The VIMOS UltraDeep Survey – a spectroscopic survey of high redshift galaxies

The VIMOS UltraDeep Survey (short: VUDS) is an observational program to gain spectroscopic measurements for ~10,000 galaxies at high redshift, when the Universe was only between 1- 3 billion years old (today, the Universe is 13.8 billion years old). This is a particularly interesting era to study in terms of galaxy evolution since astronomers expect galaxies at that epoch to look very different from today. For example, at that early time we observe that galaxies have a much more disturbed morphology compared to the beautiful structured spiral galaxies or smooth elliptical galaxies that we see in the local Universe. We expect that galaxies formed many more stars at that time partly triggered by disturbances from the merging of galaxies but also because more gas was still available to form stars in those galaxies. The time between redshift 2 to 6 (i.e. the first 1-3 billion years of the Universe’s age) is thus a major epoch of galaxy assembly.

Figure 1: Very Large Telescope in Chile, photo credit: R. Thomas.

With CANDELS, galaxies in that epoch are studied mostly based on photometry, meaning images taken at different wavelengths. We described in earlier blog posts how with photometry at many different wavelengths astronomers are able to study the properties of galaxies through comparing the observed data to model galaxy spectra.

With VUDS galaxy evolution is approached from the spectroscopic side. A spectrum of an object is created by dispersing all its emitted light by directing it through a disperser like a prism, meaning the light is split up according to its wavelength. An easy example is the creation of a rainbow where the light from the sun hits raindrops in the air which act as dispersers and split the originally "white" sunlight up by wavelength, creating the typical coloured stripes. Such spectra allow us to study the properties of galaxies in much more detail compared to the study of images alone. 

The VUDS survey covers about 1 square degree in the sky. As a comparison the diameter of the full moon is about 0.5 degrees and its area is ~0.2 square degrees, which means it’s a fifth of the area covered by the VUDS survey. However, this 1 square degree of area of the VUDS survey is split over 3 separate fields in the sky that have been observed with a lot of different instruments and at many wavelengths already, creating a unique and precious data set for astronomers to carry out their studies. The three fields are the COSMOS field (which overlaps with the CANDELS-COSMOS field), the Extended-Chandra Deep Field South (which overlaps with the CANDELS-GOODS-South field) and the VVDS-2h field. Within those 3 fields spectra of ~10,000 galaxies were taken with the VIMOS multi-object spectrograph at the Very Large Telescope (VLT) in Chile (Figure 1). We described how multi-object spectroscopy works in more detail in this recent post. In short, suffice it to say that with that instrument, astronomers are able to take a spectrum of many galaxies at the same time. VUDS is the largest spectroscopic survey of galaxies at these early cosmic times.

Two of the 3 fields covered by VUDS overlap with the CANDELS area. The spectra and spectroscopic redshifts in that overlap area (~ 700 galaxies) were just publicly released by the VUDS team.

Figure 2: Stacked spectrum of galaxies between redshift 3 to 4 with the most reliable spectroscopic redshifts in VUDS. Vertical dashed lines indicate known spectral lines which are used to determine spectroscopic redshift and galaxy properties.

For the VUDS survey, the objects which were targeted for the spectroscopy, were selected primarily based on their redshift as derived purely from photometry (again, see this blog post here). Additionally, some sources were added based on their photometric colours (i.e. the difference in brightness between two wavelength bands) which indicate a high redshift. These objects were then observed with two different grisms -- one for the blue wavelength end and one for the red wavelength end – for about 14 hours each. The resulting spectra cover a wavelength range from the blue optical to the very red optical. This means that for these high-redshift galaxies, we really observed their ultra-violet to blue optical wavelength range which are shifted due to the redshift into the optical wavelength range covered by the VIMOS instrument. This wavelength range reveals many properties of galaxies, especially with regard to their star formation. In Figure 2 we show you a stacked spectrum of some VUDS sources in which also the spectral lines are indicated. In Figure 3 you can see all the spectra of the VUDS survey compiled into a picture and sorted by redshift, where each line represents one spectrum. Emission and absorption lines in this image are nicely visible in this as bright and dark lines that stretch across the image from left bottom to top right. This also illustrates how spectral features are redshifted towards redder wavelengths. The most common spectral lines and features in these spectra are the Hydrogen Lyman-alpha, Lyman-beta and Lyman-gamma lines, the Lyman limit (below which almost all emission is absorbed by neutral Hydrogen around newly formed stars), the Carbon lines (CII, CIII and CIV, where the Roman numbers behind the letters indicate the ionization level of the element) and lines from Helium (He), Oxygen (O), Silicon (Si) and Aluminium (Al). These lines are used not only to determine the spectroscopic redshift of these galaxies (i.e., through their known rest-frame wavelength), but also other galaxy properties such as star formation and chemical composition of the galaxies. Overall in VUDS we were able to determine reliable spectroscopic redshifts for ~6000 galaxies which cover a large range of brightnesses and stellar masses. Some of the galaxies in this survey form up to 1000 solar masses per year!

