Posted on by Steve Nerlich

Journal Club: The Pulsar That Wasn’t

Today's journal article involves the mysterious case of PSR J1841-500, the pulsar that doesn't pulse

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According to Wikipedia, a Journal Club is a group of individuals who meet regularly to critically evaluate recent articles in the scientific literature. Since this is Universe Today if we occasionally stray into critically evaluating each other’s critical evaluations, that’s OK too.

And of course, the first rule of Journal Club is… don’t talk about Journal Club. So, without further ado – today’s journal article involves the mysterious case of PSR J1841-500, the pulsar that didn’t pulse.

Pulsars are neutron stars – with polar jets. They rotate quite fast and when one of those polar jets line up with Earth we detect a pulse of radio light. This is a very regular and very predictable pulse – although when measured over long time periods, the pulses slowly decline in frequency as magnetic forces create drag that slows the neutron star’s spin.

But PSR J1841-500 is an oddball – when first discovered, it pulsed every 0.9 seconds. Then for reasons unknown, it stopped pulsing for a period of over 500 days – after which it just picked up where it left off. There are at least two other pulsars known to have demonstrated such ‘switch on – switch off’ behaviour, although neither had anything like the switch-off duration of PSR J1841-500.

It is known that now and again neutron stars experience a ‘glitch’, a kind of a starquake, as the intense gravity of the star crunches its internal structure down to an even more compact state. This can change its spin and hence its pulse rate.

However, it’s not clear if glitch phenomena would help explain the extended ‘switch-off’ phenomenon observed in PSR J1841-500. Looking for other anomalies, the authors point to the unusual proximity of the magnetar IE 1841-045, which might be having some (unknown) effect on its neighbor.

The authors then conclude with the interesting suggestion that if this behavior is common, then we may be missing lots of pulsars that were in a ‘switched-off’ state when their particular region of the sky was last surveyed. This means we actually need to undertake repeated surveys – which should hopefully keep more astronomers in a job.

So – comments? Are the authors just building an overblown story out of a measurement error? Is there some weird magnetic dance going on between two proximal neutron stars? Want to suggest an article for the next edition of Journal Club?

Otherwise, happy holidays and all that stuff. SN.

Today’s article:
Camilo et al PSR J1841-0500: a radio pulsar that mostly is not there.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Special Relativity From First Principles

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Einstein’s explanation of special relativity, delivered in his 1905 paper On the Electrodynamics of Moving Bodies focuses on demolishing the idea of ‘absolute rest’, exemplified by the theoretical luminiferous aether. He achieved this very successfully, but many hearing that argument today are left puzzled as to why everything seems to depend upon the speed of light in a vacuum.

Since few people in the 21st century need convincing that the luminiferous aether does not exist, it is possible to come at the concept of special relativity in a different way and just through an exercise of logic deduce that the universe must have an absolute speed – and from there deduce special relativity as a logical consequence.

The argument goes like this:

1) There must be an absolute speed in any universe since speed is a measure of distance moved over time. Increasing your speed means you reduce your travel time between a distance A to B. A kilometre walk to the shops might take 25 minutes, but if you run it might take only 15 minutes – and if you take the car, only 2 minutes. At least theoretically you should be able to increase your speed up to the point where that travel time reaches zero – and whatever speed you are at when that happens will represent the universe’s absolute speed.

2) Now consider the principle of relativity. Einstein talked about trains and platforms to describe different inertial frame of references. So for example, you can measure someone throwing a ball forward at 10 km/hr on the platform. But put that someone on the train which is travelling at 60 km/hr and then the ball measurably moves forward at nearly 70 km/hr (relative to the platform).

3) Point 2 is a big problem for a universe that has an absolute speed (see Point 1). For example, if you had an instrument that projected something forward at the absolute speed of the universe and then put that instrument on the train – you would expect to be able to measure something moving at the absolute speed + 60 km/hr.

