Posted on by Jon Voisey

Gravitational Redshifts: Main Sequence vs. Giants

Pleiades
The Pleiades, Anglo-Australian Observatory/Royal Observatory

[/caption]

One of the consequences of Einsteins theories of relativity is that everything will be affected by gravitational potentials, regardless of their mass. The effect of this is observed in experiments demonstrating the potential for gravity to bend light. But a more subtle realization is that light escaping such a gravitational well must lose energy, and since energy for light is related to wavelength, this will cause the light to increase in wavelength through a process known as gravitational redshifting.

Since the amount of redshift is dependent on just how deeply inside a gravitational well a photon is when it starts its journey, predictions have shown that photons being emitted from the photosphere of a main sequence star should be more redshifted than those coming from puffed out giants. With resolution having reached the threshold to detect this difference, a new paper has attempted to observationally detect this difference between the two.

Historically, gravitational redshifts have been detected on even more dense objects such as white dwarfs. By examining the average amount of redshifts for white dwarfs against main sequence stars in clusters such as the Hyades and Pleiades, teams have reported finding gravitational redshifts on the order of 30-40 km/s (NOTE: the redshift is expressed in units as if it were a recessional Doppler velocity, although it’s not. It’s just expressed this way for convenience). Even larger observations have been made for neutron stars.

For stars like the Sun, the expected amount of redshift (if the photon were to escape to infinity) is small, a mere 0.636 km/s. But because Earth also lies in the Sun’s gravitational well the amount of redshift if the photon were to escape from the distance of our orbit would only be 0.633 km/s leaving a distance of only ~0.003 km/s, a change swamped by other sources.

Thus, if astronomers wish to study the effects of gravitational redshift on stars of more normal density, other sources will be required. Thus, the team behind the new paper, led by Luca Pasquini from the European Southern Observatory, compared the shift among stars of the middling density of main sequence stars against that of giants. To eliminate effects of varying Doppler velocities, the team chose to study clusters, which have consistent velocities as a whole, but random internal velocities of individual stars. To negate the latter of these, they averaged the results of numerous stars of each type.

The team expected to find a discrepancy of ~0.6 km/s, yet when their results were processed, no such difference was detected. The two populations both showed the recessional velocity of the cluster, centered on 33.75 km/s. So where was the predicted shift?

To explain this, the team turned to models of stars and determined that main sequence stars had a mechanism which could potentially offset the redshift with a blueshift. Namely, convection in the atmosphere of the stars would blueshift material. The team states that low mass stars made up the bulk of the survey due to their number and such stars are thought to undergo greater amounts of convection than most other types of stars. Yet, it is still somewhat suspect that this offset could so precisely counter the gravitational redshift.

Ultimately, the team concludes that, regardless of the effect, the oddities observed here point to a limitation in the methodology. Trying to tease out such small effects with such a diverse population of stars may simply not work. As such, they recommend future investigations target only specific sub-classes for comparison in order to limit such effects.

Posted on by Jon Voisey

Moon’s Mini-Magnetosphere

Many objects in the solar system have strong magnetic fields which deflect the charged particles of the solar wind, creating a bubble known as the magnetosphere. On Earth, this protects us from some of the more harmful solar rays and diverts them to create beautiful aurorae. Similar displays have been found to occur on the gas giants. However, many other objects in our solar system lack the ability to produce these effects, either because they don’t have a strong magnetic field (such as Venus), or an atmosphere with which the charged particles can interact (such as Mercury).

Although the moon lacks both of these, a new study has found that the moon may still produce localized “mini-magnetospheres”. The team responsible for this discovery is an international team composed of astronomers from Sweden, India, Switzerland, and Japan. It is based on observations from the Chandrayaan-1 spacecraft produced and launched by the Indian Space Research Organisation (ISRO).

Using this satellite, the team was mapping the density of backscattered hydrogen atoms that come from solar wind striking the surface and being reflected. Under normal conditions, 16-20% of incoming protons from the solar wind is reflected in this way.

