Astronomy Without A Telescope – Secular Evolution


A traditional galaxy evolution model has it that you start with spiral galaxies – which might grow in size through digesting smaller dwarf galaxies – but otherwise retain their spiral form relatively undisturbed. It is only when these galaxies collide with another of similar size that you first get an irregular ‘train-wreck’ form, which eventually settles into a featureless elliptical form – full of stars following random orbital paths rather than moving in the same narrow orbital plane that we see in the flattened galactic disk of a spiral galaxy.

The concept of secular galaxy evolution challenges this notion – where ‘secular’ means separate or isolated. Theories of secular evolution propose that galaxies naturally evolve along the Hubble sequence (from spiral to elliptical), without merging or collisions necessarily driving changes in their form.

While it’s clear that galaxies do collide – and then generate many irregular galaxy forms we can observe – it is conceivable that the shape of an isolated spiral galaxy could evolve towards a more amorphously-shaped elliptical galaxy if they possess a mechanism to transfer angular momentum outwards.

The flattened disk shape of standard spiral galaxy results from spin – presumably acquired during its initial formation. Spin will naturally cause an aggregated mass to adopt a disk shape – much as pizza dough spun in the air will form a disk. Conservation of angular momentum requires that the disk shape will be sustained indefinitely unless the galaxy can somehow lose its spin. This might happen through a collision – or otherwise by transferring mass, and hence angular momentum, outwards. This is analogous to spinning skaters who fling their arms outwards to slow their spin.

Density waves may be significant here. The spiral arms commonly visible in galactic disks are not static structures, but rather density waves which cause a temporary bunching together of orbiting stars. These density waves may be the result of orbital resonances generated amongst the individual stars of the disk.

Left: Density waves may emerge from gravitational resonances generated by the alignment of stars

It has been suggested that a density wave represents a collisionless shock which has a damping effect on the spin of the disk. However, since the disk is only braking upon itself, angular momentum still has to be conserved within this isolated system.

A galactic disk has a corotation radius – a point where stars rotate at the same orbital velocity as the density wave (i.e. a perceived spiral arm) rotate. Within this radius, stars move faster than the density wave – while outside the radius, stars move slower than the density wave.

This may account for the spiral shape of the density wave – as well as offering a mechanism for the outward transfer of angular momentum. Within the radius of corotation, stars are giving up angular momentum to the density wave as they push through it – and hence push the wave forward. Outside the radius of corotation, the density wave is dragging through a field of slower moving stars – giving up angular momentum to them as it does so.

The result is that the outer stars are flung further outwards to regions where they could adopt more random orbits – rather than being forced to conform to the mean orbital plane of the galaxy. In this way, a tightly-bound rapidly spinning spiral galaxy could gradually evolve towards a more amorphous elliptical shape.

Further reading: Zhang and Buta. Density-Wave Induced Morphological Transformation of Galaxies along the Hubble Sequence.

Taking a Galaxy’s Temperature


The role that supermassive black holes play in the formation of galaxies is a “hot” topic in astronomy. Using the Chandra X-Ray Observatory, an international team of astronomers have been able to create a temperature map of one galaxy, NGC 5813, which is located in the Virgo III Group of galaxies. The new map shows in unprecedented detail the history of various periods of activity of the Active Galactic Nucleus (AGN), which is associated with a supermassive black hole that resides at its center. They found that regular outbursts of the AGN maintained the temperature of the gas in the region of the galaxy, continually reheating the gas that would otherwise have cooled down.

Paper co-author Dr. Scott Randall of the Chandra Mission Planning Team at the Harvard-Smithsonian Center for Astrophysics said, “Although there are other systems that show AGN outburst shocks, this is still the only system where unambiguous shocks from multiple outbursts are seen. This allows us to directly measure the heating from shocks, and directly observe how often these shocks take place. Thus, at present NGC 5813 is *uniquely* well suited to the study of AGN heating.”

By studying images taken by the Chandra X-Ray Observatory, and combining these observations with those taken by the Giant Metrewave Radio Telescope (GMRT) and the Southern Astrophysical Research Telescope (SOAR), they were able to make out large cavities produced by periods of activity in the supermassive black hole. The researchers were able to determine that there were three pairs of large cavities, which corresponded to active outbursts of the galactic nucleus 3 million, 20 million and 90 million years ago (from our perspective here on Earth).