4) Einstein deduced that when you observe something moving in a different frame of reference to your own, the components of speed (i.e. distance and time), must change in that other frame of reference to ensure that anything that moves can never be measured moving at a speed greater than the absolute speed.

Thus on the train, distances should contract and time should dilate (since time is the denominator of distance over time).

The effect of relative motion. Measurable time dilation is negligible on a train moving past a platform at 60 km/hr, but increases dramatically if that train acquires the capacity to approach the speed of light. Time (and distance) will change to ensure that light speed is always light speed, not light speed + the speed of the train.

And that’s it really. From there one can just look to the universe for examples of something that always moves at the same speed regardless of frame of reference. When you find that something, you will know that it must be moving at the absolute speed.

Einstein offers two examples in the opening paragraphs of On the Electrodynamics of Moving Bodies:

  • the electromagnetic output produced by the relative motion of a magnet and an induction coil is the same whether the magnet is moved or whether the coil is moved (a finding of James Clerk Maxwell‘s electromagnetic theory) and;
  • the failure to demonstrate that the motion of the Earth adds any additional speed to a light beam moving ahead of the Earth’s orbital trajectory (presumably an oblique reference to the 1887 Michelson-Morley experiment).

In other words, electromagnetic radiation (i.e. light) demonstrated the very property that would be expected of something which moved at the absolute speed that it is possible to move in our universe.

The fact that light happens to move at the absolute speed of the universe is useful to know – since we can measure the speed of light and hence we can then assign a numerical value to the universe’s absolute speed (i.e. 300,000 km/sec), rather than just calling it c.

Further reading:
None! That was AWAT #100 – more than enough for anyone. Thanks for reading, even if it was just today. SN.

Posted on by Steve Nerlich

Astronomy Without A Telescope – How Big Is Big?

The compaison chart showing lots of large stars - but note that they are all red giants.

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You may have seen one of these astronomical scale picture sequences, where you go from the Earth to Jupiter to the Sun, then the Sun to Sirius – and all the way up to the biggest star we know of VY Canis Majoris. However, most of the stars at the big end of the scale are at a late point in their stellar lifecycle – having evolved off the main sequence to become red supergiants.

The Sun will go red giant in 5 billion years or so – achieving a new radius of about one Astronomical Unit – equivalent to the average radius of the Earth’s orbit (and hence debate continues around whether or not the Earth will be consumed). In any case, the Sun will then roughly match the size of Arcturus, which although voluminously big, only has a mass of roughly 1.1 solar masses. So, comparing star sizes without considering the different stages of their stellar evolution might not be giving you the full picture.

Another way of considering the ‘bigness’ of stars is to consider their mass, in which case the most reliably confirmed extremely massive star is NGC 3603-A1a – at 116 solar masses, compared with VY Canis Majoris’ middling 30-40 solar masses.

The most massive star of all may be R136a1, which has an estimated mass of over 265 solar masses – although the exact figure is the subject of ongoing debate, since its mass can only be inferred indirectly. Even so, its mass is almost certainly over the ‘theoretical’ stellar mass limit of 150 solar masses. This theoretical limit is based on mathematically modelling the Eddington limit, the point at which a star’s luminosity is so high that its outwards radiation pressure exceeds its self-gravity. In other words, beyond the Eddington limit, a star will cease to accumulate more mass and will begin to blow off large amounts of its existing mass as stellar wind.

It’s speculated that very big O type stars might shed up to 50% of their mass in the early stages of their lifecycle. So for example, although R136a1 is speculated to have a currently observed mass of 265 solar masses, it may have had as much as 320 solar masses when it first began its life as a main sequence star.

So, it may be more correct to consider that the theoretical mass limit of 150 solar masses represents a point in a massive star’s evolution where a certain balancing of forces is achieved. But this is not to say that there couldn’t be stars more massive than 150 solar masses – it’s just that they will be always declining in mass towards 150 solar masses.

