Globular clusters surrounding the Milky Way (artist’s impression)
ESA’s Gaia satellite
Globular cluster NGC 4147
Hubblecast 117 Light: Hubble & Gaia weigh the Milky Way
In a striking example of multi-mission
astronomy, measurements from the NASA/ESA Hubble Space Telescope and the
ESA Gaia mission have been combined to improve the estimate of the mass
of our home galaxy the Milky Way: 1.5 trillion solar masses.
The mass of the Milky Way is one of the most fundamental
measurements astronomers can make about our galactic home. However,
despite decades of intense effort, even the best available estimates of
the Milky Way’s mass disagree wildly. Now, by combining new data from
the European Space Agency (ESA) Gaia mission with observations made with the NASA/ESA Hubble Space Telescope,
astronomers have found that the Milky Way weighs in at about 1.5
trillion solar masses within a radius of 129 000 light-years from the
galactic centre.
Previous estimates of the mass of the Milky Way ranged from
500 billion to 3 trillion times the mass of the Sun. This huge
uncertainty arose primarily from the different methods used for
measuring the distribution of dark matter — which makes up about 90% of the mass of the galaxy.
“We just can’t detect dark matter directly,” explains Laura Watkins (European Southern Observatory, Germany), who led the team performing the analysis. “That’s what leads to the present uncertainty in the Milky Way’s mass — you can’t measure accurately what you can’t see!”
Given the elusive nature of the dark matter, the team had
to use a clever method to weigh the Milky Way, which relied on measuring
the velocities of globular clusters — dense star clusters that orbit the spiral disc of the galaxy at great distances [1].
“The more massive a galaxy, the faster its clusters move under the pull of its gravity” explains N. Wyn Evans (University of Cambridge, UK). “Most
previous measurements have found the speed at which a cluster is
approaching or receding from Earth, that is the velocity along our line
of sight. However, we were able to also measure the sideways motion of
the clusters, from which the total velocity, and consequently the
galactic mass, can be calculated.” [2]
The group used Gaia’s second data release
as a basis for their study. Gaia was designed to create a precise
three-dimensional map of astronomical objects throughout the Milky Way
and to track their motions. Its second data release includes
measurements of globular clusters as far as 65 000 light-years from
Earth.
"Global clusters extend out to a great distance, so
they are considered the best tracers astronomers use to measure the mass
of our galaxy" said Tony Sohn (Space Telescope Science Institute, USA), who led the Hubble measurements.
The team combined these data with Hubble’s unparalleled
sensitivity and observational legacy. Observations from Hubble allowed
faint and distant globular clusters, as far as 130 000 light-years from
Earth, to be added to the study. As Hubble has been observing some of
these objects for a decade, it was possible to accurately track the
velocities of these clusters as well.
“We were lucky to have such a great combination of data,” explained Roeland P. van der Marel (Space Telescope Science Institute, USA). “By
combining Gaia’s measurements of 34 globular clusters with measurements
of 12 more distant clusters from Hubble, we could pin down the Milky
Way’s mass in a way that would be impossible without these two space
telescopes.”
Until now, not knowing the precise mass of the Milky Way has
presented a problem for attempts to answer a lot of cosmological
questions. The dark matter content of a galaxy and its distribution are
intrinsically linked to the formation and growth of structures in the
Universe. Accurately determining the mass for the Milky Way gives us a
clearer understanding of where our galaxy sits in a cosmological
context.
Notes
[1] Globular clusters formed prior to the construction of
the Milky Way’s spiral disk, where our Sun and the Solar System later
formed. Because of their great distances, globular star clusters allow
astronomers to trace the mass of the vast envelope of dark matter
surrounding our galaxy far beyond the spiral disk.
[2] The total velocity of an
object is made up of three motions — a radial motion plus two defining
the sideway motions. However, in astronomy most often only line-of-sight
velocities are available. With only one component of the velocity
available, the estimated masses depend very strongly on the assumptions
for the sideway motions. Therefore measuring the sideway motions
directly significantly reduces the size of the error bars for the mass.
More Information
ESA's Gaia satellite was launched in 2013 to create the most precise three-dimensional map of more than one billion stars in the Milky Way. The mission has release two lots of data thus far: Gaia Data Release 1 in 2016 and Gaia Data Release 2 in 2018. More releases will follow in the coming years.
More Information
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
ESA's Gaia satellite was launched in 2013 to create the most precise three-dimensional map of more than one billion stars in the Milky Way. The mission has release two lots of data thus far: Gaia Data Release 1 in 2016 and Gaia Data Release 2 in 2018. More releases will follow in the coming years.
The study was presented in the paper “Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions”, which will be published in The Astrophysical Journal.
The international team of astronomers in this study
consists of Laura L. Watkins (European Southern Observatory, Germany),
Roeland P. van der Marel (Space Telescope Science Institute, USA, and
Johns Hopkins University Center for Astrophysical Sciences, USA), Sangmo
T. Sohn (Space Telescope Science Institute, USA), and N. Wyn Evans
(University of Cambridge, UK).
Image credit: ESA/Hubble, L. Watkins, L. Calçada
Links
Contacs
Laura Watkins
European Southern Observatory
Garching, Germany
Tel: +49 89 3200 6257
Email: lwatkins@eso.org
N. Wyn Evans
University of Cambridge
Cambridge, United Kingdom
Tel: +44-01223-765847
Email: nwe@ast.cam.ac.uk
Roeland P. van der Marel
Space Telescope Science Institute
Baltimore, USA
Tel: +1-410-338-4931
Email: marel@stsci.edu
Bethany Downer
ESA/Hubble, Public Information Officer
Garching, Germany
Email: bethany.downer@partner.eso.org
Links
Contacs
Laura Watkins
European Southern Observatory
Garching, Germany
Tel: +49 89 3200 6257
Email: lwatkins@eso.org
N. Wyn Evans
University of Cambridge
Cambridge, United Kingdom
Tel: +44-01223-765847
Email: nwe@ast.cam.ac.uk
Roeland P. van der Marel
Space Telescope Science Institute
Baltimore, USA
Tel: +1-410-338-4931
Email: marel@stsci.edu
Bethany Downer
ESA/Hubble, Public Information Officer
Garching, Germany
Email: bethany.downer@partner.eso.org
Source: ESA/Hubble/News