Figure 3: Compilation of each spectrum taken in the VIMOS UltraDeep Survey and sorted by redshift. Redshift increases from the bottom to the top, meaning the further up in the image we go, the further into the past we look and the younger the Universe is. Marked are spectral emission (bright spots in the spectrum) and absorption lines (faint spots in the spectrum) at each redshift. This figure illustrates nicely how certain spectral features seem to be present in galaxies in this survey at the various redshifts and thus across cosmic time. Figure from Le Fevre et al. 2015, A&A 576, A79

Since the completion of the observations, many researchers in the international VUDS team work on all aspects of galaxy formation and evolution, from morphology to identifying proto-galaxy-clusters and groups, from studying the ultra-violet spectroscopic properties of very young galaxies to the merging history of the Universe, in alignment with the science goals of the overall survey. If you are interested in following the results from the VUDS survey, you can find our Facebook page here and our Twitter account here.

Preparing Multi Object Spectroscopy Observations

Although CANDELS is a photometric survey, many team members have proposed for and been granted observing time for CANDELS sources to obtain spectroscopy. Such additional data not only provides us with a more accurate measurement of the distances of galaxies (aka redshift), but also with additional information to decode their properties, such as how many stars they are forming and how much dust is contained in the galaxies.

Figure 1: Example pointing for a MOS observation with the GMOS
instrument at the Gemini Telescope. The image in the background shows
the targeted sky area. The cyan outline shows the field of view of the
instrument with the gaps between the 3 CCD detectors. The dashed outlined
box shows the sky area in which the guide star needs to be placed. The red
"arm" shows the arm that holds the camera that monitors the guide star.

Classically, spectroscopy was carried out object by object, by placing one long slit where your one object is located. With this you restrict the area which lets light through to the detector to a narrow slit and blocking out everything else around it. The light that enters the prism or grism through this slit is then dispersed according to its wavelength, creating a spectrum of the object. Bright spots highlight the presence of elements that emit at this frequency/wavelength, and dark spots tell us where certain elements absorbed light and stopped it from reaching us. You can imagine though that carrying out such observations object by object is very time consuming.

In the last decades though, astronomical studies for galaxy evolution started to greatly profit from new instrumentation which allows us to observe many objects at the same time. This is not only true for taking images of the sky, but also for spectroscopic observations.

One method to take spectroscopy of many objects at the same time is grism spectroscopy, which we showed you in our post about grism spectroscopy with the Hubble Space Telescope. In that case nothing in your field of view is masked out and everything is dispersed. If your field of view is very crowded, meaning you have many many objects in your piece of sky, many spectra will overlap and will be hard to disentangle.

Figure 2: I-band image of the piece of sky to be observed with Multi Object
Spectroscopy within the mask-making software. The red outline shows the
field of view of the instrument, the blue stripes mark the gaps between the
detectors. All potential target objects are marked with different smaller
symbols according to their priority (blue triangles, green boxes, white circles
and cyan diamonds for alignment stars).

