Galaxy Overlays
Because of their various astrophysical processes and the resulting radiation mechanisms, galaxies are visible over the entire electromagnetic spectrum, from radio waves through gamma radiation. Here we show overlays of measurements onto the optical images: radio continuum, magnetic fields, neutral hydrogen (HI), carbon monoxide (CO), X-ray emission. These data have been obtained with different telescopes and prepared in such a way as to fit the optical images in an optimum way. In case of the HI and CO spectral lines the velocities of these gas components can be displayed with coloured contours and are then overlaid onto the optical images. They are measured by the Doppler effect (spectral-line shift). One can then see the galaxies' rotation: red contours show regions that are moving away from us, blue ones are regions that are coming towards us. Not enough with that: the kinematics of galaxies can be visualised even better using video films of these velocities, which can be superimposed onto the optical images. To those who wish to look at more interesting overlays or to utilize them for talks, a comprehensive PowerPoint document is recommended (ca. 1 GB, with explanations in English language), the access to which I can provide upon request (uklein@astro.uni-bonn.de). A popular article with explanations of the relevant astrophysics as well as hints at the production of such overlays has been published in Sterne und Weltraum, 07 / 2020 issue.
A spectacular example of the distribution and kinematics of the neutral atomic hydrogen (HI) in galaxies is the spiral galaxy M 101. This galaxy, which is surrounded by numerous dwarf galaxies shows their gravitational interaction quite drastically. The spiral structure is pretty irregular and is featured by a number of bifurcations and interconnections.
Here, one recognizes quite nicely the galaxy's rotation: regions in blue are moving towards us, red ones are moving away from us.
NGC 628 (M 74) opt. + HI (yellow contours).
Here we have an example of the enormous extent of the gas discs of galaxies. In this case the extent is about four times as large as that of the stellar disc. This huge reservoir of gas can still be used over billions of years to form stars out of this gas.
The galaxy is rotating towards us in the north and away from us in the south.
On the right a video can be downloaded, in which one can see the HI velocities running across the galaxy from blue to red. Also here, the enormous extent of the gas disc of this galaxy can be seen.
Also here, one realises that the gas disc extends beyond the stellar disc. In addition, it is waarped beyond the stellar disc, which in this precise edge-on view produces the shape of the mathematical integral sign. Two small neighbouring galaxies are also visible in HI.
NGC 4565 opt. + HI velocities (coloured contours).
The warp of the gas disc can also be seen in this kind of representation. Also the small neighbouring galaxies exhibit rotation.
M 31 opt. + CO (yellow contours)
Overlay of the carbon-monoxyde molecule (CO), which was observed with the 30-m radio telescope on Pico Veleta (Sierra Nevada, Spain) at a wavelength of 2.6 mm (frequency: 115 GHz) and superimposed here onto the optical image of M 31, taken with the APO refractor. The spiral structure is well visible, with the dark lanes closely following the spiral arms. It is there where interstellar gas is compressed by densiity waves circulating in the galaxy disk. This gives rise to the formation of dense molecular clouds, out of which new stars form, marking the bright spiral arms. It is evident how closely the molecular gas follows the dust lanes!
M 31 opt. + CO velocities (coloured contours).
As above: regions in blue are moving towards us, red ones are moving away from us. Below, a video can be downloaded, in which one can see the CO velocities running across the galaxy from blue to red. Also here, the precise coincidence of the CO gas and the dust regions can be seen.
Another overlay of the optical image, taken with the Skywatcher 190 mm f/5,3 - Maksutov/Newton telescope, with the radiation of the CO molecule (yellow colour). Taken with the larger focal length, this image exhibits the close correspondence of the dust lanes and the CO molecule in great detail.
The magnetic-field structure can be derived from observations of the polarized component of the synchrotron radiation. The observed magnetic-field structures can be explained by a galactic dynamo. In this complex hydrodynamic process kinetic energy is converted into magnetic one. This is supported by differential rotation of galaxies as well as by supernova activity, which causes vertical gas streams out of the disc.
The magnetic-field structure is parallel to the disc close to the disc plane, above the plane and at larger radial distances it swings away from it.
