Spirals, Tides, and M51

Spiral galaxies are undoubtedly one of the most beautiful structures in the universe. Yet, their simple elegance belies a complex nature. How do such structures not “wind up” and what causes them in the first place? The answers to these questions is a long standing challenge. Under one model, spiral structure is created by spiral density waves. In another, they are induced by tidal interactions. It is this approach that is explored in a new paper by Dobbs et al., accepted for publication in the Monthly Notices of the Royal Astronomical Society. Specifically, the authors attempted to use modeling of tidal forces to recreate the structure of the spiral arms on the grand design spiral, M51.

M51sim1To model the interaction, they began with a model of a simple galaxy with a mass distribution (broken into a disc, bulge, and halo) similar to that for M51. Their initial galaxy was initially free of spiral structure, but “gravitational instabilities in the stars [Note: as opposed to the galactic gas. Not in individual stars.] produce a multi-armed” and patchy spiral structure (known as a flocculent spiral). This flocculent nature was first predicted in a 1964 paper by Toomre and has been simulated numerous times since then. Dobbs’ team then introduced a point source to represent the smaller galaxy (NGC 5195) along the orbital parameters derived by previous simulations of Theis and Spinneker in 2003.

For the first 60 million years, significant new structure was not evidence. The disc showed some perturbation due to the approaching companion, but no new spiral structure arose. However, by 120 million years from the start of the simulation, hints of a spiral arm on the side of the galaxy closest to the companion begin forming and by 180 million years, two pronounced “grand design” spiral arms dominate the face of the galaxy, spanning over 15,000 light years.

But the arms were too good to last. By 240 million years, the arms only stretch to a mere 6,500 light years as the gravitational forces from the companion seem to shepherd the galaxy’s gas as it is pulled around in its orbit. By 300 million years, the spiral arms have grown again and the pair looks remarkably similar to the present state of the M51/NGC 5195 system.

Comparison of simulation at 300 million years to HST image.
Comparison of simulation at 300 million years to HST image.

The authors note several features their simulation has in common with the observed galaxy. On the side where the companion first approached the galaxy, they note a “kink” in one arm (labeled as A in image to left). Another similarity is a splitting of one of the spiral arms although, again, the exact positioning is different (labeled B).

Another comparison the authors made was to the strengths (or amplitude) of various arm patterns (1 arm, 2 arm, 3 arm, etc…) over time. They found that the two armed pattern was the most predominant, but from the mechanics, they determined there were underlying higher armed structures that never fully took hold. However, these higher armed patterns did come close to the strength of the 2 arm spiral. The authors note that this is consistent with the observational findings of another group studying M51 in a work that has yet to be prepared for publication.

However, there are also some differences. A plume of gas extended from the simulated M51 which has no counterpart in actual observations (labeled C). Actual observations show large amounts of gas in front of the companion galaxy which are not present to the same degree in the simulation (labeled D). Lastly, real observations show a noticeable flattening of M51’s arms closest to the companion. Again, these do not appear in the simulation. The authors suggest discrepancies may be due to the over simplistic modeling of NGC 5195 as a point source instead of an extended body, or slight differences in initial parameters when compared to the actual system.

Even with these differences, the authors suggest that their modeling of the interaction shows that spiral structure, at least in this case, is most likely the result of the tidal interaction on M51 by NGC 5195. They also note that spiral density waves are likely not the culprit since other studies have not been able to determine a consistent “pattern speed” for the galaxy (the pattern speed is the angular speed at which the arms would rotate if viewed as a coherent structure). Instead, observations showed that the arms should have different pattern speeds at different radii.

Although their work does not suggest that all spiral structure is formed by tidal interactions with companions, this work makes a strong case for the possibility in many galaxies which would have such companions and M51 in specific. Furthermore, the simulations also reveal that these tidally induced arms are a temporary phenomenon. Since they do not have a fixed speed, they will slowly wind up and as the interaction progresses, the galaxies will be further distorted and eventually merge.

(Thanks to Claire Dobbs for permission to reproduce images from the paper as well as clarification on a few points.)

Hubble Takes a New “Deep Field” Image with Wide Field Camera 3

Hubble’s latest image is another stunner — and just look at all the galaxies! Hubble has produced a new version of the Ultra Deep Field, this time in near-infrared light and taken with the newly installed Wide Field Camera 3. This is the deepest image yet of the Universe in near-infrared, and so the faintest and reddest objects in the image are likely the oldest galaxies ever identified, and they likely formed only 600–900 million years after the Big Bang. This image was taken in the same region as the visible Ultra Deep Field in 2004, but this new deep view at longer wavelengths provides insights into how galaxies grew in their formative years early in the Universe’s history.