Another method is multi-object spectroscopy (MOS) via slit-masks. With this method you can take spectra for many objects at the same time by placing slits on many objects and blocking out the rest of the sky. This requires the creation of so-called MOS-masks in which the slit areas and the blocked out areas are clearly defined. This means that for every different observation you need a custom mask. Most current instruments require these masks to be prepared well in advance of the observation and to be cut out of plastic. This process isn't feasible for a space telescope, but works very well on the ground. However, times are changing. For example, for the MOSFIRE (Multi-Object Spectrometer for InfraRed Exploration) instrument at the Keck Telescope, the masks are created on the fly and "bars" that create slits are then moved into the right position within the instrument. Also for the upcoming James Webb Space Telescope a MOS unit will be available. It is designed in such a way that little shutters open and close to produce slits and masked out areas. For many other instruments however, a mask is essentially one large piece of plastic that has lots of tiny slits cut out of it. The slits are placed exactly where you want to observe an object. To create such a mask is in principle relatively simple and I illustrate the process here with a series of images.

I recently created some MOS masks for the Gemini Multi Object Spectrograph (GMOS) instrument at the Gemini Telescope to observe CANDELS galaxies and will use one of the masks I created as an example here to illustrate the process. Firstly, an image of the desired piece of sky in which the positions of the objects you want to observe are measured (Figure 2) and a list of objects, i.e. a catalogue, are required. From that list we  picked our desired targets. Often these are selected based on specific properties and limited by their brightness to ensure the maximum success with the granted observation time. Then we also need a list of stars to guide the telescope and to align the mask properly. Guide stars are used to correct for the rotation of the Earth throughout the observation so that the telescope is pointing at the same portion of the sky the entire time. You can see an example pointing in the first figure.

Figure 3: Zoom in to show the placement of slits on some targets. Objects with blue triangles have highest priority, next are objects with green boxes, and then those with white circles. The yellow vertical stripes overlaid on an object show where the slit will be placed and cut out of the mask. The horizontal white lines mark the extension of the dispersed light, i.e. the spectrum of the object. Basically, all the light that hits the disperser when it comes through the vertically extended slit, is dispersed in the horizontal direction.

Alignment stars are included on the mask to make sure all the slits are on the selected objects and not on some other piece of empty sky when the telescope operators define the pointing of the telescope. Then we take this image and list of targets and run them through the provided software for the given instrument.  Usually, the original list of targets leaves room for other objects to be placed on the mask as well, so we basically work with a prioritized list of objects. The highest priority objects are "forced" onto the mask into the space left after placements of the alignment stars to observe as many as possible of the desired targets. Then any available gaps are filled with objects of lower priority. In Figures 3 and 4 you can see all the slits that were placed on this particular mask and a zoom in that shows you a slit.

Figure 4: The finished mask. The red outline is the field of view of the instrument, the blue vertical lines mark the gaps in the detector. Each rectangle box shows where the spectrum of that object will extend. Yellow vertical lines mark the position of the slit on the selected object. The cyan rectangle boxes mark the position of the alignment stars.

After this, the observer can manually remove objects that received a slit if he/she wants the software to pick out a different object for example, one that might be more optimally placed. Then there are usually a few iterations in which the slit placement is refined a bit more and the maximum amount of objects are placed on the mask. And that's it, the mask is finished. All that is left to do is create all the masks for all the pointings in the same manner and then sending them off to the telescope and instrument support team for checking and approval. Once a mask is approved, all the necessary information is send to the mask cutting team who cut the mask, meaning all the tiny slits are cut out. After masks are cut, they will be installed in the instrument and then it's anxious waiting for us for the completion of your observations if they are carried out by the support astronomers at the observatory (Figure 5) or hoping for good weather if we go to the telescope ourselves to carry out the observations. 

The CANDELS fields are currently targeted by astronomers all over the world with many observational programs on instruments such as DEIMOS (on the Keck Telescope), MOSFIRE (on the Keck Telescope), GMOS (on the Gemini Telescopes, described in this post) and VIMOS (at the VLT, for example with the VIMOS UltraDeep Survey). 