Here one can see how far the hot X-ray emitting gas reaches out of the galaxy disc. A salient feature is the constriction of this gas bubble immediately in the north-west along the disc. This is caused by the absorption of the X-ray emission by heavy elements embedded in the interstellar gas of the disc. One can utilize the location of this absorption zone to determine the orientation of the galaxy unambiguously: we are looking at it 'from underneath'! If we were looking at it 'from above', then the absorption zone would lie in the south-east below the disc.
This image of the radio continuum at 327 MHz shows the synchrotron radiation produced by relativistic electrons. The image has a very high dynamic range and hence shows a multitude of structures, which have been produced over several epochs. The oldest one reaches over 250,000 ly (in projection) and possesses a well-defined outer edge. This is caused by magnetic fields, which inhibit the formation of so-called Kelvin-Helmholtz instabilities and hence any fraying of the relativistic plasma. A younger structure shows up about 30,000 to 40,000 ly east and west of the active centre of the galaxy in the form of bubble structures. The youngest activity of the central supermassive black hole has led to the innermost, very bright (roughly triangular) structure.
NGC 3034 (M 82) opt. + soft X-rays (left).
Also here the very hot gas (10 million K) extends far beyond the galaxy disc. One can even make out a hot shock front (called 'polar cap') to the north of the galaxy. The structure has a projected distance of about 30,000 ly from the centre of M 82, so the hot gas has been expelled already thus far!
NGC 4631
(top middle), NGC 4627 (immediately above), and NGC 4656 (bottom left), are three gravitationally interacting galaxies, which orbit each other over a period of several billion years. The influence of this interaction is evident in the two bigger galaxies. NGC 4656 is strongly warped, while the most massive galaxy, NGC 4631, shows a very high star formation rate, which is also responsible for its blue colour. The small elliptical galaxy NGC 4627 appears structureless as it lacks any ongoing star formation. Its interstellar gas has almost completely been turned into stars.
NGC4631 group, opt. + HI (yellow contours).
Here we see the distribution of the neutral atomic hydrogen (the so-called HI line at 21 cm wavelength) in the galaxy group around NGC 4631. The contour lines represent the HI column densities. These reach from about 5x10^19 to about 10^23 atoms per square centimeter.
This representation allows to find very weak structures.
In the top right, a video can be downloaded in which the velocities running across the optical image of the galaxy from blue to red are displayed. One can see the galaxies' opposite rotation. Furthermore, one can see the complex kinematics of the intergalactic gas, which has been torn out of NGC 4631 by the strong tidal forces.
NGC 4631 group, opt. + HI velocities (coloured contours).
In this image the coloured contours represent the velocities of HI gas, measured via the Doppler effect of the 21-cm line. Red regions are moving away from us at 130 km/s, blue ones are moving towards us at 130 km/s. The rotation of the galaxy disc of NGC 4631 (top) and NGC 4656 (lower left) as well as the complex kinematics of the gas that has been pulled out into intergalactic space by the tidal forces of the gravitational interaction can be readily seen. The elliptical galaxy NGC 4627 (just above NGC 4631) does not possess any neutral gas and is hence invisible in HI.
The M81 Group
The 'Gang of Four': the galaxy group around the spiral galaxy M81, which I managed to capture in one image. M81 (above image centre), M82 (top), NGC3077 (left), and NGC2976 (bottom right). In the first collage, the optical foto, taken with the Skywatcher Evostar 72 (f = 420 mm, D = 72 mm) and Canon EOS 1100Da is shown leftmost. The central image is an overlay of the atomic, neutral hydrogen (the most abundant element in the universe), measured by Erwin de Blok und Fabian Walter at 21 cm wavelength with the VLA in New Mexico (yellow colour). On the right, the velocities of the gas relative to the earth are shown as coloured contours: red bordered regions are moving away from us at about 170 km/s, blue ones are moving towards us at about 240 km/s. It is evident that the mutual gravitational interaction of the galaxies has torn out a substantial amount of gas in the course of their 'dance', which has spread over intergalactic space. The rotation of the galaxies is well visible, in particular in case of M81. One revolution takes about 200 million years. The motion of the galaxies about each other takes a couple of billion years.