“Hubble has now re-visited the Ultra Deep Field which we first studied 5 years ago, taking infrared images which are more sensitive than anything obtained before,” said Dr. Daniel Stark, a postdoctoral researcher from Cambridge University. “We can now look even further back in time, identifying galaxies when the Universe was only 5 percent of its current age – within 1 billion years of the Big Bang.”

A portion of the Hubble Ultra Deep Field showing the location of a potentially very distant galaxy (marked by crosshairs).   Credit: Oxford University
A portion of the Hubble Ultra Deep Field showing the location of a potentially very distant galaxy (marked by crosshairs). Credit: Oxford University

The image was taken during a total of four days in August 2009, with 173,000 seconds of total exposure time. Since infrared light is invisible to the human eye and therefore does not have colors that can be perceived, the image is a “natural” representation that in shorter infrared wavelengths are represented as blue and the longer wavelengths as red. The faintest objects are about one billion times fainter than the dimmest visible objects seen with the naked eye.

Click here for a video zooming into the Ultra Deep Field.

“The expansion of the Universe causes the light from very distant galaxies to appear more red, so having a new camera on Hubble which is very sensitive in the infrared means we can identify galaxies at much greater distances than previously possible,” said Stephen Wilkins, from Oxford University.

Where is the new Ultra Deep Field in the sky?  Credit: HubbleSite
Where is the new Ultra Deep Field in the sky? Credit: HubbleSite

The team that took this image in August of 2009 have made it available for research by astronomers worldwide, and a multitude of astronomers have been furiously searching through the data for the most distant galaxies yet discovered. In just three months, twelve scientific papers on these new data have been submitted.

As well as identifying potentially the most distant objects yet, these new HST observations present an intriguing puzzle. “We know the gas between galaxies in the Universe was ionized (or fried) early in history, but the total light from these new galaxies may not be sufficient to achieve this,” said Andrew Bunker, from the University of Oxford.

Installation of Wide Field Camera 3 by astronauts as part of servicing mission 4. Courtesy of NASA.
Installation of Wide Field Camera 3 by astronauts as part of servicing mission 4. Courtesy of NASA.

“These new observations from HST are likely to be the most sensitive images Hubble will ever take, but the very distant galaxies we have now discovered will be studied in detail by Hubble’s successor, the James Webb Space Telescope, which will be launched in 2014,” said Professor Jim Dunlop at the University of Edinburgh.

1. By R.J. McLure, J.S. Dunlop, M. Cirasuolo, A.M. Koekemoer, E. Sabbi, D.P. Stark, T.A. Targett, R.S. Ellis,

2. By Stephen M. Wilkins, Andrew J. Bunker, Richard S. Ellis, Daniel Stark, Elizabeth R. Stanway, Kuenley Chiu, Silvio Lorenzoni, Matt J. Jarvis

3. By Bunker, Andrew; Wilkins, Stephen; Ellis, Richard; Stark, Daniel; Lorenzoni, Silvio; Chiu, Kuenley; Lacy, Mark; Jarvis, Matt; Hickey, Samantha,

Sources: Oxford University, Space Telescope Center

How Galaxies Lose Their Gas

Galaxy mergers, such as the Mice Galaxies will be part of Galaxy Zoo's newest project. Credit: Hubble Space Telescope

As galaxies evolve, many lose their gas. But how they do this is a point of contention. One possibility is that it is used to form stars when the galaxies undergo intense periods of star formation known as starburst. Another is that when large galaxies collide, the stars pass through one another but the gas gets left behind. It’s also possible that the gas is pulled out in close passes to other galaxies through tidal forces. Yet another possibility involves a wind blowing the gas out as galaxies plunge through the thin intergalactic medium in clusters through a process known as ram pressure.

A new paper lends fresh evidence to one of these hypotheses. In this paper, astronomers from the University of Arizona were interested in galaxies that displayed long gas tails, much like a comet. Earlier studies had found such galaxies, but it was unclear whether or not this gas tail was pulled out from tidal forces, or pushed out from ram pressure.