For those excited above 150 electron volts, the team found a region near the Crisium antipode (the region directly opposite the Mare Crisium on the moon). This region was previously discovered to have magnetic anomalies in which the local magnetic field strength reached several hundred nanotesla. The new team found that the result of this was that incoming solar wind was deflected, creating a shielded region some 360 km in diameter surrounded by a “300-km-thick region of enhanced plasma flux that results from the solar wind flowing 23 around the mini-magnetosphere.” Although the flow bunches up, the team finds that the lack of a distinct boundary means that there is not likely to be a bow shock, which would be created as the buildup becomes sufficiently strong to directly interact with additional incoming particles.

Below energies of 100 eV, the phenomenon seems to disappear. The researchers suggest this points to a different formation mechanism. One possibility is that some solar flux breaks through the magnetic barrier and is reflected creating these energies. Another is that, instead of hydrogen nuclei (which composes the majority of the solar wind) this is the product of alpha particles (helium nuclei) or other heavier solar wind ions striking the surface.

Not discussed in the paper is just how valuable such features could be to future astronauts looking to create a base on the moon. While the field is relatively strong for local magnetic fields, it it still around two orders of magnitude weaker than that of Earth’s. Thus, it is unlikely that this effect would be sufficiently strong to protect a base, nor would it provide protection from the x-rays and other dangerous electromagnetic radiation that is provided by an atmosphere.

Instead, this finding poses more in the way of scientific curiosity and can help astronomers map local magnetic fields as well as investigate the solar wind if such mini-magnetospheres are located on other bodies. The authors suggest that similar features be searched for on Mercury and asteroids.

Posted on by Jon Voisey

The Atmosphere of WASP-17b

One of the greatest potentials of transiting exoplanets is the ability to monitor the spectra and examine the composition of the planet’s atmosphere. This has been done already for HD 18733b and HD 209458b. In a new article by a team of astronomers at Keele University in the UK, absorption spectroscopy has been applied to the unusual exoplanet WASP-17b, which is known to orbit retrograde.

Not only does the spectra tell astronomers the atmospheric composition, but can also give an understanding of the the composition, but can also be indicative of how the atmosphere absorbs the light from the star and how heat is transferred around the planet. Additionally, since the atmosphere will absorb differently at different wavelengths, this gives differences in the timing of the eclipse and can be used to probe the radius of the planet more tightly as well as potentially examining the layering of the atmosphere.

For their investigation, the team concentrated on the sodium doublet lines at 5889.95 and 5895.92 Å. Observations were taken by the Very Large Telescope in Chile to observe 8 transits of the planet in June of 2009. The planet itself has a short orbit of 3.74 days.

Applying these spectroscopic techniques to WASP-17b, the team discovered the presence of sodium in the atmosphere. Yet the absorption wasn’t as strong as expected based on models using formation mechanisms from a nebula with solar composition and forming a planet with a cloudless atmosphere. Instead, the team describes 17b’s atmosphere as “sodium-depleted” similar to HD 209458b.

An additional observation was that the depth of seeing dropped off when using certain filters with different bandwidths (ranges of allowed wavelengths). The team noted that at bandwidths greater than 3.0 Å, the amount of sodium absorption seen nearly disappeared. Since this property is related to how much atmosphere the light travels through, this allowed the team to speculate that this may be indicative of clouds in the upper layers of the atmosphere.

Lastly, the team speculated as to the reason on the lack of sodium in the atmosphere. They proposed that energy from the star ionizes sodium on the day side. The motion of the atmosphere carrying it to the night side would then allow it to condense and be removed from the atmosphere. Since giant exoplanets in such tight orbits would likely be tidally locked, the sodium would have little chance to return to the day side and be brought back into the atmosphere.

While the examination of extrasolar atmospheres is undoubtedly new and will certainly be revised as the number of explored atmospheres increases, these pioneering studies are among the first that can allow astronomers directly test predictions of planetary atmospheres which, until recently have been solely based on observations of our own solar system. More generally, this will allow us to develop a fuller understanding of how planets evolve.