What makes the galaxy NGC 5813 especially suited to this study is its relative isolation from other galaxies that could influence the formation of these cavities – it is an older galaxy that is relatively undisturbed, allowing for these cavities in the gas to persist over such a long time period.

Current models of galaxy formation must take into account just how much of an influence the output of the supermassive black hole at the center of a galaxy has on the formation of stars within the galaxy, and the evolution of the shape and size of the galaxy as a whole. This process of “AGN feedback” has a dramatic influence on how the galaxy takes shape. The research by Dr. Randall, et. al shows an intimate portrait of this process.

Dr. Randall explained, “This is an important result for stellar formation and galaxy evolution. The AGN heats the gas, preventing it from cooling and forming large amounts of stars. There have been several galaxy evolution models proposed that require this kind of “AGN feedback” near the centers of galaxies to explain the observed differences in galaxies. Here we show explicitly that this kind of feedback can and does take place, at least in this system.”

A labeled image of the various shock waves and cavities formed by the activity of the AGN. Image Credit: Credit: NASA/CXC/SAO/S.Randall et al.

As you can see in the image directly above, various outbursts of the AGN create shock waves in the gas near the center of the galaxy. As these shock waves expanded and the galaxy evolved over millions of years, the heat generated by the shocks spread outwards and into the gas surrounding NGC 5813. The gas between all of the galaxies in a cluster is called the intracluster medium (ICM). The heat – which is produced by the friction of the gases at the edge of each of the shock waves – radiates outward into the surrounding gas, increasing its temperature.

The output of the jets streaming from the supermassive black hole in the center vary over a span of roughly 10 million years, and the amount of energy that each outburst puts out is rather variable – the difference between the last two largest outbursts, for example, is almost an order of magnitude.

This process is cyclical, though the details of the mechanisms involved are still a topic that isn’t completely understood.

Dr. Randall explained this process as follows:

“…the gas cools radiatively, and flows in towards the AGN. The cool gas is rapidly accreted by the black hole, dirving [sic] an energetic outburst. The outburst heats the gas (via shocks), stopping the inflow and starving the AGN. The gas is then able to cool once more, and the cycle repeats, with, in this case, a period of about 10 million years. However, the fine details of how the jet and the ICM interact are not currently well uderstood [sic], and it is not clear how well this simple model describes reality. Our goal with the upcoming deep Chandra observation is to better understand the details of this process, most likely through comparisons with detailed numerical simulations.”

Further observations of NGC 5813 in the fall of 2011 using Chandra are in the works, Dr. Randall said. The results of their analysis will be published in the Astrophysical Journal. A preprint version of the paper, “Shocks and Cavities from Multiple Outbursts in the Galaxy Group NGC 5813: A Window to AGN Feedback,” is available on Arxiv.

Sources: Chandra press release, Arxiv paper, email interview with Dr. Scott Randall

Astronomy Without A Telescope – Black Hole Evolution


While only observable by inference, the existence of supermassive black holes (SMBHs) at the centre of most – if not all – galaxies remains a compelling theory supported by a range of indirect observational methods. Within these data sources, there exists a strong correlation between the mass of the galactic bulge of a galaxy and the mass of its central SMBH – meaning that smaller galaxies have smaller SMBHs and bigger galaxies have bigger SMBHs.

Linked to this finding is the notion that SMBHs may play an intrinsic role in galaxy formation and evolution – and might have even been the first step in the formation of the earliest galaxies in the universe, including the proto-Milky Way.

Now, there are a number of significant assumptions built into this line of thinking, since the mass of a galactic bulge is generally inferred from the velocity dispersion of its stars – while the presence of supermassive black holes in the centre of such bulges is inferred from the very fast radial motion of inner stars – at least in closer galaxies where we can observe individual stars.

For galaxies too far away to observe individual stars – the velocity dispersion and the presence of a central supermassive black hole are both inferred – drawing on the what we have learnt from closer galaxies, as well as from direct observations of broad emission lines – which are interpreted as the product of very rapid orbital movement of gas around an SMBH (where the ‘broadening’ of these lines is a result of the Doppler effect).