The Wolf-Rayet star WR 124 and its wind nebulae (actually denoted M1-67). The mass of WR 124 is estimated at a moderate 20 solar masses, although this is after it has already lost much of its initial mass to create the wind nebula around it. Credit: ESO.

Having unloaded a substantial proportion of their initial mass such massive stars might continue as sub-Eddington blue giants if they still have hydrogen to burn, become red supergiants if they don’t – or become supernovae.

Vink et al model the processes in the early stages of very massive O type stars to demonstrate that there is a shift from optically thin stellar winds, to optically thick stellar winds at which point these massive stars can be classified as Wolf-Rayet stars. The optical thickness results from blown off gas accumulating around the star as a wind nebulae – a common feature of Wolf-Rayet stars.

Lower mass stars evolve to red supergiant stage through different physical processes – and since the expanded outer shell of a red giant does not immediately achieve escape velocity, it is still considered part of the star’s photosphere. There’s a point beyond which you shouldn’t expect bigger red supergiants, since more massive progenitor stars will follow a different evolutionary path.

Those more massive stars spend much of their lifecycle blowing off mass via more energetic processes and the really big ones become hypernovae or even pair-instability supernovae before they get anywhere near red supergiant phase.

So, once again it appears that maybe size isn’t everything.

Further reading: Vink et al Wind Models for Very Massive Stars in the Local Universe.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Could Dark Matter Not Matter?

The rotation curve of the Andromeda Galaxy - actual (white line) and rotational velocities of outer stars that would be expected based on the estimated mass of visible matter in the galaxy. From this we conclude up to 90% of the mass must be in the form of dark matter.

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You probably want to put on your skeptical goggles and set them to maximum for this one. An Italian mathematician has come up with some complex formulae that can, with remarkable similarity, mimic the rotation curves of spiral galaxies without the need for dark matter.

Currently, these galactic rotation curves represent key evidence for the existence of dark matter – since the outer stars of spinning galaxies often move around a galactic disk so fast that they should fly off into intergalactic space – unless there is an additional ‘invisible’ mass present in the galaxy to gravitationally hold them in their orbits.

The issue can be appreciated by considering the Keplerian motion of the planets in our Solar System. Mercury orbits the Sun at an orbital velocity of 48 kilometers a second – while Neptune orbits the Sun at an orbital velocity of 5 kilometers a second. In the Solar System, a planet’s proximity to the substantial mass of the Sun is a function of its orbital velocity. So, hypothetically, if the Sun’s mass was reduced somehow, Neptune’s existing orbital velocity would move it outwards from its current orbit – potentially flinging it off into interstellar space if the change was significant enough.

The physics of the Milky Way Galaxy is different from the Solar System, since its mass is distributed more evenly across the galactic disk, rather than 99% of its mass being concentrated centrally – the way it is in the Solar System.

Nonetheless, as this past Universe Today article explains, if we assume a similar relationship between the cumulative mass of the Milky Way and the orbital velocity of its outer stars, we must acknowledge that the visible objects within the Milky Way only have 10-20% of the mass that is required to contain the orbital velocity of stars in its outer disk. So we conclude that the rest of that galactic mass must be dark (invisible) matter.

This is the contemporary consensus view of how galaxies work – and a key component of the current standard model of the cosmology of the universe. But Carati has come along with a seemingly implausible idea that the rotational curves of spiral galaxies could be explained by the gravitational influence of faraway matter, without needing to appeal to dark matter at all.

Left image: the rotation curve of spiral galaxy NGC 3198 showing the actual velocities of its outer stars (plotted points), then the velocities that would be expected given the mass of visible matter in its disk - overlaid by the assumed contribution of the mass of a dark matter halo. Right image: Carati's theoretical curve calculated from the effect of faraway matter and its remarkable fit to observed values from NGC 3198.

Conceptually the idea makes little sense. Positioning gravitationally significant mass outside of the orbit of stars might draw them out into wider orbits, but it’s difficult to see why this would add to their orbital velocity. Drawing an object into a wider orbit should result in it taking longer to orbit the galaxy since it will have more circumference to cover. What we generally see in spiral galaxies is that the outer stars orbit the galaxy within much the same time period as more inward stars.

