Where is the Dust in Distant Galaxies?

In a previous post we wrote about the morphology of a galaxy's star light. Most galaxies have matter that we can see in 3 forms: stars, gas, and dust.

Figure 1: M51 at optical wavelengths of light. Credit: NASA, ESA, 

S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA).

Figure 1 shows a picture of M51 at optical wavelengths of light. The yellow, red, and blue parts of the picture are the regions hosting M51's stars which are visible to us. Along the spiral arms we also see dark structures. The dark parts of the picture are the regions hosting M51's stars which are invisible to us --- the stars which are obscured by dust.

Dust grains in M51 absorb light from these stars and reemit that light at infrared wavelengths. Figure 2 shows a picture of M51 at an infrared wavelength of light. The spiral arms in the infrared picture line up with the dark structure in the optical picture. We know where the dust is in M51. What about the dust in other galaxies?

Figure 2: M51 at an infrared wavelength of light. Credit: IRSA.

As we study galaxies that are further and further away from our own, we lose information on where the dust in these galaxies is. Infrared telescopes cannot produce pictures of a distant galaxy at the same resolution as pictures of M51. We can guess at where the dust is by looking at dark structures in pictures at optical wavelengths. Your eye is good at picking out dark structures in a many-color image. What is a dark structure in a two-color image?

Figure 3 shows a zoomed in picture of M51. The dark structure is black in many places. In many other places the dark structure is adjacent to a red spot, a spot missing blue and yellow colors. Dust grains in M51 are good at obscuring blue and yellow light and less good at obscuring red light. If we measure the brightness of a spot in a red image, and the brightness at the same location in a blue image, many galaxies will have the same ratio between those brightnesses. The ratio comes from two aspects of the dust: the sizes of the grains and the number of grains. A dark structure in a distant galaxy might be a red spot with a weak blue spot.

Figure 3: a cropped and zoomed-in view of M51 at optical
wavelengths of light. Credit: NASA, ESA, S. Beckwith (STScI),
and The Hubble Heritage Team (STScI/AURA).

The red color in the image of M51 is due to light from Hydrogen atoms. The Hubble Space Telescope has an instrument allowing us to see the light from Hydrogen atoms in distant galaxies; it has another instrument allowing us to see blue light from distant galaxies. I wrote a paper using CANDELS data to compare the brightnesses of the light at the two wavelengths. We conclude that we need more data!

The ratio of brightnesses between red spots and blue spots for distant galaxies is different from the ratio for local galaxies. Dust grains in distant galaxies might have different sizes compared to their sizes in M51, which would make them more or less good at obscuring red light compared to how they obscure blue and yellow light. We cannot distinguish between this hypothesis and the one saying that the number of grains differs.

NASA has a plan to launch several telescopes into space and connect them, which would solve the problem of resolution that prevents us from having detailed pictures at infrared wavelengths of distant galaxies. You can find out more about the Far-IR Surveyor here:

http://science.nasa.gov/science-committee/subcommittees/nac-astrophysics-subcommittee/astrophysics-roadmap/

The Geminid Meteor Shower

We may be in the grip of some Arctic weather here in the US right now, but if you can stand to venture outside then the skies have a treat in store this week, in the shape of the Geminid meteor shower. Hyped as the best meteor shower of any given year, the Geminids reaches its peak this weekend (13/14 December).

So what is a meteor?

They’re also known colloquially as ‘shooting stars,’ but have nothing to do with stars. A lot of things in astronomy that are basically the same are given different names depending on how and where we see them. Space debris is no different. A small, solid body moving within the Solar System is known as a meteoroid. If that meteoroid happens to cross paths with the Earth, it burns up in the atmosphere, creating the distinctive streak that we know as a meteor. In exceptional cases, a large rock might not entirely burn up, and survives intact to hit the ground. The solid remains that hit the Earth’s surface are known as meteorites.

Space is full of debris. A quick look at the crater-covered moon is a good indicator of what the Earth might look like if we didn’t have the atmosphere to burn up most of what might impact us, and erosion on the ground to smooth over the damage caused by those that do. On any clear night, if you watch a patch of sky for long enough, chances are you’ll see a meteor.

What makes meteor showers different to these random occurrences is that they’re highly concentrated – a lot of meteors all originating in the same place – and that they occur regularly, at the same time each year. This is because the Earth is moving through space as it orbits the Sun, carving out the same path every year, and so at the same time each year we hit the same particularly intense patches of debris.

Most of these debris patches have been left by comets. Comets have been observed for as long as astronomical observations have been recorded, and often viewed as divine messengers or omens. One of the earliest recorded sightings was in China in 240BC. The same comet was also recorded by the Babylonians and in medieval Europe, and is even featured on the Bayeux Tapestry. It wasn’t until 1705 that Edmund Halley realised these sightings were of the same object: it now bears his name, Halley’s Comet, and is due to pass the Earth again in 2061.

