May 6, 2010

Herschel Reveals Galaxies In The GOODS Fields

The discovery of a previously unresolved population of galaxies in the GOODS fields and the first measurements of properties of galaxies in the almost unexplored far-infrared domain are among the first exciting scientific results achieved by Herschel's PACS and SPIRE instruments. These findings confirm the extraordinary capabilities of ESA's new infrared space observatory to investigate the formation and evolution of galaxies.

These are some of the many discoveries presented this week at the Herschel First Results Symposium, ESLAB 2010, held at the European Space Research and Technology Centre, Noordwijk, The Netherlands.

Understanding the details of how galaxies formed and evolved throughout cosmic history is one of the main goals of current astrophysical research. ESA's Herschel space observatory has begun to address this issue by joining in GOODS - the Great Observatories Origins Deep Survey - an ambitious project conceived to shed new light on this open topic.

Two carefully selected regions of the northern and southern sky, centered on the Hubble Deep Field North and the Chandra Deep Field South, have been the target of deep surveys conducted during the past decade over an extremely broad wavelength range, by means of ESA's and NASA's space observatories and the foremost ground-based telescopes. The GOODS fields, each measuring 10 by 16 arc minutes, are ideal for studying galaxies out to very high redshifts, as they do not contain any bright star and are not contaminated by strong emission coming from the Milky Way.

"Although both GOODS fields have been the object of extensive observations in the past, they have not yet been explored in the far-infrared region of the electromagnetic spectrum," explains Göran Pilbratt, Herschel Project Scientist. "Exploiting Herschel's powerful infrared eye and its broad wavelength coverage, we are now in the process of revealing some of the secrets that have been concealed until now."

In October 2009, the Photodetector Array Camera & Spectrograph (PACS) and the Spectral Imaging and Photometric Receiver (SPIRE) aboard Herschel observed the GOODS-North field. Three months later it was the turn of the GOODS-South field to be scrutinized by PACS. These PACS and SPIRE observations have yielded the most sensitive images yet obtained by Herschel. Thanks to these highly sensitive instruments and to the telescope's large mirror, astronomers have been able, for the first time, to pinpoint a large fraction of the galaxies responsible for a diffuse light emission in the infrared, the so-called Cosmic Infrared Background (CIB).

The CIB is a relic, isotropic emission distinct from the Cosmic Microwave Background (CMB) and is associated with the formation of galaxies. Predicted in the mid-1960s, it was first detected, only 30 years later, with the Cosmic Background Explorer (COBE). The CIB is particularly hard to probe, as all galaxies at all redshifts contribute to it and hence it has no characteristic signature.

Infrared observations carried out with the ISO and Spitzer space telescopes already detected a handful of galaxies producing part of this background radiation, in the mid-infrared, but they could not access the bulk of it. "Since the CIB peaks around 100-200 microns, the majority of the galaxies contributing to this emission remained unidentified in the pre-Herschel era," says Dieter Lutz from the Max Planck Institute for extraterrestrial Physics and leader of the PACS Evolutionary Probe Key Program which includes the GOODS fields amongst its targets.

With both the wavelength coverage and the technical characteristics required to resolve this 'cosmic fog' into individual galaxies, Herschel's PACS has isolated the sources of about a half of the CIB in the GOODS fields. "Thanks to the wealth of complementary data available for these fields, we have also studied how many of these galaxies are to be found at various epochs in cosmic history," adds Stefano Berta, who led this study. In fact, most of these galaxies are located at relatively low redshifts (z<1), their light having travelled less than 8000 million years before reaching us.

In peering into the GOODS-North field, SPIRE has yielded the first observations of this region of the sky at this instrument's characteristic wavelengths (250-500 microns). This new spectral window allows astronomers to look at star-forming galaxies at even higher redshifts. These are normally hidden from observations at shorter wavelengths because most of the energy emitted in star formation processes is absorbed by dust and re-emitted in the far-infrared domain.

"We know that the energy we receive from galaxies directly is roughly as much as the energy we receive from them after it has been reprocessed by dust," says Seb Oliver from the University of Sussex and leader of the HerMES Key Program which probes galaxy evolution. "Thanks to SPIRE and PACS, we can finally explore how the population of obscured galaxies has evolved over cosmic time."

The true power of Herschel is unleashed when the images from both instruments are studied together. As a result of this synergy, observations of galaxies made by SPIRE on the GOODS-North field complement the studies on the CIB performed with PACS data. "A significant fraction of galaxies contributing to the CIB has also been resolved by SPIRE," comments Oliver. "This population of galaxies is dominated by sources at z~1, demonstrating that most of the infrared background radiation is emitted at low redshifts," adds Steve Eales from Cardiff University.

By sampling galaxies at the peak of their far-infrared emission, Herschel allows also a better understanding of the star-forming processes taking place within them. "The properties of galaxies in these new PACS and SPIRE images appear surprisingly uniform over the last 10 billion years of cosmic history, even for those galaxies harboring an active nucleus," says David Elbaz from Laboratoire AIM-Paris-Saclay. This finding suggests that the history of star formation in the Universe is governed by simple, universal mechanisms. "This is only a first step, since a new window on the GOODS fields has just been opened with the GOODS-Herschel Open Time Key Program. This will push Herschel to its ultimate limits in terms of depth," adds Elbaz.

And it is clear that much more is in store when placing Herschel data in the broader context of the wider electromagnetic spectrum.  Some interesting results have already arisen when comparing Herschel data with radio observations of the GOODS-North field performed by the Very Large Array.  "We have revealed tantalizing signs of an evolution of the famous (and surprisingly strong) correlation between infrared emission and radio emission," notes Rob Ivison from UK Astronomy Technology Centre at the Royal Observatory in Edinburgh.


Image 1: This Herschel-PACS image shows the GOODS-South field at far-infrared wavelengths of 70, 100 and 160 µm. This color composite image has been constructed by mapping 70 µm to blue, 100 µm to green, and 160 µm to red. The data for this image were obtained in January 2010. Credits: ESA/PACS Consortium/PEP Key Program Consortium

Image 2: This Herschel-PACS image shows the GOODS-North field (in the constellation of Ursa Major) at far-infrared wavelengths of 100 and 160 µm. Galaxies at high redshift (i.e. larger cosmological distance) or with colder dust are displayed in red, while nearby galaxies appear in blue. The data for this image were obtained in October 2009. Credits: ESA/PACS Consortium/PEP Key Program Consortium

Image 3: This image of the GOODS-North field is made from the three SPIRE bands, with red, green and blue corresponding to 500 microns, 350 microns and 250 microns, respectively. Every fuzzy blob in this image is a very distant galaxy, seen as they were 3-10 thousand million years ago when star formation was very widely spread throughout the Universe. This is a subset of the entire SPIRE GOODS-N field, which covers an area of 30 x 30 arc minutes. The data for this image were obtained in October 2009. Credits: ESA/SPIRE Consortium/HerMES Key Program Consortium


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