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/

Coming Out of the Dark Ages

Until about 400,000 years after the Big Bang,  the Universe was mostly full of electrons and protons, zipping in random directions. It was only when the Universe cooled down enough, because of expansion, that electrons and protons had a chance to combine to form neutral hydrogen (the lightest element in the Universe) for the first time. This epoch is known as the epoch of recombination. The Universe then enters and remains in what we call the Dark Ages until the formation of the first luminous sources -- first stars, first galaxies, quasars, and so on. During this period, the Universe was full of neutral hydrogen, and thus completely opaque to any ultra-violet (UV) radiation because neutral hydrogen is very efficient at absorbing UV radiation. Intense UV ionizing photons from the first stars and first galaxies then start to ionize their surrounding, forming ionized bubbles. These bubbles grow with time, and eventually the entire Universe was filled with ionized bubbles. The epoch during which this change of phase or transition occurred i.e., the ionization of most of the neutral hydrogen to ionized hydrogen -- is called the epoch of reionization (see Figure below). This was the last major transition in the history of the Universe, and had a significant impact on the large scale structure of the Universe. Therefore, this is one of the frontier research areas in modern observational cosmology.

Time line history of the Universe from Big Bang (left) to the present day Universe (right). Before the process of reionization, the Universe was completely filled with neutral hydrogen. It is only after the formation of first sources including first stars, first galaxies, that the neutral hydrogen in the Universe started ionizing, and by about one billion years after the Big Bang, most of the neutral hydrogen in the Universe was vaporized marking the end of the epoch of reionization (Image credit: NASA, ESA, A. Fields (STScI).

Probing the Epoch of Reionization

One of the most powerful and practical tools to probe the epoch of reionization is the Lyman-alpha emission test. Lyman-alpha photons are a n=2 to n=1 transition in neutral hydrogen which emits a photon with a wavelength of lambda=1215.67 Angstroms. In the presence of neutral hydrogen, Lyman-alpha photons are scattered again and again and eventually many of the Lyman-alpha photons are  scattered away form our line of sight . As a result, we expect to see fewer and fewer galaxies with Lyman-alpha emission as we probe higher and higher redshifts (closer to the Big Bang).

To study the epoch of reionization, we did exactly this using a large sample of very distant (high-redshift) galaxy candidates selected from the Hubble Space Telescope (HST) CANDELS survey -- the largest galaxy survey ever undertaken using  HST.  To know the exact distance of a galaxy, it is critical to obtain spectroscopic observations of these galaxies. We did this using a near-infrared spectrograph, MOSFIRE, on the Keck Telescope located at 13,000 ft on top of Mauna Kea, a dormant-volcano mountain in Hawaii.

To our surprise, we discovered that most of the galaxies we observed did not show Lyman-alpha emission. The figure below shows our results combined with previous studies. This figure shows the Lyman-alpha equivalent width, the ratio of strength of Lyman-alpha emission from a galaxy to its underlying blue stellar light continuum (non Lyman-alpha light), as a function of redshift (or age of the Universe on the top axis), as we probe closer and closer to the Big Bang. As can be seen, there are fewer galaxies,  and at the same time the strength of Lyman-alpha emission also decreases as we go to higher redshifts. While this can be a result of a few different things, upon careful inspection, we think that this is likely because of the Universe becoming more neutral as we go beyond redshift ~7, and we are witnessing the epoch of reionization in-progress.

This Figure shows the evolution of strength of Lyman-alpha emission in galaxies, as we get closer and closer to the Big Bang. As can be seen, the strength of Lyman-alpha emission appears to be decreasing or in other words we are missing vetry strong Lyman-alpha emitting galaxies as we go towards higher redshifts. This is likely a consequence of increasing neutral hydrogen, as expected from theoretical studies (Image credit: Tilvi et al 2014).

Currently, Lyman-alpha emission provides the best tool to discover and confirm very distant galaxies. While there are a few other emission lines that could be used to confirm distance to a galaxy, their strengths compared to the Lyman-alpha emission is much weaker.  Despite this, we have made quite a significant progress in understanding the first billion years of the Universe.

The figure below shows the summary of progress astronomers have made over the past few years, understanding the transition of Universe from a completely neutral to an ionized phase. Below redshift of about 6, that is about 1 billion years after the Big Bang, the Universe is almost completely full of ionized hydrogen—only one part in 10,000 is neutral. At redshifts greater than 6, the Universe becomes more and more neutral. The James Webb Space Telescope (JWST) will be very instrumental in discovering galaxies within the first 600 Myrs, and will help us gain even more insight into the details of the crucial epoch.

This figure shows the evolution of neutral hydrogen fraction as a function of redshift (or age of the Universe shown on top axis). Only one part in 10,000 is neutral below redshift of about 6 which implies that the Universe is mostly ionized and the process of reionization has occurred at redshifts greater than six, where the Universe is becoming increasingly neutral (Image credit: V. Tilvi).

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.

Astronomer of the Month: Amber Straughn

Each month we will highlight a member of the CANDELS team by presenting an interview introducing them and what it's like to be an astronomer. This month's Astronomer is Amber Straughn.

Tell us a little about yourself!