Comets are small bodies made up of ice, rock and dust, thought to originate in the outer reaches of the Solar System. Some of these ‘dirty snowballs’ are pushed towards the centre of the Solar System, where they enter into highly elliptical orbits that see them pass close to the Sun before shooting off back to the outer reaches of the Solar System. As they approach the center of the Solar System, radiation from the Sun causes some of the material in the comet to melt and vaporize; this gives rise to the characteristic tail. Consequently, the tail always points away from the Sun.

Forging paths through the entire length of the Solar System is dangerous work, and not all comets survive the journey intact. In 1994, Comet Shoemaker-Levy 9 collided spectacularly with Jupiter. More recently, you may have seen Comet ISON in the news as it ventured into the inner Solar System. ISON made its closest approach to the Sun (called perihelion) on November 28^th 2013, but is believed to have disintegrated as it whipped around the Sun.

Those that do survive their journeys, however, are not good at cleaning up after themselves. There are still debris trails from several comets that have crossed the path of the Earth’s orbit in the past, and each time the Earth reaches that point in its orbit – once per year – we collide with this debris, which burns up in the atmosphere to produce the streaks of light we call meteors. Comet Halley mentioned above actually intersects the Earth’s orbit twice, and its trail is believed to give rise to both the Eta Aquarids in May and the Orionids in late October.

The Geminids are unusual for a meteor shower in that the origin is not actually a comet, but an asteroid known as 3200 Phaethon. The asteroid is on an unusual orbit that brings it closer to the Sun than Mercury, and it sheds enough material to generate the most intense meteor shower of the year. The video below from NASA Science Casts explains more about the origin of the Geminids.

           

Meteors can be seen all over the sky, but most will appear to originate at a single point, known as the radiant. For the Geminids, this radiant is in the constellation Gemini (which gives the shower its name), close to the star Castor. This effect is caused by the fact that the Earth is moving into the debris; this is the same effect used to demonstrate spaceships moving at faster-than-light speeds in science fiction.

The Eastern sky as seen from Austin, Texas at 9pm on Friday December 13th 2013.
The Geminid meteors appear to radiate from the constellation Gemini, near to the
star Castor. Gemini can be most easily located by finding Orion with its distinctive belt.
A little way over Orion's left shoulder (the red star Betelgeuse) are the two bright
stars Castor and Pollux. (Image credit: Stellarium)

The Geminid meteor shower will peak on Friday and Saturday nights (December 13^th – 14^th), but meteors can be seen for a few days either side. The best thing about meteor showers is that no equipment is required (save something to keep you warm) – just pick a dark location and lie back so that you can see as much of the sky as possible.

The Geminids regularly peak in intensity around mid-December and seem to have increased in strength in the past years. This year, astronomers expect 120-160 meteors per hour during the peak, which would be early in the morning on Dec. 14. However, the moon is close to  full during the peak so only the brightest meteors will be easy to spot. For the truly dedicated, the best time to watch is an hour before dawn, when the moon will have set leaving the sky much darker. Reports say that we can still expect around 50 per hour under the best observing conditions (clear skies, no light pollution, etc.) and we may even be able make out their different colors (mainly white and yellow and a few being blue, green or red).

And if you’re not brave enough to venture out into the cold, you can even watch online. Now that’s astronomy for the 21^st century.

Hubble's Law and Hubble's Legacy

In astronomy, everything in the Universe is moving relative to everything else. The Earth moves around the Sun, the Sun around the Milky Way, the Milky Way moves relative to the other Local Group galaxies, and the Local Group relative to more distant galaxies and galaxy clusters. Within the large-scale cosmic web we find bulk motions in every direction on the sky. 

Such motions can be measured using a variety of techniques depending on the objects of interest. For galaxies, this is typically achieved through the identification of known "lines" in their spectra, which shift from where they should be, where we measure them at rest in the lab. This is simply a "light" version of the known Doppler Effect for sound, where, for example, the pitch of a train goes up when approaching, then down when moving away, compared to the pitch you hear when you're on it.

In astronomy, the degree of this shift is known as either redshift (for galaxies moving away from us) or blueshift (for galaxies moving towards), as explained previously.

It was then a curious set of observations in the early 1900's that revealed that the majority of objects outside our own galaxy (then called nebulae, now known as other galaxies) were all moving away from us (i.e. redshifted), and in approximate proportion to their distance. This was explicitly seen in the pioneering work of Vesto Slipher in 1917, Knut Lundmark in 1924, and Edwin Hubble in 1929, amongst others of the time. 

In the figure below we reproduce the original "discovery" plot by Edwin Hubble, which has since come to be known as Hubble's Law, written as:

galaxy recession velocity = H0 x galaxy distance

The proportionality constant, H0, is called Hubble's constant and was determined by fitting a straight line through the data. Hubble estimated H0 = 500 km/s/Mpc at the time.

Edwin Hubble's "discovery" plot from 1929, showing that the distance to a galaxy (on the x-axis) is correlated with the speed at which it's moving away from us (called redshift, on the y-axis). Such a distance-redshift relation is strong evidence supporting the idea that the Universe is expanding.