Figure 5: Example observation from one of the GMOS masks. Each horizontal package of lines is the dispersed light from one slit. The bright vertical lines (a few are highlighted by the violet arrows) are emission lines caused by the night sky, meaning elements in our atmosphere emit light at certain wavelengths which are also detected and then overlap with the spectrum of the target object. The spectral traces of the target objects are highlighted by red arrows and are faint horizontal lines. In the red box, we can clearly see 2 bright dots, these are emission lines in the target object which we can use to determine its redshift and other properties. The green arrows point towards high energy cosmic rays that hit the detector and cause a detection. In order to retrieve the spectra for the target objects, astronomers have to remove the cosmic rays and subtract the spectrum of the night sky, so that ideally only the spectra of the real targets are left in the end.

Using Our Largest Set of "Eyes"

While our data from the CANDELS program is nothing short of revolutionary, occasionally we need to step back and remember that Hubble is "only" a 2.4 meter telescope (meaning that the diameter of the primary mirror, which sets the telescopes light-gathering power, is 2.4 meters). Since Hubble is in space, above the atmosphere, its still able to do great work. However, some tasks are best left to larger, ground-based telescopes. One of these tasks is to followup some of the CANDELS sources with spectroscopic observations.

Spectroscopy spreads the light from a source out into its component wavelengths; much like placing a prism in a beam of sunlight. Through this technique, we can study a variety of properties about very distant galaxies, including their distance. In previous blog posts, we talked about how we can find very distant galaxies using their colors (we call this "photometric redshifts"). However, this technique provides only an approximate distance, and there's always a (small) chance that an object identified as being distant is really not quite so far (we call these "catastrophic outliers"). To ensure that our color-based distances are accurate, its always a good idea to measure exact distances for some subset of your distant galaxy sample.

Because these galaxies are so distant, we can just barely see them in our Hubble images. If we tried to use Hubble to do spectroscopy, splitting the light up into its colors, we would no longer be able to see them; essentially, it would be like taking all the light from the CANDELS image, and splitting it up into multiple images. Since we can just barely see it when all the light is added together, we would not be able to see it if we split the light with a spectrograph. So, to spectroscopically study the distant universe, I obtained time with the largest optical telescope on the ground, the Keck 10 meter telescope.

Most of the time on this telescope goes to its two major partners, CalTech, and the University of California.  I'm at the University of Texas at Austin, and we don't have our own access. However, NASA typically has 10-20 heavily contested nights to get away, and we wrote a proposal, and managed to get two nights with Keck in April.  We used these nights to observe distant galaxies with a brand new spectrograph, called MOSFIRE. This instrument is revolutionary, its it is highly sensitive, can observe many objects at once, and it observes in the near-infrared, all of which are crucial to observe our very faint, very distant galaxies.

Keck is one of my favorite observatories to go to, since although the telescope is located at 13,500 feet on top of Mauna Kea, we observe from the town of Waimea, at an elevation of 1000 feet, and only 15 minutes from the beach (and about 100 feet from Starbucks!). Joining me on the run was Mimi Song, a graduate student working with me at UT Austin, and Vithal Tilvi, a postdoctoral research from Texas A&M. Over our two nights, we obtained data on 43 very distant galaxies. I will write another post soon on the results from our run, so stay tuned!

Myself, Tilvi and Mimi in the Keck control room.

Life as an Observer

The Keck Telescopes on Mauna Kea
Courtesy W. M. Keck Observatory

One of the best aspects of being an astronomer is actually using a telescope and collecting data. Tonight, I have the opportunity to use one of the best telescopes in the world along with an exciting new instrument. Fellow CANDELS team member Mark Dickinson and I are observing on one of the Keck telescopes atop Mauna Kea on the Big Island of Hawaii. Mauna Kea is one of the best observing sites on the planet because the peak of the mountain where the telescopes are located sits above a substantial portion of the atmosphere. Because of this, the atmosphere has less of an effect on the images of astronomical objects than it would from an observatory at sea level. While the Keck telescopes themselves are at the summit of the 14,000 foot mountain, observers work from a remote observing facility located in Waimea. Being at this lower altitude makes it much easier to work and all of the instruments can be controlled remotely. 

The instrument that we are using is called MOSFIRE (Multi-Object Spectrometer for InfraRed Exploration) and is very new. It is a top of the line instrument that allows us to obtain sensitive, high resolution near-infrared spectroscopy of many objects at the same time. Most spectrographs in the near-infrared observe one object at a time. There are now several available that can observe many objects at once but they are often difficult to use for very faint galaxies. MOSFIRE is still brand new but so far has been working well. CANDELS team member Jonathan Trump recently published a paper on some first results! 