Posted on by Jon Voisey

Catching Planets in the Womb

Young stars have a disk of gas and dust around them called a protoplanetary disk. Credit: NASA/JPL-Caltech

[/caption]

Awhile ago I wrote on the difficulty of finding young planets. There, I mentioned one team announcing the potential discovery of a planet a mere 1-5 million years old. But what are astronomers to do if they want to find even younger planets?

The chief difficulty in this instance is that such planets would still be hidden in the circumstellar disks from which they formed, hiding them from direct observation. Additionally, depending on how far along the process had advanced, they may not yet have accreted sufficient mass to show up in radial velocity surveys, if such surveys could even been conducted with interference from the disc.

One way astronomers have proposed to detect forming planets is to observe their effects on the disc itself. This could come in a number of ways. One would be for the planet to carve out grooves in the disc, clearing its orbit as it sweeps up matter. Another possibility is to look for the “shadows” caused by the local overdensity an accreting planet would cause.

But recently, another new method caught my eye. In this one, proposed by astronomers at the Crimean National Observatory in the Ukraine, astronomers could potentially look for again turns to the characteristics of the parent star. Earlier, astronomers had made a link between the properties of the disc around classes of protostars (such as T Tauri and Herbig Ae stars) and the variable luminosity of the star itself.

The authors suggest that, “[t]wo different mechanisms can be involved in interpretation of these results: 1) circumstellar extinction and 2) accretion.” In either scenario, a body present in the disc itself concentrating the material would be necessary to explain these results. In the first case, a protoplanet would draw a swarm of material around it again creating a local overdensity in the disc which would be dragged around with the planet, creating a dimming of the star as it passed near the line of sight. In the second, the planet would draw out tidal structures in the disc in much the same way tidal interactions can draw out spiral structure in galaxies. As these veins of matter fall onto the star, it feeds the star, temporarily causing an outburst and increasing the brightness.

The team conducted an analysis of periodicity in several protostellar systems and found several instances in which the periods were similar to those of planetary systems discovered around mature stars. Around one star, V866 Sco, they discovered, “two distinct periods in light variations, 6.78 and 24.78 days, that persist over several years.” They note that the shorter period is likely “due to axial rotation of the star” but could not offer an explanation for the longer period which leaves it open to the possibility of being a forming planet and they suggest that spectral observations may be possible. Other systems the team analyzed had periods ranging from 25 – 120 days also hinting at the possibility for young planetary systems.

The advantage to this method is that finding candidate systems can be done relatively easily using photometric systems which can survey great numbers of stars at once whereas radial velocity measurements generally require dedicated observations on a single object. This would allow astronomers to discriminate against candidates unlikely to harbor forming planets. Ultimately, finding young systems with forming planets will help astronomers understand how these systems form and evolve and why our own system is so different than many others found thus far.

Posted on by Jon Voisey

Dissolving Star Systems Create Mess in Orion

For young stars, stellar outflows are the rule. T Tauri stars and other young stars eject matter in generally collimated jets. However, a region in Orion’s giant molecular cloud known as the Becklin-Neugebauer/Kleinmann-Low (BN/KL) region, appears to have a clumpy, scattered set of outflows with “finger-like” projections in numerous directions. A new study, led by Luis Zapata at the National Autonomous University of Mexico, explores this odd region.

To conduct their study, the team used the Submillimeter Array to trace the motion of carbon monoxide gas in the area. Flying away from this region are three massive and young stars. Tracing their paths back, astronomers had previously determined that these stars likely had a common origin as members of a multiple system that for some reason, broke apart an estimated 500 years ago. Likely related to this, the new study discovered several new fingers of gas moving away as well with velocities that implied they came from the same point of origin near the same time. But what could send stars and gas hurtling outwards?

Nearby, the team also discovered a “hot core” of material as well as a “bubble” of empty space near the point of origin of the event. To explain the combination of these three events, the team proposes that an close interaction between the three stars (or perhaps more) occurred. At that time, the interaction tore apart any potential binary system throwing the stars outwards.