But although the proposed mechanism seems a little implausible, what is remarkable about Carati’s claim is that the math apparently deliver galactic rotation curves that closely fit the observed values of at least four known galaxies. Indeed, the math delivers an extraordinarily close fit.

With skeptical goggles firmly in place, the following conclusions might be drawn from this finding:
• There are so many galaxies out there that it’s not hard to find four galaxies that fit the math;
• The math has been retro-fitted to match already observed data;
• The math just doesn’t work; or
• While the author’s interpretation of the data may be up for discussion, the math really does work.

The math draws on principles established in the Einstein field equations, which is problematic as the field equations are based on the cosmological principle, which assumes that the effect of faraway matter is negligible – or at least that it evens out at a large scale.

Perplexingly, Carati’s paper also notes two further examples where the math can also fit galaxies with declining rotational velocities in their outer stars. This is achieved by switching the sign of one of the formulae components (which can be + or -). Thus, on the one hand the effect of faraway matter is to induce a positive pressure that contains the rapid rotation of stars, preventing them from flying off – and on the other hand, it can induce a negative pressure to encourage an atypical decay in a galaxy’s rotation curve.

As the saying goes, if something seems too good to be true – it probably isn’t true. All comments welcome.

Further reading:
Carati Gravitational effects of the faraway matter on the rotation curves of spiral galaxies.

Posted on by Steve Nerlich

Astronomy Without A Telescope – The Progenitor Problem

DEM L71 - a Type 1a supernova remnant. Analysis a the outer shockwave and inner ejecta indicate the explosion contains a mass not much exceeding 1 solar mass, which contains a high iron to silicon/ oxygen ratio. This all very suggestive that the progenitor star was a compact white dwarf. Apart from that the steps leading up to the explosion are a mystery (Credit: NASA/Chandra)

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With so much of our current understanding of the universe based on Type 1a supernovae data, a good deal of current research is focused upon just how standard these supposed standard candles are. To date, the weight of analysis seems reassuring – apart from a few outliers, the supernovae do all seem very standard and predictable.

However, some researchers have come at this issue from a different perspective by considering the characteristics of the progenitor stars that produce Type 1a supernovae. We know very little about these stars. Sure, they are white dwarfs that explode after accumulating extra mass – but just how this outcome is reached remains a mystery.

Indeed, the final stages preceding an explosion have never been definitively observed and we cannot readily point to any stars as likely candidates on a pathway towards Type Ia-ness. In comparison, identifying stars that are expected to explode as core collapse supernovae (Types Ib, Ic or II) is easy – core collapse should be the destiny of any star bigger than 9 solar masses.

Popular theory has it that a Type 1a progenitor is a white dwarf star in a binary system that draws material off its binary companion until the white dwarf reaches the Chandrasekhar limit of 1.4 solar masses. As the already compressed mass of predominantly carbon and oxygen is compressed further, carbon fusion is rapidly initiated throughout the star. This is such an energetic process that the comparatively small star’s self-gravity cannot contain it – and the star blows itself to bits.

Surprisingly, the white dwarf merger scenario seems the more likely cause of Type 1a supernovae, based on current (though largely circumstantial) evidence (Credit: Bad Astronomy/Discovery).

But when you try to model the processes leading up to a white dwarf achieving 1.4 solar masses, it seems to require a lot of ‘fine tuning’. The rate of accretion of extra mass has to be just right – too fast a flow will result in a red giant scenario. This is because adding extra mass quickly will give the star enough self-gravity so that it can partially contain the fusion energy – meaning that it will expand rather than explode.

Theorists get around this problem by proposing that a stellar wind arising from the white dwarf moderates the rate of infalling material. This sounds promising, although to date studies of Type 1a remnant material have found no evidence of the dispersed ions that would be expected from a pre-existing stellar wind.