To help determine the cause of this the team used new observations from Spitzer to look for subtle differences in the causes of a tail following the galaxy ESO 137-001. In cases where tails are known to be pulled out tidally (such as in the M81/M82 system), there “is no physical reason why the gas would be preferentially stripped over stars.” Stars from the galaxy are pulled out as well and often large amounts of new star formation are induced. Meanwhile, ram pressure tails should be largely free of stars although some new star formation may be expected if there is turbulence in the tail which causes regions of higher density (think like the wake of a boat).

Examining the tail spectroscopically, the team was unable to detect the presence of large numbers of stars suggesting tidal processes were not responsible. Furthermore, the disk of the galaxy seemed relatively undisturbed by gravitational interactions. To support this, the team calculated the relative strengths of the forces acting on the galaxy. They found that, between the tidal forces acting on the galaxy from its parent cluster, and its own centripetal forces, the internal forces where greater, which reaffirmed that tidal forces were an unlikely cause for the tail.

But to confirm that ram pressure was truly responsible, the astronomers looked at other parameters. First they estimated the gravitational force for the galaxy. In order to strip the gas, the force generated by the ram pressure would have to exceed the gravitational one. The energy imparted on the gas would then be measurable as a temperature in the gas tail which could be compared to the expected values. When this was observed, they found that the temperature was consistent with what would be necessary for ram stripping.

From this, they also set limits on how long gas could last in such a galaxy. They determined that in such circumstances, the gas would be entirely stripped from a galaxy in ~500 million to 1 billion years. However, because the density of the gas through which the galaxy would slowly become denser as it passed through the more central regions of the cluster, they suggest the timescale would be much simpler. While this timescale say seem long, it is still shorter than the time it takes such galaxies to make a full orbit in their cluster. As such, it is possible that even in one pass, a galaxy may lose its gas.

If the gas loss occurs on such short timescales, this would further predict that tails like the one observed for ESO 137-001 should be rare. The authors note that an “X-ray survey of 25 nearby hot clusters only discovered 2 galaxies with X-ray tails.”

Although this new study in no way rules out other methods of removing a galaxy’s gas, this is one of the first galaxies for which the ram stripping method is conclusively demonstrated.


A Warm Molecular Hydrogen Tail Due to Ram Pressure Stripping of a Cluster Galaxy

Try Your Hand At Galaxy Zoo’s New “Slot Machine”

Galaxy mergers, such as the Mice Galaxies will be part of Galaxy Zoo's newest project. Credit: Hubble Space Telescope

Here’s your chance to play online slot machines without risking your life savings. Plus it’s an opportunity to contribute to a citizen science project that is sure to revolutionize our understanding of galaxy mergers. Galaxy Zoo’s newest project asks for help in looking at colliding galaxies, and uses a tool akin to a cosmic slot machine to compare images of galactic pile-ups with millions of simulated mergers.

“The analogy I’ve been using is that it is like driving past a car crash,” said Galaxy Zoo team member Chris Lintott from Oxford University. “You get a snapshot of the action, but there are two things you want to know: what caused the crash (or what did things look like before it all went wrong), and you want to know what the outcome is going to be. We’re doing the same thing. We want to know what the galaxies looked like before the mergers started disrupting them, and we want to know how they are going to end up. Just like our other Galaxy Zoo projects, humans are much better at doing this than computers, and lots of humans are even better.”

The Galaxy Zoo mergers project goes live on November 24 at http://mergers.galaxyzoo.org

“This is another classic Galaxy Zoo problem,” Lintott told Universe Today. “We found 3,000 galaxy mergers from Galaxy Zoo 1, and we don’t have a good understanding of the processes that take place during and after the collisions. This new project will help us work that out.”

On the Galaxy Zoo Mergers page, there will be a real image of a galactic merger in the center and with eight randomly selected merger simulations filling the other eight ‘slots’ around it. Visitors to the site pick which animation best demonstrates what is happening in that collision. But if they don’t see a good simulation, they can “spin the wheel again,” Lintott said, until a good depiction of the merger shows up.

A Grazing Encounter Between Two Spiral Galaxies (NGC 2207 and IC2163).  Credit: HubbleSite
A Grazing Encounter Between Two Spiral Galaxies (NGC 2207 and IC2163). Credit: HubbleSite

“By randomly cycling through the millions of simulated possibilities and selecting only the very best matches the users are helping to build up a profile of what kind of factors are necessary to create the galaxies we see in the Universe around us — and, hopefully, having fun too,” Lintott said.

