Measuring the velocity of stars inside galaxies is a difficult task, but it gives fundamental information about the properties of these systems, in particular their total mass, which might reveal the presence of the mysterious Dark Matter. The Chinese Large sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), located at the Xinglong Observatory northeast of Beijing, is one of the few facilities that is allowing astronomers to measure the central motion of stars in tens of thousands of galaxies distant out to about 4.5 Giga light years from us in all the Northern Hemisphere.

An International team led by the Sun Yat-sen University and other Chinese Institutes, including the Chinese Academy of Sciences, has obtained, for the first time, a measurement of internal stellar motions of about 90,000 galaxies (see figure) observed with the Large sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). The results of this research have been published on the international journal Monthly Notice of Royal Astronomy Society. And the catalog of the galaxy internal motions has been delivered to the international community to provide new information about the internal structure and evolution of galaxies and their dark matter content, which are expected to shed new light on the formation and Dark Matter content of these systems. .

LAMOST is a Chinese national scientific research facility operated by National Astronomical Observatories of Chinese Academy of Sciences (NAOC). It is a 4m effective aperture telescope located at the Xinglong Observatory northeast of Beijing, China, which is observing all the Northern Sky to obtain a map of millions of stars of our Milky Way and to measure the receding velocities of hundreds of thousands of far away galaxies to reconstruct the large scale structure of the Universe and identify galaxy clusters and groups. This latter program has been dubbed LAMOST Extra-GAlactic Survey (LEGAS) and it has joined the long tradition of large spectroscopic surveys that have observed the sky to map the 3D distribution of galaxies in the Universe in the last decades.

 
“Since the commissioning year, back in 2009, LAMOST has been extremely successful in carrying out a spectral survey of millions of objects in a large part of the northern sky,” says Prof. Zhao of NAOC, Principal Investigator of the LAMOST project and co-author of the publication, “We knew that the quality of the measurements of our facility was very good and indeed LAMOST has produced many scientific papers both on Milky Way structure and on Extragalactic science. However, none tried to measure the internal velocity of individual galaxies, so far, as this is a quite difficult task,” added Prof. Zhao.

 
“In the last decade I have been interested in studying the dark matter content of galaxies,” says Prof. Napolitano, of the Sun Yat-sen University, leading author of the article reporting the new catalog of galaxy internal motions with LAMOST, who added that “and the first difficult step to detect the presence of dark matter is to measure the velocity of the stars inside them. If indeed suck a dark mass is in galaxies, stars have to move fast to balance the gravitational attraction of these unseen and mysterious form of mass.”

 
Dr. D’Ago, of the Pontificia Universidad Católica de Chile adds: “In particular, we have performed the measurement of the so called velocity dispersion, which is the velocity related to the chaotic motions that characterize the spheroidal components of galaxies. So far, only few other international teams have provided similar measurements, for instance the Sloan Digital Sky Survey, but now we could add thousands of new galaxies out to about 3 Giga light years far from us with LAMOST observations and we will continue to provide more of such measurements with future data releases.”

 
“We are very excited to study these galaxies in more details and determine how the dark matter has been assembled in these systems. One of the open questions in galaxy formations is indeed the understanding of how stars have been formed in the dark matter haloes, how they move inside this dark component and which is the mutual gravitational effect of stars and dark matter particles. These data will allow us also to prepare to interpret data from future projects, like the Chinese Space Station Telescope, that will carry even more powerful instrumentation to study the internal galaxy motions and their dark matter content,” concludes Prof. Napolitano.

 
These new results obtained at the Sun Yat-sen University in collaboration with NAOC, the University of Chinese Academy of Science and the Nanjing Institute of Astronomical Optics & Technology, the Pontifica Universidad Catolica de Chile and the Italian National Institute of Astrophysics, will be crucial for different very important scientific applications, like the understanding of the super-massive black hole formation inside galaxies, the search of gravitational lenses and the understanding of the evolution of the galaxy scaling relation in time. Most of all, they will become part of the legacy of the LAMOST project.

Figure: Sky distribution (RA, DEC) of LAMOST galaxy targets from the LEGAS survey. In red all sources labeled as galaxies in the LAMOST observations, in blue the ones that have high quality spectroscopic data, and in green the galaxies with measured velocity dispersion in this work. (Credit: Nicola R. Napolitano/Rui Li & LAMOST)

This paper can be accessed at https://academic.oup.com/mnras/article/498/4/5704/5902399