Whale Galaxy (NGC 4631) and Hockey Stick Galaxy (NGC 4656)

Whale Galaxy (NGC 4631) and Hockey Stick Galaxy (NGC 4656)

The Whale Galaxy, located in the Canes Venatici constellation, is a spiral galaxy smaller than the Milky Way and Andromeda Galaxy. The shapes of this galaxy and the Hockey Stick Galaxy, seen in the lower left from the Whale Galaxy, are distorted, indicating that the two galaxies are gravitationally interacting. These two galaxies are likely to merge into a larger galaxy. The Subaru Telescope’s Hyper Suprime-Cam (HSC), the ultra-wide field-of-view camera, successfully observed the two galaxies in one shot.
(Credit: Tohoku University/NAOJ). High Res.（ 8.34 MB)

Distance from Earth: About 25 million light-years
Instrument: Hyper Suprime-Cam (HSC)

Relevant Links:

• Uncovering the Origins of Galaxies' Halos

Source: Subaru Telescope

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Giant Magnetic Ropes in a Galaxy’s Halo

Composite image of the galaxy NGC 4631, the "Whale Galaxy," revealing large magnetic structures.

Credit: Composite image by Jayanne English of the University of Manitoba, with NRAO VLA radio data from Silvia Carolina Mora-Partiarroyo and Marita Krause of the Max-Planck Institute for Radioastronomy. The observations are part of the project Continuum HAlos in Nearby Galaxies -- an EVLA Survey (CHANG-ES). The optical data were from the Mayall 4-meter telescope, collected by Maria Patterson and Rene Walterbos of New Mexico State University. Arpad Miskolczi of the University of Bochum provided the software code for tracing the magnetic field lines. Hi-Res File

This image of the “Whale Galaxy” (NGC 4631), made with the National Science Foundation’s Karl G. Jansky Very Large Array (VLA), reveals hair-like filaments of the galaxy’s magnetic field protruding above and below the galaxy’s disk.

The spiral galaxy is seen edge-on, with its disk of stars shown in pink. The filaments, shown in green and blue, extend beyond the disk into the galaxy’s extended halo. Green indicates filaments with their magnetic field pointing roughly toward us and blue with the field pointing away. This phenomenon, with the field alternating in direction, has never before been seen in the halo of a galaxy.

“This is the first time that we have clearly detected what astronomers call large-scale, coherent, magnetic fields far in the halo of a spiral galaxy, with the field lines aligned in the same direction over distances of a thousand light-years. We even see a regular pattern of this organized field changing direction,” said Marita Krause, of the Max-Planck Institute for Radioastronomy in Bonn, Germany.

An international team of astronomers who are part of a project called the Continuum HAlos in Nearby Galaxies — an EVLA Survey (CHANG-ES), led by Judith Irwin of Queen’s University in Ontario, said the image indicates a large-scale, coherent magnetic field that is generated by dynamo action within the galaxy and spirals far outward in the form of giant magnetic ropes perpendicular to the disk.

“We are a little bit like the blind men and the elephant, since each time we look at the galaxy in a different way we reach a different conclusion about its nature! However, we seem to have one of those rare occasions where a classical theory, about magnetic generators called dynamos, predicted the observations of NGC 4631 quite well. Our dynamo model produces spiralling magnetic fields in the halo that are a continuation of the normal spiral arms in the galaxy’s disc,” said Richard Henriksen, of Queen’s University.

The scientists are continuing their work to further refine their understanding of the galaxy’s full magnetic structure.

The image was made by combining data from multiple observations with the VLA’s giant dish antennas arranged in different configurations to show both large structures and finer details within the galaxy. The naturally-emitted radio waves from the galaxy were analyzed to reveal the magnetic fields, including their directions.

The scientists said the techniques used to determine the direction of the magnetic field lines, illustrated by this image, now can be used on this and other galaxies to answer important questions about whether coherent magnetic fields are common in galactic halos and what their shapes are.

Building such a picture, they said, can answer important questions such as how galaxies acquire magnetic fields, and whether all such fields are produced by a dynamo effect. Can these galaxy halo fields illuminate the mysterious origin of the even larger intergalactic magnetic fields that have been observed?

NGC 4631, 25 million light-years from Earth in the constellation Canes Venatici, is about 80,000 light-years across, slightly smaller than our own Milky Way. It was discovered by the famous British astronomer Sir William Herschel in 1787. This image also shows a companion, NGC 4627, a small elliptical galaxy, just above NGC 4631.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

The results were reported in the journal Astronomy & Astrophysics.

The theoretical models are described in Woodfinden et al. 2019 MNRAS, 487, 1498.