Furthermore, a Type 1a explosion within a binary should have a substantial impact on its companion star. But all searches for candidate surviving companions – which would presumably possess anomalous characteristics of velocity, rotation, composition or appearance – have been inconclusive to date.

An alternative model for the events that lead up to a Type 1a are that two white dwarfs are drawn together, inexorably inspiralling until one or the other achieves 1.4 solar masses. This is not a traditionally favoured model as the time required for two such comparatively small stars to inspiral and merge could be billions of years.

However, Maoz and Mannucci review recent attempts to model the rate of Type 1a supernovae within a set volume of space and then align this with the expected frequency of different progenitor scenarios. Assuming that between 3 to 10 % of all 3-8 solar mass stars eventually explode as Type 1a supernovae – this rate does favour the ‘when white dwarfs collide’ model over the ‘white dwarf in a binary’ model.

There is no immediate concern that this alternate formation process would affect the ‘standardness’ of a Type 1a explosion – it’s just not the finding that most people were expecting.

Further reading:
Maoz and Mannucci Type-Ia supernova rates and the progenitor problem. A review.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Mass Is Energy

The USS Enterprise in 1964 (pre Zephram Cochran era), which sailed around the world in 64 days without refuelling to demonstrate the capability of nuclear-powered surface ships. Credit: US Navy.

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Some say that the reason you can’t travel faster than light is that your mass will increase as your speed approaches light speed – so, regardless of how much energy your star drive can generate, you reach a point where no amount of energy can further accelerate your spacecraft because its mass is approaching infinite.

This line of thinking is at best an incomplete description of what’s really going on and is not a particularly effective way of explaining why you can’t move faster than light (even though you really can’t). However, the story does offer useful insight into why mass is equivalent to energy, in accordance with the relationship e=mc2.

Firstly, here’s why the story isn’t complete. Although someone back on Earth might see your spacecraft’s mass increase as you move near light speed – you the pilot aren’t going notice your mass change at all. Within your spacecraft, you would still be able to climb stairs, jump rope – and if you had a set of bathroom scales along for the ride you would still weigh just the same as you did back on Earth (assuming your ship is equipped with the latest in artificial gravity technology that mimics conditions back on Earth’s surface).

The change perceived by an Earth observer is just relativistic mass. If you hit the brakes and returned to a more conventional velocity, all the relativistic mass would go away and an Earth observer would just see you retaining with same proper (or rest) mass that the spacecraft and you had before you left Earth.

The Earth observer would be more correct to consider your situation in terms of momentum energy, which is a product of your mass and your speed. So as you pump more energy in to your star drive system, someone on Earth really sees your momentum increase – but interprets it as a mass increase, since your speed doesn’t seem to increase much at all once it is up around 99% of the speed of light. Then when you slow down again, although you might seem to be losing mass you are really offloading energy – perhaps by converting your kinetic energy of motion into heat (assuming your spacecraft is equipped with the latest in relativistic braking technology).

As the ratio of your velocity to light speed approaches 1, the ratio of your relativistic mass to your rest mass grows asymptotically - i.e. it approaches infinite.

From the perspective of the Earth-based observer, you can formulate that the relativistic mass gain observed when travelling near light speed is the sum of the spacecraft’s rest mass/energy plus the kinetic energy of its motion – all divided by c2. From that you can (stepping around some moderately complex math) derive that e=mc2. This is a useful finding, but it has little to do with why the spacecraft’s speed cannot exceed light speed.

The phenomenon of relativistic mass follows a similar, though inverse, asymptotic relationship to your speed. So as you approach light speed, your relativistic time approaches zero (clocks slow), your relativistic spatial dimensions approach zero (lengths contract) – but your relativistic mass grows towards infinite.

But as we’ve covered already, on the spacecraft you do not experience your spacecraft gaining mass (nor does it seem to shrink, nor its clocks slow down). So you must interpret your increase in momentum energy as a genuine speed increase – at least with respect to a new understanding you have developed about speed.

