Fig. 1:
Eagle Nebula imaged by Hubble Space Telescope.
Credit: NASA, ESA/Hubble and the Hubble Heritage Team (STScI/AURA)
Credit: NASA, ESA/Hubble and the Hubble Heritage Team (STScI/AURA)
Bottom: The optical image of one of the galaxies in the sample, NGC 5457. Coloured squares show grids cells, with 1 kpc on a side, in the arm (green), interarm (yellow) and bulge (red) regions.
The star formation rate in galaxies varies greatly both across different galaxy types and over galactic time scales. MPA astronomers have been trying to gain insight into how the interstellar medium may change in different galaxies by studying molecular gas in a wide variety of galaxies, ranging from gas-poor, massive ellipticals to strongly star-forming irregulars, and in environments ranging from inner bulges to outer disks. They find that the gas depletion time depends both on the strength of the local gravitational forces and the star formation activity inside the galaxy.
Molecular clouds are clouds in galaxies consisting predominantly of
molecular hydrogen. They are stellar nurseries where the gas reaches
high enough densities to form new stars and planetary systems. Molecular
clouds are highly complex structures. Figure 1 shows a Hubble Space
Telescope image of the Eagle Nebula, a nearby molecular cloud with a
highly filamentary and irregular structure.
In the neighbourhood of our Sun, molecular clouds make up only 1 % of
the total volume of the interstellar medium and form stars at modest
rates of a few solar masses per year. In the early Universe, however,
there is mounting evidence that galaxies contain much more molecular gas
and therefore they can form stars at rates up to a thousand times
higher than in our Milky Way. The densities and pressures in the
interstellar media of these early galaxies are also orders of magnitude
higher than in the solar neighbourhood, and it is unlikely that
molecular clouds in these systems are the same as the very well-studied
Eagle nebula.
In recent work, the MPA group studied variations in the relation between
the local density of molecular gas and newly formed stars. They used
this as a diagnostic of changing conditions within the interstellar
medium. According to standard theory, molecular clouds exist in a
balance between gravitational forces, which work to collapse the cloud,
and pressure forces (primarily from the gas), which work to keep the
cloud from collapsing. When these forces fall out of balance, such as
can happen in a supernova shock wave, the cloud begins to collapse and
fragment into smaller and smaller pieces. The smallest of these
fragments begin contracting and become proto-stars.
Gravitational forces vary significantly from one galaxy to the next, as
well as in different regions of the same galaxy. At the centre of a
giant elliptical galaxy, gravity is much higher than in the outskirts of
a small dwarf irregular. Likewise, the incidence of supernova
explosions can differ drastically between different galaxies and between
different locations within the same galaxy. Variations in the ratio of
the density of molecular gas to young stars (commonly referred to as the
depletion time of the molecular gas) may thus be expected as a
consequence of these changing conditions.
The main result (see Figure 2) from the MPA group's analysis is that the
rate at which molecular gas forms new stars is set BOTH by gravity (as
measured by the local surface density of stars in the galaxy) and by the
local star formation activity level in the galaxy, which in turn will
determine the incidence of supernova-driven shock waves in the
interstellar medium. Molecular gas depletion times are shortest in
regions where gravity is strong and where the star formation activity is
high, particularly in galaxy bulges with gas and ongoing star
formation.
Reaching this conclusion required very careful analysis of a variety of
data sets at different wavelengths. In particular, star formation rates
derived from the combination of infrared images that trace young stars
embedded inside dusty clouds and far-ultraviolet images that trace stars
that have migrated outside these clouds, are crucial for pinpointing
these relations as accurately as possible. In future, new
state-of-the-art interferometric radio telescopes, in particular the
Atacama Large Millimeter/submillimeter Array (ALMA), will allow us to
understand the detailed structure of molecular clouds in regions of high
gravity in much more detail.
Guinevere Kauffmann and Mei-Ling Huang
Guinevere Kauffmann and Mei-Ling Huang
Publications:
Huang M.-L., Kauffmann G., 2014, MNRAS, 443, 1329Huang M.-L., Kauffmann G., 2015, MNRAS, 450, 1375