Hi! I’m Amber Straughn. I work at NASA’s Goddard Space Flight Center in Greenbelt, MD as a Civil Servant Scientist (my formal title is “Research Astrophysicist”) and as the Deputy Project Scientist for James Webb Space Telescope Science Communications. I’m also on Goddard’s WFIRST science team. I grew up in a tiny rural farming town in north-central Arkansas (Bee Branch, to be specific, not that anyone ever knows where that is!). I got my B.S. in Physics at University of Arkansas (Go Razorbacks!) and my M.S. and Ph.D. in Physics at Arizona State University, all the while focusing on astrophysics. I did my first postdoc at Goddard through the NASA Postdoctoral Program, and was hired by NASA in 2011.

What is your specific area of research? What is your role within the CANDELS team? 

I am broadly interested in galaxy evolution, and specifically how galaxies gain their mass over time; as well as the interplay between galaxy interactions, star formation, and supermassive black hole growth. I’ve done work on both galaxy morphologies and also looking at emission-line galaxies using HST grism spectra. I am an original co-I on the CANDELS proposal, which was submitted back when I was a postdoc at Goddard.

What made you want to become an astronomer? At what age did you know you were interested in astronomy? 

I really have always known I wanted to be an astronomer. As I mentioned above, I grew up in an extremely rural part of the US. There wasn’t a lot to do in my hometown, but the night sky was -- and still is -- breathtaking. I was pulled in by the night sky from as early as I can remember. I would drag my family outside to watch meteor showers and eclipses, and I remember asking my parents ridiculous questions about how the Universe worked… I distinctly remember at one point when I was very young and asked my mom something that she didn’t know the answer to, that she told me: “I don’t know. But you can find out the answer yourself someday.” That gave me the initial motivation I needed to pursue this very privileged path of studying the Universe for a living.

What obstacles have you encountered on your path to becoming an astronomer and how did you overcome them? 

Aside from the very real obstacles of getting through the first year of grad school (and qualifying exams, and full loads of classes and TA’ing, etc., that everyone goes through!), I would say that I’m lucky to not have had any huge obstacles. I am, however, a first generation college student. So that did present its own challenges. Coming from a small town, with a tight-knit extended blue-collar family where nobody really ventures too far from home, I did encounter some skepticism and negative feedback from people close to me that didn’t understand what I wanted to do. It was a weird thing to “leave”…leave your hometown, your family. But I’m grateful that my immediate family -- especially my mom -- has always been extremely supportive of me! And of course being a woman in a male-dominated field has at times been challenging. I’m grateful that I’ve never experienced overt harassment or discrimination, but as others have more eloquently elaborated on…sometimes it’s the constant “small” things that add up.

 

Who has been your biggest scientific role model and why? 

I’m very grateful for many role models and mentors I’ve had along my career path. My undergrad academic physics advisor at the University of Arkansas (Lin Oliver) was one of my earliest and most influential mentors. He helped convince me that I could succeed on this path very early on as a not-very-well prepared college student, when I was sometimes worried about my capabilities (imposter syndrome is real!). I’m happy to say that we’re still in contact! 

What is it like to be an astronomer? What is your favorite aspect? 

Is there anything better than doing something you love as a career? It’s wonderful. In my current job at NASA, I do a lot of work on future space missions that enable astronomy, and science communications work, in addition to my own research. I mostly use Hubble data for my research. And for me, Hubble’s always “been up there” (it was launched when I was in elementary school). Working at Goddard, I get to see hardware for the James Webb Space Telescope as it’s being developed, and there’s something that’s so cool about that. 

What motivates you in your research? 

I think it’s generally just the drive to find out something new, and to feel like I’ve contributed -- even if it’s only a tiny bit -- to this grand endeavor of understanding our Universe. 

What is your favorite astronomical facility? 

Well, that would have to be Hubble! I think it’s amazing that not only has Hubble so profoundly changed the way we understand the Universe, but it’s also completely captivated the imagination of the public.

Where do you see yourself in the future? What are your career aspirations? 

Right now, I can’t imagine a place I’d rather work than NASA. But maybe…astronaut? Who knows!

If you could have any astronomy related wish, what would it be? 

Sort of unrelated to actual astronomy research, but if I could wave the magic wand, I’d totally go to Mars.

What is your favorite, most mind-boggling astronomy fact? 

That our physical bodies are literally made of exploded stars. There’s something so poetic about that…and it’s actually literal fact. To get a bit more philosophical…I think it speaks to our interconnectedness as human beings -- to each other, and to the cosmos itself! 

Is there anything else you would like for the public to know about you or astronomy in general? 

I think sometimes the public, and/or kids who think about becoming scientists, think that scientists are these super-intelligent socially-awkward genius loners who spend all their time in the lab or “doing science”. And it’s not surprising that people think that way…that’s often the way that scientists are portrayed in the media. But the reality is that the vast majority of us are regular, everyday people (who do have an aptitude for science and math, and certainly an increased interest in it) -- people who have families, outside hobbies (I’m both a pilot and a faithful yoga practitioner!), and hopes and dreams unrelated to science. Science is a big part of our lives, to be sure, but feel free to talk to us…we’re a lot like you!