For you, the pilot, when you approach light speed and keep pumping more energy into your drive system, what you find is that you keep reaching your destination faster – not so much because you are moving faster, but because the time you estimated it would take you to cross the distance from point A to Point B becomes perceivably much less, indeed the distance between point A to Point B also becomes perceivably much less. So you never break light speed because the distance over time parameters of your speed keep changing in a way that ensures that you can’t.

In any case, consideration of relativistic mass is probably the best way to derive the relationship e=mc2 since the relativistic mass is a direct result of the kinetic energy of motion. The relationship does not easily fall out of consideration of (say) a nuclear explosion – since much of the energy of the blast derives from the release of the binding energy which holds a heavy atom together. A nuclear blast is more about energy transformation than about matter converting to energy, although at a system level it still represents genuine mass to energy conversion.

Similarly you might consider that your cup of coffee is more massive when it’s hot – and gets measurably less massive when it cools down. Matter, in terms of protons, neutrons, electrons …and coffee, is largely conserved throughout this process. But, for a while, the heat energy really does add to the mass of the system – although since it’s a mass of m=e/c2, it is a very tiny amount of mass.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Orphan Supernovae?

Supernova G292.0+1.8. Like most supernovae it detonated within a host galaxy - in fact ours. Credit: Chandra.

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For some years now astronomers have been scratching their heads over the appearance of supernovae that detonate out in the middle of nowhere – rather than within a host galaxy.

Various hypotheses have been proposed, notably that they might be hypervelocity stars – which are stars flung out of their host galaxy due to an unfortunate coincidence of gravitational interactions. It’s thought that such interactions may accelerate those stars up to a velocity of more than 100 kilometers a second – that is, more than the escape velocity of your average galaxy.

But Zinn et al suggest a more mundane suggestion for their particular orphan supernovae of interest, which is SN 2009z. They propose that it is in a galaxy, it’s just a galaxy that is very difficult to see.

They propose the supernova actually detonated within a low surface brightness galaxy, N271. From the images they have produced, this seems a reasonable claim – it’s just that low surface brightness galaxies (or LSBs) aren’t meant to have supernovae.

Left frame: Sloan Digital Sky Survey image showing the location of the Type IIB supernovae SN 2009z. Right frame: Close-up of the rectangular area taken by the New Technology Telescope (ESO), showing the location of SN 2009z at the cross marks. It seems closely associated with the small galaxy N271, even though such a galaxy is not usually thought capable of supporting massive star formation. Credit: Zinn et al.

Since galaxies can appear as extended objects, rather than as point-like stars, we refer to them as having ‘surface brightness’ – which can vary across the object’s apparent surface. LSB’s are generally isolated field galaxies, rather than being grouped in amongst dense galaxy clusters. They are most often dwarf galaxies as well, but at least one spiral LSB has been identified.

The dimness of LSB galaxies is suggestive of them having almost no active star formation – either being too old, with no free hydrogen remaining for new star formation – or just not dense enough for much star formation to ever have taken off.

But here you have supernova SN 2009z that was most likely was contained within LSB galaxy N271. And SN 2009z was a Type II supernova – a massive and short-lived star that underwent core collapse. Indeed, it was a Type IIb with only a small shell of hydrogen when it detonated. Type IIb supernovae are probably massive stars which lose most, but not all, of their hydrogen shell through having it stripped off by a companion star in a binary system.

This all seems quite unusual behaviour for a galaxy that does not support active star formation. Zinn et al propose that LSB galaxies must go through short bursts of active star formation followed by long quiescent phases of almost no activity. This then suggests that the progenitor star of supernova SN 2009z was formed in the previous starburst period, before N271 quietened down again.

Of course, none of this need suggest that hypervelocity stars don’t exist – indeed several have been discovered since the first confirmed finding in 2005. All those known are associated with the Milky Way, since finding a single isolated hypervelocity star ejected by a distant galaxy is probably beyond the detection of our current technology – unless of course they go supernovae.

