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Monthly Archives: June 2015

Dark Lenses Magnify Star Formation in Dusty Galaxies

Dusty star-forming galaxies (DSFGs) are found in abundance in the early universe. They are especially bright because they are experiencing a large burst of high-rate star formation. Since they are mainly at higher redshifts, we are seeing them well in the past; the high star formation rates occur typically during the early life of a galaxy.

The optical light from new and existing stars in such galaxies is heavily absorbed by interstellar dust interior to the galaxy. The dust is quite cold, normally well below 100 Kelvins. It reradiates the absorbed energy thermally at low temperatures. As a result the galaxy becomes bright in the infrared and far infrared portions of the spectrum.

Dark matter has two roles here. First of all, each dusty star-forming galaxy would have formed from a “halo” dominated by dark matter. Secondly, dark matter lenses magnify the DSFGs significantly, allowing us to observe them and get decent measurements in the first place.

An international team of 27 astronomers has observed half a dozen DSFGs at 3.6 micron and 4.5 micron infrared wavelengths with the space-borne Spitzer telescope. These objects were originally identified at far infrared wavelengths with the Herschel telescope. Combining the infrared and far infrared measurements allows the researchers to determine the galaxy stellar masses and the star formation rates.

The six DSFGs observed by the team have redshifts ranging from 1.0 to 3.3 (corresponding to  look back times of roughly 8 to 12 billion years). Each of the 6 DSFGs has been magnified by “Einstein” lenses. The lensing effect is due to intervening foreground galaxies, which are also dominated by dark matter, and thus possessing sufficient gravitational fields that are able to significantly deflect and magnify the DSFG images. Each of the 6 DSFGs is therefore magnified by a lens that is mostly dark.

The lenses can result in the images of the DSFGs appearing as ring-shaped or arc-shaped. Multiple images are also possible. The magnification factors are quite large, ranging from a factor of 4 to a factor of more than 16 times. (Without dark matter’s contribution the magnification would be very much less).

It is a delicate process to subtract out the foreground galaxy, which is much brighter. The authors build a model for the foreground galaxy light profile and gravitational lensing effect in each case. They remove the light from the foreground galaxy computationally in order to reveal the residual light from the background DSFG. And they calculate the magnification factors so that they can determine the intrinsic luminosity of the DSFGs.

The stellar masses for these 6 DSFGs are found to be in the range of 80 to 400 billion solar masses, and their star formation rates are in the range of 100 to 500 solar masses per year.

One of the 6 galaxies, nicknamed HLock12, is shown in the Spitzer infrared image below, along with the foreground galaxy. The model of the foreground galaxy is subtracted out, such that in the rightmost panes, the DSFG image is more apparent. There are two rows of images, the top row shows measurements at 3.6 microns, and the bottom row is for observations at 4.5 microns.

This particular DSFG among the six was found to have a stellar mass of 300 billion solar masses and a total mass in dust of 3 billion solar masses. So the dust component is just about 1% of the stellar component. The estimated star formation rate is 500 solar masses per year, which is hundreds of times larger than the current star formation rate in our own Milky Way galaxy.

It is only because of the significant magnification through gravitational lensing (“dark lenses”) that researchers are able to obtain good measurements of these DSFGs. This lensing due to intervening dark matter allows astronomers to advance our understanding of galaxy formation and early evolution, much more quickly than would otherwise be possible.

HLock12

The figure 6 is from the paper referenced below. The top row shows (a) a Hubble telescope image of the field in the near infrared at 1.1 microns, and (b) the field at 3.6 microns from the Spitzer telescope. The arc is quite visible in the Hubble image in the upper right quadrant just adjacent to the foreground galaxy in the center. The model for the foreground galaxy is in column (c) and after subtraction the background galaxy image is in column (d), along with several other faint objects. The corresponding images in the bottom row are from Spitzer observations at 4.5 microns.

Reference

B. Ma et al. 2015, “Spitzer Imaging of Strongly-lensed Herschel-selected Dusty Star Forming Galaxies” http://arxiv.org/pdf/1504.05254v3.pdf

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Dusty Star-Forming Galaxies Brightened by Dark Matter

The first galaxies were formed within the first billion years of the Universe’s history. Our Milky Way galaxy contains very old stars with ages indicating formation around 500 or 600 million years after the Big Bang.

Astronomers are very eager to study galaxies in the early universe, in order to understand galaxy formation and evolution. They can do this by looking at the most distant galaxies. With the expanding universe of the Big Bang, the farther away a galaxy is, the farther back in time we are looking. Astronomers often use redshift to measure the distance, and hence age, of a galaxy. The larger the redshift, z, the farther back in time, and the closer to a galaxy’s birth and the universe’s birth.

The interstellar medium of a galaxy consists of gas and dust. The gas can be hot or cold, and in atomic or molecular form. Atomic gas may be ionized by ultraviolet starlight, or X-radiation from neutron stars or black holes (not the black holes themselves, but hot matter near the black hole), from cosmic rays or from other astrophysical mechanisms. Our Milky Way galaxy is rich in gas and dust, and contains thousands of molecular clouds. These are very cold clouds composed mainly of molecular hydrogen but also many other molecular species. Molecular clouds are the primary sites of new star formation. The Horsehead Nebula is an example of a molecular cloud in the constellation of Orion.

