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!".

The Theoretical Astrophysical Observatory

This post is not going to be about CANDELS directly, but about work that, in the long run, could play an enormous part in helping CANDELS astronomers analyse and interpret their data.

At Swinburne University in Australia, myself and my group are developing a new tool, called the Theory Astrophysical Observatory (TAO), which will make access to cutting edge supercomputer simulations of galaxy formation almost trivial. TAO will put the latest theory data in to the "cloud" for use by the international astronomy community, plus add a number of science enhancing eResearch tools. It is part of a larger project funded by the Australian Government called the All Sky Virtual Observatory (ASVO).

TAO boasts a clean and intuitive web interface. It avoids the need to know a database query language (like SQL) by providing a custom point-and-click web-form to select virtual galaxies and their properties, which auto-generates the query code in the background. Query results can then be funneled through additional "modules" and sent to a local supercomputer for further processing and manipulation. These include the ability to:

• Construct observer light-cones (i.e. with the geometry of the sky) from simulated data cubes (the default format of the models and which assume a cartesian geometry);
• Generate complete spectral energy distributions for model galaxies to provide true multi-wavelength galaxy luminosities;
• Produce custom mock images of the sky in each simulated universe;
• Add a virtual telescope simulator, through which the theory data can be "observed" by current or future telescopes.

TAO is already making it easy for the scientific community to apply the latest theoretical models. With an early TAO prototype, a group of Australian astronomers constructed a representation of the deep night sky that will be seen by the newly commissioned Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope, which is based in the outback of Western Australia.

To do this, the TAO prototype was used to build a light-cone of many hundreds of thousands of simulated galaxies. These galaxies were selected from the much larger millions contained in the TAO database. The selection was based on the neutral hydrogen properties of each galaxy, as predicted by one of my galaxy formation models (Croton et al. 2006). After fine tuning to match the sensitivity of the ASKAP telescope, the researchers were able to reproduce the ASKAP night sky even before a single photon in the radio part of the electromagnetic spectrum had ever been collected by the telescope.

The night sky, as seen by the ASKAP radio telescope, and generated by a new eResearch tool, the Theoretical Astrophysical Observatory (TAO). Similar representations of the CANDELS fields are currently under development.

In the visualisation shown above one can see the light-cone of galaxies produced by TAO. Two surveys were constructed from the simulated data. The first replicates the shallower and wider WALLABY galaxy survey and is expected to find approximately 600,000 galaxies. The second is the much deeper and  narrower DINGO galaxy survey, and will find approximately 100,000 galaxies according to TAO. More information on these surveys can be found at the ASKAP science page.

The work has recently been published in the prestigious UK journal The Monthly Notices of the Royal Astronomical Society. Don't forget to check out the amazing movie here!

These tools are being applied to the CANDELS survey to do similarly exciting science. Watch this space!

The Trials and Tribulations of Finding the Most Distant Galaxy Clusters

OK, now for something a little different. In this post I'm not going to talk about stuff we already know or get right, but the stuff we think we may be getting wrong. That may seem a little negative, but is in fact what astronomers spend much of their time doing. We constantly check, question, argue, ponder … it's only when we've run out of ways to challenge an idea or result that we then tend to think we may have just learnt something.

We've heard before about galaxy environment, and more specifically, the relative importance of galactic nature vs. nurture. Measuring environment (or nurture) has become increasingly popular in recent years, mainly because it's become increasingly possible to do.

In particular, large space-bound telescopes, like Hubble, and the galaxy surveys they produce, like CANDELS, have been especially important in the search for, and classification of, rare yet impressive cosmic structures called galaxy clusters. This has sparked a kind of friendly competition within the community to find increasingly distant clusters in the early Universe. For astronomers, it's a bit like searching for the earliest fossilised pre-humans: a trophy when you can beat the previous record, and (more importantly) juicy new clues to early galaxy life within the cosmos.