But despite the assumptions built on assumptions nature of this work, ongoing observations continue to support and hence strengthen the theoretical model. So, with all that said – it seems likely that, rather than depleting its galactic bulge to grow, both an SMBH and the galactic bulge of its host galaxy grow in tandem.

It is speculated that the earliest galaxies, which formed in a smaller, denser universe, may have started with the rapid aggregation of gas and dust, which evolved into massive stars, which evolved into black holes – which then continued to grow rapidly in size due to the amount of surrounding gas and dust they were able to accrete.

Distant quasars may be examples of such objects which have grown to a galactic scale. However, this growth becomes self-limiting as radiation pressure from an SMBH’s accretion disk and its polar jets becomes intense enough to push large amounts of gas and dust out beyond the growing SMBH’s sphere of influence. That dispersed material contains vestiges of angular momentum to keep it in an orbiting halo around the SMBH and it is in these outer regions that star formation is able to take place. Thus a dynamic balance is reached where the more material an SMBH eats, the more excess material it blows out – contributing to the growth of the galaxy that is forming around it.

The almost linear correlation between the SMBH mass (M) and velocity dispersion (sigma) of the galactic bulge (the 'M-sigma relation') suggests that there is some kind of co-evolution going on between an SMBH and its host galaxy. The only way an SMBH can get bigger is if its host galaxy gets bigger - and vice versa. The left chart shows data points derived from different objects in a galaxy - the right chart shows data points derived from different types of galaxies. Credit: Tremaine et al. (2002).

To further investigate the evolution of the relationship between SMBHs and their host galaxies – Nesvadba et al looked at a collection of very red-shifted (and hence very distant) radio galaxies (or HzRGs). They speculate that their selected group of galaxies have reached a critical point – where the feeding frenzy of the SMBH is blowing out about as much material as it is taking in – a point which probably represents the limit of the active growth of the SMBH and its host galaxy.

From that point, such galaxies might grow further by cannibalistic merging – but again this may lead to a co-evolution of the galaxy and the SMBH – as much of the contents of the galaxy being eaten gets used up in star formation within the feasting galaxy’s disk and bulge, before whatever is left gets through to feed the central SMBH.

Other authors (e.g. Schulze and Gebhardt), while not disputing the general concept, suggest that all the measurements are a bit out as a result of not incorporating dark matter into the theoretical model. But, that is another story…

Further reading: Nesvadba et al. The black holes of radio galaxies during the “Quasar Era”: Masses, accretion rates, and evolutionary stage.

Astronomy Without A Telescope – Not So Ordinary


Sorry – a bit of southern sky bias in this one. But it does seem that our favourite down under naked eye objects are even more unique than we might have thought. The two dwarf galaxies, the Large and Small Magellanic Clouds, orbit the Milky Way and have bright star forming regions. It would seem that most satellite galaxies, in orbit around other big galaxies, don’t. And, taking this finding a step further, our galaxy may be one of a declining minority of galaxies still dining on gas-filled dwarf galaxies to maintain a bright and youthful appearance.

We used to think that the Sun was an ordinary, unremarkable star – but these days we should acknowledge that it’s out of statistical mid-range, since the most common stars in the visible universe are red dwarfs. Also, most stars are in binary or larger groups – unlike our apparently solitary one.

The Sun is also fortunately positioned in the Milky Way’s habitable zone – not too close-in to be constantly blasted with gamma rays, but close-in enough for there to be plenty of new star formation to seed the interstellar medium with heavy elements. And the Milky Way itself is starting to look a bit out of the ordinary. It’s quite large as spiral galaxies go, bright with active star formation – and it’s got bright satellites.

The Lambda Cold Dark Matter (CDM) model of large scale structure and galaxy formation has it that galaxy formation is a bottom-up process, with the big galaxies we see today having formed from the accretion of smaller structures – including dwarf galaxies – which themselves may have first formed upon some kind of dark matter scaffolding.

Through this building-up process, spinning spiral galaxies with bright star forming regions should become common place – only dimming if they run out of new gas and dust to feast on, only losing their structure if they collide with another big galaxy – first becoming a ‘train wreck’ irregular galaxy and then probably evolving into an elliptical galaxy.

