now be used to better understand its nature.”

This information-dense figure from the study shows some of the team's findings. The top panel shows the layout of the Inclined Disc feature, with the Fermi bubbles outlined in purple for reference. The bottom panel shows the spectral close-ups of two of WHAM's objectives. Each lower panel shows the presence of alpha hydrogen, beta hydrogen, as well as sulfur II, nitrogen II and oxygen III. Image credit: Krishnarao et al, 2020.
This information-dense figure from the study shows some of the team’s findings. The top panel shows the layout of the Inclined Disc feature, with the Fermi bubbles outlined in purple for reference. The bottom panel shows the spectral close-ups of two of WHAM’s targets. Each lower panel shows the presence of alpha hydrogen, beta hydrogen, as well as sulfur II, nitrogen II and oxygen III. Image credit: Krishnarao et al, 2020.

Prior to this work, scientists knew only neutral or non-ionized gases in the central region. They now have a better understanding of ionized gas and also know that it changes as it moves away from the galactic center. This is a critical result, because it shows for the first time that the Milky Way is similar to other spiral galaxies called LINER.

“Near the core of the Milky Way,” explained Krishnarao, “the gas is ionized by newly formed stars, but as we move away from the center, things become more extreme and the gas becomes similar to a class of galaxies called LINER “or low ionization (nuclear) regions”.

LINERS are galactic nuclei identified by their emission of spectral lines, which show the presence of weakly ionized or neutral atoms such as O, O+, N+and S+. About a third of the nearby galaxies are LINER. They are more radiant than galaxies whose only source of energy is star formation, but less radiative than galaxies that have a supermassive black hole that actively feeds.

“Before this WHAM discovery, the Andromeda galaxy was the closest LINER spiral to us.”

L. MATTHEW HAFFNER, CO-AUTHOR OF THE STUDY, EMBRY-RIDDLE AERONAUTICAL UNIVERSITY

Now that we know that our Milky Way galaxy is a LINER, it means astronomers can now study a LINER up close and personal.

The Andromeda galaxy is also a LINER spiral galaxy. Although it is close in astronomical terms, it is still very distant and difficult to study. Image credit: NASA / ESA / Hubble
The Andromeda galaxy is also a LINER spiral galaxy. Although it is close in astronomical terms, it is still very distant and difficult to study. Image credit: NASA / ESA / Hubble

“Before this WHAM discovery, the Andromeda galaxy was the closest LINER spiral to us,” said Haffner. “But there are still millions of light years left. With the Milky Way core only tens of thousands of light years away, we can now study a LINER region in more detail. Studying this extensive ionized gas should help us learn more about the environment current and past at the center of our Galaxy. “

There are still many questions of course. Although we now know that the Milky Way is a LINER and that the bubble structure identified by Benjamin who started all this seems to move only towards us because of its elliptical orbit, the key question remains unanswered: what is the source of energy that is driving all this ionization?

“WHAM was an excellent tool for producing the first open-air detection of this gas, but now we are hungry for more detail.”

L. MATTHEW HAFFNER, CO-AUTHOR OF THE STUDY, EMBRY-RIDDLE AERONAUTICAL UNIVERSITY

This question may have to wait for the planned but still unnamed WHAM successor.

“In the coming years, we hope to build WHAM’s successor, which would give us a clearer view of the gas we study,” said Haffner. “Right now the pixels on our map are twice as large as the full moon. WHAM was an excellent tool for producing the first open-air detection of this gas, but now we are hungry for more detail.”

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