There’s also the “enhance” option, where you can take control. “Once you have picked a simulation, you can take control of it directly, and change the parameters by hand such as the size, mass, the speed, for example. So, if you get impatient you can take control and see if you can do a better job than the slot machine,” Lintott explained.

For some of mergers, there will be a unique solution – only one way to get the merger we see today. For others there may be many different simulations that could provide the answer.

The Mergers project is a bit different than the regular Galaxy Zoo in that there will be, initially, just one daily challenge. “We’re aiming for one a day, but obviously if everyone who reads Universe Today turns up, we’ve got an idea of how many people we need to look at each one, so then we’ll change them out quicker,” Lintott said. “The more that people do, the more galaxies they’ll get to see.”

Of course, galaxy mergers are beautiful and amazing astronomical objects to behold, so the Galaxy Zoo team is hoping this will be a popular project.

“The point of Galaxy Zoo is to try and understand how we got the mix of galaxies that we see today,” Lintott said. “One of the mysteries is trying to work out how the ellipticals formed. We know that one way to form elliptical is to smash two spirals together. There’s the famous simulation of the Milky Way and Andromeda colliding and everyone assumes it will end up as a big elliptical that has used up all its gas. But actually it’s not clear how often that happens, and it’s not clear that you always get elliptical when you smash spirals together. In fact we know that in some cases they don’t. There is a lot of debate as to how important mergers are in this process.”

Right now, 3% of galaxies are in the process of merging, so, Lintott said, if most big galaxies undergo a merger every million years or so, this is clearly an important process.

“But we don’t understand what affects it has, and that’s what we hope to realize in this project.”

And Lintott admitted this newest Galaxy Zoo project is supposed to be fun and addictive. “Some people will love it, and some people will probably prefer the regular Galaxy Zoo. But it’s nice to have a range of scientific tasks that we have to work through.”

For more information:

Galaxy Zoo Mergers

Galaxy Zoo

“X” Marks Puzzling Galactic Bulge

Looking at a galaxy edge-on provides astronomers the opportunity to study different aspects of galaxies than a face-on view offers. This Hubble image of NGC 4710 is part of a survey conducted to provide more information about the puzzling bulges that form around the middle of some galaxies. Have these galaxies been “eating” too much, or is it just part of a “middle-age spread” similar to what humans experience? Astronomers aren’t sure why bulges evolve and become a substantial component of most spiral galaxies.

This image was taken by the Advanced Camera for Surveys in 2006, before the recent Hubble Servicing Mission.

A faint, ethereal “X”-shaped structure is also visible. Such a feature, which astronomers call a “boxy” or “peanut-shaped” bulge, is due to the vertical motions of the stars in the galaxy’s bar and is only evident when the galaxy is seen edge-on. This curiously shaped puff is often observed in spiral galaxies with small bulges and open arms, but is less common in spirals with arms tightly wrapped around a more prominent bulge, such as NGC 4710.

Click here to watch a movie zooming into this galaxy.

When targeting spiral galaxy bulges, astronomers often seek edge-on galaxies, as their bulges are more easily distinguishable from the disc. This exceptionally detailed edge-on view of NGC 4710 taken by the Advanced Camera for Surveys (ACS) aboard Hubble reveals the galaxy’s bulge in the brightly coloured centre. The luminous, elongated white plane that runs through the bulge is the galaxy disc. The disc and bulge are surrounded by eerie-looking dust lanes.

A wide-field image of the region around NGC 4710 constructed from Digitized Sky Survey 2 data. The field of view is approximately 2.8 degrees x 2.9 degrees.  Credit: NASA, ESA and Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)
A wide-field image of the region around NGC 4710 constructed from Digitized Sky Survey 2 data. The field of view is approximately 2.8 degrees x 2.9 degrees. Credit: NASA, ESA and Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)

NGC 4710 is a member of the giant Virgo Cluster of galaxies and lies in the northern constellation of Coma Berenices (the Hair of Queen Berenice). It is not one of the brightest members of the cluster, but can easily be seen as a dim elongated smudge on a dark night with a medium-sized amateur telescope. In the 1780s, William Herschel discovered the galaxy and noted it simply as a “faint nebula”. It lies about 60 million light-years from the Earth and is an example of a lenticular or S0-type galaxy – a type that seems to have some characteristics of both spiral and elliptical galaxies.

