The new version of Hubble’s deep image is shown above. In dark grey you can see the new light that has been found surrounding nearly every galaxy in this field. That light corresponds to the brightness of more than one hundred billion suns. It took researchers at the Instituto de Astrofísica de Canarias almost three years to produce this deepest image of the Universe ever taken from space by recovering a large quantity of ‘lost’ light around the largest galaxies in the iconic Hubble Ultra-Deep Field (HUDF).
“The Time Machine”
To produce the image, a group of researchers from the Instituto de Astrofísica de Canarias (IAC), led by author of this article Alejandro S. Borlaff, used original HUDF images from the Hubble Space Telescope. After improving the process of combining several images the group was able to recover a large quantity of light from the outer zones of the largest galaxies in the HUDF. Recovering this light, emitted by the stars in these outer zones, was equivalent to recovering the light from a complete galaxy (“smeared out” over the whole field) and for some galaxies this missing light shows that they have diameters almost twice as big as previously measured.
In an email to the Daily Galaxy, Borlaff provided an additional graphic and explanation, “the HUDF is one of the most amazing efforts ever done to probe into the past of the Universe. It is, in itself, a time machine. Located away from bright stars or nearby galaxies, the only light that we receive from this region of the sky comes from objects that lived 7 to 10 billion years ago” (a significant fraction of the 13.8 billion year age of the Universe). Borlaff continued, “The HUDF was designed to detect the first galaxies that existed in the Universe, heavily dimmed and reddened by the effects of the expansion of the Universe while the light was traveling to our telescopes. In the color ABYSS HUDF image below, these galaxies are the tiny red smudges that we can see in the background.”
The HUDF is the result of combining hundreds of images taken with the Wide Field Camera 3 (WFC3) of the HST during over 230 hours of observation which, in 2012, yielded the deepest image of the Universe taken until then. But the method of combining the individual images was not ideally suited to detect faint extended objects. To do this, Borlaff explains “What we have done is to go back to the archive of the original images, directly as observed by the HST, and improve the process of combination, aiming at the best image quality not only for the more distant smaller galaxies but also for the extended regions of the largest galaxies.”
The ABYSS HUDF –Centered around the HUDF-5 elliptical galaxy
The WFC3 with which the data were taken was installed by astronauts in May 2009, when the Hubble had already been in space for 19 years. This was a major challenge for the researchers because the complete instrument (telescope + camera) could not be tested on the ground, which made calibration more difficult. To overcome the problems they analyzed several thousand images of different regions on the sky, with the aim of improving the calibration of the telescope on orbit.
Borlaff expanded further in his email to the Daily Galaxy, “the gif animation below shows the improvement between the different versions of the HUDF. In order, the animation shows the F105W filter images of the eXtreme Deep Field (Illingworth et al. 2013), the HUDF 2012 (Koekemoer et al. 2013), and last, our version ABYSS HUDF (Borlaff et al. 2019), centered around the HUDF-5 elliptical galaxy. All images are scaled to the same color scale, and the black contour represents the 29 mag arcsec^-2 isophote. Notice how in order, the XDF and the HUDF12 show a smaller size of this 29 mag arcsec^-2 boundary, meaning that the stellar halo in the new ABYSS version is much brighter, and therefore, the galaxy is bigger than previously thought! (from 50 kpc to 140 kpc!).”
Mysterious Outer Envelopes of Light
“Interestingly, not all galaxies in this picture are that far away. The relatively larger galaxies are closer to us than the previous ones, and for these, we can use the fantastic sensitivity of Hubble to detect the mysterious outer envelopes of light that surround them.”
Borlaff continued, “Galaxies are formed by the accretion of gas and stars from their surroundings. Sometimes, because of their enormous gravitational field, they absorb some of their own kind. That is called galactic merging (or cannibalism, to add some dramatism). During the process of galactic accretion, the smallest galaxy usually is elongated, shredded, and ultimately destroyed around the largest one, leaving a characteristic trace of gas and stars on its way, which is very dim, and only detectable with the most advanced telescopes and imaging techniques (reference to tidal tail image). If a sufficient amount of accretion events happen, we (the astronomers) suspect that a galactic halo might form around the cannibal galaxy. And that’s what we were looking for in the Hubble Ultra Deep Field.”
HUDF-5 –One of the brightest objects in the field of view
“For example, one of the largest galaxies in this image is HUDF-5. HUDF-5 is an elliptical galaxy with approximately 150 billion stars, and one of the brightest objects in the field of view. In the earlier version of the HUDF images (eXtreme Deep Field, Illingworth et al. 2013), the light of the stellar halo extended over ~50 kpc from the galactic center. That means that it would take 320,000 years for a photon traveling at the speed of light to cross it from side to side, almost the time that separates us from the first Homo Sapiens. That is an astonishing size, right? Well, apparently that was not all.”
“Thanks to the Hubble Space Telescope archive and our new HUDF version, we could enhance the sensitivity of the telescope and look even deeper into the surroundings of HUDF-5. In the new images, the HUDF-5 halo can be detected out to 140 kpc from the galactic center, compared to the previous 50 kpc. That means that we were not seeing more than half of the galaxy! This outer halo is made of stars, possibly accreted during the formation of this enormous elliptical galaxy more than 7 billion years ago.”
Why the galaxies bigger on the new update
Borlaff concluded, “But why are the galaxies bigger on the new update of this image? The reason is that our capability to detect astronomical objects is completely limited by how well we know our telescope, and how good we are at correcting the problems that a 30-year old space telescope might have in orbit. We call these problems systematic effects, and in this article (Borlaff et al. 2019) we provide a three-year-long effort to track, characterize, and correct many of these. Once the systematic effects are well-known, they can be subtracted from the individual exposures, improving the quality of the final mosaics. HUDF-5 is only one example of the amazing sensitivity power to be recovered from the Hubble Space Telescope using new imaging processing techniques on archival observations.”
The image of the universe which is now the deepest “has been possible thanks to a striking improvement in the techniques of image processing which has been achieved in recent years, a field in which the group working in the IAC is at the forefront”, says Borlaff.
Image Credits: A. S. Borlaff et al.