Since the stars are young and still embedded in a nebula, the team suggests it was likely they also contained circumstellar disks that had not yet formed planets. During the interaction, the outer portions which would be least strongly bound, were thrown outwards, creating the finger-like projections. Material that was bound more tightly but just enough to be torn off, “would find itself with an excess of kinetic energy, and will start to expand” creating the apparent bubble. If that bubble, expanding supersonically for the local medium, encountered a region that was overly dense, it would collide, heating the region and potentially forming the hot core.

This new discovery presents a potential first for the discovery of one or more destroyed circumstellar disks. Such findings could help impose new constraints on how planetary systems form since most stars form in open clusters and associations in which such interactions may be commonplace. Yet, the very fact that such destroyed systems have never been found until now imply that interactions sufficiently close to cause such disruption are rare. Regardless, such things will help astronomers form a better picture of the formation of planets.

Posted on by Jon Voisey

Spectroscopy in 1881

Instrument for imaging solar spectra on photographic plate. Also contains electric arc lamp which can be focused above solar spectra to allow for comparison.

[/caption]

Presently, I’ve been reading a lot of very old papers and books in astronomy. The work I’m currently reading a portion of, is from 1881, and is a summary of all the findings of the year in all fields of Science. For those that aren’t familiar with that time period in astronomy, the big thing was spectroscopy. It was only ~30 years earlier that chemists and astronomers had begun to work out methods by which to investigate spectra and with the newly developed tools in hand, astronomers were pointing them at anything they could find sufficiently bright to get a spectra. Obviously, this meant the first target was the Sun. This work provides an interesting snapshot at a developing era in astronomical history.

The article describes a brief bit of background, noting that the pioneering work of spectroscopy was done by Fraunhofer, Kirchoff, Angstrom, and Thalen (but manages to leave out Kirchoff’s colleague, Robert Bunsen!). These early explorers noted that, although spectral lines may appear unique, several had lines that would appear in very nearly the same positions.

Another discovery around that time was the phenomenon of emission lines from the Sun’s corona. This had officially been discovered in 1868 during a solar eclipse, but now that astronomers knew about the occurrence, they began to explore it further and discovered that many of the features had no apparent explanation as the chemicals causing them had yet to be discovered on Earth. Incidentally, it would be a year following this publication that helium, one of the chief components of the Sun, would be found and isolated on Earth.

As the astronomers explored the corona, they inspected the various layers and found a bizarre thing: Magnesium appeared higher in the corona than sodium despite magnesium having a higher atomic weight which astronomers realized, should cause it to sink. While this is not explained, I should note that spectra often play tricks like this. It may well have been that magnesium simply emits better at the temperatures in that region given an overestimation of the abundance. This odd behavior, as well as the inconstant nature of the spectra on various portions of the Sun was described as “a great screw loose”.

Another portion of the paper provides another somewhat humorous snapshot of this moment in history as the writer remarks just how different the Sun is from the Earth. He states, “It was difficult to imagine a stronger difference to exist between any two masses of matter than the chemical constitution of the incandescent sun, and of the earth, which is now cooling.” He wonders if perhaps planets evolved from failed stars in which the Sun’s “immense temperature had not allowed a complex evolution of higher complex forms of chemical matter to take place”. While this may seem quaint, the periodic table had only been developed 12 years prior and the creation of heavy elements would not be well understood until the 1950’s.

Similarly, the confusion on the varying spectral lines between stars is apparent although the author shows that the answers were already being developed, although still not fully fleshed out. He cites Angstrom stating: “In increasing successively the temperature I have found that the lines of the spectra vary in intensity in an exceedingly complicated way, and consequently new lines even may present themselves if the temperature is raised sufficiently high.”

In this single flash of insight, Angstrom had predicted a methodology by which astronomers could have begun to classify stars. Unfortunately, the standard of classification had already been set and it would take until the next century for astronomers to begin classifying stars by temperature (thanks to the work of Annie Jump Cannon). However, the author demonstrates that investigation was underway as to the relationship between temperature and line intensity. This work would eventually connect to our modern understanding of stellar temperatures.