Astronomers are scrutinizing these systems to determine how many globular clusters they host. Globular clusters are thought to represent an indication of the processes that can build bulges. Two quite different processes are believed to be at play regarding the formation of bulges in spiral galaxies: either they formed rather rapidly in the early Universe, before the spiral disc and arms formed; or they built up from material accumulating from the disc during a slow and long evolution. In this case of NGC 4710, researchers have spotted very few globular clusters associated with the bulge, indicating that its assembly mainly involved relatively slow processes.

Source: STSci

Do “Skeleton” Filaments Give Structure to the Universe?

This 3D illustration shows the position of the galaxies and reveals the extent of this gigantic structure. The galaxies located in the newly discovered structure are shown in red. Galaxies that are either in front or behind the structure are shown in blue. Credit: ESO

Are there “skeletons” out in the Universe –structures that form the framework of how galaxies are distributed? Astronomers have tracked down a gigantic, previously unknown assembly of galaxies located almost seven billion light-years away from us, which seems to point to a prominent galaxy structure in the distant Universe, providing further insight into the cosmic web and how it formed. “Matter is not distributed uniformly in the Universe,” says Masayuki Tanaka from ESO, who led the new study. “In our cosmic vicinity, stars form in galaxies and galaxies usually form groups and clusters of galaxies. The most widely accepted cosmological theories predict that matter also clumps on a larger scale in the so-called ‘cosmic web’, in which galaxies, embedded in filaments stretching between voids, create a gigantic wispy structure.”

The filament is located about 6.7 billion light-years away from us and extends over at least 60 million light-years. The newly uncovered structure does probably extend further, beyond the field probed by the team, and hence future observations have already been planned to obtain a definite measure of its size.

These filaments are millions of light years long and constitute the skeleton of the Universe: galaxies gather around them, and immense galaxy clusters form at their intersections, lurking like giant spiders waiting for more matter to digest. Scientists are struggling to determine how they swirl into existence. Although massive filamentary structures have been often observed at relatively small distances from us, solid proof of their existence in the more distant Universe has been lacking until now.

The galaxies located in the newly discovered structure are shown in red. Galaxies that are either in front or behind the structure are shown in blue.  Credit: ESO
The galaxies located in the newly discovered structure are shown in red. Galaxies that are either in front or behind the structure are shown in blue. Credit: ESO

The team led by Tanaka discovered a large structure around a distant cluster of galaxies in images they obtained earlier. They have now used two major ground-based telescopes to study this structure in greater detail, measuring the distances from Earth of over 150 galaxies, and, hence, obtaining a three-dimensional view of the structure. The spectroscopic observations were performed using the VIMOS instrument on ESO’s Very Large Telescope and FOCAS on the Subaru Telescope, operated by the National Astronomical Observatory of Japan.

With these and other observations, the astronomers were able to make a real demographic study of this structure, and have identified several groups of galaxies surrounding the main galaxy cluster. They could distinguish tens of such clumps, each typically ten times as massive as our own Milky Way galaxy — and some as much as a thousand times more massive — while they estimate that the mass of the cluster amounts to at least ten thousand times the mass of the Milky Way. Some of the clumps are feeling the fatal gravitational pull of the cluster, and will eventually fall into it.

Image of the assembly of galaxies. Credit: ESO
Image of the assembly of galaxies. Credit: ESO

“This is the first time that we have observed such a rich and prominent structure in the distant Universe,” says Tanaka. “We can now move from demography to sociology and study how the properties of galaxies depend on their environment, at a time when the Universe was only two thirds of its present age.”

Source: ESO

Straight From the Island of Misfit Galaxies: Barnard

By galactic standards, Barnard is a misfit. This galaxy is diminutive, oddly shaped, and hard to see. But give the little box-shaped guy credit: this dwarf galaxy has no shortage of stellar splendor and pyrotechnics. Some feisty star formation is taking place, and a few curious nebulae dot the landscape with scorching stars sending waves of matter smashing into the surrounding stellar material. Plus, Barnard has a storied past: it likely was the victim of cannibalization. But cosmic misfits like Barnard’s Galaxy help researchers understand how galaxies interact, evolve and occasionally feed on each other, leaving behind radiant, star-filled scraps.

