Even Small Galaxies Can Have Big Black Holes

Astronomers detected supermassive black holes in 28 distant, low-mass galaxies, including the four shown in these Hubble Space Telescope images. Image credit: A. Koekemoer, Space Telescope Science Institute.


The Hubble Space Telescope has done it again. By utilizing a slitless grism, the Wide Field Camera 3 has uncovered evidence that supermassive black holes are right at home in some very small galaxies. Apparently these central black holes began their life when their host galaxies were first forming!

“It’s kind of a chicken or egg problem: Which came first, the supermassive black hole or the massive galaxy? This study shows that even low-mass galaxies have supermassive black holes,” said Jonathan Trump, a postdoctoral researcher at the University of California, Santa Cruz. Trump is first author of the study, which has been accepted for publication in the Astrophysical Journal.

It’s another cosmic conundrum. As we’ve learned, large galaxies are host to central supermassive black holes and many of them are the AGN variety. But the real puzzle is why do some smaller galaxies contain them when most do not? By taking a closer look at dwarf galaxies some 10 billion light-years away, astronomers are reaching back in time to when the Universe was about an estimated quarter of its current age.

“When we look 10 billion years ago, we’re looking at the teenage years of the universe. So these are very small, young galaxies,” Trump said.

If your mind is still wondering what a “slitless grism” is, then wonder no more. It’s part of Hubble’s WFC3 infrared camera that provides spectroscopic information. Thanks to highly detailed information on the different wavelengths of light, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) team could achieve separate spectra from each sector of the candidate galaxies and identify emissions from black hole sources.

“This is the first study that is capable of probing for the existence of small, low-luminosity black holes back in time,” said coauthor Sandra Faber, University Professor of astronomy and astrophysics at UC Santa Cruz and CANDELS principal investigator. “Up to now, observations of distant galaxies have consistently reinforced the local findings–distant black holes actively accreting in big galaxies only. We now have a big puzzle: What happened to these dwarf galaxies?”

It’s possible they are forerunners of the massive galaxies we see today. “Some may remain small, and some may grow into something like the Milky Way,” Trump said. But this theory is a juxtaposition in itself. According to Faber, “To become big galaxies today, the dwarf galaxies would have to grow at a rate much faster than standard models predict. If they remain small, then nearby dwarf galaxies should also have central black holes. There might be a large population of small black holes in dwarf galaxies that no one has noticed before.”

But these distant little dwarfs aren’t quiet – they are actively forming new stars. According to Trump, “Their star formation rate is about ten times that of the Milky Way. There may be a connection between that and the active galactic nuclei. When gas is available to form new stars, it’s also available to feed the black hole.”

But the Hubble wasn’t the only instrument interested in the 28 small galaxy studies. The team also employed x-ray data acquired by NASA’s Chandra X-ray Observatory. To help refine their information on such small, faint objects, the data was combined to improve the signal-to-noise ratio.

“This is a powerful technique that we can use for similar studies in the future on larger samples of objects,” Trump said. “Together the compactness of the stacked OIII spatial profile and the stacked X-ray data suggest that at least some of these low-mass, low-metallicity galaxies harbor weak active galactic nuclei.”

Original Story Source: University of Santa Cruz News. For Further Reading: A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation.

Hubble Takes a New “Deep Field” Image with Wide Field Camera 3

Hubble’s latest image is another stunner — and just look at all the galaxies! Hubble has produced a new version of the Ultra Deep Field, this time in near-infrared light and taken with the newly installed Wide Field Camera 3. This is the deepest image yet of the Universe in near-infrared, and so the faintest and reddest objects in the image are likely the oldest galaxies ever identified, and they likely formed only 600–900 million years after the Big Bang. This image was taken in the same region as the visible Ultra Deep Field in 2004, but this new deep view at longer wavelengths provides insights into how galaxies grew in their formative years early in the Universe’s history.

“Hubble has now re-visited the Ultra Deep Field which we first studied 5 years ago, taking infrared images which are more sensitive than anything obtained before,” said Dr. Daniel Stark, a postdoctoral researcher from Cambridge University. “We can now look even further back in time, identifying galaxies when the Universe was only 5 percent of its current age – within 1 billion years of the Big Bang.”

A portion of the Hubble Ultra Deep Field showing the location of a potentially very distant galaxy (marked by crosshairs).   Credit: Oxford University
A portion of the Hubble Ultra Deep Field showing the location of a potentially very distant galaxy (marked by crosshairs). Credit: Oxford University

The image was taken during a total of four days in August 2009, with 173,000 seconds of total exposure time. Since infrared light is invisible to the human eye and therefore does not have colors that can be perceived, the image is a “natural” representation that in shorter infrared wavelengths are represented as blue and the longer wavelengths as red. The faintest objects are about one billion times fainter than the dimmest visible objects seen with the naked eye.

Click here for a video zooming into the Ultra Deep Field.

“The expansion of the Universe causes the light from very distant galaxies to appear more red, so having a new camera on Hubble which is very sensitive in the infrared means we can identify galaxies at much greater distances than previously possible,” said Stephen Wilkins, from Oxford University.

Where is the new Ultra Deep Field in the sky?  Credit: HubbleSite
Where is the new Ultra Deep Field in the sky? Credit: HubbleSite

The team that took this image in August of 2009 have made it available for research by astronomers worldwide, and a multitude of astronomers have been furiously searching through the data for the most distant galaxies yet discovered. In just three months, twelve scientific papers on these new data have been submitted.

As well as identifying potentially the most distant objects yet, these new HST observations present an intriguing puzzle. “We know the gas between galaxies in the Universe was ionized (or fried) early in history, but the total light from these new galaxies may not be sufficient to achieve this,” said Andrew Bunker, from the University of Oxford.

Installation of Wide Field Camera 3 by astronauts as part of servicing mission 4. Courtesy of NASA.
Installation of Wide Field Camera 3 by astronauts as part of servicing mission 4. Courtesy of NASA.

“These new observations from HST are likely to be the most sensitive images Hubble will ever take, but the very distant galaxies we have now discovered will be studied in detail by Hubble’s successor, the James Webb Space Telescope, which will be launched in 2014,” said Professor Jim Dunlop at the University of Edinburgh.

1. By R.J. McLure, J.S. Dunlop, M. Cirasuolo, A.M. Koekemoer, E. Sabbi, D.P. Stark, T.A. Targett, R.S. Ellis,

2. By Stephen M. Wilkins, Andrew J. Bunker, Richard S. Ellis, Daniel Stark, Elizabeth R. Stanway, Kuenley Chiu, Silvio Lorenzoni, Matt J. Jarvis

3. By Bunker, Andrew; Wilkins, Stephen; Ellis, Richard; Stark, Daniel; Lorenzoni, Silvio; Chiu, Kuenley; Lacy, Mark; Jarvis, Matt; Hickey, Samantha,

Sources: Oxford University, Space Telescope Center