Posted on by Jon Voisey

Galaxy Zoo Searches for Supernovae

Aside from categorizing galaxies, another component of the Galaxy Zoo project has been asking participants to identify potential supernovae (SNe). The first results are out and have identified “nearly 14,000 supernova candidates from [Palomar Transient Factory, (PTF)] were classified by more than 2,500 individuals within a few hours of data collection.”

Although the Galaxy Zoo project is the first to employ citizens as supernova spotters, the background programs have long been in place but were generating vast amounts of data to be processed. “The Supernova Legacy Survey used the MegaCam instrument on the 3.6m Canada-France-Hawaii Telescope to survey 4 deg2” every few days, in which “each square degree would typically generate ~200 candidates for each night of observation.” Additionallly, “[t]he Sloan Digital Sky Survey-II Supernova Survey used the SDSS 2.5m telescope to survey a larger area of 300 deg2” and “human scanners viewed 3000-5000 objects each night spread over six scanners”.

To ease this burden, the highly successful Galaxy Zoo implemented a Supernova Search in which users would be directed through a decision tree to help them determine what computer algorithms were proposing as transient events. Each image would be viewed and decided on by several participants increasing the likelihood of a correct appraisal. Also, “with a large number of people scanning candidates, more candidates can be examined in a shorter amount of time – and with the global Zooniverse (the parent project of Galaxy Zoo) user base this can be done around the clock, regardless of the local time zone the science team happens to be based in” allowing for “interesting candidates to be followed up on the same night as that of the SNe discovery, of particular interest to quickly evolving SNe or transient sources.”

To identify candidates for viewing, images are taken using the 48 inch Samuel Oschin telescope at the
Palomar Observatory. Images are then calibrated to correct instrumental noise and compared automatically to reference images. Those in which an object appears with a change greater than five standard deviations from the general noise are flagged for inspection. While it may seem that this high threshold would eliminate other events, the Supernova Legacy Survey, starting with 200 candidates per night, would only end up identifying ~20 strong candidates. As such, nearly 90% of these computer generated identifications were spurious, likely generated by cosmic rays striking the detector, objects within our own solar system, or other such nuisances and demonstrating the need for human analysis.

Still, the PTF identifies between 300 and 500 candidates each night of operation. When exported to the Galaxy Zoo interface, users are presented with three images: The first is the old, reference image. The second is the recent image, and the third is the difference between the two, with brightness values subtracted pixel for pixel. Stars which didn’t change brightness would be subtracted to nothing, but those with a large change (such as a supernova), would register as a still noticeable star.

Of course, this method is not flawless, which also contributes to the false positives from the computer system that the decision tree helps weed out. The first question (Is there a candidate centered in the crosshairs of the right-hand [subtracted] image?) eliminates misprocessing by the algorithm due to misalignment. The second question (Has the candidate itself subtracted correctly?) serves to drop stars that were so bright, they saturated the CCD, causing odd errors often resulting in a “bullseye” pattern. Third (Is the candidate star-like and approximately circular?), users eliminate cosmic ray strikes which generally only fill one or two pixels or leave long trails (depending on the angle at which they strike the CCD). Lastly, users are asked if “the candidate centered in a circular host galaxy?” This sets aside identifications of variable stars within our own galaxy that are not events in other galaxies as well as supernovae that appear in the outskirts of their host galaxies.

Each of these questions is assigned a number of positive or negative “points” to give an overall score for the identification. The higher the score, the more likely it is to be a true supernova. With the way the structure is set up, “candidates can only end up with a score of -1, 1 or 3 from each classification, with the most promising SN candidates scored 3.” If enough users rank an event with the appropriate score, the event is added to a daily subscription sent out to interested parties.

To confirm the reliability of identifications, the top 20 candidates were followed up spectroscopically with the 4.2m William Herschel Telescope. Of them, 15 were confirmed as SNe, with 1 cataclysmic variable, and 4 remain unknown. When compared to followup observations from the PTF team, the Galaxy Zoo correctly identified 93% of supernova that were confirmed spectroscopically from them. Thus, the identification is strong and this large volume of known events will certainly help astronomers learn more about these events in the future.