In this new image from ESO, Barnard’s Galaxy glows beneath a sea of foreground stars in the direction of the constellation of Sagittarius (the Archer). Also known as NGC 6822, the nicknamed comes its discoverer, American astronomer Edward Emerson Barnard, who spied it with his 125-millimeter aperture refractor in 1884. At the relatively close distance of about 1.6 million light-years, Barnard’s Galaxy is a member of the Local Group (ESO 11/96), the archipelago of galaxies that includes our home, the Milky Way.

Astronomers obtained this latest portrait using the Wide Field Imager (WFI) attached to the 2.2-metre MPG/ESO telescope at ESO’s La Silla Observatory in northern Chile.

At only about a tenth of the Milky Way’s size, Barnard’s Galaxy fits its dwarfish classification. All told, it contains about 10 million stars — a far cry from the Milky Way’s estimated 400 billion. In the Local Group, as elsewhere in the Universe, however, dwarf galaxies outnumber their larger, shapelier cousins, such as the Milky Way, the Andromeda and the Triangulum galaxies.

Even though Barnard’s Galaxy lacks the majestic spiral arms and glowing, central bulge that grace its big galactic neighbors, the Milky Way, the Andromeda and the Triangulum galaxies, there is a lot going on in this dwarf galaxy.

Reddish nebulae in this image reveal regions of active star formation, where young, hot stars heat up nearby gas clouds. Also prominent in the upper left of this new image is a striking bubble-shaped nebula. At the nebula’s centre, a clutch of massive, scorching stars send waves of matter smashing into the surrounding interstellar material, generating a glowing structure that appears ring-like from our perspective. Other similar ripples of heated matter thrown out by feisty young stars are dotted across Barnard’s Galaxy.

Irregular dwarf galaxies like Barnard’s Galaxy get their random, blob-like forms from close encounters with or “digestion” by other galaxies. Like everything else in the Universe, galaxies are in motion, and they often make close passes or even go through one another. The density of stars in galaxies is quite low, meaning that few stars physically collide during these cosmic dust-ups. Gravity’s fatal attraction, however, can dramatically warp and scramble the shapes of the passing or crashing galaxies. Groups of stars are pulled or flung from their galactic home, in turn forming irregularly shaped dwarf galaxies like NGC 6822.

Click here to see a zoom-in video (choose from various formats) of Barnard’s Galaxy.

The image was made from data obtained through four different filters (B, V, R, and H-alpha). The field of view is 35 x 34 arcmin. North is up, East to the left.

Source: ESO

New Hubble Release: Dramatic Galaxy Collision

At first glance, this latest image release from Hubble appears to be one really bizarre-looking galaxy. But actually, this is a pair of spiral galaxies that resemble our own Milky Way smashing together at breakneck speeds. The centers have already merged into one nucleus, and the two tidal tails stretching out from the center are sparkling with active star formation, prompted by the exchange of mass and gases from the dramatic collision. This object, NGC 2623, or Arp 243, is about 250 million light-years away in the constellation of Cancer (the Crab), and is in the late stages of the merging process.

The prominent lower tail is richly populated with bright star clusters — 100 of them have been found in these observations. The large star clusters that the team has observed in the merged galaxy are brighter than the brightest clusters we see in our own vicinity. These star clusters may have formed as part of a loop of stretched material associated with the northern tail, or they may have formed from debris falling back onto the nucleus. In addition to this active star-forming region, both galactic arms harbor very young stars in the early stages of their evolutionary journey.

Watch this video for more information on NGC 2623:

Some mergers (including NGC 2623) can result in an active galactic nucleus, where one of the supermassive black holes found at the centers of the two original galaxies is stirred into action. Matter is pulled toward the black hole, forming an accretion disc. The energy released by the frenzied motion heats up the disc, causing it to emit across a wide swath of the electromagnetic spectrum.

NGC 2623 is so bright in the infrared that it belongs to the group of very luminous infrared galaxies (LIRG) and has been extensively studied as the part of the Great Observatories All-sky LIRG Survey (GOALS) project that combines data from Hubble, the Spitzer Space Telescope, Chandra X-ray Observatory and the Galaxy Evolution Explorer (GALEX). The combination of resources is helping astronomers characterize objects like active galactic nuclei and nuclear star formation by revealing what is unseen at visible wavelengths.