An early forming galaxy cluster, but is this a real over-density or
artificial? Credit: Papovich et al. 2010

A perfect example is the 2008 discovery of a early forming cluster at redshift 1.6 (about 9 billion light years distant) by CANDELS team member Casey Papovich and collaborators. This cluster was first identified using the Spitzer Space Telescope in the infrared, and the galaxies within were recently studied using CANDELS data to measure their structural properties, types, masses and colours. However, to date, only 13 galaxies in the cluster have been confirmed to be at the distance of the cluster centre to any precision.

This highlights a problem: when clusters become distant and hard to find, how do we know if we've actually found one? Let me propose that you have to answer three challenges before you get to pop the champagne cork on a new cluster distance record:

1. Demonstrate that the galaxies making up the cluster aren't just a chance alignment of unassociated galaxies along the line-of-sight. When you only have very rough distances, which is often the case (called photometric redshifts in astronomy-speak), such alignments can be a very common occurrence, and tricky to recognise.
2. Show that the "reality" of the cluster doesn't depend on the method used to find it. There are many environment "metrics" employed by astronomers, typically based on counting galaxies projected on the sky within a fixed radius, or by an association of nearest neighbours. But the measurement techniques are varied, the uncertainties large, and the agreement between them approximate at best.
3. Even when you've found a genuine cluster, how do you actually know what it will evolve in to? Simulations have shown that some structures grow fast and some slow, and there's no way to know in advance. It may be that the future for this particular object is rather boring and not the "Australopithecus" you were expecting.

At Swinburne, my graduate student Genevieve Shattow and I have been looking into these issues using CANDELS-based simulations of galaxy and cluster formation. These simulations are useful because we have "perfect" information for every modelled galaxy, unlike the real observations. By perfect I mean that every property of each dark matter halo and galaxy are precisely described (because we made them!), and the history of each is fully identified. We can use this information to our advantage.

With this simulation we looked at the most massive cluster at redshift 2 (i.e. when the Universe was 1/3 its present age, and where the current distance record is) and asked:

• How accurately can we identify the cluster if we degrade our information to be comparable to that of real observations?
• When we apply different environment measurement techniques, do they paint a consistent picture? Or do they return different answers?
• What will this object evolve into? Is it actually interesting? Is there a way to separate the interesting objects in the distant Universe from the more mundane ones without knowing in advance?

We're still working on these problems, but let me show you a few early, yet slightly disturbing results.

The above figure shows how galaxies in the larger environment around this simulated cluster appear on the sky if observed at four different times, ranging from the present day (far left) to redshift 2 (far right). We've coloured the galaxies: purple shows the actual gravitationally bound cluster members, while black are the non-cluster galaxies. The circle represents the typical radius astronomers would count within to look for over-densities, signalling the presence of a cluster.

As you can see, finding a cluster in the present-day Universe is fairly easy as there are many galaxies around to characterise it. In the early Universe though it's not so clear. Even worse, the ratio of cluster to non-cluster galaxies tips in an uncomfortable direction, making it hard to distinguish the actual cluster members from the background of field galaxies.

Different viewing angles of the same
cluster return different cluster
membership due to projection effects

We can play an even more illuminating game with the simulation. Let's again look at this single cluster and its surroundings at redshift 2, again projected onto the sky, but now from many different angles. Since galaxies in three dimensions will project differently on to the two dimensional sky depending on the viewing angle, the measured environmental over-density within the circle will vary. But by how much?

The distribution of counted galaxies based solely on changing the viewing angle is shown in the figure on the right. As can be seen, the environment measure may find 60 cluster members, or it may find 80 cluster members. That's a big difference for what is in essence exactly the same object! Given that we can't correct for such projection effects in real observations, this highlights a substantial uncertainty in any such measurement.

This is just the beginning of our work, but important to quantify if we're to have an honest accounting of the issues around measuring galaxy environment in real data. In doing this we'll be looking for ways to correct such biases, hopefully making the job of cluster identification easier and more accurate. This will enhance the value of data from galaxy surveys such as CANDELS and the science that follows from them.