Tracing the spiral arms of the Milky Way

Our galaxy, the Milky Way, is thought to be a huge spiral galaxy like many such galaxies we see across the Universe. One of the tasks that modern-day astronomers are trying to achieve is to map the size and structure of our galaxy so we can better understand how it formed and how it will evolve in the future.

The spiral galaxy Messier 100 - similar to our own Milky Way? (Credit: ESO)

One of the key tasks in such work is to map the spiral arms of our galaxy. This is important because spiral arms are thought to be where the majority of dense gas is found in galaxies, and therefore where the majority of star formation takes place. Spiral arms aren't fixed objects though, the stars in our galaxy actually move in and out of the spiral arms as they orbit within our galaxy. Spiral arms are actually thought to be density waves that rotate around our galaxy, independently of the stars in our galaxy, just like waves in the ocean move independently of the water in them.

Spiral arm model of the Milky Way with four arms.
The Sun is located towards the top of this image.
(Credit: Georgelin & Georgelin 1976)

Identifying spiral arms is easy when you're outside of a galaxy and looking at it face on, but its much harder when you're embedded within the galaxy and all you can see is the plane of our galaxy. We can't directly see the spiral arms of our galaxy, but we can trace their presence by looking for signposts that identify them. Signposts such as giant molecular clouds, star forming regions, and bright young stars are all indicators of where spiral arms are found.

The Milky Way was first identified as a spiral galaxy thanks to the work of William Morgan from Yerkes Observatory who showed that the distribution of bright and hot OB stars, which are known to be very young objects, appear to be distributed in spiral arms. Morgan identified three spiral arms, which he labelled the Perseus, Orion and Sagittarius arms.

Later studies that attempted to discern the spiral structure of the Milky Way used the radio emission from hydrogen gas to trace its structure, but it can be tricky to determine the distance to such gas, making it hard to reveal the 3-dimensional structure.

A major breakthrough came in the 1970s when scientists combined radio measurements of hydrogen gas with optical measurements of the distances to the young stars associated with the gas. This work lead to a model made up of four spiral arms called the Norma, Scutum-Centaurus, Sagittarius and Perseus arms. While many researchers debated the distances to the various star forming regions used for this model (and therefore the exact structure and number of spiral arms the model predicted), this picture was for over 30 years the standard model of the spiral structure of the Milky Way.

The model changed again in 2008 thanks to data from NASA's infrared Spitzer Space Telescope, which allowed astronomers to count the number of stars all the way across our galaxy. The number of stars they counted suggested that there weren't four spiral arms, but only two, with a number of smaller spiral arms lying in between them.

Artist's conception of our new view of the Milky Way's structure thanks to results from the Spitzer Space Telescope.
The Sun's position is marked towards the bottom of this image.
(Credit: NASA)

This new model suggests that the Perseus and Scutum-Centaurus arms are the two major arms, while the Norma and Sagittarius arms are actually relatively minor arms. The two major arms connect up with the inner Galactic Bar, which dominates the central part of our Milky Way and may also play a role in the origin of the spiral arms.

Recently a flurry of results have taken this work even further with suggestions of a new and distant spiral arm that wraps completely around one side of the galactic centre, while other researchers have started using the distribution of star clusters to trace the structure of the Milky Way. Further improvements in the model of our galaxy's structure have come thanks to improved distance estimates for many of the stars and clusters in our galaxy, allowing the exact size and extent of the galaxy to be better determined.

Upcoming missions such as the Gaia observatory that will determine the distances to a billion stars across our galaxy will dramatically improve our understanding of our galaxy's size and shape. The motions that the Gaia spacecraft will measure will allow astronomers to study the orbits of these stars as well, improving our understanding of our galaxy from a purely structural model to a more advanced dynamical model.

Where the wild things are: Cygnus OB2

This week I want to tell you about one of my favourite astronomical objects, and one which I have spent many years studying, the Cygnus OB2 association.

Cygnus OB2 is a loose group of young stars known as an association, a sort of less-dense version of a star cluster. As the name would suggest, the Cygnus OB2 association is the 2nd OB association catalogued in the constellation of Cygnus. This constellation lies very prominently in the plane of our galaxy where the majority of stars and nearly all young stars in our galaxy are found.

The Milky Way, showing the constellation of Cygnus and the position of the Cygnus OB2 association
(Original image credit: Axel Mellinger)

From the image above you wouldn't believe anything particularly exciting was happening in Cygnus, as the region appears mostly dark, particularly around Cygnus OB2 itself. The reason for this is that between us and Cygnus OB2 there is a vast cloud of obscuring dust that absorbs the majority of starlight emitted by the association, hiding it from us. This cloud of dust is sometimes referred to as the Cygnus Rift or the Great Rift, and can be seen extending across much of the Galactic Plane, as the above image shows.

Optical photons are readily absorbed by dust particles, but infrared photons are less susceptible to this problem and can penetrate these obscuring dust clouds. An infrared view of the Galactic Plane, such as that shown below thanks to Japan's Akari satellite, reveals the prominent sites of star-formation activity in our galaxy, radiating brightly thanks to the many young stars forming within them.