The second image provides the same representation, with the optical image this time taken wth the Maksutov-Newton telescope (f = 1000 mm, D = 190 mm), again with a Canon EOS 1100Da. The two telescopes track the rotating sky precisely, with the two cameras being controlled simultaneously.
In this overlay the yellow bars indicate the structure of the magnetic field in the edge-on galaxy NGC 4631. These were obtained from measurements of the radio continuum emission with the 100-m telescope in Effelsberg at 3 cm wavelength. It is obvious that the magnetic field emerges from the disc and runs into the halo. This is connected to the strong star-forming activity and the resulting supernova rate, which leads to a high pressure in the galaxy disc, thus transporting material into the halo. In this process the magnetic field is pulled along with this material. Its basic structure results from a complex electromagnetic process, the so-called galactic dynamo.
NGC 4258 alias M 106 is not a normal galaxy. What cannot be seen at first glance is that there is a supermassive black hole lurking in the centre of this galaxy. This was revealed early on in the radio domain, when M 106 turned out to be a strong radio source. Jets with highly energetic particles (electrons and protons) thrust through the interstellar medium and induce radiation so that one can see the action in the Hα line and in X-rays. Also early on, a number of water masers were found, the fast motion of which within a very small region around the central massive object allowed only one conclusion: a supermassive black hole. In the montage shown here I have superimposed in the left image the synchrotron radiation (yellow colour), which has been measured with the Very Large Array (VLA) in New Mexico. The image on the right shows the X-ray emission in blue colour, observed with the Chandra space observatory, overlaid onto my optical image. One can see how the jets strive to make their way through the interstellar medium. The north-western jet propagates behind the galaxy disc as seen by us, the south-eastern one is moving in front of the disc. One can see that the jets are deflected and bifurcated during their propagation. They heat up the gas of the interstellar medium to a couple of million K, which gives rise to the X-ray emission.
NGC 4826 (M 64) Is a conspicuous galaxy at first glance: a kidney-shaped dark structure of dust is catching the eye (dust in galaxies consists of silicates and graphites). Inspecting the kinematics of this galaxy in the central region yields a surprise: the centre of NGC 4826 is counter-rotating, i.e. the gas there rotates about the galactic centre in an opposite sense relative to the outer regions! There is only one plausible explanation for such a scenario: the galaxy has swallowed another (smaller) one. Such a 'dry meal' (the 'digested' dwarf galaxy most likely possessed a lot of dust and gas) is ongoing for a couple of billion years.
On the left we see the optical image of NGC 4826. Coloured contour lines have been overlaid, which represent the velocities of the neutral atomic hydrogen, observed with the VLA. The colours indicate the velocities: regions encompassed by red contours are moving away from us, regions with blue contours are coming towards us. It is quite obvious that the sense of rotation of this gas is reversed in the centre of NGC 4826. In this regime the gas streams pass each other with relative velocities of more than 200 km/s! This must have an impact: the stirred-up gas becomes very turbulent, which leads to strong star formation.
The extent and orientation of the gas disc of the dwarf galaxy NGC 6822 are a surprise! Its full extent of about 30,000 ly is nearly the size of our Milky Way. Presuming at first glance that the bar-shaped structure in the optical regime represents the main axis, the 21-cm radiation of the neutral atomic hydrogen reveals, however, that the principal axis is inclined with respect to the bar-shaped structure by about 120°. The gas reservoir of the galaxy is enormous - it is sufficient for star formation to take place over billions of years to come.
Below, a video can be downloaded, which shows the HI velocities running across the optical image of the galaxy from from low (moving towards us) to high (moving away from us) velocities.
The Centre of the Virgo Cluster
(click on images!)
Here we see several overlays onto my optical image of a field close to the centre of the Virgo Cluster. The top row first shows the original image, and on the right a b/w version of this image, annotated with names of galaxies visible in this field is displayed.
