Category Archives: Interacting Galaxies

Starburst spiral galaxy NGC 3310 with Gemini North

Last Tuesday 1st of March the famous NASA webpage Astronomy Picture of the Day (APOD) released a very nice image of the galaxy NGC 3310 obtained with the 8.2m Gemini North Telescope in Hawaii (U.S.A.).

Nice image of the starburst spiral galaxy NGC 3310 in the Ursa Major obtained with the 8.2m Gemini North Telescope in Hawaii (U.S.A.). This image was obtained for the “Cosmic Poll” contest organized by the International Telescopes Support Office at the Australian Astronomical Observatory (AAO) and appeared as APOD on 1st March 2016. Colours codify the light received in blue (B, blue) and red (R, green) filters, plus the emission of the ionized gas (Hα filter) coded in red.AAO ITSO Office, Gemini Obs./AURA & T. A. Rector (U. Alaska Anchorage).

NGC 3310 lies at a distance of around 50 million years from us, within stars of the Northern constellation of the Ursa Mayor (meaning we cannot see it here from Australia, well, it has a maximum elevation of ~5 degrees from Siding Spring Observatory). The spiral structure in NGC 3310 looks like what we expect for our own Milky Way galaxy, with plenty of star-forming regions (in red-pink colours tracing the Hα emission). However in the case of NGC 3310, the star-formation activity seems to be more extreme.

It seems that NGC 3310 started experiencing an interaction with a dwarf galaxy around 100 million years ago. This interaction has triggered a very strong star-formation event (that is why NGC 3310 is defined as a starburst galaxy), and has “broken” the external areas of the galaxy as consequence of the intense tidal forces. In the image, all regions showing red-pink colours (tracing Hα emission) are nebulae. These regions are found almost everywhere within NGC 3310, sometimes even forming some peculiar alignments of red-pink-ish regions as that “ray” that goes from the centre of the galaxy till the upper left corner. It is interesting to note that, although the interaction with the dwarf galaxy happened ~100 million years ago, the fact of finding this large amount of Hα emission informs that the star-formation activity is still important today. The colliding dwarf galaxy was probably engulfed by NGC 3310, its remaining debris could be that diffuse arc-like structure we observed in the outskirts of the galaxy in the upper part of the image.

These are the kind of objects (starburst galaxies) and the kind of features (enhanced Hα activity, tidal distortions of the stellar component of the galaxy, tails, rays…) I studied in a sample of dwarf galaxies for my PhD Thesis (I still have to tell all of that here…).

Beside tracing the nebular (Hα) emission, the image also allows to distinguish that the majority of the stars in NGC 3310 have blue colours, even in its external areas. Again, this fact informs that the dominant stellar populations in this galaxy are relatively young, as only young stars emit a lot of light in blue and ultraviolet colours.

Although it was not said in the APOD I would like to remark that the idea of observing this galaxy in the 8.2m Gemini North Telescope came from the International Telescopes Support Office at the Australian Astronomical Observatory (AAO). In particular, mi colleagues Elaina Hyde, Richard McDermid, Caroline Foster-Guanzon and Stuart Ryder (AAO) organized a very interesting outreach initiative, the Cosmic Poll, asking the people to emit a vote for which kind of object would they like to be observed at the 8.2m Gemini Telescope. The winning entry was “an individual galaxy”, and later they decided to observe NGC 3310. Furthermore they organised an on-line event,a live-stream with the Gemini North Telescope (is available on YouTube) explaining how the telescope works and giving details of the observatory. The Gemini Observatory website also included this in its blog. After processing and cleaning the images, the final result is that you see in APOD.

I couldn’t help myself, though, and decided to play a bit with the colours, levels, contrast and lights of the image to try to get an enhanced image of this very nice object. In my opinion, the central part of the galaxy is a bit too bright (it should be, of course, the real difference in brightness between the central part of NGC 3310 and the diffuse stellar streams in its outskirts is several orders of magnitude, but for illustration purposes I have found that it is a good idea to minimize that) and the outskirts of the galaxy are not that easy to see. So here it goes my enhanced image of NGC 3310 with Gemini North:

Comparison between the image of the starburst spiral galaxy NGC 3310 obtained by the 8.2m Gemini North Telescope and published in APOD on 1st March 2016 (left) and the same image enhanced by myself (right). Credit: AAO ITSO Office, Gemini Obs./AURA & T. A. Rector (U. Alaska Anchorage), Enhancement: Ángel R. López-Sánchez (AAO/MQ)

What do you think? What image do you like more?

