February 8, 2016
The VIMOS UltraDeep Survey (short: VUDS) is an observational program to gain spectroscopic measurements for ~10,000 galaxies at high redshift, when the Universe was only between 1- 3 billion years old (today, the Universe is 13.8 billion years old). This is a particularly interesting era to study in terms of galaxy evolution since astronomers expect galaxies at that epoch to look very different from today. For example, at that early time we observe that galaxies have a much more disturbed morphology compared to the beautiful structured spiral galaxies or smooth elliptical galaxies that we see in the local Universe. We expect that galaxies formed many more stars at that time partly triggered by disturbances from the merging of galaxies but also because more gas was still available to form stars in those galaxies. The time between redshift 2 to 6 (i.e. the first 1-3 billion years of the Universe’s age) is thus a major epoch of galaxy assembly.
With CANDELS, galaxies in that epoch are studied mostly based on photometry, meaning images taken at different wavelengths. We described in earlier blog posts how with photometry at many different wavelengths astronomers are able to study the properties of galaxies through comparing the observed data to model galaxy spectra.
With VUDS galaxy evolution is approached from the spectroscopic side. A spectrum of an object is created by dispersing all its emitted light by directing it through a disperser like a prism, meaning the light is split up according to its wavelength. An easy example is the creation of a rainbow where the light from the sun hits raindrops in the air which act as dispersers and split the originally “white” sunlight up by wavelength, creating the typical coloured stripes. Such spectra allow us to study the properties of galaxies in much more detail compared to the study of images alone.
The VUDS survey covers about 1 square degree in the sky. As a comparison the diameter of the full moon is about 0.5 degrees and its area is ~0.2 square degrees, which means it’s a fifth of the area covered by the VUDS survey. However, this 1 square degree of area of the VUDS survey is split over 3 separate fields in the sky that have been observed with a lot of different instruments and at many wavelengths already, creating a unique and precious data set for astronomers to carry out their studies. The three fields are the COSMOS field (which overlaps with the CANDELS-COSMOS field), the Extended-Chandra Deep Field South (which overlaps with the CANDELS-GOODS-South field) and the VVDS-2h field. Within those 3 fields spectra of ~10,000 galaxies were taken with the VIMOS multi-object spectrograph at the Very Large Telescope (VLT) in Chile (Figure 1). We described how multi-object spectroscopy works in more detail in this recent post. In short, suffice it to say that with that instrument, astronomers are able to take a spectrum of many galaxies at the same time. VUDS is the largest spectroscopic survey of galaxies at these early cosmic times.
Two of the 3 fields covered by VUDS overlap with the CANDELS area. The spectra and spectroscopic redshifts in that overlap area (~ 700 galaxies) were just publicly released by the VUDS team.
For the VUDS survey, the objects which were targeted for the spectroscopy, were selected primarily based on their redshift as derived purely from photometry (again, see this blog post here). Additionally, some sources were added based on their photometric colours (i.e. the difference in brightness between two wavelength bands) which indicate a high redshift. These objects were then observed with two different grisms — one for the blue wavelength end and one for the red wavelength end – for about 14 hours each. The resulting spectra cover a wavelength range from the blue optical to the very red optical. This means that for these high-redshift galaxies, we really observed their ultra-violet to blue optical wavelength range which are shifted due to the redshift into the optical wavelength range covered by the VIMOS instrument.
This wavelength range reveals many properties of galaxies, especially with regard to their star formation. In Figure 2 we show you a stacked spectrum of some VUDS sources in which also the spectral lines are indicated. In Figure 3 you can see all the spectra of the VUDS survey compiled into a picture and sorted by redshift, where each line represents one spectrum. Emission and absorption lines in this image are nicely visible in this as bright and dark lines that stretch across the image from left bottom to top right. This also illustrates how spectral features are redshifted towards redder wavelengths. The most common spectral lines and features in these spectra are the Hydrogen Lyman-alpha, Lyman-beta and Lyman-gamma lines, the Lyman limit (below which almost all emission is absorbed by neutral Hydrogen around newly formed stars), the Carbon lines (CII, CIII and CIV, where the Roman numbers behind the letters indicate the ionization level of the element) and lines from Helium (He), Oxygen (O), Silicon (Si) and Aluminium (Al). These lines are used not only to determine the spectroscopic redshift of these galaxies (i.e., through their known rest-frame wavelength), but also other galaxy properties such as star formation and chemical composition of the galaxies. Overall in VUDS we were able to determine reliable spectroscopic redshifts for ~6000 galaxies which cover a large range of brightnesses and stellar masses. Some of the galaxies in this survey form up to 1000 solar masses per year!
Since the completion of the observations, many researchers in the international VUDS team work on all aspects of galaxy formation and evolution, from morphology to identifying proto-galaxy-clusters and groups, from studying the ultra-violet spectroscopic properties of very young galaxies to the merging history of the Universe, in alignment with the science goals of the overall survey. If you are interested in following the results from the VUDS survey, you can find our Facebook page here and our Twitter account here.
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About the CANDELS blog
In late 2009, the Hubble Space Telescope began an ambitious program to map five carefully selected areas of the sky with its sensitive near-infrared camera, the Wide-Field Camera 3. The observations are important for addressing a wide variety of questions, from testing theories for the birth and evolution of galaxies, to refining our understanding of the geometry of the universe.
This is a research blog written by people involved in the project. We aim to share some of the excitement of working at the scientific frontier, using one of the greatest telescopes ever built. We will also share some of the trials and tribulations of making the project work, from the complications of planning and scheduling the observations to the challenges of trying to understand the data. Along the way, we may comment on trends in astronomy or other such topics.
CANDELS stands for the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. It builds on the legacy of the Hubble Deep Field, as well as the wider-area surveys called GOODS, AEGIS, COSMOS, and UKIDSS UDS. The CANDELS observations are designed to search for galaxies within about a billion years of the big bang, study galaxies at cosmic high-noon about 3 billion years after the big bang – when star-formation and black hole growth were at their peak intensity – and discover distant supernovae for refining our understanding of cosmic acceleration. You can find more details, and download the CANDELS data, from the CANDELS website.
You can also use the Hubble Legacy Archive to view the CANDELS images.