Mark Dickinson and I observing in the Keck I Remote Observing
Control Room

We spent the last several days carefully selecting targets to observe in two of the CANDELS fields, GOODS-S and COSMOS. Our primary targets of interest are distant luminous infrared galaxies detected by the Herschel Space Observatory at z~2. At this redshift, many of the interesting optical emission lines (such as the Hydrogen line known as H-alpha) are shifted into the near infrared. We can use these various lines to measure precisely how far away the galaxies are, whether or not an AGN might be present, how important that AGN is to the energy output of the galaxy, how much star formation is taking place in the galaxy, as well as many other things.

In addition to these targets, we are also observing AGN selected in other ways, such as through X-ray detections or from the shape of their SED in the near-infrared. If there is any space left we are also looking at other types of galaxies at these high redshifts to see if we can detect lines. 

Once our target selection was complete, we waited anxiously to see what the weather would do. The forecast called for clear skies but starting last night the summit of Mauna Kea became foggy. We awoke this morning (morning for an observing astronomer is really about 2 PM) to find that the fog had not cleared and some clouds had rolled in. We were really starting to get nervous! However, we proceeded as planned and began our afternoon setup. Around sunset, things were really looking dicey and it started to snow! Luckily for us that didn't last too long and the road to the summit remained clear for the night crew to head up and check things out. After a couple of hours, the fog cleared and the humidity dropped to a level low enough to open. We had a good time learning how to operate this new instrument and at 9:30 PM we started our first exposure on a set of galaxies in GOODS-S!

 

Video from a webcam at CFHT (the Canada-France-Hawaii Telescope) on Mauna Kea showing the clouds roll in Wednesday night and ice starting to form on the camera itself. Video courtesy of CFHT Observatory

Over the course of the night clouds have come and gone but the weather has steadily been improving. Right now we are observing galaxies in COSMOS and things are looking good. We are anxious to analyze our data and see how many of our galaxies have been detected. With any luck, we will soon be writing a blog post about our results! 

This is our last blog post before the holidays but we look forward to discussing more CANDELS science in January. Happy Holidays! 

Follow the Keck Observatory on Twitter!  

How Far Away is this Galaxy?

In previous posts you have read a lot about redshift and the distance of galaxies from us. You have read how to find and select very distant galaxies and how to distinguish between different types of galaxies from their spectral energy distributions. What you haven’t read in more detail is how astronomers determine a galaxy’s distance or redshift. I’ll tell you a bit more about this now.

The electromagnetic spectrum: wavelengths get smaller from the radio to gamma rays.
A wavelength is here the distance between two peaks of the red wave. Credit: NASA

First of all, let’s recap again what redshift means. I guess all of you have heard an ambulance with its siren switched on passing by. You might have noticed that the pitch of the siren changes as the ambulance moves. In particular it sounds higher when it approaches you and deeper when it drives away from you. (The same is true for the sound of passing by race cars which make the popular "brrrmmmmm" sound kids love so much.) This effect is called the Doppler Effect. While the ambulance moves, the sound waves coming from the siren are squeezed when it drives in our direction and stretched when it moves away from us, resulting in a different sound.

The same happens to the light of distant galaxies. The Universe is expanding and so distant galaxies are moving away from us. The light emitted by these galaxies is stretched out on its way to us. If the galaxies were moving towards us, their light would be squished. Because light can be described as a wave just like sound, this stretch/squish means that its wavelength is changing. Blue light has a shorter wavelength than red light, so light that is blue when it is emitted turns redder on its way to us when the galaxy is moving away, hence we say it is redshifted. The light from distant galaxies takes a long time to reach us. The further away the galaxy is the longer it takes, because light travels with a particular speed and nothing is quicker than light. So if we know the redshift of a galaxy, we know its distance to us and with this we know how long it took the light we see today to get to us. Hence, we see distant galaxies not as they are right now (this light is just emitted and hasn't reached us yet), but as they were a long time ago. With a particular redshift astronomers can assign a so-called look-back time which is the time the light of a galaxy at this redshift took to reach us. For example, the light of the sun takes 8 minutes to reach Earth, but it takes about 5 hours to reach Pluto. The light of distant galaxies needs billions of years to reach us which means they existed when the Universe was very young.