Kathryn’s Wheel: A ring of fireworks around a nearby galactic collision

Story based on the news release about Kathryn’s Wheel I prepared for the Australian Astronomical Observatory webpage.

The majority of the galaxies in the Universe can be classified into two well-distinguished classes: spiral galaxies (as our own Milky Way Galaxy) or elliptical galaxies. Spiral galaxies have on-going star-formation activity, possess a lot of young, blue stars, and are rich in gas and dust. However elliptical galaxies are just made up of old stars, with no signs of star formation, gas and dust. Besides these two large galaxy classes, some galaxies are found to have irregular or disturbed morphologies. That is certainly the case of many dwarf galaxies. A disturbed morphology is typically indicating a galaxy that has experienced an interaction with a nearby companion object. Indeed, all galaxies are experiencing interactions and mergers with other galaxies during their life time: the theory currently accepted about how galaxies grow and evolve naturally explains the building of spiral galaxies as mergers of dwarf galaxies, and the birth of an elliptical galaxy after the merger of two spiral galaxies. This will actually be the final destiny of our Milky Way, when it is colliding and merging with the Andromeda galaxy in around 4.5 billions years from now.

When galaxies collide, stars and gas are pulled out from them by gravity, and long tails of material stripped from the parent galaxies are formed. Famous galaxies in interaction developing these long “tidal tails” are the Mice Galaxies (NGC 4676) and the Antennae Galaxies (NGC 4038/4039). Very rarely, the geometry of the galaxy encounter is such that a small galaxy passes through the centre of a spiral galaxy creating a collisional ring galaxy. The ring structure is created by a powerful shock wave that sweeps up gas and dust, triggering the formation of new stars as the shock wave moves outwards. The most famous ring galaxy is the Cartwheel (ESO 350-40) galaxy, which is located at 500 million light-years away in the Southern constellation of the Sculptor. However complete ring galaxies are extremely rare in the Universe, only 20 of these objects are known.

Images of the interacting galaxies The Mice (NGC 4676), the Antennae Galaxies (NGC 4038/4039), and the Cartwheel (ESO 350-40) galaxy. Credit: The Mice: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA, Antennae Galaxies: Robert Gendler, The Cartwheel: ESA/Hubble & NASA.

An international team of astronomers led by Prof. Quentin Parker (The University of Hong Kong / Australian Astronomical Observatory) has discovered a nearby ring galaxy which in some ways is similar to the Cartwheel galaxy but 40 times closer. The system was discovered as part of the observations of the AAO/UK Schmidt Telescope (UKST) Survey for Galactic H-alpha emission. Completed in late 2003, this survey used the 1.2m UKST at Siding Spring Observatory (NSW, Australia) to get wide-field photographic data of the Southern Galactic Plane and the Magellanic Clouds using a H-alpha filter. This special filter is able to trace the gaseous hydrogen (and not the stellar emission) within galaxies, allowing astronomers to detect the ionized gas from nebulae. The survey films were scanned by the SuperCosmos measuring machine at the Royal Observatory, Edinburgh (UK), to provide the online digital atlas “SuperCOSMOS H-alpha Survey” (SHS). When using this survey to search for new, undiscovered planetary nebulae (dying stars which often show ring morphologies in nebular emission) in the Milky Way, the team realised that a very peculiar of these structures was actually found around a nearby galaxy, ESO 179-13, located in the Ara (the Altar) constellation. The reason why this magnificent collisional ring structure has been unknown by astronomers is that it is located behind a dense star field (this area of the sky is very close to the Galactic plane, where the majority of the Milky Way stars are located) and very close to a bright foreground star.

Discovery images of the “Kathryn’s Wheel” using the data obtained at the 1.2m UKST by the “SuperCOSMOS H-alpha Survey” (SHS). The left panel (SR) shows the red image tracing mainly the stars. The three main components of the system are labelled. The central panel shows the image using the H-alpha filter (Hα), which sees both the diffuse ionized gas and the stars. The right panel (Hα-SR) shows the continuum-substracted image of the system, revealing for the very first time the intense collisional star-forming ring. Image credit: Quentin Parker / the research team.