The observation that every distant galaxy in the Universe appears to be red and not blueshifted is itself remarkable. In effect, it tells us that the motions of all galaxies beyond our local volume are in a direction away from us, and Hubble's Law tells us that the further away a galaxy is, the faster its moving away. This was, in essence, the first observational evidence of an expanding Universe!

That the Universe could be expanding was predicted by Einstein's equations of general relativity, as many of you may know. A somewhat crazy idea when first proposed, Einstein himself was unsatisfied with the concept of a dynamic space-time, which led him to update his equations with the famous cosmological constant, Lambda.

Although Hubble is solely credited with the discovery of his Law, closer examination of the literature shows a more complex history with no one single eureka moment by any individual. In fact, the redshifts that Hubble used above were entirely borrowed from Slipher's earlier work, and the distances that Hubble measured himself were unfortunately significantly flawed. The modern value of H0 is 67.3 km/s/Mpc, measured to about 2% accuracy by the Planck cosmic microwave background satellite.

As Edinburgh Royal Observatory Professor John Peacock recently argued, Hubble was perhaps somewhat fortunate to be able to demonstrate the relation given the data he had on hand at the time. However he was already an important figure in the community, very good at promoting the result, and the community of the day was equally as excited to accept it.

Regardless, suffice it to say that once the Law was established its ramifications changed our understanding of the Universe. The measurement of the Hubble expansion (and repeated confirmation over the years) heralded in the age of modern cosmology. It underpins our modern cosmological paradigm. And it is a key component to many of the observations and results that CANDELS produces using the Hubble Space Telescope.

More discussion of the Hubble constant and its use (and misuse) in astronomy data analysis can be found in my recently published paper, "Damn you little h!".

2013 - The Year of Comets

Comets are small solar-system objects. They are often referred to as dirty snowballs because they are believed to mainly consist of ice and dust. But they also contain things like methane, ammonia, carbon dioxide etc.

Illustration of a comet's tails. Image credit: NASA

When comets come close to the sun, the radiation from the sun causes some of the comet material to be released, i.e. the ice is turned into gas and any dust within it is freed. This forms a coma of material around the comet's core; it's a little bit like an atmosphere. Due to the solar wind this material is pushed away from the comet and leaves a visible tail behind. In fact comets can have more than one tail. There is a gas tail that points in the opposite direction of the sun which consists mostly of gas atoms that are ionized by the suns radiation and a tail of dust grains that leans a little bit more towards the comets trajectory (see the illustration). The core of a comet is thought to be a few tens of miles/kilometers in size or smaller, the coma on the other hand can reach a million miles or more; that's about the size of the sun! Some comets can also have very long tails with some of the longest reported tails being as long as about 1 Astronomical Unit; that's the distance between the Earth and the sun!  

The tail and coma are what makes comets easily distinguishable from asteroids. However, every time a comet passes by our Sun it loses some of its material until eventually all the ice has gone and the only remainder might be a piece of rock. 

It is believed that most comets originate from the formation of our solar system. They are left-overs that didn't make it into a planet or moon. They mostly live in the Kuiper Belt and what is called the Oort Cloud. The Oort Cloud is a described as a sort of spherical area far out around our solar system that harbors a vast number of icy objects. Occasionally some of these collide or encounter other massive objects (such as the gas planets) that disturb their regular orbit. And sometimes the new path the comet adopts will lead through the inner solar system. When these objects come close to the Sun we observe them as comets. Some comets come by on a regular basis, such as the famous Halley's comet. They found a stable new orbit that can take the comet anything from a few years to more than 100,000 years to complete once. Comets with periods shorter than about 200 years are called short-period comets and are believed to come from the Kuiper Belt, those with longer periods on the other hand are called long-period comets coming most likely from the Oort Cloud. Rarer are those comets that only pass by once and are kicked out of the solar system forever. These comets are called hyperbolic comets, named after the shape of their trajectory.

Path of Comet PanSTARRS, Image credit: NASA

In the past we have had the pleasure to see many great comets in the night sky, the greatest ones even with the naked eye. This year, 2013, promises to be another great year for bright comets. There will be 2 very bright comets, one moderately bright one (Comet C/2012 F6 Lemmon) and one regular visitor (Comet 2P/Encke). Let me tell you here about the 2 brightest ones. The first one predicted to be relatively bright is comet PanSTARRS (official designation C/2011 L4 PanSTARRS) this month. It's got its name from the PanSTARRS survey which discovered this comet. Scientists predict that the brightness of this comet is going to be around as bright as the stars in the big dipper but brightness predictions are difficult. While being visible from the Southern Hemisphere already, comet PanSTARRS will be visible from the Northern Hemisphere starting March 7th (this Thursday), just above the horizon after sunset.

 

Comet McNaught in 2007, Image credit: ESA/NASA

Another great show will be put on at the end of November by comet C/2012 S1 ISON, which some say is expected to be brighter than the full moon!! This is indeed a rare occasion. Two Russian amateur astronomers discovered Comet ISON while observing for the International Scientific Optical Network (ISON) which gave the comet its name. Calculations of the comet's orbit revealed that it will pass very close to the sun (less than 1 million miles distance). Comets that come this close to the sun are called sungrazers. In the case of surviving this close pass to the sun, the view of this comet should be spectacular, possibly similar to that of Comet C/2006 P1 McNaught in 2007 (see picture). I will sure keep an eye out this year and try to spot one of these passers-by!