If you’d like to join, head over to their website and register. Presently, all supernovae candidates have been processed, but the next observing run is coming up soon!

Posted on by Jon Voisey

ω Centauri’s Red Giants Confirm Stellar Evolution Models

As the get older, Sun-like stars become red giants. 30-50 percent of these red giants exhibit a strange variability in their brightness that has so far eluded explanation. Image Credit: ESO/S. Steinhofel

[/caption]

While science education often focuses on teaching the scientific method (or at least tries to), the real process of science is often far less linear. Theories tie together so many points of data, that making singular predictions that confirm or refute a proposition is often challenging. Such is the case for stellar evolution. The understanding is woven together from so many independent pieces, that the process is more of a roaring sea than a directed river.

Realizing this, I’ve been keen on instances in which necessary predictions are observationally confirmed later. A new study, led by Mariela Vieytes from the University of Buenos Aires and accepted in an upcoming publication of Astronomy & Astrophysics, does just that by demonstrating one of the necessary conditions for predictions of post main sequence evolution. Specifically, astronomers need to establish that stars undergo significant amounts of mass loss (~0.1-0.3 M☉) during their red giant branch evolution. This requirement was set forth as part of the expected behavior necessary to explain: “i) the very existence of the horizontal branch (HB) and its morphology, ii) the pulsational properties of RR Lyrae stars, iii) the absence of asymptotic giant branch (AGB) stars brighter than the red giant branch (RGB) tip, and the chemistry and characteristics in the AGB, post-AGB and planetary nebula evolutionary phases, iv) the mass of white dwarf (WD) stars.”

Astronomers expected to find confirmation of this mass loss by detecting gas congregating in the cores of globular clusters after being shed by stars evolving along the RGB. Yet searches for this gas came up mostly empty. Eventually astronomers realized that gas would be stripped relatively quickly as globular clusters plunged through the galactic plane. But this left them with the need to confirm the prediction in some other manner.

One way to do this is to look at the stars themselves. If they show velocities in their photospheres greater than the escape velocity, they will lose mass. Just how much higher will determine the amount of mass lost. By analyzing the Doppler shift of specific absorption lines of several stars in the cluster ω Centauri, the team was able to match the amount of mass being lost to predictions from evolutionary models. From this, the team concluded that their target stars were losing between the rates of mass loss are estimated as a few 10-9 and 10-10 M☉ yr-1. This is in general agreement with the predictions set forth by evolutionary models.

Posted on by Jon Voisey

PSA: Bars Kill Galaxies

Barred Spiral Galaxy NGC 6217
Barred Spiral Galaxy NGC 6217

[/caption]

Many spiral galaxies are known to harbor bars. Not the sort in which liquor is served as a social lubricant, but rather, the kind in which gas is served to the central regions of a galaxy. But just as recent studies have identified alcohol as one of the most risky drugs, a new study using results from the Galaxy Zoo 2 project have indicated galactic bars may be associated with dead galaxies as well.

The Galaxy Zoo 2 project is the continuation of the original Galaxy Zoo. Whereas the original project asked participants to categorize galaxies into Hubble Classifications, the continuation adds the additional layer of prompting users to provide further classification including whether or not the nearly quarter of a million galaxies showed the presence of a bar. While relying on only quickly trained volunteers may seem like a risky venture, the percentage of galaxies reported to have bars (about 30%) was in good agreement with previous studies using more rigorous methods.

The new study, led by Karen Masters of the Institute of Cosmology and Gravitation at the University of Portsmouth, analyzed the presence or lack of bars in relation to other variables, such as “colour, luminosity, and estimates of the bulge size, or prominence.” When looking to see if the percent of galaxies with bars evolved over the redshifts observed, the team found no evidence that this had changed in the sample (the GZ2 project contains galaxies to a lookback time of ~6 billion years).