   redshift                  look-back time                   age of the Universe

   0                                         0                                      13.7 billion years

   0.0043                      59 million years                         13.6 billion years

   0.01                        140 million years                         13.5 billion years

   0.1                           1.3 billion years                          12.4 billion years

   0.5                              5 billion years                            8.6 billion years

   1.0                           7.7 billion years                            5.9 billion years

   2.0                         10.3 billion years                            3.3 billion years

   3.0                          11.5 billion years                           2.2 billion years

   4.0                          12.1 billion years                           1.6 billion years

   5.0                          12.5 billion years                           1.2 billion years

   7.0                          12.9 billion years                          800 million years

   10                           13.2 billion years                          480 million years

I've listed the look-back times and ages of the Universe for some redshifts above (for the very cosmologically interested: using a Hubble constant of 71 km/s/Mpc and Omega Matter, describing the density of matter in the Universe, of 0.27 assuming a flat Universe, but you can play yourself with these values over here). Redshift 0.0043 is the redshift of the closest large galaxy cluster to us. Up until redshift 0.5 the galaxy population is pretty well mapped out and studied by surveys such as SDSS. The range between redshift 1 and 3 is interesting to researchers to study how galaxies assemble their mass as we told you in this previous post. Redshift 7 is close to the redshift of the most distant galaxy known to date. With CANDELS we hope to detect many galaxies at this redshift and even further away. We hope to discover galaxies at redshift 10 someday, which were perhaps the first galaxies to form in the early Universe.

Redshift Determination

Example of emission lines in a galaxy spectrum. Credit: Jeyhan Kartaltepe

There are two ways to determine the redshift or distance of a galaxy, either from the spectrum of that galaxy or from its spectral energy distribution (SED) and colours. If astronomers have spectra available, they look for particular features in each spectrum, such as emission lines (those spikes in the figure to the right) or absorption lines. Emission and absorption lines are caused by elements in the galaxy that emit or absorb the light of that galaxy’s stars at particular wavelengths. From lab experiments we know at which wavelength the lines of various elements should be when not moving, we call this the rest-frame wavelength. We also know what their separation in a spectrum is. In order to determine the redshift of a distant galaxy one can look for the spectral lines caused by elements in that galaxy. Since the elements are located within that distant galaxy, the wavelengths of the spectral lines are redshifted by the same amount as all other light coming from the galaxy. We say these observed spectral lines are in observed-frame. The difference between the rest-frame and observed-frame wavelength of a particular spectral line lets you calculate the redshift of the galaxy. The difficulty with getting redshifts from spectra lies in the identification of the spectral lines, you need to be sure that the line you are seeing comes from the element that you think it is. It is easier if spectra have several lines that can be used to determine the redshift because then you can identify the pattern of lines, and know without a doubt what the observed lines are. A redshift determined from a spectrum is called 'spectroscopic redshift'.

SEDs of typical galaxies at different redshifts. The solid curves show the SED in
the rest-frame or at z=0, meaning how it looks when the galaxy is very close to us. The
dotted and dashed lines show the same SED at redshifts z=1 and z=2, meaning the
light from such galaxies traveled about 8 billion and about 10 billion years to reach us!!
The bottom panel shows various filter bands from U (blue curve, visibly blue light) to
infrared bands (black curves), the filter bands are the same ones we showed you in
the SED post. Galaxy templates are taken from Polletta et al.