Measuring the Universe

In a lot of previous posts you have read about redshift and the distance between the Milky Way and other galaxies. In this post, we step back a little bit and explore the size scales in the Universe and how distances can be measured.

First off, let's start in our Solar System, on planet Earth. Assume the size of the Earth is represented by a peppercorn (a size of about 0.08 inch). Using the same size scaling, the Sun can be represented by blowing up a balloon until it is 8 inches in diameter. In reality the Earth's diameter is about 8000 miles wide, the Sun's diameter is 800 thousand miles. This means that in the peppercorn model we assumed a single inch stands for 100 thousand miles. A yard (or 36 inches) then represent 3.6 million miles. If we rank all other planets in the solar system accordingly, Venus, our sister planet, is also a peppercorn. Both Mars and Mercury are smaller than Earth and Venus. They can be represented by the head of a pin (about 0.03 inches wide). For the size of Jupiter, the first gas planet we would encounter when travelling out of the solar system, we can use a walnut (about 0.9 inches wide). Saturn is a little smaller, about the size of an acorn (0.7 inches). Uranus and Neptune are even smaller still, so a peanut for each is adequate (0.3 inches). If you still count Pluto as a planet, than you would want to represent it by another pin head. 

Size comparisons between Solar System planets and other stars. Image source here and here.

So now, that we have established the sizes of the planets using peppercorns and nuts and so on, what about the distance between them? Well, if you take a huge step that is about 1 yard wide, you travel those 3.6 million miles described above. If you take 10 steps away from the Sun, you reached your pinhead Mercury (it's about 36 million miles away from the Sun). After another 9 steps you'll find Venus, one of the peppercorns. Take another 7 steps and you finally reached your Earth peppercorn. So the distance between the Sun and the Earth is 93 million miles (also called 1 Astronomical Unit) means taking 26 steps from the balloon Sun and your Earth peppercorn. From Earth you have to take another 14 steps to reach Mars. From there now, it is a much larger distance to reach Jupiter. You have to walk 95 steps. Remember, each step has to be the size of about 1 yard! In comparison to this distance, look at the size of Jupiter, which we represented by a walnut. From now on you have to take more and more steps between the planets to reach the next one. Saturn takes another 112 steps. From there Uranus is another 249 steps away. And to get to Neptune from Uranus, walk another 281 steps. That is nearly 3 football fields! And if you still care about Pluto, then walk another 242 steps to reach it.

Now you have walked more than 1000 yards, or across about 10 football fields and the planets have merely the size of nuts or smaller! There is a lot of space between them! 

To reach the next star, Proxima Centauri, from the Sun, a travel of 4.21 light years is required (or 1.20 parsecs). A light year is the distance light travels in a year, which is 5,878,625 million miles or 63,241 times the distance between the Earth and the Sun. In the scale we assumed for the solar system above this is about 16443 football fields or the distance between Tucson and Houston. So to reach the next star in that scale you have to travel this distance more than 4 times. You could reach Oslo in Norway from Washington D.C. and that would still be not quite far enough to go.

Our Solar System is located in the Milky Way, our home galaxy. But the Sun is only one star among 100 billion in it. And there are hundreds of billions of galaxies in the Universe. 

 

The cosmological distance ladder

So how do astronomers measure distances to so many far away objects, may it be other stars or other galaxies? Well, we use what is called the cosmological distance ladder. In order to reach the next step on the ladder you have to be sure about the step you are standing on. The principle of the ladder is based on the fact that each method to measure distances overlaps with another method, so that the next can be calibrated with the previous. 

Measuring distances to other stars via the Parallax and the distance R between the Earth and the Sun.

The distance ladder starts in our Solar System (or even on Earth if you want). In this previous post about the Venus transit we explained how some 100 years ago astronomers measured the distance between the Earth and the Sun and the size of the planets. Nowadays distances in our Solar System can be determined using radar, the same technique with which ships try to find the location of other ships on the ocean. Once we know the distance between the Earth and the Sun, we can determine the distance to other nearby stars using the Parallax method (see figure above). The Parallax method works out to distances of about 100 light years. The Milky Way has a diameter of about 100,000 light years. So with the Parallax we can measure only our more local neighbourhood.

For the next step on the ladder, the so-called Main Sequence Fitting, is used. With this method the distance to star clusters can be determined by exploiting the relation between brightness and colour of the cluster stars, i.e. their position in the Hertzsprung-Russell Diagram. Stars like our Sun line up on the Main Sequence in this diagram and by measuring the properties of stars, their absolute brightness can be estimated and used as distance indicator. Main Sequence Fitting can be applied across the Milky Way.