When comparing the fraction with bars to the overall color of the galaxy, the team saw strong trends. In blue galaxies (which have more ongoing star formation) only about 20% of galaxies contained bars. Meanwhile, red galaxies (which contain more older stars) had as many as 50% of their members hosting bars. Even more striking, when the sample was further broken down into grouping by overall galaxy brightness, the team found that dimmer red galaxies were even more likely to harbor bars, peaking at ~70%!

Before considering the possible implications, the team stopped to consider whether or not there was some inherent biasing in the selection based on color. Perhaps bars just stood out more in red galaxies and the ongoing star formation in blue galaxies managed to hide their presence? The team referenced previous studies that determined visual identification for the presence of bars was not hindered in the wavelengths presented and only dipped in the ultraviolet regime which was not presented. Thus, the conclusion was deemed safe.

While the findings don’t establish a causal relationship, the connection is still apparent: If a galaxy has a bar, it is more likely to lack ongoing star formation. This discovery could help astronomers understand how bars form in the first place. Given both structure, such as bars and spiral arms, and star formation are associated with galactic interactions, the expectation would be that we should observe more bars in galaxies in which interactions have caused them to form as well as triggering star formation. As such, this study helps to constrain modes of bar formation. Another possible connection is the ability of bars to assist in movement of gas, potentially shuttling and shielding it from being accessible for formation. As Masters states, “It’s not yet clear whether the bars are some side effect of an external process that turns spiral galaxies red, or if they alone can cause this transformation. We should get closer to answering that question with more work on the Galaxy Zoo dataset.”

Posted on by Jon Voisey

Missing Milky Way Dark Matter

A composite image shows a dark matter disk in red. From images in the Two Micron All Sky Survey. Credit: Credit: J. Read & O. Agertz.

[/caption]

Although dark matter is inherently difficult to observe, an understanding of its properties (even if not its nature) allows astronomers to predict where its effects should be felt. The current understanding is that dark matter helped form the first galaxies by providing gravitational scaffolding in the early universe. These galaxies were small and collapsed to form the larger galaxies we see today. As galaxies grew large enough to shred incoming satellites and their dark matter, much of the dark matter should have been deposited in a flat structure in spiral galaxies which would allow such galaxies to form dark components similar to the disk and halo. However, a new study aimed at detecting the Milky Way’s dark disk have come up empty.


The study concentrated on detecting the dark matter by studying the luminous matter embedded in it in much the same way dark matter was originally discovered. By studying the kinematics of the matter, it would allow astronomers to determine the overall mass present that would dictate the movement. That observed mass could then be compared to the amount of mass predicted of both baryonic matter as well as the dark matter component.

The team, led by C. Moni Bidin used ~300 red giant stars in the Milky Way’s thick disk to map the mass distribution of the region. To eliminate any contamination from the thin disc component, the team limited their selections to stars over 2 kiloparsecs from the galactic midplane and velocities characteristic of such stars to avoid contamination from halo stars. Once stars were selected, the team analyzed the overall velocity of the stars as a function of distance from the galactic center which would give an understanding of the mass interior to their orbits.

Using estimations on the mass from the visible stars and the interstellar medium, the team compared this visible mass to the solution for mass from the observations of the kinematics to search for a discrepancy indicative of dark matter. When the comparison was made, the team discovered that, “[t]he agreement between the visible mass and our dynamical solution is striking, and there is no need to invoke any dark component.”

While this finding doesn’t rule out the presence of dark matter, it does place constraints on it distribution and, if confirmed in other galaxies, may challenge the understanding of how dark matter serves to form galaxies. If dark matter is still present, this study has demonstrated that it is more diffuse than previously recognized or perhaps the disc component is flatter than previously expected and limited to the thin disc. Further observations and modeling will undoubtedly be necessary.

Yet while the research may show a lack of our understanding of dark matter, the team also notes that it is even more devastating for dark matter’s largest rival. While dark matter may yet hide within the error bars in this study, the findings directly contradict the predictions of Modified Newtonian Dynamics (MOND). This hypothesis predicts the apparent gain of mass due to a scaling effect on gravity itself and would have required that the supposed mass at the scales observed be 60% higher than indicated by this study. Continue reading “Missing Milky Way Dark Matter”