Taking spectra of many galaxies is very time-consuming and requires a lot of telescope time, which can be difficult to get. Instead, with imaging, you can observe thousands of galaxies at once and construct an SED by using multiple filters. In this case you can still determine the redshift of a galaxy by using SED-fitting. Astronomers take advantage of the fact that the SEDs of galaxies have particular shapes and features at specific wavelengths. For example the Lyman-break that is used to find distant galaxies and about which Russell told you in his post is always located at the same rest-frame wavelength and shifts to redder wavelengths the further away the galaxy is. Another important feature is the so-called 4000 Angstrom break. The 4000 Angstrom break (at 0.4 micron in the figure to the left) occurs because many heavy elements in the atmospheres of old stars in the galaxy absorb some of the starlight around this particular wavelength. The brightness of a galaxy changes noticeably from the left to the right of this break, so in one filter band the galaxy will be brighter and in the other one it will be fainter. A comparison with model galaxies and the location of these jumps in brightness allows us to estimate the redshift of the galaxy. The better the shape of the SED and features such as the mentioned spectral breaks are sampled with the images, the better we can determine the redshift. A redshift determined from galaxy images where no spectrum is available is called a 'photometric redshift'.

For large galaxy surveys, the latter method is used to measure redshifts for many objects at the same time. Although the uncertainties for photometric redshifts are larger than those for spectroscopic redshifts and photometric redshifts can be wrong for single objects, researchers gain better statistics with large sample sizes. Furthermore, the redshift distribution of that sample will be correct on average. While CANDELS itself is focused on imaging data, many spectroscopic surveys have been undertaken in these deep fields and new surveys are always ongoing to obtain more spectra. A lot of CANDELS science you will read about here will make use of photometric redshifts.

CANDELS Spectroscopy: The Infrared Grism

Many people think of Hubble as a telescope that primarily takes striking high-resolution images; and most astronomers think of the CANDELS project as a deep imaging survey. However, Hubble also has a strong capability for spectroscopy, dispersing the light from astronomical objects as a function of wavelength to measure features in the spectrum, and the CANDELS survey itself also has a spectroscopic component. Hubble's main spectroscopic capability is at ultraviolet (UV) wavelengths, unique because ground based telescopes cannot see UV, as it is blocked by the atmosphere. But CANDELS spectroscopy, like the backbone of our imaging, is in the near-infrared, and that's what this post is about.

CANDELS imaging covers contiguous areas that are relatively large for HST, tiling many fields together, but the spectroscopy is in smaller fields, one at a time. Why? The primary goal of the spectroscopic program is to follow up high redshift supernovae, an effort led by Adam Riess and Steve Rodney. CANDELS discovers supernovae by comparing images taken about two months apart, looking for the appearance of new point sources, caused by exploding stars in distant galaxies. Some of these sources are at high redshift, and some have colors consistent with Type Ia supernovae, caused by the explosion of an overloaded, collapsing white dwarf. These are very useful as a measure of distance, one of the cornerstones of the evidence for the accelerating universe and dark energy. 

But the supernovae and their host galaxies are very faint and difficult to follow up with ground based spectroscopy. We take grism spectra to attempt to measure the redshifts of the host galaxies, and to get spectra of the supernova, where in the best cases we can measure spectral features to determine if they are really Type Ia, exploding white dwarfs, rather than exploding massive stars. As a side result, we get spectra with the grism for most other galaxies in the Hubble field of view, and that is scientifically very useful in its own right. (Although CANDELS spectroscopy follows up supernovae one pointing at a time with relatively long exposures, there are separate Hubble grism programs that cover most of the CANDELS fields with shorter exposures, one led by me - AGHAST, and one led by Pieter van Dokkum at Yale University - 3D-HST).

How a grism works: a combination of a diffraction grating to spread out the wavelengths of light, and a prism to direct the rays back into the rest of the camera so we can see them. Image Credit: Benjamin Weiner

What is a grism, anyway, and how do we get spectra for a few hundred objects at a time? "Grism" is a word used mostly by astronomers, and it simply means a combination of a diffraction grating and a prism. Both are used to disperse light, bending it as a function of wavelength, like a prism you may have used to project sunlight into a rainbow on the wall. The grism is inserted into Hubble's Wide Field Camera 3 (WFC3) - Infrared instrument, where ordinarily a filter would go to make images in a broad range of infrared wavelengths. The grating is a series of parallel ridges deposited on the glass surface of the prism; the grating disperses the light at an angle that spreads out the wavelengths, and the prism bends that bundle of rays back so it goes straight into the rest of the camera, making an image on the detector. The image of a galaxy through a normal filter looks like a galaxy, but the image made through the grism is a spread-out spectrum, just like that rainbow on your wall is a spread-out spectrum of the Sun.