Beyond the Milky Way out to other close-by galaxies (within 10 million light years) a relation between the period of variability of Cepheid stars and their luminosity serves to measure the distance to these stars. Cepheid stars are so-called standard candles. This means that their brightness is very well known. From the difference between the observed and the known brightness, the distance can then be measured. The calibration for this method can be achieved using stars in the Small Magellanic Cloud for which main sequence fitting is still possible.

Similarly to the Cepheids, certain types of Supernovae, can also be used as standard candles. Supernovae are exploding dying stars and we already told you a lot about them here. For the calibration of this method one looks for a Supernovae in a galaxy for which the distance could be determined with Cepheid stars. Again the comparison between the absolute and the observed brightness of the Supernova allows us to calculate the distance. With Supernovae distances out to about 10 billion light years (or a redshift of about 1) can be measured.

Beyond Supernovae, Hubble's law and redshifts are used for distance measurements. Hubble's law relates the distance of an object to the speed with which it moves away from us due to the expansion of the Universe. In modern astronomy most distances beyond our local galaxy group system are given in terms of redshift. 

Having reached for out in the Universe at the end of this blog post, we have long outgrown the simple scaling that we used at the beginning to describe the distances and sizes of planets in the Solar System. And even my astronomer mind is regularly boggled by the truly astronomical scales I am confronted with every day.

Unveiling Obscured Supermassive Black Hole Growth

In a previous post, we discussed the class of objects known as Active Galactic Nuclei (AGN). AGN are actively growing black holes with masses as large as 100 billion times that of our Sun. These supermassive black holes were once thought to be rare, but are now known to live at the centers of nearly all galaxies. While the majority of supermassive black holes are inactive (at least in the present day), their extreme masses exert a strong gravitational pull on their surroundings capable of ripping a star apart if it wanders too close. While black holes occasionally grow by this violent process, they are more commonly fed by interstellar gas that first settles into a rapidly rotating disk around the black hole before being accreted (eaten). 

Artist's illustration of the accretion disk and jets surrounding
a black hole. Credit: Cosmovision, Dr. Wolfgang Steffen.
For an animated version, click here.

It is this accretion disk and other features in its vicinity that make actively accreting supermassive black holes visible and that make AGN among the most luminous objects in the Universe across much of the electromagnetic spectrum. The accretion disk itself gives off bright ultraviolet and optical/visible emission capable of outshining the light from the AGN’s host galaxy. A corona surrounding the accretion disk is thought to be responsible for the bright X-rays emitted by AGN, and in approximately 15% of cases, jets launched near the supermassive black hole give rise to bright radio emission. Incidentally, it was this combination of bright radio emission and bright, yet extremely compact, optical emission that gave AGN their other well-known name: quasars, or “quasi-stellar radio sources”.

Obscured AGN

The combination of bright ultraviolet, optical, X-ray, and occasionally radio emission should give astronomers plenty of ways to identify AGN. However, actively accreting supermassive black holes appear to be surrounded not only by an accretion disk of hot gas, but also by a donut-shaped torus of colder gas and dust (astronomer lingo for very small solid particles in space). This torus lies further from the black hole than the accretion disk, and while its origin and properties are still an active field of research, one thing is clear. If we are lucky enough to view an AGN from above or below the torus (e.g., though the hole in the donut), we get a clear view of the accretion disk and its surroundings. If, however, the torus is positioned so that we must look through it to see the accreting black hole, the picture changes. 

Artist's illustration of the dusty torus that surrounds an
AGN's accretion disk.  Credit: NASA/CXC/M.Weiss

Dust, it turns out, is very good at blocking UV and optical light, and gas is very good at blocking X-ray light. When we view an AGN through the torus, much of the light we would normally see is therefore missing, or significantly weakened. We call such unfortunately aligned sources ‘obscured AGN’. If these sources were rare, this would not be a big problem for our study of supermassive black hole growth. However, in the local Universe, obscured AGN are four times more common than unobscured AGN, and some studies point to an even higher fraction of obscured AGN at earlier times. In order to study how and why supermassive black holes grow, we must first find ways to identify these obscured AGN.

Finding Obscured AGN

Thankfully, not all obscured AGN remain entirely hidden. Radio emission is relatively insensitive to dust and gas, leading to fairly complete samples for the 15% of AGN with radio jets. Furthermore, while X-rays can be blocked by gas, it takes a lot to fully block the X-ray emission observed by the current generation of X-ray satellites. Deep X-ray observations by Chandra and XMM-Newton can therefore detect many sources that would be missed in the UV and optical, and recent X-ray missions like Swift, Integral, Suzaku and NuSTAR are opening our window on the heavily obscured Universe by probing more energetic X-rays that are even harder to obscure. Certain lines in the optical spectra of AGN are also emitted in a region beyond the torus, and can be used to identify obscured AGN missed by other techniques. 

And then there is the torus itself. When the dust in the torus absorbs the UV and optical light, it heats up to temperatures as high as 1500K (~2200 degrees Fahrenheit). While this may seem quite hot, it is several times cooler than the surface of the Sun. As such, the torus does not emit in the optical like a star might, but at lower energies in the infrared. (Note: at approximately 100 degrees Fahrenheit, we also emit our own infrared or ‘thermal’ emission). While warm dusty objects in the Universe are therefore faint in the UV and optical, they can be bright in the infrared, and the same is true for AGN. The torus that obscures our view of the AGN in the UV, optical, and X-ray in fact provides us with a way of identifying luminous AGN using the infrared satellites Spitzer and WISE. 