The Milky Way in infrared light, as seen by the Akari satellite, with major infrared-bright regions labelled
(Credit: ESA)

The brightest and most prominent of all these star forming regions is probably the Cygnus region, despite it being more distant than many of the other star forming regions shown on this image. This brightness is testament to the intense amount of star formation that has, and still is occurring in this region. In fact when this region was first studied at radio wavelengths the bright structure was so prominent as to be marked with an 'X', hence leading to the name of Cygnus X for the whole star forming complex.

Detailed infrared view of the Cygnus X region, with
Cygnus OB2 in the centre (Credit: NASA)

Zooming into this image we can begin to see some of the amazing structures present within the Cygnus X giant molecular cloud. Vast clouds of gas and dust can be seen collapsing to form young stars, while huge pillars are constantly being sculpted and eroded by the recently-formed stars. This is a place where star formation takes place at the extremes!

Right in the centre of the Cygnus X giant molecular cloud is the Cygnus OB2 association, a massive group of young stars, as populous as some of the most massive young star clusters in our galaxy, yet nowhere near as compact.

The diffuseness of Cygnus OB2, coupled with the obscuring dust clouds, led to it being maligned for many decades. Only in the last 15 years, thanks to the revolutions in infrared and X-ray astronomy, have researchers been able to penetrate the extinction and uncover the thousands of massive, young stars in this huge OB association, including some of the most massive and luminous stars known!

As I said at the beginning of this post, Cygnus OB2 is a region I have studied for many years, hoping to better understand its origins and its content, and therefore to appreciate its role in the continual evolution of our galaxy. Next week I hope to share some exciting news and discoveries about Cygnus OB2 that we will be publishing very soon, so stay tuned!

The largest astronomical catalogs ever made! (Part II of II)

In a previous post I introduced the 'Top 10' largest astronomical catalogs that are currently and publicly available for astronomers (and anyone) to download and use. For a quick reminder, here's the list:

1. The US Naval Observatory (USNO-B1) all-sky catalog, 1046 million entries (2003)
2. The Guide Star Catalog (GSC v2.3) all-sky catalog, 946 million entries (2006)
3. The Sloan Digital Sky Survey (SDSS DR9), 933 million entries (2012)
4. The Wide-field Infrared Survey Explorer (WISE) all-sky catalog, 748 million entries (2013)
5. The United Kingdom Infrared Deep Sky Survey (UKIDSS), 727 million entries (2012)
6. The 2 Micron All Sky Survey (2MASS), 471 million entries (2003)
7. The Deep Near Infrared Survey (DENIS) of the southern sky, 355 million entries (2005)
8. The INT Photometric H-Alpha Survey (IPHAS), 219 million entries (2014)
9. The USNO CCD Astrograph Catalog v4 (UCAC4), 114 million entries (2012)
10. The Galactic Legacy Infrared Mid-Plane Survey (GLIMPSE), 104 million entries (2008)

In that post I discussed five of these surveys (including the IPHAS survey that I work on - read more about that here) that all include astronomical objects observed in the visible part of the electromagnetic spectrum (that is the part that we can see with our own eyes). The other five of these surveys all include measurements made outside of this part of the spectrum, and its those I'd like to talk about today.

The electromagnetic spectrum, with wavelength increasing to the right, and frequency (or energy) increasing to the left. (Credit: NASA)

These five surveys are WISE, UKIDSS, 2MASS, DENIS, and GLIMPSE, and they're all infrared photometric surveys, which means they measure how bright astronomical sources are in the infrared. The infrared part of the electromagnetic spectrum includes radiation with a longer wavelength than that of the light we can see, though not as long as microwave radiation or radio waves. You may be aware that infrared radiation is what we commonly know of as heat or warmth. Any object with a temperature around body temperature or up to a few hundred Celsius will radiate energy in the infrared.

Astronomers often divide up the infrared part of the electromagnetic spectrum into the near-, mid- and far-infrared regions, ordered by how far they are from the visible part of the spectrum. The definition of these three regions isn't important, but originates in the different technologies required to detect radiation in each region (such as different CCD detectors).

The near-infrared is the most easily accessible part of the infrared spectrum, and three of these surveys cover this region. Perhaps the most commonly-used of these is 2MASS, an all-sky near-infrared survey, which operated from 1997 to 2001 at two observatories in Arizona and Chile. The 2MASS all-sky view of the Milky Way is a stunning panorama of our galaxy, aided by the ability of near-IR radiation to penetrate the dust that would otherwise absorb optically visible radiation.

The 2MASS all-sky view of the Milky Way (Credit: IPAC)

The DENIS survey was also a deep near-IR survey that finished in 2001, but which only covered the southern sky. The UKIDSS survey(s) are a series of very deep near-IR surveys (deeper than both 2MASS and DENIS) on the United Kingdom Infra-Red Telescope (UKIRT) in Hawaii that targeted certain areas of the sky for different science goals. Some of these areas include regions of our galaxy where stars are forming and other areas look away from our galaxy out into the darkest regions of space where other galaxies are easily studied.

The remaining two surveys, WISE and GLIMPSE, are mid-infrared surveys, operating at slightly longer wavelengths than the near-IR surveys.

