Posted on by Tammy Plotner

Galactic Archaeology: NGC 5907 – The Dragon Clash

NGC 5907 - Credit: R. Jay Gabany

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The sprawling northern constellation of Draco is home to a monumental galactic merger which left a singular spectacle – NGC 5907. Surrounded by an ethereal garment of wispy star trails and currents of stellar material, this spiral galaxy is the survivor of a “clash of the dragons” which may have occurred some 8 to 9 billion years ago. Recent theory suggests galaxies of this type may be the product of a larger galaxy encountering a smaller satellite – but this might not be the case. Not only is NGC 5907 a bit different in some respects, it’s a lot different in others… and peculiar motion is just the beginning.

“If the disc of many spirals is indeed rebuilt after a major merger, it is expected that tidal tails can be a fossil record and that there should be many loops and streams in their halos. Recently Martínez-Delgado et al. (2010) have conducted a pilot survey of isolated spiral galaxies in the Local Volume up to a low surface brightness sensitivity of ~28.5 mag/arcsec2 in V band. They find that many of these galaxies have loops or streams of various shapes and interpret these structures as evidence of minor merger or satellite infall.” says J. Wang of the Chinese Academy of Sciences. “However, if these loops are caused by minor mergers, the residual of the satellite core should be detected according to numerical simulations. Why is it hardly ever detected?”

The “why” is indeed the reason NGC 5907 is being intensively studied by a team of six scientists of the Observatoire de Paris, CNRS, Chinese Academy of Sciences, National Astronomical Observatories of China NAOC and Marseille Observatory. Even though NGC 5907 is a member of a galactic group, there are no galaxies near enough to it to be causing an interaction which could account for its streamers of stars. It is truly a warped galaxy with gaseous and stellar disks which extend beyond the nominal cut-off radius. But that’s not all… It also has a peculiar halo which includes a significant fraction of metal enriched stars. NGC 5907 just doesn’t fit the patterns.

“For some of our models, we assume a star formation history with a varying global efficiency in transforming gas to stars, in order to preserve enough gas from being consumed before fusion.” explains the research team. “Although this fine-tuned star formation history may have some physical motivations, its main role is also to ensure the formation of stars after the emergence of the gaseous disc just after fusion.”

On left, the NGC 5907 galaxy. It is compared to the simulations, on right. Both cases show an edge-on galactic disk surrounded by giant loops of old stars, which are witnessing of a former, gigantic collision. (Jay Gabany, cosmotography.com / Observatoire de Paris / CNRS / Pythéas / NAOC)

Now enter the 32- and 196-core computers at the Paris Observatory center and the 680-core Graphic Processor Unit supercomputer of Beijing NAOC with the capability to run 50000 billion operations per second. By employing several state of the art, hydrodynamical, and numerical simulations with particle numbers ranging from 200 000 to 6 millions, the team’s goal was to show the structure of NGC 5907 may have been the result of the clash of two dragon-sized galaxies… or was it?

“The exceptional features of NGC 5907 can be reproduced, together with the central galaxy properties, especially if we compare the observed loops to the high-order loops expected in a major merger model.” says Wang. “Given the extremely large number of parameters, as well as the very numerous constraints provided by the observations, we cannot claim that we have already identified the exact and unique model of NGC 5907 and its halo properties. We nevertheless succeeded in reproducing the loop geometry, and a disc-dominated, almost bulge-less galaxy.”

In the meantime, major galaxy merger events will continue to be a top priority in formation research. “Future work will include modelling other nearby spiral galaxies with large and faint, extended features in their halos.” concludes the team. “These distant galaxies are likely similar to the progenitors, six billion years ago, of present-day spirals, and linking them together could provide another crucial test for the spiral rebuilding disc scenario.”

And sleeping dragons may one day arise…

Original Story Source: Paris Observatory News. For Further Reading: Loops formed by tidal tails as fossil records of a major merger and Fossils of the Hierarchical Formation of the Nearby Spiral Galaxy NGC 5907.

Posted on by Jon Voisey

More to Meets the Eye in M33

The spiral galaxy M33 is one of the largest galaxies in our local group. This spiral galaxy is moderately tilted when viewed from Earth, displaying a lack of a distinct central bulge but prominent spiral arms. It has only one potential companion galaxy (the Pisces Dwarf) and its spiral arms are so pristine, they have been thought to be unperturbed by the accretion of dwarf galaxies that constantly occurs in the Milky Way and Andromeda galaxy. Yet these features are what has made M33 so hard to explain. Since larger galaxies are expected to form from the merger of smaller galaxies it is expected that M33 should show some scars from previous mergers. If this picture is true, where are they?