Because the grism is in the light path just like a filter, the image made with a Hubble WFC3 grism has a dispersed spectrum for everything in the field of view. That's both a strength and a weakness. It means that one exposure gets spectra for many objects, but also that the spectra sometimes overlap, and that the light of the night sky also falls on all of the detector. This is why the infrared grisms on Hubble are uniquely efficient. From the ground, the atmosphere emits bright infrared radiation that would clobber any signal from galaxies and stars, and we have to get around this by using entrance slits in spectrographs to block most of the skylight. In orbit, the infrared background that Hubble sees is much lower (it's mostly from the zodiacal dust cloud), which allows Hubble to do "slitless" grism observations, getting spectra for many objects at once without the complexity of mechanically adjustable slits.

A section of a CANDELS grism observation, with the regular imaging on the left and the grism spectra on the right. The grayscale is reversed, so bright galaxies appear dark. For each galaxy on the left that is bright enough, there is a spectrum on the right. Some show bright spots, which are emission lines. (For the experts, many of the bright spots are true emission lines, but a few are undispersed images of very bright objects.) Image Credit: Benjamin Weiner

The spectra of galaxies and stars are valuable for doing astrophysics, because they reveal features at discrete wavelengths, called absorption and emission lines. These are features from stars and from hot gas heated by young stars - the emission from hot gas comes out at discrete wavelengths, like the light from the gas in a neon sign. These can be used to measure a galaxy's redshift, or how far away it is, and the strengths of the lines tell us about the physical conditions in the stars and gas, from which we infer many facts about distant galaxies. In the 1860s, Gustav Kirchhoff and Robert Bunsen used absorption lines found in the spectra of the Sun (and later in other stars) to show that the Sun and stars are made of the same elements that exist on Earth; this was essentially the birth of astrophysics, and it still boggles my mind that we have only known this for 150 years.

With the grism, instead of a field of galaxy images, we see a field of galaxy spectra, running from blue to red, or for CANDELS, from infrared to more infrared.  This means we get many spectra at once, although sometimes they overlap. When a galaxy has strong emission at a single wavelength, we see a bright spot in the spectrum. Image Credit: Benjamin Weiner

In grism data, the spectrum of a galaxy looks like a streak because the light is spread out in wavelength. An emission line shows up as a bright dot on that streak, a lot of light at a single wavelength. By identifying these lines, measuring the wavelengths and comparing to their known wavelengths at rest, we can get the galaxy's redshift. The distance then allows us to compute its true brightness. Because each grism exposure includes the spectra of many galaxies, we can get many redshifts - sometimes over a hundred - in a single HST pointing. This is great for us, although it also makes a lot of work to visually inspect all the spectra! The large numbers of redshifts also allow us to measure which galaxies are in densely or sparsely populated environments.

A zoomed in view of part of the grism image above. Each streak is a 
spectrum, and I've drawn green circles around some of the noticeable 
emission lines. Image Credit: Benjamin Weiner

Another strength of the WFC3-IR grism is that it gives us spectra in the near-infrared, beyond 1 micron. It's been difficult to take large numbers spectra from the ground in the infrared due to the sky brightness. The near-infrared is particularly interesting because for distant galaxies, the redshift moves their strongest emission lines from optical wavelengths into the near-IR. The WFC3-IR grism data are good for measuring emission from galaxies at redshifts 1-2, or when the universe was about a half to a quarter of its present age. 

We use the strength of emission lines, mostly from ionized hydrogen and oxygen, to understand the nature of galaxies at this long-ago epoch. The strength of the lines and the ratios of their intensities are a probe of the amount of star formation in a galaxy, the amount of metals that have been formed by stars and ejected back into the interstellar gas, and whether the gas is being ionized primarily by ultraviolet light from young stars, or from an active nucleus - a massive black hole. So far, CANDELS has used this data in a couple of papers that look at very young star forming galaxies with strong emission lines (see Arjen van der Wel's post), and there will be more to come.