New Method to Study Barred Spiral Galaxies

(Upper Left) The distribution of stars (psudocolor) and gas (contour lines) for two barred spiral galaxies in this study, NGC 2903 and NGC 4303. (Lower Left) The velocity of the gas in the galaxies. Blue indicates motion towards the viewer; red indicates motion away from the viewer. (Right) The Nobeyama 45-m Radio Telescope used for the COMING (CO Multi-line Imaging of Nearby Galaxies) survey.  Credit: Upper Left psudocolor images: 2MASS J-band, Jarrett et al. 2003, contour and Lower Left images: COMING project; Right: Dragan Salak.  Original size (1.2MB)

Analysis of gas motion in 20 nearby spiral galaxies has revealed a clear difference between those with bars and those without bars. This suggests that already available data on gas motion can be used to study bars in spiral galaxies, even in the absence of high-resolution imaging data.

In spiral galaxies, a large disk of stars and gas rotates around a central bulge. Spiral galaxies take their name from bright swirls (spiral arms) in the disk where stars are more densely concentrated. Many different types of spirals have been observed, including some with straight sections known as bars.

But a galactic disk is not a solid object. Different parts of the disk rotate at different speeds, similar to the clouds in a typhoon or soap suds spinning around a drain. In fact, the motion in a galactic disk isn’t limited to pure circular rotation, parts moving radially towards or away from the center can also be observed.

To better understand motion within the disk, a team led by Dragan Salak (at that time an assistant professor at Kwansei Gakuin University and now a postdoctoral researcher at the University of Tsukuba) analyzed the gas motion in the disks for a sample of 20 nearby spiral galaxies, including 7 barred-spirals. They found a clear difference between the kinematics of barred and non-barred galaxies. Non-barred spiral galaxies show very little radial motion at all locations. In contrast, barred spirals have on average 1.5-2 times more radial motion than non-barred spirals out to the end of the bar, but beyond the end of the bar the motion is close to circular. This result matches theoretical models where the bar structure helps to channel gas towards the center of the galaxy. The team found that the radius where the motion towards the center stops is closely related to the length of the bar, ranging between 0.8 to 1.6 times the length. This suggests that using the gas motion as a proxy for the bar could allow researchers to use modest-resolution, wide-field velocity data which are more easily available than high-resolution image data. For example, this study used the COMING survey of gas properties in nearby galaxies from the Nobeyama 45-m Radio Telescope in Japan.

Then by correlating the properties of the bar with the properties of the host galaxy, the team found that bars in more massive galaxies tend to be larger and rotate slower. This agrees with simulations where more massive galaxies provide more material for the bars to grow, but the mass of the galaxy exerts a torque which slows the rotation of the bar.

These results appeared as Salak et al. “CO Multi-line Imaging of Nearby Galaxies (COMING). VII. Fourier Decomposition of Molecular Gas Velocity Fields and Bar Pattern Speed” in December 2019 in Publications of the Astronomical Society of Japan Special Issue: Nobeyama 45 m Telescope: Legacy Projects and Receiver FOREST.

Related Links

• New Method to Study Barred Spiral Galaxies (Nobeyama Radio Observatory)

Source: National Astronomical Observatory of Japan (NOAJ)/News/Science

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First Light for VIRUS-W spectrograph

Fig. 1: "First Light" for VIRUS-W: This image (from the Sloan Digital Sky Survey) shows the galaxy NGC2903 and the field of view of the spectrograph. Credit: SDSS

Fig. 2: These are the first observational data taken by VIRUS-W at the beginning of November. The false colour image in the bottom row, left, shows an enlarged area of the galaxy NGC2903 shown in Fig.1. The bottom right image shows the reconstructed image by VIRUS-W, combining the total light received in each fibre. The top images show the velocities of the stars inside the galaxy. The left image gives the mean velocity, where blue indicates that the stars are moving towards us and red indicates that they are moving away. As all stars in lower half move away and all stars in the upper half move towards us, this means that the galaxy is rotating. The top right image shows the velocity dispersion, which increases towards the centre. This indicates that the motion of the stars becomes more chaotic the closer they are to the core of the galaxy.

Credit: M. Fabricius, MPE

The new observing instrument VIRUS-W, built by the Max Planck Institute for Extraterrestrial Physics and the University Observatory Munich, saw "first light" on 10th November at the Harlan J. Smith Telescope of the McDonald observatory in Texas. Its first images of a spiral galaxy about 30 million light-years away where an impressive confirmation of the capabilities of the instrument, which can determine the motion of stars in near-by galaxies to a precision of a few kilometres per second.