The discovery SHS images of the system reveal 3 main structures (A, B and C) plus tens of H-alpha emitting knots making the ring. Component A is the remnant of the main galaxy, the collisional ring is centered on it. Component A does not possess ionized gas (that is, it does not have star-formation at the moment). On the other hand, component B seems to be the irregular, dwarf galaxy (“the bullet”) that impacted with the main galaxy. Component B does possess a clumpy and intense H-alpha emission.

Astronomers have dubbed this ring galaxy as “Kathryn’s Wheel” in honour of the wife of one of the discoverers, Prof. Albert Zijlstra, (University of Manchester, UK). Kathryn’s Wheel lies at a distance of 30 million light years away, and therefore it is an ideal target for detailed studies aiming to understand how these rare collisional ring galaxies are formed, the physics behind these structures, and their role in galaxy evolution. Interestingly, the collisional ring is not very massive: its mass is only a few thousand million Suns. This is less than ~1% of the Milky Way mass, indicating that ring galaxies can be formed around small galaxies, something that was not considered so far.

(Left) Colour image of the collision, made by combining data obtained at the Cerro-Tololo InterAmerican Observatory (CTIO) 4-metre telescope in Chile. The H-alpha image is shown in red and reveals the star-forming ring around the galaxy ESO 179-13, that has been dubbed “Kathryn’s Wheel”. Image credit: Ivan Bojicic / the research team. (Right) Image showing only the pure H-alpha emission of the system highlighting just the areas of active star formation. For clarity any remaining stellar residuals have been removed. Image credit: Quentin Parker / the research team.

Furthermore, the galaxy possesses a lot of diffuse, neutral hydrogen in its surroundings. This cold gas is the raw fuel that galaxies need to create new stars. Observations using the 64-m Parkes radiotelescope (“The Dish”, Parkes, NSW) as part of the “HI Parkes All-Sky Survey” (HIPASS) revealed that the amount of neutral gas around Kathryn’s Wheel is similar to the amount of mass found in stars in the system. Astronomers are unsure about from where this cold gas is coming from, although they suspect it mainly belonged to the main galaxy before the collision started. However, as the remnant of the galaxy (component A) does not have star-formation at the moment, it seems that the diffuse gas has been expelled from the centre of the system to the outskirts of the galaxy.

The results were published in MNRAS in August 2015.
MNRAS 452, 3759–3775 (2015) doi:10.1093/mnras/stv1432
Kathryn’s Wheel: a spectacular galaxy collision discovered in the Galactic neighbourhood
Authors: Quentin A. Parker, Albert A. Zijlstra, Milorad Stupar, Michelle Cluver, David J. Frew, George Bendo and Ivan Bojicic

Gas, star-formation and chemical enrichment in the spiral galaxy NGC 1512

How do galaxies grow and evolve? Galaxies are made of gas and stars, which interact in very complex ways: gas form stars, stars die and release chemical elements into the galaxy, some stars and gas can be lost from the galaxy, some gas and stars can be accreted from the intergalactic medium. The current accepted theory is that galaxies build their stellar component using their available gas while they increase their amount of chemical elements in the process. But how do they do this?

That is part of my current astrophysical research: how gas is processed inside galaxies? What is the chemical composition of the gas? How does star-formation happen in galaxies? How galaxies evolve? Today, 21st May 2015, the prestigious journal “Monthly Notices of the Royal Astronomical Society”, publishes my most recent scientific paper, that tries to provide some answers to these questions. This study has been performed with my friends and colleagues Tobias Westmeier (ICRAR), Baerbel Koribalski (CSIRO), and César Esteban (IAC, Spain). We present new, unique observations using the 2dF instrument at the 3.9m Anglo-Australian Telescope (AAT), in combination with radio data obtained with the Australian Telescope Compact Array (ATCA) radio-interferometer, to study how the gas in processed into stars and how much chemical enrichment has this gas experienced in a nearby galaxy, NGC 1512.

Deep images of the galaxy pair NGC 1512 and NGC 1510 using optical light (left) and ultraviolet light (right).Credit: Optical image: David Malin (AAO) using photographic plates obtained in 1975 using de 1.2m UK Schmidt Telescope (Siding Spring Observatory, Australia). UV image: GALEX satellite (NASA), image combining data in far-ultraviolet (blue) and near-ultraviolet (red) filters.