The centre of our Milky Way galaxy seen in the mid-infrared from GLIMPSE (Credit: Spitzer GLIMPSE team)

GLIMPSE was a survey of the Milky Way galaxy with NASA's Spitzer Space Telescope, and WISE is also a NASA space telescope that performed an all-sky infrared survey. Because the Earth's atmosphere absorbs most mid-infrared radiation, mid-infrared telescopes have to operate from above the atmosphere.

An artist's impression of NASA's
Spitzer Space Telescope in orbit
(Credit: IPAC)

A further complication for mid-infrared telescopes is that the telescopes themselves have to be cooled to very low temperatures to prevent them from radiating infrared photons themselves! This is because, even in space objects will radiate infrared radiation depending on their temperature. These photons would contaminate the measurements made, and so the entire telescope has to be cooled to incredibly low temperatures, much lower than anything in our solar system, to prevent them from radiating significantly large amounts of infrared radiation. Both space telescopes used cryostats filled with liquid hydrogen to achieve this, allowing them to be cooled to less than -263 C (under 10 K).

That brings our list of the largest astronomical catalogs to a close, for the time being at least. Astronomy is moving ever so rapidly towards bigger and bigger catalogs and it is almost certain that these catalogs will be eclipsed in the near future. The most likely candidates to take the throne of largest astronomical catalog will probably come from either ESA's Gaia satellite or the Large Synoptic Survey Telescope (LSST). More on these in a future post!

A new catalogue of over 200 million stars in the Milky Way galaxy

Recently one of the surveys that I'm a member of made a huge data release including over 200 million stars from our own galaxy, the Milky Way. We've made the data available to the entire world, not just the astronomical community, but everyone in the world! Astronomers love to share data!

The Isaac Newton Telescope on La Palma (Credit: ING)

The survey in question is the INT Photometric H-Alpha Survey (IPHAS), which has, over the last 11 years been using the Isaac Newton Telescope on the island of La Palma to survey our own galaxy to an unprecedented level of detail.

IPHAS is a photometric survey, meaning the goal of the survey is to measure photometry for all the sources it observes (astronomers refer to most objects as 'sources' until they're sure exactly what they are: stars, planets or galaxies). Photometry is the easiest measurement astronomers can make, since all you have to do is measure how bright an object is using a certain filter. IPHAS uses two 'broad' filters and one 'narrow' Hα filter (more on the awesomeness of this filter and what you can do with it in a future post).

The Milky Way (Credit: Wikimedia Commons)

Our own galaxy is visible to us as a glowing band of light stretching across the sky, known as the Milky Way. It appears this way to us because we (the Earth, the Sun and the entire solar system) are within the galaxy, and so we see the galaxy all around us, but concentrated in a band of light that makes up the disk of our galaxy. Astronomers refer to this as the Galactic Plane.

Surveying our galaxy is not easy because the stars can often be very crowded together (hence why they often appear blurred as a bright band of light instead of individual stars), so to do the job properly requires a high-resolution telescope and instrument, but also one that could invest a lot of time and effort into a single project such as this.

In 2003 the Isaac Newton Telescope was just such a telescope and so the IPHAS collaboration was born, led by Janet Drew (currently at the University of Hertfordshire where I am also based). I joined the collaboration in 2004 when I started my PhD at University College London and have been involved in the survey ever since, participating in the telescope observations, analysing data, and writing papers. Its been great to be involved in such a large and exciting project such as this.

After 11 years of taking data, sorting and organising the data, and doing some exciting science as well, the catalog is finally finished. It contains information on 219 million detected objects, the vast majority of which are stars in our own galaxy. It's an example of what astronomers (and data scientists) refer to as 'big data': huge catalogs of data, observations and measurements that can be sorted, filtered and analysed en masse.

One example of this is the image shown below, which was put together by IPHAS astronomer Hywel Farnhill. This map shows part of the Galactic Plane that IPHAS has been surveying, mostly covering the constellation of Cygnus, but stretching to Sagitta on the right.

IPHAS stellar density map (Credit: Hywel Farnhill)

This map is a stellar density map, showing the density of stars across the Galactic Plane, i.e. the number of stars detected by the IPHAS survey in each area of the plane, and its really detailed! Click here to see a high-resolution version of the image, zoom into it, and have a look at the exquisite level of detail that can be seen. The brightest parts are where we detect the most stars and the darkest parts are where we detect the least stars.

Most of this structure is actually due to the obscuring effects of interstellar dust, and not actually the distribution of stars in our galaxy (though this does contribute). Interstellar dust, which is mostly very small silicate rocks (similar to those found on Earth), absorbs starlight, particularly at visible wavelengths, and leads to the fine structure that you can see in this image. Tracing the distribution of this dust is really important to astronomers because of the huge influence it can have when it absorbs the light from the stars and galaxies we are trying to study.

You can read more about the survey on the IPHAS website, learn about the data release itself in this published paper by IPHAS astronomer and catalog maestro Geert Barentsen, or read the press release from the Royal Astronomical Society. If you'd like to access the data (and please do) you can get the entire set of data, for free, from the Vizier web archive.