The role of galaxy accretion in our own galaxy was first revealed in 1994 with the discovery of the Sagittarius stellar stream. With the completion of the first Sloan Digitised Sky Survey, many more tidal streams were revealed in our own galaxy. Modeling of the kinematics of these streams suggested they should last billions of years before fading into the rest of the galaxy. Deep imaging of the Andromeda galaxy revealed stellar streams as well as a notable warping of the disc of the galaxy.

Yet M33 seems to lack obvious signs of these structures. In 2006, a spectroscopic study analyzed the bright red giants in the galaxy and found three distinct populations. One could be attributed to the disc, one to the halo, but the third was not immediately explicable. Could this be the relic of an ancient satellite?

Another potential clue on missing mergers was discovered in 2005 when a radio survey around M33 was conducted with the Arecibo telescope. This study uncovered large clouds with a thousand to a million solar masses worth of raw hydrogen suspended around the galaxy. Might these be incomplete dwarf galaxies that never merged into M33? A new study uses the Subaru telescope atop Mauna Kea to study these regions as well as the outskirts of M33 to better understand their history.

The team, led by Marco Grossi at the Observatório Astronómico de Lisboa in Portugal, did not find evidence of a stellar population in these clouds suggesting they were not likely to be galaxies in their own right. Instead, they suggest that these clouds may be analogous to hydrogen clouds around the Milky Way and Andromeda which are “often found close to stellar streams or disturbances in the stellar disc” where gas is pulled from a former satellite galaxy through tidal or ram-pressure stripping. This would constitute another piece of indirect evidence that M33 once underwent mergers of some sort.

Outside of these clouds, in the outskirts of the galaxy, the team uncovered a diverse population of stars beyond the main disc. The overall metallicity of these stars was lower, but it also included some younger stars. At such a distance, these young stars would not be expected unless accreted.

While this finding doesn’t fully answer the question of how M33 may have formed, it does reveal that this galaxy has likely not evolved in the isolation previously assumed.

Posted on by Jon Voisey

Testing the Spiral Density Wave Theory

M66 from Hubble
M66 from Hubble

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Spiral galaxies are one of the most captivating structures in astronomy, yet their nature is still not fully understood. Astronomers currently have two categories of theories that can explain this structure, depending on the environment of the galaxy, but a new study, accepted for publication in the Astrophysical Journal, suggests that one of these theories may be largely wrong.

For galaxies with nearby companions, astronomers have suggested that tidal forces may draw out spiral structure. However, for isolated galaxies, another mechanism is required in which galaxies form these structures without intervention from a neighbor. A possible solution to this was first worked out in 1964 by Lin & Shu in which they suggested that the winding structure is merely an illusion. Instead, these arms weren’t moving structures, but areas of greater density which remained stationary as stars entered and exited them similar to how a traffic jam remains in position although the component cars travel in and out. This theory has been dubbed the Lin-Shu density wave theory and has been largely successful. Previous papers have reported a progression from cold, HI regions and dust on the inner portion of the spiral arms, that crash into this higher density region and trigger star formation, making hot O & B class stars that die before exiting the structure, leaving the lower mass stars to populate the remainder of the disk.

One of the main questions on this theory has been the longevity of the overdense region. According to Lin & Shu as well as many other astronomers, these structures are generally stable over long time periods. Others suggest that the density wave comes and goes in relatively short-lived, recurrent patterns. This would be similar to the turn signal on your car and the one in front of you at times seeming to synch up before getting out of phase again, only to line up again in a few minutes. In galaxies, the pattern would be composed of the individual orbits of the stars, which would periodically line up to create the spiral arms. Teasing out which of these was the case has been a challenge.

To do so, the new research, led by Kelly Foyle from McMaster University in Ontario, examined the progression of star formation as gas and dust entered the shock region produced by the Lin-Shu density wave. If the theory was correct, they should expect to find a progression in which they would first find cold HI gas and carbon monoxide, and then offsets of warm molecular hydrogen and 24 μm emission from stars forming in clouds, and finally, another offset of the UV emission of fully formed and unobscured stars.

The team examined 12 nearby spiral galaxies, including M 51, M 63, M 66, M 74, M 81, and M 95. These galaxies represented several variations of spiral galaxies such as grand design spirals, barred spirals, flocculent spirals and an interacting spiral.

When using a computer algorithm to examine each for offsets that would support the Lin-Shu theory, the team reported that they could not find a difference in location between the three different phases of star formation. This contradicts the previous studies (which were done “by eye” and thus subject to potential bias) and casts doubt on long lived spiral structure as predicted by the Lin-Shu theory. Instead, this finding is in agreement with the possibility of transient spiral arms that break apart and reform periodically.