The data used for this color composite were actually taken in 2007 by the Advanced Camera for Surveys (ACS) aboard Hubble, but is just being released now, as a team of over 30 astronomers, led by Aaron S. Evans, recently published an overview paper, detailing the first results of the GOALS project. Observations from ESA’s X-ray Multi-Mirror Mission (XMM-Newton) telescope contributed to the astronomers’ understanding of NGC 2623.

NGC 2623 paper
GOALS Overview paper
GOALS website

Source: European Hubble website

Hubble Sees Galaxies Stripped by Ram Pressure


Strange forces of nature are stripping away gas from galaxies in the Virgo cluster. An extremely hot X-ray emitting gas known as the intra-cluster medium permeates the regions between galaxies inside clusters and, as fast moving galaxies whip through this medium, strong winds tear through galaxies distorting their shape and even halting star formation with a process known as “ram pressure stripping.” Hubble spied two galaxies “losing it” to these forces.

Ram pressure is the drag force that results when something moves through a fluid — much like the wind you feel in your face when bicycling, even on a still day — and occurs in this context as galaxies orbiting about the centre of the cluster move through the intra-cluster medium, which then sweeps out gas from within the galaxies.

The two galaxies — NGC 4522 and NGC 4402 – were imaged by the old Advanced Camera for Surveys on Hubble before it suffered from a power failure in 2007. Astronauts on Servicing Mission 4 in May 2009 were able to restore ACS during their 13-day mission.

This image shows NGC 4522 within the context of the Virgo Cluster.   Credit: NASA, ESA and the Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)
This image shows NGC 4522 within the context of the Virgo Cluster. Credit: NASA, ESA and the Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)

The spiral galaxy NGC 4522 is located some 60 million light-years away from Earth and it is a spectacular example of a spiral galaxy currently being stripped of its gas content. Astronomers estimate the galaxy is moving at more than 10 million kilometers per hour, and its rapid motion within the cluster results in strong winds across the galaxy as the gas within is left behind. A number of newly formed star clusters that developed in the stripped gas can be seen in the Hubble image.

The image provides a vivid view of the ghostly gas being forced out of it. Bright blue pockets of new star formation can be seen to the right and left of centre. The image is sufficiently deep to show distant background galaxies.

The image of NGC 4402 also highlights some telltale signs of ram pressure stripping such as the curved, or convex, appearance of the disc of gas and dust, a result of the forces exerted by the heated gas. Light being emitted by the disc backlights the swirling dust that is being swept out by the gas. Studying ram pressure stripping helps astronomers better understand the mechanisms that drive the evolution of galaxies, and how the rate of star formation is suppressed in very dense regions of the Universe like clusters.

Source: Hubble Science Center

Barred Spiral Galaxy

As its name implies, a barred spiral galaxy is a spiral galaxy with a bar through the center.

Hubble introduced the ‘tuning fork’ scheme for describing the shapes of galaxies (“morphologies” in astronomer-speak) in 1936. In this, the two arms of the fork are barred spirals (from SBa to SBc) and spirals without bars (from Sa to Sc); the S stands for spiral, B for ‘it’s got a bar’, and a/b/c for how tightly wound the spiral arms are. This was later extended to a fourth type, SBm and Sm, for irregular barred spiral galaxies which have no bulge.

In 1959, Gérard de Vaucouleurs extended the scheme to the one perhaps the most commonly used by astronomers today (though there’ve been some mods since). In this scheme spirals without bars are SA, and those which have really weak bars are SAB; barred spirals remain SB. He also added a ‘d’ (SAd, SBd), and a few other things, like rings.

About half of spiral galaxies are barred; examples include M58 (SBc), M61 (SABbc), the Large Magellanic Cloud (LMC, Sm), … and our own Milky Way galaxy!

The bars are mostly stars (usually), unlike spiral arms (which have lots of gas and dust besides stars). The formation and evolution of bars is an active area of research in astronomy today; they seem to form from close encounters of the galaxy kind (galaxy near-collisions), funnel gas into the central bulge (where the super-massive black holes there snack on it), and are sustained by the same density waves which keep the arms alive.

Why not join the Galaxy Zoo project, and have some fun classifying spiral galaxies into whether they have bars or not (and getting to see some amazing sights too)?

Hubble Early Release Observation of Barred Spiral NGC 6217, Two Galaxies Walk Into a Bar…, and The Milky Way Has Only Two Spiral Arms; just some of the Universe Today stories on barred spiral galaxies.

Astronomy Casts featuring barred spiral galaxies include The Story of Galaxy Evolution, and Galaxies.