But given what we know so far:
• a hypervelocity star arises from a binary system’s unfortunate interaction with a galaxy’s central supermassive black hole;
• one binary member is captured, the other flung violently outwards at escape velocity.
• but, massive stars that go supernovae only have a main sequence life span of the order of millions of years;
• so, even at more than 100 kilometers a second, it’s unlikely that any are going to make it across the many light years distance from the center of a galaxy to its outer boundary before they detonate.

Putting all this together… orphan supernovae? Busted (well, unless we find one anyway).

Further reading: Zinn et al. Supernovae without host galaxies? The low surface brightness host of SN 2009Z.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Inconstant Supernovae?

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Given the importance of Type 1a supernovae as the standard candles which demonstrate that the universe’s expansion is actually accelerating – we require a high degree of confidence that those candles really are standard.

A paper released on Arxiv, with a list of authors reading like a Who’s Who in cosmology and including all three winners of this year’s Nobel Prize in Physics, details an ultraviolet (UV) analysis of four Type 1a supernovae, three of which represent significant outliers from the standard light curve expected of Type 1a supernovae.

Some diversity in UV output has already been established from observing distant high red-shift Type 1a supernovae, since their UV output is shifted into optical light and can hence be observed through the atmosphere. However, to gain detailed observations in UV, you need to look at closer, less red-shifted Type 1a supernovae and hence you need space telescopes. These researchers used data collected by the ACS (Advanced Camera for Surveys) on the Hubble Space Telescope.

The supernovae studied were SN 2004dt, SN 2004ef, SN 2005M and SN 2005cf. SN 2005cf is considered a ‘gold standard’ Type 1a supernovae – while the other three show considerable diversion from the standard UV light curve, even though their optical light output looks standard.

The left side diagrams show the three anomalous Type 1a supernovae light curves in UV light through three filters. The three outlier supernovae are mapped against the light curve of SN 2005cf (solid line), considered a 'gold standard' light curve. The diversity of the other three supernovae is apparent in UV, but not in optical - as shown in the frames on the right side. Credit: Wang et al.

The researchers also looked at a slightly larger dataset of UV supernovae observations made by the Swift spacecraft – which also showed a similar diversity in UV light, that was not apparent in optical light.

This is a bit of a worry, since the supernovae dataset from which we conclude that the universe is expanding is largely based on observations in optical light which, unlike UV, can make it through the atmosphere and be collected by ground-based telescopes.

Nonetheless, if you are thinking that three outliers isn’t a lot – you’d be right. The paper’s aim is to indicate that there are minor discrepancies in the current data set upon which we have built our current model of the universe. The academic muscle that is focused on this seemingly minor issue is some indication of the importance of isolating and characterising the nature any such discrepancies, so that we can continue to have confidence in the Type 1a supernovae standard candle dataset – or not.

The researchers acknowledge that the UV excess – not seen at all in SN 2005cf, but seen in varying degrees in the other three Type 1a supernovae – with the most pronounced difference seen in SN 2004dt – is a problem, even if it is not a huge problem.

As standard candles, Type 1a supernovae (or SNe1a) are key to determining the distance of their host galaxies. But one key consideration in determining their absolute luminosity is the reddening caused by the dust in the host galaxy. A higher than expected UV flux in some SNe1a could lead to an underestimate of this normal reddening effect, which dims the visible light of the star irrespective of its distance. Such an atypical SNe1a would then be picked up in ground-based SNe1a sky surveys as misleadingly dim – and their host galaxies would be determined as being further away from us than they really are.

The researchers call this another possible systematic error within the current SNe1a-based calculations of the nature of the universe – those other possible systematic errors including the metallicity of the supernovae themselves, as well as the size, density and chemistry of their host galaxy.

The key question to take forward now is what proportion of the total population of SNe1a in the universe might have this high UV flux. To answer that we will need to get more space telescope data.