Source: National Radio Astronomy Observatory (NRAO)/News

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Media Contact:

Dave Finley, Public Information Officer
(575) 835-7302
dfinley@nrao.edu

Science Contact:

Jayanne English
(204) 474-7105
Jayanne.English@umanitoba.ca

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Uncovering the Origins of Galaxies' Halos

Figure 1: Eleven dwarf galaxies and two star-containing halos were identified in the outer region of the nearby Whale Galaxy. (Credit: Tohoku University/NAOJ)

Using the Subaru Telescope atop Maunakea, researchers have identified 11 dwarf galaxies and two star-containing halos in the outer region of a large spiral galaxy 25 million light-years away from Earth. The findings, published in The Astrophysical Journal, provide new insight into how these 'tidal stellar streams' form around galaxies.

Researchers from Tohoku University and colleagues used an ultra-wide field of view camera on the Subaru Telescope to develop a better understanding of stellar halos. These ring-shaped collections of stars orbit large galaxies and can often originate from smaller dwarf galaxies nearby.

The team focused their attention on Galaxy NGC 4631, otherwise known as the Whale Galaxy because of its shape. They identified 11 dwarf galaxies in its outer region, some of which were already known. Dwarf galaxies are not easily detected because of their small sizes, masses and low brightness. The team also found two tidal stellar streams orbiting the galaxy: one, called Stream SE, is located in front of it and the other, called Stream NW, is nestled behind it.

Based on calculations aiming to estimate the metallic content of the stellar streams, the team believes it's possible that they originated as a result of a gravitational interaction between the Whale Galaxy and a dwarf galaxy orbiting it.The team focused their attention on Galaxy NGC 4631, otherwise known as the Whale Galaxy because of its shape. They identified 11 dwarf galaxies in its outer region, some of which were already known. Dwarf galaxies are not easily detected because of their small sizes, masses and low brightness. The team also found two tidal stellar streams orbiting the galaxy: one, called Stream SE, is located in front of it and the other, called Stream NW, is nestled behind it.

The team also found that both streams are relatively fainter than other stellar streams that have been studied around galaxies close to the Milky Way. Stream NW is the brighter of the pair, and has a more concentrated core. The researchers hypothesize that this brightness is due to a dwarf galaxy, possibly embedded within it, and that this dwarf had a gravitational interaction with the Whale Galaxy to form Stream SE.

Figure 2: Dwarf galaxies discovered by observation. Three color composites are characterized by HSC-g and HSC-i images. A pseudo-image with intermediate color is created from the averaged image of HSC-g and HSC-i images. The top right column (3) was previously thought to be a dwarf galaxy from an earlier study, but from the high resolution image taken by HSC, it is apparent that what was observed was an overlapping of foreground stars and background galaxies. (Credit: Tohoku University/NAOJ)

It is thought that stellar halos are less common when a galaxy's total stellar mass is smaller than the stellar mass of larger galaxies, such as the Triangulum Galaxy. As a result of their calculations, the researchers believe that the Whale Galaxy, although large, has a smaller mass than the Milky Way. Nonetheless, it is still in an active growth phase, and so are its surrounding halos. Future studies could help further clarify how stellar halos form around galaxies with relatively small masses, the researchers conclude.

This research is published in the Astrophysical Journal (Tanaka et al. 2017, "Resolved Stellar Streams around NGC 4631 from a Subaru/Hyper Suprime-Cam Survey", The Astrophysical Journal, 842, 127).

Link

• Research News from Tohoku University

Source: Subaru Telescope

The Belly of the Cosmic Whale

NGC 4631
Credit: NASA & ESA

The NASA/ESA Hubble Space Telescope has peered deep into NGC 4631, better known as the Whale Galaxy. Here, a profusion of starbirth lights up the galactic centre, revealing bands of dark material between us and the starburst. The galaxy’s activity tapers off in its outer regions where there are fewer stars and less dust, but these are still punctuated by pockets of star formation.

The Whale Galaxy is about 30 million light-years away from us in the constellation of Canes Venatici (The Hunting Dogs) and is a spiral galaxy much like the Milky Way. From our vantage point, however, we see the Whale Galaxy edge-on, seeing its glowing centre through dusty spiral arms. The galaxy's central bulge and asymmetric tapering disc have suggested the shape of a whale or a herring to past observers.

Many supernovae — the explosions of hot, blue, short-lived stars at least eight times the mass of the Sun — have gone off in the core of the Whale Galaxy. The stellar pyrotechnics have bathed the galaxy in hot gas, visible to X-ray telescopes like ESA’s XMM–Newton. Comparing the optical and near-infrared observations from Hubble with other telescopes sensitive to different wavelengths of light helps astronomers gather the full story behind celestial phenomena.

From such work, the triggers of the starburst in the Whale Galaxy and others can be elucidated. The gravitational "feeding" on intergalactic material, as well as clumping caused by the gravitational interactions with its galactic neighbours, creates the areas of greater density where stars start to coalesce. Just as blue whales, the biggest creatures on Earth, can gorge themselves on comparatively tiny bits of plankton, so the Whale Galaxy has become filled with the gas and dust that powers a high rate of star formation.