Visible (left) and infrared (right) images of a person with their hand in a bag.  While the visible light is blocked by the bag, the man's infrared emission passes through the bag.  Similarly, the ultraviolet and optical light absorbed by an AGN's torus warms the dust in the torus itself, which then produces infrared radiation. Credit: NASA/IPAC

Why Obscured AGN Matter

Obscured AGN make up the biggest fraction of the total AGN population, so it is crucial that we be able to detect and study these elusive sources if we hope to understand when, where, and why supermassive black holes formed. However, obscured AGN provide astronomers with another advantage. Unlike bright unobscured AGN, whose UV and optical light outshines the light from their host galaxies, the host galaxies of obscured AGN can often be seen quite clearly with minimal interference from the AGN itself. This is crucial, as one of the major open questions in astronomy concerns the relationship between AGN and their hosts. 

The Antennae Galaxies, the nearest example of a major
galaxy merger.  Credit: NASA/ESA and the Hubble Heritage Team 

As recently as 12 years ago, astronomers believed that AGN and their host galaxies grew and evolved independently of one another. However, the surprising discovery of a tight correlation between the mass of a supermassive black hole and the mass of its host galaxy’s bulge (the so-called 'M-sigma relation') indicates that the evolution of galaxies and their supermassive black holes are tightly coupled. One possible explanation for this connection is the merger of two or more near-equal-mass galaxies, a common outcome, particularly in the early Universe. These major galaxy mergers are thought to drive gas and dust into the central regions of a galaxy, fueling both star-formation and black hole growth. If emission from the supermassive black hole then shuts off both of these processes when the AGN reaches a certain luminosity (a process astronomers call ‘feedback’), the bulge and supermassive black hole cease to grow and end up with masses that are related to one another. However, there is increasing evidence that this scenario may only be important at high luminosities and/or at high redshifts (e.g., in the early Universe). This is where CANDELS come in. 

CANDELS

Not only were AGN most active when the Universe was only ~3 billion years old, but the major merger scenario proposed to explain the correlation between bulge and black hole mass may also be most relevant at this time. If light traveled infinitely fast, we would have no way of knowing what AGN or their hosts looked like ~11 billion years ago. Thankfully, however, light has a finite speed, so the more distant an object is, the longer its light has been traveling to us, and the younger it was when that light was emitted. Along the way, light also gets stretched by the expanding Universe, so that light that left a distant galaxy in the optical is shifted into the near-infrared by the time it reaches us. In practice, this means that the CANDELS deep near-infrared data gives us a snapshot of the optical light that was emitted by distant AGN and their host galaxies when the Universe was still quite young. For bright unobscured AGN, CANDELS therefore allows us to study accreting supermassive black holes in the early Universe. For fainter AGN and luminous obscured AGN, however, CANDELS is providing us with the first optical images of AGN hosts in the early Universe. What will we see?  Are AGN hosts undergoing major galaxy mergers, or are other processes driving the growth of supermassive black holes and their host galaxies? Our first study of the host galaxy properties of X-ray selected AGN in CANDELS is discussed here. Stay tuned for more results! 

Where and When do Galaxies Form their Stars?

One of the biggest questions in galaxy formation is how, where, and why galaxies form their stars. The CANDELS project is working towards a complete census of galaxies in five deep regions of the sky, probing significant numbers of galaxies from the first billion years of the Universe, all the way to the very dim galaxies in the nearby Universe. We are fundamentally interested not just in completing the census, but in developing theoretical models that explain how this diversity of galaxy properties arises.

Within the last 15-20 years, we have developed a very compelling cosmological framework, called "Lambda-CDM," which explains the contents of the Universe and its evolution on large scales. This model has been tested by a wide range of data including data from the cosmic microwave background, which provides a detailed picture of the tiny fluctuations in the early Universe that gave rise to all of the structure we see today; the evolution of the most massive objects in the Universe, galaxy clusters, which provide a way to follow how this structure grows with time; and stellar explosions called supernovae, which provide a way to probe the expansion history of the Universe. Within this framework, most of the mass of the Universe is not in stars at all, or even in the gas that fuels galaxies. Most of the mass is "dark matter", made of a particle which does not absorb or emit light.

Although we can't see dark matter directly, we can probe its effects on the light emitted by galaxies, and in fact can use galaxies to trace where and how dark matter gets put together over time.  In our modern picture of galaxy formation, every galaxy in the Universe forms at the center of a gravitationally bound clump of dark matter, called a dark matter halo. We can use large computer simulations to determine how dark matter halos get built up over time, providing a scaffolding on which galaxy formation, galaxy growth, and galaxy evolution take place.

Formation of a Massive Galaxy Cluster from Risa Wechsler on Vimeo.

