Astronomy

An early stellar nursery

Searching for distant objects in our Universe is equivalent to looking back in time, to the early origins of stars and galaxies. The most distant object known shows the earliest evidence of star formation.

In the astronomical hunt for ever more distant objects, and hence a window on the early Universe, the current leader1 is J1148 + 5251 at a redshift of 6.4. The light we now see from this particularly luminous object was emitted only 800 million years after the Big Bang, making it the youngest object known. J1148 + 5251 is a quasar (from 'quasi-stellar' object). Whereas stars are powered by nuclear fusion, a quasar's power is derived from gravitation: quasars are powered by matter from rotating accretion disks spiralling into massive black holes at the centres of galaxies.

Observations of an object as young as J1148 + 5251 could reveal much about the evolution of galaxies early in the history of the Universe, and about the relation between the formation of stars and massive black holes. New data are now reported, by Walter et al.2 on page 406 of this issue, and by Bertoldi et al.3 in a complementary article in Astronomy and Astrophysics. These authors have detected radiation at millimetre wavelengths from molecules of carbon monoxide, indicating the presence of a large mass of interstellar molecular gas, and from which the presence of molecular hydrogen (the dominant component in molecular clouds) can be inferred. This is the raw material from which stars form. These measurements, combined with the observation of strong emission at far-infrared wavelengths from interstellar dust4 in this very young galaxy, point to an ongoing burst of star formation that began only a short time after the Big Bang.

The combination of high luminosity at far-infrared wavelengths and a large mass of molecular gas and dust is an accepted signature of star formation in galaxies. Young stars embedded in the molecular clouds heat the interstellar dust, which then radiates at infrared wavelengths. In the local Universe, many spiral galaxies show significant infrared luminosity from star formation, and the most powerful galaxies — ultraluminous infrared galaxies5 — all have large masses of molecular gas6 and CO emission similar to that seen in the distant J1148 + 5251. Most ultraluminous galaxies seem to have formed from collisions between separate galaxies5, and their molecular gas is found concentrated in rotating disks or rings a few thousand light years across7 — a central region much smaller than the galaxies but much larger than a quasar accretion disk. An analysis of the CO emission from a quasar at lower redshift than J1148 + 5251 showed that the molecular gas is also distributed in a star-forming ring with a size of about 6,000 light years8.

From the observed infrared luminosity of J1148 + 5251, Walter et al.2 and Bertoldi et al.3 have calculated the rate of star formation in its galaxy. It is equivalent to the creation of about 3,000 Sun-like stars each year — a thousand times greater than the star formation rate in the entire Milky Way, and a few times greater even than that of ultraluminous galaxies. Star formation on this scale is a major event in the evolution of a galaxy.

It is possible that some of the far-infrared radiation originates from dust heated by the quasar itself. However, there is another very strong line of evidence in favour of a major episode of star formation in this galaxy. The very existence of a large mass of interstellar molecules and dust is proof that substantial star formation is taking place, or occurred at an even earlier time. In addition to carbon monoxide gas, the small dust particles are composed of what astronomers refer to as heavy elements or 'metals', such as carbon, oxygen, silicon and magnesium. All of these elements were produced after the Big Bang, inside stars, by a process known as nucleosynthesis9, and then ejected into the interstellar medium. This interstellar matter provides the raw material for a new generation of stars and planets. There are already indications from the spectrum of the optical-wavelength emission1 that high-redshift quasars show strong emission from metals, but this does not by itself indicate the amount or extent of star formation. The molecular gas and dust around J1148 + 5251 clearly show that the process of chemical enrichment was well advanced only 800 million years after the Big Bang. For this to occur in such a short time implies that the enrichment is due to the formation and evolution of high-mass stars, the same type of star responsible for the heating of the dust.

J1148 + 5251 is not the first high-redshift quasar found to have strong far-infrared radiation from warm dust. More than 20 quasars at redshift, z, higher than 3.8 have been identified10 whose high far-infrared luminosity is associated with star formation. CO emission has been detected in more than 13 quasar host galaxies with z larger than 2, including the previous high-redshift record-holder11 at z = 4.7. In addition, a large number of far-infrared sources have been found from 'blank field' surveys12,13, most of which are also very luminous and have redshifts in the range 1 to 3.5 (ref. 14). For only two of these have CO spectral lines been reported15, but it is likely that CO emission will be found in most of them now that their redshifts are known. These dust-enshrouded molecular-gas-rich galaxies account for about half of all star formation in the Universe.

Among all of these, the galaxy hosting J1148 + 5251 is the most distant and the earliest example of a star-forming region. That also makes it the only example so far of star formation occurring at a time before the hydrogen between galaxies (the intergalactic medium) was completely ionized by ultraviolet radiation from other galaxies and quasars, a time in the evolution of the Universe known as the epoch of reionization. The presence of even a few per cent of unionized intergalactic hydrogen atoms is a clear indicator that the Universe was in the early stages of lighting up, following the cooling off of the intergalactic medium a few hundred thousand years after the Big Bang.

The ability to observe molecular emission from such distant objects is due to the remarkable sensitivity of the instruments used by Walter et al.2 and Bertoldi et al.3 — the National Radio Astronomy Observatory's Very Large Array and the Plateau de Bure Interferometer of the Institut de Radio Astronomie Millimétrique (Fig. 1). In some cases there is also help from magnification of the signal by gravitational lensing along the line of site, although this is probably a small effect1 in J1148 + 5251. In addition, there is what astronomers refer to as a 'negative K correction' for the highly redshifted radiation from thermally excited rotational energy levels16 of molecules. For a given telescope operating at an approximately fixed frequency, it is possible to observe CO emission from redshifted galaxies simply by observing a spectral line from a higher rotational energy level and higher rest frequency, instead of lowering the frequency to match the redshift. This switching of spectral lines means that it is no harder to detect CO when the source is at a redshift of 6 than when it is at a redshift of 1 (provided that the source redshift is well known and the gas is not extremely cold).

Figure 1: East meets west.
figure1

K. GATLIN/P. SMILEY/NRAO/AUI; A. RAMBAUD/IRAM

The Very Large Array (left), on the plains of New Mexico, USA, is a Y-shaped configuration of 27 radio telescopes. In the Hautes Alpes of France, an array of five (now six) antennas forms the Plateau de Bure Interferometer (right). Arrays of telescopes such as these, operating synchronously, are equivalent to a single antenna dish up to many kilometres in diameter, and hence offer more power for resolving distant objects. Walter et al.2 and Bertoldi et al.3 have used data from both arrays in their study of the most distant astronomical object known, a quasar called J1148 + 5251, and have uncovered evidence for star formation at an early stage in the history of the Universe.

Nonetheless, these measurements are right at the limit of existing telescopes. The constraints will be lifted by the new generation of instruments, particularly the Atacama Large Millimeter Array of telescopes being built by an international collaboration in Chile. With increased resolution, sensitivity and bandwidth, this array will produce images of interstellar molecules and dust with the clarity that the Hubble Space Telescope has brought to optical astronomy. A fuller understanding of the formation of galaxies and stars in the early Universe awaits us.

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Solomon, P. An early stellar nursery. Nature 424, 382–383 (2003). https://doi.org/10.1038/424382a

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