"Hubble Sees a Horsehead of a Different Color" by ESA/Hubble. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Hubble_Sees_a_Horsehead_of_a_Different_Color.jpg#/media/File:Hubble_Sees_a_Horsehead_of_a_Different_Color.jpg

“Hubble Sees a Horsehead of a Different Color” by ESA/Hubble. Licensed under CC BY 3.0 via Wikimedia Commons 

During their most active phase of star formation, a large galaxy might give birth to over 1000 solar masses worth of stars per year. By comparison, in the Milky Way galaxy, the new star formation rate is only of order 1 solar mass per year, the equivalent of 1 Sun, or, say, 2 stars with half the mass of our Sun, per annum. Over its entire 13 billion year life the Milky Way has formed many hundreds of billions of stars, so clearly the star formation rate was higher in the past.

Before the first stars and galaxies form, the universe contains only hydrogen and helium, and no heavier elements. Those are produced by thermonuclear reactions in stellar interiors. This is a wonderful thing, because carbon, oxygen and other heavy elements are essential to life.

After a galaxy produces its first generation of massive stars, its interstellar medium will begin to contain carbon, nitrogen, oxygen and other heavy elements (heavy means anything above helium, in this context). Massive stars (above a few solar masses) evolve rapidly, with timescales in the millions of years, rather than billions, and explode as supernovae at the end of their lives. A large portion of their material, now containing heavy elements as well as hydrogen and helium, is expelled at high velocity and mixed into the interstellar medium. The carbon, nitrogen and oxygen which is then in the respective galaxy’s interstellar medium can be detected in atomic (including ionized) or molecular forms. The relative abundance of heavy elements grows with time as more stars are formed, evolve, and recycle matter into the interstellar medium.

High-redshift (z > 2) galaxies with active star formation are best observed in the infrared. The gas and dust in molecular clouds is quite cold, usually less than 100 K (100 degrees above absolute zero). And their radiation is shifted further toward the far infrared and sub-millimeter portions of the spectrum by the redshift factor of 1+z. So radiation emitted at 100 microns is detected at the Earth at 400 microns for a source at z = 3.

These are difficult measurements to make, because if the galaxy is very distant, it is also very faint. However the possibility of getting good measurements is helped by two things. One is that galaxies with very active star formation are intrinsically brighter.

And the other reason is that intervening clusters of galaxies are massive and contain mostly dark matter. As we look far back through the universe toward an early galaxy, there is a good chance that the line of sight passes through a cluster of galaxies. Clusters of galaxies contain hundreds or even thousands of galaxies, and are dominated by dark matter. Most of the infrared radiation can pass through the intracluster medium – the space between galaxies – without being absorbed; it does not interact with dark matter. The clusters are sufficiently massive to bend the light, however, according to general relativity. As the background galaxy’s light passes through the cluster during its multi-billion year journey to the Earth and our telescopes, the cluster’s gravitational potential modifies the light ray’s path. Actually the intervening cluster of galaxies does more than displace the light, it acts as a lens, causing the image to brighten by as much as 10 times or more. This makes it much easier to gather enough photons from the target galaxy to obtain good quality results.

An international research team with participants from Germany, the U.S., Chile, the U.K. and Canada has identified 20 high redshift “dusty star forming galaxies” at very high redshift (DSFG is a technical term for galaxies with high star formation rates and lots of dust) from the South Pole Telescope infrared galaxy survey. They have been able to further elucidate the nature of 17 of these early galaxies by measuring C II emission from singly ionized atomic carbon, and CO emission from carbon monoxide molecules for 11 of those. They have also determined the total far infrared luminosity for these target galaxies. Their results allow them to place constraints on the nature of the interstellar medium and the properties of molecular clouds.

The galaxies’ high redshifts, ranging from z = 2.1  to 5.7, actually makes it possible to make Earth-bound measurements in most cases. At lower redshifts the observations would not be possible from Earth because the Earth’s atmosphere is highly opaque at the observation frequencies. But it is much more transparent at longer wavelengths, so as the redshift exceeds z = 3, Earth-based observations are possible from favorable locations, in this case the Chilean desert. For three sources with redshifts around 2 the atmosphere prohibits ground-based observations and the team therefore made observations from the orbiting Herschel Space Telescope, designed for infrared work.

The figure labelled Figure 3 below is taken from their paper. It indicates the redshift z on the x-axis (logarithmically) and the far infrared luminosity of the galaxy on the y-axis (as the log) as well. The 17 galaxies studied by the authors are indicated with red dots and labelled “SPT DSFGs”. Their very high luminosities are in the range of 10 to 100 trillion times the Sun’s luminosity. Note that the luminosities must be very high for detection at such a high redshift (distance from Earth). Also these luminosities are uncorrected for the lensing magnification, so the true luminosities are around an order of magnitude lower.

CIIEmissionDSFGfig3

The redshift range covered in this research corresponds to ages for the universe of around 1 billion years old (z = 5.7) to a little over 3 billion years old (z = 2.1). So the lookback time is roughly 11 to 13 billion years.

For those of us interested in dark matter, their findings regarding the degree of magnification by dark matter are also interesting. They find “strong lensing” or magnification in the range of 5 to 21 times for 4 sources that allowed for lens modeling. The other sources do not have magnifications measured, but they are presumed to be of the same order of magnitude of around 10 times or so, to within a factor of 2 either way.

It is only because the lensing is so substantial that they are able to measure these galaxies with sufficient fidelity to arrive at their results. So not only is dark matter key to galaxy formation and evolution, it is key to allowing us to study galaxies in the early universe. Dark matter forms galaxies and then helps us understand how they form!

Reference

B. Gullberg et al. 2015, ”The nature of the [CII] emission in dusty star-forming galaxies from the SPT survey” to be published, Monthly Notices of the Royal Astronomical Society, http://arxiv.org/pdf/1501.06909v2.pdf

C.M. Casey, D. Narayanan, A. Cooray 2015, “Dusty Star-Forming Galaxies at High Redshift”, http://arxiv.org/abs/1402.1456