As imaging field spectrograph, VIRUS-W can simultaneously produce 267 individual spectra - one for each of its glass fibres. By dispersing the light into its constituent colours, astronomers thus are able to study properties such as the velocity distribution of the stars in a galaxy. For this they use the so called Doppler shift, which means that the light from stars moving towards or away from us is shifted to blue or red wavelengths, respectively. This effect can also be observed on Earth, when a fast vehicle, such as a racing car, is driving past: the sound of the approaching car is higher, while for the departing car it is lower.

VIRUS-W´s unique feature is the combination of a large field of view (about 1x2 arcminutes) with a relatively high spectral resolution. With the large field of view astronomers can study near-by galaxies in just one or few pointings, while the high spectral resolution permits a very accurate determination of the velocity dispersion in these objects. In this way the astronomers obtain the large-scale kinematic structure of near-by spiral galaxies, which gives important insight into their formation history.

Most galaxies are too distant and the separation between the billions upon billions of stars is too small to resolve it with even the best, cutting-edge instruments. The astronomers therefore cannot study individual stars but only the average motion along a specific line of sight.

The measured velocity distributions are characterised by two parameters: The mean velocity reveals the large-scale motion of the stars along the line of sight. The velocity dispersion measures how much the velocities of the individual stars differ from this mean velocity. If the stars have more or less the same velocity, the dispersion is small, if they have very different velocities, the dispersion is broad. For spiral galaxies, where the stars travel in fairly regular circular orbits, the velocity dispersion is mostly small. In elliptical galaxies, however, the stars have rather disordered orbits and so the dispersion is broad.

With the high spectral resolution of VIRUS-W, the astronomers can investigate relatively small velocity dispersions, down to about 20 km/s. This was impressively confirmed by the first images taken by VIRUS-W of the near-by spiral galaxy NGC2903 (see Figure). "When we attached VIRUS-W around midnight on the 10th of November to the 2.7m telescope, we were very happy to see that the data delivered by VIRUS-W was of science quality virtually from the first moment on," says Maximilian Fabricius from the Max-Planck-Institute for Extraterrestrial Physics. "As the first galaxy to observe we had selected the strongly barred galaxy NGC2903 at a distance of about 30 million lightyears - right in front of our doorstep. The data we collected reveal a centrally increasing velocity dispersion from about 80 km/s to 120 km/s within the field of view of the instrument. This was a very exciting moment and only possible because of the remarkable teamwork during the commissioning with a lot of support by the observatory staff!" The observing time at the telescope was made available by the VENGA project, to which VIRUS-W will be contributing from the beginning of 2011 onwards. It will then provide detailed kinematic data to this study.

The main instrument for VENGA is VIRUS-P, a spectrograph operating at the 2,7m Harlan J. Smith-Teleskope of the McDonald observatory since 2007. This instrument is a prototype of the VIRUS spectrographs being developed for the HETDEX project led by the University of Texas in Austin. For a study of the large scale distribution of galaxies, HETDEX will combine about 100 spectrographs at the 9.2m Hobby-Eberly Telescope of the McDonald observatory to form one large instrument. VIRUS-W (where the W stands for a later mission at the Wendelstein telescope of the Munich Observatory) is based on the same basic VIRUS design. Because of its broader spectral coverage and despite its much lower resolution, the prototype VIRUS-P already gives interesting insight into the age and chemical composition of stars and the interstellar medium as well as information about the star formation rate.

Links :

Press Release of the McDonald Observatory
VENGA Projekt
MPE Press Release June 2010

Dr. Hannelore Hämmerle
Press Officer
Max-Planck-Institut für extraterrestrische Physik
phone: +49 89 30000-3980
email: hanneh@mpe.mpg.de

Maximilian Fabricius
Max-Planck-Institut für extraterrestrische Physik, Garching
phone: +49 89 30000-3694
email: mxhf@mpe.mpg.de

Prof. Dr. Ralf Bender , Dr. Frank Grupp
Max-Planck-Institut für extraterrestrische Physik, Garching
Universitätssternwarte München
email: bender@mpe.mpg.de , fug@usm.lmu.de

Dr. Roberto Saglia
Max-Planck-Institut für extraterrestrische Physik, Garching
email: saglia@mpe.mpg.de

Dr. Niv Drory
Instituto de Astronomia, Universidad Nacional Autonoma de Mexico (UNAM)
phone: (+52 55) 5622 4014
fax: (+52 55) 5616 0653

Contact: MPE public outreach department