NGC 1512 and NGC 1510 is an interacting galaxy pair composed by a spiral galaxy (NGC 1512) and a Blue Compact Dwarf Galaxy (NGC 1510) located at 9.5 Mpc (=31 million light years). The system possesses hundreds of star-forming regions in the outer areas, as it was revealed using ultraviolet (UV) data provided by the GALEX satellite (NASA). Indeed, the UV-bright regions in the outskirts of NGC 1512 build an “eXtended UV disc” (XUV-disc), a feature that has been observed around the 15% of the nearby spiral galaxies. However these regions were firstly detected by famous astronomer David Malin (AAO) in 1975 (that is before I was born!) using photographic plates obtained with the 1.2m UK Schmidt Telescope (AAO), at Siding Spring Observatory (NSW, Australia).

The system has a lot of diffuse gas, as revealed by radio observations in the 21 cm HI line conducted at the Australian Telescope Compact Array (ATCA) as part of the “Local Volume HI Survey” (LVHIS) and presented by Koribalski & López-Sánchez (2009). The gas follows two long spiral structures up to more than 250 000 light years from the centre of NGC 1512. That is ~2.5 times the size of the Milky Way, but NGC 1512 is ~3 times smaller than our Galaxy! One of these structures has been somehow disrupted recently because of the interaction between NGC 1512 and NGC 1510, that it is estimated started around 400 million years ago.

Multiwavelength image of the NGC 1512 and NGC 1510 system combining optical and near-infrared data (light blue, yellow, orange), ultraviolet data from GALEX (dark blue), mid-infrared data from the Spitzer satellite (red) and radio data from the ATCA (green). The blue compact dwarf galaxy NGC 1510 is the bright point-like object located at the bottom right of the spiral galaxy NGC 1512.
Credit: Ángel R. López-Sánchez (AAO/MQ) & Baerbel Koribalski (CSIRO).

Our study presents new, deep spectroscopical observations of 136 genuine UV-bright knots in the NGC 1512/1510 system using the powerful multi-fibre instrument 2dF and the spectrograph AAOmega, installed at the 3.9m Anglo-Australian Telescope (AAT).

2dF/AAOmega is generally used by astronomers to observe simultaneously hundreds of individual stars in the Milky Way or hundreds of galaxies. Without considering observations in the Magellanic Clouds, it is the first time that 2dF/AAOmega is used to trace individual star-forming regions within the same galaxy, in some way forming a huge “Integral-Field Unit” (IFU) to observe all the important parts of the galaxy.

Two examples of the high-quality spectra obtained using the AAT. Top: spectrum of the BCDG NGC 1510. Bottom: spectrum of one of the brightest UV-bright regions in the system. The important emission lines are labelled.
Credit: Ángel R. López-Sánchez (AAO/MQ), Tobias Westmeier (ICRAR), César Esteban (IAC) & Baerbel Koribalski (CSIRO).

The AAT observations confirm that the majority of the UV-bright regions are star-forming regions. Some of the bright knots (those which are usually not coincident with the neutral gas) are actually background galaxies (i.e., objects much further than NGC 1512 and not physically related to it) showing strong star-formation activity. Observations also revealed a knot to be a very blue young star within our Galaxy.

Using the peak of the H-alpha emission, the AAT data allow to trace how the gas is moving in each of the observed UV-rich region (their “kinematics”), and compare with the movement of the diffuse gas as provided using the ATCA data. The two kinematics maps provide basically the same results, except for one region (black circle) that shows a very different behaviour. This object might be an independent, dwarf, low-luminosity galaxy (as seen from the H-alpha emission) that is in process of accretion into NGC 1512.

Map showing the velocity field of the galaxy pair NGC 1512 / NGC 1510 as determined using the H-alpha emission provided by the AAT data. This kinematic map is almost identical to that obtained from the neutras gas (HI) data using the ATCA, except for a particular region (noted by a black circle) that shows very different kinematics when comparing the maps.
Credit: Ángel R. López-Sánchez (AAO/MQ), Tobias Westmeier (ICRAR), César Esteban (IAC) & Baerbel Koribalski (CSIRO).

The H-alpha map shows how the gas is moving following the optical emission lines up to 250 000 light years from the centre of NGC 1512, that is 6.6 times the optical size of the galaxy. No other IFU map has been obtained before with such characteristics.