Another option, one that salvages the density wave theory is that there are multiple “pattern speeds” producing more complex density waves and thus blurs the expected offsets. This possibility is supported by a 2009 study which mapped these speeds and found that several spiral galaxies are likely to exhibit such behavior. Lastly, the team notes that the technique itself may be flawed and underestimating the emission from each zone of star formation. To settle the question, astronomers will need to produce more refined models and explore the regions in greater detail and in more galaxies.

Posted on by Jean Tate

Magnetic Fields in Spiral Galaxies – Explained at Last?

M51 (Hubble) overlaid by 6cm radio intensity contours and polarization vectors (Effelsberg and VLA) Credit: MPIfR Bonn

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That spiral galaxies have magnetic fields has been known for well over half a century (and predictions that they should exist preceded discovery by several years), and some galaxies’ magnetic fields have been mapped in great detail.

But how did these magnetic fields come to have the characteristics we observe them to have? And how do they persist?

A recent paper by UK astronomers Stas Shabala, James Mead, and Paul Alexander may contain answers to these questions, with four physical processes playing a key role: infall of cool gas onto the disk, supernova feedback (these two increase the magnetohydrodynamical turbulence), star formation (this removes gas and hence turbulent energy from the cold gas), and differential galactic rotation (this continuously transfers field energy from the incoherent random field into an ordered field). However, at least one other key process is needed, because the astronomers’ models are inconsistent with the observed fields of massive spiral galaxies.

“Radio synchrotron emission of high energy electrons in the interstellar medium (ISM) indicates the presence of magnetic fields in galaxies. Rotation measures (RM) of background polarized sources indicate two varieties of field: a random field, which is not coherent on scales larger than the turbulence of the ISM; and a spiral ordered field which exhibits large-scale coherence,” the authors write. “For a typical galaxy these fields have strengths of a few μG. In a galaxy such as M51, the coherent magnetic field is observed to be associated with the optical spiral arms. Such fields are important in star formation and the physics of cosmic rays, and could also have an effect on galaxy evolution, yet, despite their importance, questions about their origin, evolution and structure remain largely unsolved.”

This field in astrophysics is making rapid progress, with understanding of how the random field is generated having become reasonably well-established only in the last decade or so (it’s generated by turbulence in the ISM, modeled as a single-phase magnetohydrodynamic (MHD) fluid, within which magnetic field lines are frozen). On the other hand, the production of the large-scale field by the winding of the random fields into a spiral, by differential rotation (a dynamo), has been known for much longer.

The details of how the ordered field in spirals formed as those galaxies themselves formed – within a few hundred million years of the decoupling of baryonic matter and radiation (that gave rise to the cosmic microwave background we see today) – are becoming clear, though testing these hypotheses is not yet possible, observationally (very few high-redshift galaxies have been studied in the optical and NIR, period, let alone have had their magnetic fields mapped in detail).

“We present the first (to our knowledge) attempt to include magnetic fields in a self-consistent galaxy formation and evolution model. A number of galaxy properties are predicted, and we compare these with available data,” Shabala, Mead, and Alexander say. They begin with an analytical galaxy formation and evolution model, which “traces gas cooling, star formation, and various feedback processes in a cosmological context. The model simultaneously reproduces the local galaxy properties, star formation history of the Universe, the evolution of the stellar mass function to z ~1.5, and the early build-up of massive galaxies.” Central to the model is the ISM’s turbulent kinetic energy and the random magnetic field energy: the two become equal on timescales that are instantaneous on cosmological timescales.

The drivers are thus the physical processes which inject energy into the ISM, and which remove energy from it.

“One of the most important sources of energy injection into the ISM are supernovae,” the authors write. “Star formation removes turbulent energy,” as you’d expect, and gas “accreting from the dark matter halo deposits its potential energy in turbulence.” In their model there are only four free parameters – three describe the efficiency of the processes which add or remove turbulence from the ISM, and one how fast ordered magnetic fields arise from random ones.

Are Shabala, Mead, and Alexander excited about their results? You be the judge: “Two local samples are used to test the models. The model reproduces magnetic field strengths and radio luminosities well across a wide range of low and intermediate-mass galaxies.”

And what do they think is needed to account for the detailed astronomical observations of high-mass spiral galaxies? “Inclusion of gas ejection by powerful AGNs is necessary in order to quench gas cooling.”

SKA central region with separate core stations for the two aperture arrays for low and mid frequencies and for the dish array. Graphics: Xilostudios and SKA Project Development Office

It goes without saying that the next generation of radio telescopes – EVLA, SKA, and LOFAR – will subject all models of magnetic fields in galaxies (not just spirals) to much more stringent tests (and even enable hypotheses on the formation of those fields, over 10 billion years ago, to be tested).

Source: Magnetic fields in galaxies: I. Radio disks in local late-type galaxies