The movie above shows this process, just for the dark matter in a region of the Universe that becomes a massive galaxy cluster today. The bright spots here are just dense clumps of dark matter, but in our Universe this is where we expect to form galaxies.  (If you'd like to learn more about our movies, check out this recent Science Bytes special "Dark Matters" on pbs.org!)

My group has been developing models of how galaxies build up on this dark matter scaffolding, and using observational data, from CANDELS and elsewhere, to infer the process of how galaxies grow and form stars within their dark matter halos. We have compiled a large amount of data from several observational surveys, and used this to say something about the average star formation histories of galaxies over time, as related to the growth of the dark matter clumps that they live in.  On average, the star formation rate in all galaxies peaked nearly 10 billion years ago, and has been declining since then. However, the rate at which star formation peaks in galaxies depends on their mass.  In a recent study, we found something very interesting. Although galaxies come with a range of shapes, sizes, masses, and star formation rates, we found that the efficiency of turning gas into stars in galaxies is nearly constant with time, and is peaked at a specific mass.  At all times over the last 10 billion years, stars form most efficiently in galaxies roughly the mass of our own galaxy the Milky Way.  This efficiency is also nearly constant over this whole time period. This means that nearly 2/3 of all stars ever formed in the Universe were formed in systems roughly the size of our own galaxy!

Halo mass as a function of time since the Big Bang. Colored bands
represent different star formation efficiencies; the yellow region indicates
the mass at which halos turn the largest fraction of their gas into stars.
Credit: Behroozi, Wechsler, and Conroy 2012

This basic picture helps us understand a number of galaxy properties. In the picture on the left, the yellow band is the region in halo mass where galaxies are able to form stars most efficiently. Small things (bottom white line), though very numerous, are not very efficient at forming stars. Today, they are generally blue, because they are still forming stars today (but slowly!). Big things, like the giant elliptical galaxies at the centers of clusters, formed a lot of stars early on (when they were roughly the mass of our own galaxy), but their halos continued to grow (top white line) and they stopped forming stars efficiently once they got more massive than the Milky Way. They are red because they haven't formed stars recently. Galaxies similar to our own actually spend a lot of their history in the region that is most efficient, and thus they form more stars relative to their total mass.

So far, this study compiled a range of data from various studies in the literature, and did not use data directly from CANDELS. The exciting thing about CANDELS is that we will be able to study galaxy star formation rates and galaxy masses for both very small and very large galaxies in the same way across more than 10 billion years of cosmic time. For the same galaxies, we will also have information about their environments, which are related to the mass of the dark matter halos that they live in, and their morphologies, which are likely related to the merger histories of the galaxies in those halos. This will allow us to build up a full picture of the diversity of galaxy growth, from the first billion years of the Universe until today.  Theorists will have a lot more work to do to understand these data, but we are looking forward to it!

Meet Christina Williams

My name is Christina Williams, and I am a grad student in Astronomy at the University of Massachusetts in Amherst. I study galaxy evolution with CANDELS data as part of my PhD thesis. I'm working with Mauro Giavalisco as my thesis adviser, studying compact, massive, elliptical galaxies and their evolution. How and why I arrived at an astronomy department for work is in some ways similar to other astronomers, and in some ways different. Like many, I had an early fascination with the world around me, how nature works, and in particular the night sky, which is what led me here! I grew up in Washington, DC, a city full of culture and people from all over the world, but not so much in the way of nature and dark skies. I was lucky to have extremely interesting and inspiring science teachers in elementary and middle school, who showed us all sorts of fascinating gadgets in the lab and taught us about things like volcanoes and tornadoes, which you don't find in Washington. But I was especially lucky to have science teachers and advisers throughout my life, who took a special interest in my ambition to learn science, and made sure to foster it. By high school, I was convinced Astronomy was my route in life, giving in to my hunger to learn more and more fundamental aspects of science. The universe, it seemed to me, was about as fundamental as it gets!

I went to college to study Physics, not too far from home at Johns Hopkins University (JHU) in Baltimore, MD, which had a very research oriented department of Physics and Astronomy. With its location across the street from Space Telescope Science Institute, it seemed like the perfect place to be introduced to the world of astronomical research. I immediately joined a research group focused on low-mass stars and brown dwarfs, with whom I worked for all my four years there. Since JHU is a part of the Sloan Digital Sky Survey, we had loads of telescope time at its host observatory, Apache Point, in New Mexico. I traveled there several times for observing runs and also observed remotely (through a computer) from Baltimore.