Using the emission lines detected in the optical spectra, which includes H I, [O II], [O III], [N II], [S II] lines (lines of hydrogen, oxygen, nitrogen and sulphur), we are able to trace the chemical composition -the “metallicity”, as in Astronomy all elements which are not hydrogen or helium as defined as “metals”- of the gas within this UV-bright regions. Only hydrogen and helium were created in the Big Bang. All the other elements have been formed inside the stars as a consequence of nuclear reactions or by the actions of the stars (e.g., supernovae). The new elements created by the stars are released into the interstellar medium of the galaxies when they die, and mix with the diffuse gas to form new stars, that now will have a richer chemical composition than the previous generation of stars. Hence, tracing the amount of metals (usually oxygen) within galaxies indicate how much the gas has been re-processed into stars.

Metallicity map of the NGC 1512 and NGC 1510 system, as given by the amount of oxygen in the star-forming regions (oxygen abundance, O/H). The colours indicate how much oxygen (blue: few, green: intermediate, red: many) each region has. Red diamonds indicate the central, metal rich regions of NGC 1512. Circles trace a long, undisturbed, metal-poor arm. Triangles and squares follow the other spiral arms, which is been broken and disturbed as a consequence of the interaction between NGC 1512 and NGC 1510 (blue star). The blue pentagon within the box in the bottom right corner represents the farthest region of the system, located at 250 000 light years from the centre.
Credit: Ángel R. López-Sánchez (AAO/MQ), Tobias Westmeier (ICRAR), César Esteban (IAC) & Baerbel Koribalski (CSIRO).

The “chemical composition map” or “metallicity map” of the system reveals that indeed the centre of NGC 1512 has a lot of metals (red diamonds in the figure), in a proportion similar to those found around the centre of our Milky Way galaxy. However the external areas show two different behaviours: regions located along one spiral arm (left in the map) have low amount of metals (blue circles), while regions located in other spiral arm (right) have a chemical composition which is intermediate between those found in the centre and in the other arm (green squares and green triangles). Furthermore, all regions along the extended “blue arm” show very similar metallicities, while the “green arm” also has some regions with low (blue) and high (orange and red) metallicities. The reason of this behaviour is that the gas along the “green arm” has been very recently enriched by star-formation activity, which was triggered by the interaction with the Blue Compact Dwarf galaxy NGC 1510 (blue star in the map).

When combining the available ultraviolet and radio data with the new AAT optical data it is possible to estimate the amount of chemical enrichment that the system has experienced. This analysis allows to conclude that the diffuse gas located in the external regions of NGC 1512 was already chemically rich before the interaction with NGC 1510 started about 400 million years ago. That is, the diffuse gas that NGC 1512 possesses in its outer regions is not pristine (formed in the Big Bang) but it has been already processed by previous generations of stars. The data suggest that the metals within the diffuse gas are not coming from the inner regions of the galaxy but very probably they have been accreted during the life of the galaxy either by absorbing low-mass, gas-rich galaxies or by accreting diffuse intergalactic gas that was previously enriched by metals lost by other galaxies.

In any case this result constrains our models of galaxy evolution. When used together, the analysis of the diffuse gas (as seen using radio telescopes) and the study of the metal distribution within galaxies (as given by optical telescopes) provide a very powerful tool to disentangle the nature and evolution of the galaxies we now observe in the Local Universe.

More information

Scientific Paper in MNRAS: “Ionized gas in the XUV disc of the NGC 1512/1510 system”. Á. R. López-Sánchez, T. Westmeier, C. Esteban, and B. S. Koribalski.“Ionized gas in the XUV disc of the NGC1512/1510 system”, 2015, MNRAS, 450, 3381. Published in Monthly Notices of the Royal Astronomical Society (MNRAS) through Oxford University Press.

AAO/CSIRO/ICRAR Press Release (AAO): Galaxy’s snacking habits revealed

AAO/CSIRO/ICRAR Press Release (ICRAR): Galaxy’s snacking habits revealed

Royal Astronomical Society (RAS) Press Release: Galaxy’s snacking habits revealed

Article in Galaxy’s snacking habits revealed

Article in EurekAlert!: Galaxy’s snacking habits revealed

Article in Galaxy’s snacking habits revealed

Article in Open Science World: Galaxy’s snacking habits revealed

ATNF Daily Astronomy Picture on 21st May 2015.