The  Lambda CDM model suggests that other bright spiral galaxies should also be surrounded by lots of gas-filled satellite galaxies, being slowly draw in to feed their host. Otherwise how is it that these spiral galaxies get so big and bright? But, at least for the moment, that’s not what we are finding – and the Milky Way doesn’t seem to be a ‘typical’ example of what’s out there.

The relative lack of satellites observed around other galaxies could mean the era of rapidly accreting and growing galaxies is coming to a close – a point emphasised by the knowledge that we observe distant galaxies at various stages of their past lives anyway. So the Milky Way may already be a relic of a bygone era – one of the last of the galaxies still growing from the accretion of smaller dwarf galaxies.

Supernova 1987a, which exploded near the Tarantula Nebula of the Large Magellanic Cloud. Credit: Anglo-Australian Observatory.

On the other hand – maybe we just have some very unusual satellites. To a distant observer, the Large MC would have nearly a tenth of the luminosity of the Milky Way and the Small MC nearly a fortieth – we don’t find anything like this around most other galaxies. The Clouds may even represent a binary pair which is also fairly unprecedented in any current sky survey data.

They are thought to have passed close together around 2.5 billion years ago – and it’s possible that this event may have set off an extended period of new star formation. So maybe other galaxies do have lots of satellites – it’s just that they are dim and difficult to observe as they are not engaged in new star formation.

Either way, using our galaxy as a basis for modelling how other galaxies work might not be a good idea – apparently it’s not so ordinary.

Further reading: James, P. A. And Ivory C.F. On the scarcity of Magellanic Cloud-like satellites.

The Case of the Missing Bulges

The Hubble sequence is astronomer’s main tool for classifying galaxies. On one side, you have elliptical galaxies with defined structure. As you progress, the galaxies become more stretched out, but still lack definition until suddenly, there’s a bulge in the center and spiral arms! Oh yeah, and then there’s the cousins that no one really likes to hang out with, the “irregular” galaxies, hanging out in the corner.

But there’s another class of galaxies that seems to have fallen off the Hubble wagon. Some spiral galaxies seem to lack defined bulges. These oddities pose a challenge to our understanding of galactic formation.

The current understanding of galactic formation is one of hierarchical merging. Small dwarf galaxies form first, and then form bigger galaxies which merge and continue to eat more dwarf galaxies until a fully fledged galaxy is formed. However, the collisional nature of this formation tends to scatter stars, favoring random orbits towards the center of flattened galaxies, which should create a classical bulge. Galaxies that do not have a bulge, or have a “pseudobulge” (small bulges created by gravitational sorting of stars within an already formed galaxy) don’t seem to fit this picture.

A recent review suggests that galaxies without true bulges are in fact common and include many well-known galaxies such as M101 (the Pinwheel Galaxy) and M33. The team, led by John Kormendy of the University of Texas, Austin, conducted a survey of spiral galaxies in the Local Group to determine just how common they were. To determine the status of the bulge, the team analyzed the physical size of the bulge, its luminosity as a fraction of the overall light output, and the color/age of the stars therein. Bulges that were small, indistinct, and contained stars similar to the color/age of the stars found in the disk were considered examples of the psuedobulges. Ones with significant, bright, and distinctly redder/older bulges were indicative of what would be expected in the classical merger bulge.

The team determined that as much as 58-74% of their sample did not contain a classical bulge. Furthermore, they state, “Almost all of the classical bulges that we do identify – some with substantial uncertainty – are smaller than those normally made in simulations of galaxy formation.” Indeed, included among these galaxies is our own Milky Way which has a very odd, box shaped bulge. The team notes that the velocity distribution of the apparent bulge merges seamlessly into the disk portion of the galaxy as opposed to a discontinuous fit in classical bulges.

Kormendy’s team finds that one way to form such “pure-disk” galaxies is to allow for the possibility of early star formation. According to the paper, this would “give the halo time to grow without forming a classical bulge.”

These findings stand in strong contrast with a study published by the same group in 2009, analyzing the Virgo cluster of galaxies. In that study they found that classical bulge galaxies (including in this study, elliptical galaxies) seemed to dominate. As such, they suggest that the formation of bulges is somehow related to the local environment. Although the question cannot yet be answered, it begs the question for future study: What about our environment is so special that we can form galaxies in a non-merger process? The answer to this question will require further study.

Death in the Sky: M31 Shreds its Satellites

An international team of astronomers has identified two new tidal streams in M31, the Andromeda galaxy. They are more-or-less intact remnants of dwarf galaxies that M31 has otherwise ripped to shreds.

One team – using the Suprime-Cam camera on Subaru – discovered two new dwarf galaxy shards by mapping the sky density of red giants in M31’s outskirts; the other – using the DEIMOS spectrograph on Keck II – separated the M31 red giant wheat from the Milky Way chaff.

In a project led by collaborators Mikito Tanaka and Masashi Chiba of Tohoku University, Japan, the astronomers used the Subaru 8-meter telescope and Suprime-Cam camera to map the density of red giants in large portions of M31, including the hitherto uncharted north side. This led to the discovery of two tidal streams to the northwest (streams E and F) at projected distances of 60 and 100 kiloparsecs (200,000 and 300,000 light-years) from M31’s nucleus. The study also confirmed a few previously known streams, including the little-studied diffuse stream to the southwest (stream SW), which lies at a projected distance of 60 to 100 kiloparsecs (200,000 to 300,000 light years) from M31’s nucleus.

The Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo (SPLASH) collaboration, a large survey of red giants in M31 lead by Puragra Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz, has followed up with a spectroscopic survey of several hundred red giants in Streams E, F, and SW, using the Keck II 10-meter telescope and DEIMOS spectrograph at the W. M. Keck Observatory in Hawaii. Analysis of the spectra from this survey yields estimates of the line-of-sight velocity of the stars, which in turn allows M31 red giants to be distinguished from foreground stars (in the Milky Way). The spectral data confirmed the presence of coherent groups of M31 red giants moving with a common velocity.

Distribution of line-of-sight velocities in the Stream SW field (Raja Guhathakurta)

Stars spread over the vast reaches of a halo in a big galaxy like the Milky Way or M31 are characterized by old age, few elements other than helium and hydrogen (i.e. low metallicities; astronomers call all elements other than hydrogen and helium “metals”), and high velocities. The exceptional nature of these halo stars, when compared to stars in a galaxy’s disk, reflects the early dynamics and element formation of the galaxy when its appearance differed significantly from what we see today. Consequently, the halo provides important insights into the processes involved in the formation and evolution of a massive galaxy. In the best Big Bang model we have today – ΛCDM (Lambda Cold Dark Matter) – the outer halos are built up through the merger and dissolution of smaller, dwarf, satellite galaxies. “This process of galactic cannibalism is an integral part of the growth of galaxies,” said Guhathakurta.

The smooth, well-mixed population of halo stars in these large galaxies represents the aggregate of the dwarf galaxy victims of this cannibalism process, while the dwarf galaxies that are still intact as they orbit their large parent galaxy are the survivors of this process.

“The merging and dissolution of a dwarf galaxy typically lasts for a couple billion years, so one occasionally catches a large galaxy in the act of cannibalizing one of its dwarf galaxy satellites,” Guhathakurta said. “The characteristic signature of such an event is a tidal stream: an enhancement in the density of stars, localized in space and moving as a coherent group through the parent galaxy.”

Tidal streams are important because they represent a link between the victims and survivors of galactic cannibalism – an intermediate stage between the population of intact dwarf galaxies and the well-mixed stars dissolved in the halo.

The Andromeda galaxy is a unique test bed for studying the formation and evolution of a large galaxy, said Guhathakurta, “Our external vantage point gives us a global perspective of the galaxy, and yet the galaxy is close enough for us to obtain detailed measurements of individual red giant stars within it.”

One of the next steps will be to measure the detailed elemental compositions (“chemical properties”, in astronomer-speak) of red giants in these newly discovered tidal streams in M31. Comparing the chemical properties of tidal streams, intact dwarf satellites, and the smooth halo will be of particular significance, Guhathakurta said. Mikito Tanaka put it this way: “Further observational surveys of an entire halo region in Andromeda will provide very useful information on galaxy formation, including how many and how massive individual dwarf galaxies as building blocks are and how star formation and chemical evolution proceeded in each dwarf galaxy.”

At the present time, detailed studies of the chemical properties of tidal streams, intact dwarf satellites, and smooth stellar halos are possible only in the Milky Way and M31 galaxies and their immediate surroundings. Existing telescopes and instruments are simply not powerful enough for astronomers to carry out such studies in more distant galaxies. This situation will improve greatly with the advent of the planned Thirty Meter Telescope later in this decade, Guhathakurta said.

Tanaka’s team published their survey results in a recent Astrophysics Journal (ApJ) paper (the preprint is arXiv:0908.0245), and Guhathakurta’s team presented their results on the newly discovered tidal streams earlier this month at the 215th meeting of the American Astronomical Society in Washington, D.C.; they hope to have an ApJ paper on these results published later this year. You can read an earlier SPLASH paper, “The SPLASH Survey: A Spectroscopic Portrait of Andromeda’s Giant Southern Stream”, published in ApJ (the preprint is arxiv:0909.4540).

Sources: University of California, Santa Cruz, National Astronomical Observatory of Japan.

Quasar Caught Building Future Home Galaxy

The birth of galaxies is quite a complicated affair, and little is known about whether the supermassive black holes at the center of most galaxies formed first, or if the matter in the galaxy accreted first, and formed the black hole later. Observations of the quasar HE0450-2958, which is situated outside of a galaxy, show the quasar aiding a nearby galaxy in the formation of stars. This provides evidence for the idea that supermassive black holes can ‘build’ their own galaxies.

The quasar HE0450-2958 is an odd entity: normally, supermassive black holes – also known as quasars – form at the center of galaxies. But HE0450-2958 doesn’t appear to have any host galaxy out of which it formed. This was a novel discovery in its own right when it was made back in 2005. Here’s our original story on the quasar, Rogue Supermassive Black Hole Has No Galaxy.

The formation of the quasar still remains a mystery, but current theories suggest that it formed out of cold interstellar gas filaments that accreted over time, or was somehow ejected from its host galaxy by a strong gravitational interaction with another galaxy.

The other oddity about the object is its proximity to a companion galaxy, which it may be aiding to form stars. The companion galaxy lies directly in the sights of one of the quasar’s jets, and is forming stars at a frantic rate. A team of astronomers from France, Germany and Belgium studied the quasar and companion galaxy using the Very Large Telescope at the European Southern Observatory. The astronomers were initially looking to find an elusive host galaxy for the quasar.

The phenomenon of ‘naked quasars’ has been reported before, but each time further observations are made, a host galaxy is found for the object. Energy streaming from the quasars can obscure a faint galaxy that is hidden behind dust, so the astronomers used the VLT spectrometer and imager for the mid-infrared (VISIR). Mid-infrared observations readily detect dust clouds. They combined these observations with new images obtained from the Hubble Space Telescope in the near-infrared.A color composite image of the quasar in HE0450-2958 obtained using the VISIR instrument on the Very Large Telescope and the Hubble Space Telescope. Image Credit: ESO

Observations of HE0450-2958, which lies 5 billion light years from Earth, confirmed that the quasar is indeed without a host galaxy, and that the energy and matter streaming out of the jets is pointed right at the companion galaxy. This scenario is ramping up star formation in that galaxy: 340 solar masses of stars a year are formed in the galaxy, one-hundred times more than for a typical galaxy in the Universe. The quasar and the galaxy are close enough that they will eventually merge, finally giving the quasar a home.

David Elbaz of the Service d’Astrophysique, who is the lead author of the paper which appeared in Astronomy & Astrophysics, said “The ‘chicken and egg’ question of whether a galaxy or its black hole comes first is one of the most debated subjects in astrophysics today. Our study suggests that supermassive black holes can trigger the formation of stars, thus ‘building’ their own host galaxies. This link could also explain why galaxies hosting larger black holes have more stars.”

‘Quasar feedback’ could be a potential explanation for how some galaxies form, and naturally the study of other systems is needed to confirm whether this scenario is unique, or a common feature in the Universe.

Source: ESO, Astronomy & Astrophysics