The middle row shows this field with overlays of the radio continuum
(left)
and atomic neutral hydrogen
(right). This reveals rather different properties. There are some galaxies which radiate mainly radio continuum (synchrotron) emission, others show strong emission in the 21-cm line of the neutral hydrogen (HI). Strong radio continuum indicates intense star formation because this also produces supernova explosions, which create relativistic particles that produce synchrotron radiation in the magnetic fields of galaxies. An exception is NGC 4374 (M 84). Here, the synchrotron radiation is produced by two jets transporting highly energetic electrons, which are ejected from the surroundings of the supermassive black hole in the centre of this galaxy, forming bubbles of relativistic gas at the periphery of this elliptical galaxy. Some galaxies do not possess any neutral hydrogen at all, but rather only hot gas radiating in the X-ray regime (s.b.). In this case, the gas has been almost completely turned into stars. The smaller galaxies lacking neutral hydrogen have lost this gas to their surroundings because every time they dive through the centre of the Virgo Cluster they experience strong ram pressure, due to the intergalactic gas that has accumulated there.
The bottom row shows an overlay of the soft X-ray emission in the form of blue (left) and yellow colours (right). The dominant hot (about 10 million K) gas of the galaxy NGC 4406 (M 86) and its surroundings is immediately evident. This galaxy is rushing towards the centre of the galaxy cluster from a direction back in the upper right of this field, thereby strongly heating the surrounding gas. NGC 4374 (M 84) contains very little hot gas. Most of the spiral disc galaxies visible here exhibit more or less strong X-ray emission, which implies that they also possess hot gas.
The Perseus Cluster of Galaxies
The Perseus Cluster of Galaxies has also been a frequent target in the radio frequency domain. A study by Dr.
Marie-Lou Gendron-Marsolais is dedicated to an investigation of the synchrotron radiation of galaxies in this cluster. Synchrotron radiation is produced when relativistic electrons are moving in a magnetic field. The energy of the electrons moving in the Perseus Cluster is about 10 GeV,
the magnetic-field strength is a few μG.
The image shows an overlay of the radio emission of the Perseus Cluster at 85 cm wavelength, observed with the VLA in New Mexico (USA). Two structures are most salient, viz. an extended halo centered on the galaxy NGC1275 (3C84), and a so-called head-tail source in the north, centered on the galaxy NGC 1265 (3C83.1). This tailed structure is caused by the fast motion of the galaxy relative to the ambient gas of the galaxy cluster. In the centre of the galaxy a super-massive black hole produces ultra-fast particles in its surroundings, which are ejected from the galaxy as diametrical jets from the galaxy. Upon leaving the shielding gas of the galaxy, these particles 'feel the headwind' and are hence quickly deflected; they thus leave behind a trail that sort of paints the motion of the galaxy on the sky. Two more such tailed radio sources are seen moving through the cluster at high speed.
The radio emission around the galaxy NGC 1275 also results from the production of relativistic electrons in the surroundings of a super-massive black hole lurking in the centre of this galaxy. Since the surrounding gas of the galaxy cluster is denser there, the jets are quickly decelerated and don't make it to the more tenuous medium outside of NGC 1275, as is the case for NGC 1265. They thus form an extended structure, which is called 'halo' by astrophysicists.
The head-tail radio galaxies NGC1265 (left) and IC310 (above), which are moving through the intergalactic gas of the Perseus Cluster at high speed, thereby leaving behind synchrotron-emitting tails of relativistic particles.
Owing to the higher gas density in the centre of the Perseus Cluster, the jets of relativistic particles ejected from the active centre of the galaxy NGC 1275 (left) do not make it out into intergalactic space, thereby producing an S-shaped structure of radio emission, similar to that around the active galaxy M 87 in the Virgo Cluster. NGC 1272 (right) also exhibits a spatially confined jet.
In the X-ray regime, the Perseus Cluster exhibits a bright halo of hot (100 Million K) gas, which is centered on NGC 1275 and extends out to more than 200 kpc. It is a relic of the merging epoch, during which multiple supernovae expelled the gas from the galaxies into intergalactic space. The heating was then furnished by gas compression, galaxy motions and AGN activity.