Aurora Borealis over my cabin in Fairbanks, AK.
Photo credit: Christina Williams

The summer before my senior year, I decided to try something new for one summer, and received an internship studying the polar ice caps of Mars at the Geophysical Institute at the University of Alaska in Fairbanks (UAF), as part of the Research Experience for Undergraduates program (REU; see this recent blog post by a CANDELS REU student). This was an important scientific, and life changing, experience. It was there that I first learned how to make physical models with a computer, which is what many astronomers spend much of their time doing. And it was then that I fell in love with Alaska, with its big mountains, glaciers, and vast wilderness to explore. Not ready to leave Alaska, but desiring still to continue with scientific research, I took a brief hiatus from astronomy and enrolled in a masters program in Geophysics at UAF. While there, I wrote a thesis on the formation of the Arctic Ocean and its tectonic history, (something totally new for me), which is a part of the Earth that is still very poorly understood because the sea ice in the Arctic makes studying the ocean floor difficult. Living year-round in Alaska offers many exotic experiences for an exploratory spirit. Like many other Fairbanksans, I lived in a cabin without running water. There were Northern Lights to photograph, hot springs to ski to in the winter, and remote and wild rivers to float in the summer.

Fieldwork on the sea ice near Barrow, AK

I finished my masters degree mid-(academic) year, so before returning to the world of Astronomy and starting my PhD the next fall, I accepted a job with the arctic sea ice research group at the Geophysical Institute at UAF. They hired me to work with the marine radars they had set up on the Arctic coast, as part of an ice observatory, which monitors the real-time motion and other changes in the coastal sea ice. I also went to Barrow, Alaska, for fieldwork out on the ice. To get out there, we rode snow-machines on landfast sea ice (sea ice which is grounded off the coast after winter), testing the electric conductivity and albedo of the ice, and taking core samples to learn about how the ice composition changes over time. One of many goals of this constant monitoring is to learn about how the ice patterns have been changing in recent decades. This is increasingly important in part because Inupiat Eskimo communities, who have relied on knowledge of seasonal landfast ice patterns for subsistence hunting for centuries, are now faced with the need to adapt to changes in the arctic.

ASTE site, in the Atacama Desert, Chile

The next fall I started my PhD in Astronomy at UMass. The astronomy program here is structured such that you have two different research projects before choosing a thesis topic, and in this way get exposed to a variety of sub-fields within astronomy. In my first few years here, I studied sub-millimeter galaxies (SMGs), and their clustering (see this recent blog post, and also this one). These are galaxies which are so dust-obscured, they are often only observable at long wavelengths. The dust blocks and absorbs the starlight which heats the dust, and the dust re-radiates in the far-infrared part of the spectrum. By the time the light reaches us, it is red-shifted to the sub-millimeter part of the spectrum. The wavelength we observed in is 1.1 mm, and is extremely sensitive to the amount of water vapor in the atmosphere. This means observations need to be taken from extremely dry regions of the Earth. We were using a Japanese telescope located in the Atacama Desert in Chile, called the Atacama Submillimeter Telescope Experiment (ASTE), which at the time was using a sub-millimeter detector called AzTEC (acronym translation available here) that was developed here at UMass. (This detector is now in Mexico being tested on the new Large Millimeter Telescope (LMT), which will be the biggest single dish sub-millimeter telescope on Earth!) To balance all the time I end up sitting in front of a computer, I went down to Chile for observing and general observatory maintenance at ASTE for a month!

Sub-millimeter astronomy is intriguing because very little is known about the galaxies that produce this kind of light because detectors and observatories in this wavelength regime are relatively new advances in technology. So there are many unanswered questions, which hopefully other new observatories such as the Atacama Large Millimeter Array (ALMA) in the Atacama desert and the LMT in Mexico will help us understand. Part of my goal in studying clustering of SMGs was to understand if they have an evolutionary connection to massive elliptical galaxies. But the unfortunate thing about current sub-millimeter observatories is that they have very low resolution imaging, which means you rarely get to see the shape and morphology of what you're looking at in detail. Perhaps that's one of the reasons that led me to Hubble Space Telescope (HST) and working with CANDELS data for the rest of my PhD thesis. I definitely love looking at the beautiful high-resolution images from HST that show the morphologies of galaxies!

Climbing Pigeon Spire, Bugaboo Provincial Park,
British Columbia

But, I might be a pretty boring scientist if science was all I was interested in. Getting the mind off work periodically is really important, not only because we are human, but also because it allows the brain some perspective for solving problems and can result in small epiphanies. What better way to gain a little different perspective than to climb hundreds of feet off the ground? Probably my biggest passion outside of science is climbing, which I started in college, and it has taken me to remote corners of the world. But even exploring the climbing here in New England has been a wonderful opportunity, because Amherst is centrally located between many world-class climbing areas (which are much more accessible than much of the climbing in Alaska!).

Where in the world I go from here is anyone's guess! I hope to graduate soon, and find a good postdoctoral position where i can continue exploring unanswered questions about high-redshift galaxies. Some serious hurdles currently face junior astronomers on the job market. The number of PhDs in astronomy exceeds the number of available permanent positions in astronomy, so competition for jobs is fierce and the prospects can be quite daunting. Astronomers typically do two postdoctoral positions before finding a permanent position. These are things I will face soon enough. But its important to keep in mind that often a big hurdle to success is lack of confidence in ones own abilities. Among all the science I've learned in graduate school, one of the most important lessons I've learned thus far is that believing in yourself is not just a cliche phrase, but has some serious truth in it. None of us arrived where we are by doubting ourselves. And this lesson I'll take with me no matter where I go.