Further reading:
Wang et al. Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope.

Posted on by Steve Nerlich

Astronomy Without A Telescope – Dark Matter Science

Dark matter has been found to be (unexpectedly) evenly distributed across dwarf galaxies, rather than clumping together in the centre - in the way we that we had expected of 'cold' dark matter.

Dark matter – there’s a growing feeling that we are getting closer to finding out the true nature of this elusive stuff. At least we are running a number of experiments that seem (on theoretical grounds) to have the capacity to identify it – and if they don’t… well, maybe it’s time for a rethink of the whole ball game.

There are two arguably quite separate requirements for dark matter to make sense of our current dataset and our theoretical schema for the universe. Firstly, the Standard Model of cosmology (Lambda-Cold Dark Matter) requires that 96% of the universe is composed of stuff of an unknown nature that cannot be directly observed.

Continue reading “Astronomy Without A Telescope – Dark Matter Science”

Posted on by Steve Nerlich

Astronomy Without A Telescope – Green Peas

A Green Pea galaxy - which may be an local analogue of the univere's first galaxies. Credit: Galaxy Zoo/SDS.

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The ground-breaking discovery of a new class of galaxies, Green Peas, in 2009 by a group of Galaxy Zoo volunteers – have recently been followed up by further observations in the radio spectrum.

The Green Peas were first identified from Sloane Digital Sky Survey data – and then in Hubble Space Telescope archive images. Now radio observations of Green Pea galaxies (from GMRT and VLA) have led to some new speculation on the role of magnetic fields in early galaxy formation.

Green Pea galaxies were so named from their appearance as small green blobs in Galaxy Zoo images. They are low mass galaxies, with low metallicity and high star formation rates – but, surprisingly, are not all that far away. This is surprising given that their low metallicity means they are young – and being not very far away means they formed fairly recently (in universal timeframe terms).

Most nearby galaxies reflect the 13.7 billion year old age of the universe and have high metallicity resulting from generations of stars building elements heavier than hydrogen and helium through fusion reactions.

But Green Peas do seem to have formed from largely unsullied clouds of hydrogen and helium that have somehow remained unsullied for much of the universe’s lifetime. And so, Green Peas may represent a close analogue of what the universe’s first galaxies were like.

Their green color comes from strong OIII (ionised oxygen) emission lines (a common consequence of lots of new star formation) within a redshift (z) range around 0.2. A redshift of 0.2 means we see these galaxies as they were when the universe was about 2.4 billion years younger (according to Ned Wright’s cosmology calculator). Equivalent early universe galaxies are most luminous in ultraviolet at a redshift (z) between 2 and 5 – when the universe was between 10 and 12 billion years younger than today.

Spectroscopic data from Green Pea galaxy 587739506616631548 - demonstrating the prominent OIII emission lines which are characteristic of Green Pea galaxies. Credit: Galaxy Zoo.

Anyhow, studying Green Peas in radio has yielded some interesting new features of these galaxies.

With the notable exception of Seyfert galaxies, where the radio output is dominated by emission from supermassive black holes, the bulk radio emission from most galaxies is a result of new star formation, as well as synchrotron radiation arising from magnetic fields within the galaxy.

Based on a number of assumptions, Chakraborti et al are confident they have discovered that Green Peas have relatively powerful magnetic fields. This is surprising given their youth and smaller size – with magnetic field strengths of around 30 microGauss, compared with the Milky Way’s approximately 5 microGauss.

They do not offer a model to explain the development of Green Pea magnetic fields, beyond suggesting that turbulence is a likely underlying factor. Nonetheless, they do suggest that the strong magnetic fields of Green Peas may explain their unusually high rate of star formation – and that this finding suggests that the same processes existed in some of the first galaxies to appear in our 13.7 billion year old universe.

Further reading:
Chakraborti et al Radio Detection of Green Peas: Implications for Magnetic Fields in Young Galaxies
Cardamone et al Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies.