Faint Galaxies in Virgo

Welcome to the newly relaunched Hoku. Like the previous incarnation of Hoku, this page is devoted to information for educators, parents, families and those of all ages interested in astronomy. The biggest change is Hoku has transformed into a blog, which allows us to update more frequently. To this end, we will have regular postings that detail the comings and goings of CFHT. For those of you that follow the CFHT Facebook page, this space will have a similar feel, but the blog allows us to delve into more details than we can on Facebook. We’ll also be including a monthly stargazing column and other fun posts. Thanks for joining us!

And now our very first post….

On May 26th, Jonathan Grossauer and James Taylor at the University of Waterloo, announced the discovery of hundreds of new galaxies in the Virgo Cluster, the nearest large cluster of galaxies to our home in the Milky Way. Most of the galaxies are extremely faint dwarf galaxies. Dwarf galaxies are hundreds of thousands of times smaller than our own Milky Way galaxy. Many of these are among the faintest galaxies known in the Universe.

The discovery was announced by the “Next Generation Virgo Cluster Survey” (NGVS) team and is based on data collected at CFHT over the course of 6 years using Megacam, CFHT’s 340 Megapixel camera. NGVS observed the entire Virgo cluster, covering an area of the sky equivalent to over 400 full moons. Astronomers created a mosaic of the image, almost 40 billion pixels in size, making NGVS the deepest, widest, continuous field imaged in such detail.

Very faint dwarf galaxies in the Virgo cluster field.  The inset is a zoomed in picture of the red box on the field
Very faint dwarf galaxies in the Virgo cluster field. The inset is a zoomed in picture of the red box on the field

To utilize the full power of the data, Laura Ferrarese, Lauren McArthur and Patrick Cote of the National Research Council of Canada developed a sophisticated data analysis technique that allowed them to discover many times more galaxies than were known previously, including some of the faintest and most diffuse objects ever detected.

The sheer number of dwarf galaxies discovered intrigued astronomers. Studying galaxy formation is complicated. Unlike watching plants grow or the life cycles of fruit flies in biology, astronomers do not have the opportunity to watch one galaxy form start to finish. Instead they observe many galaxies across the universe at different stages of their development and try to fill in the gaps. Galaxy clusters are ideal to study because they contain galaxies of varying sizes and shapes. Some are huge spiral galaxies like our own Milky Way while others are smaller faint dwarfs. Astronomers study these clusters and create computer simulations to try and understand how galaxies form.

These computer models predict that hundreds or thousands of faint dwarf galaxies should have formed within our Local Group, the galaxy cluster containing the Milky Way and our large neighbor, the Andromeda Galaxy. However, astronomers have discovered fewer than 100. Compare this with the new results from NGVS and a disconnect emerges between what we see in our Local Group and what astronomers observed in the Virgo Cluster and predict with their models. The models also show a relationship in the Virgo Cluster between dark matter mass and the brightness of galaxies. This relationship is missing in our Local Group. Why are the Virgo Cluster and the Local Group so different? Astronomers aren’t sure. The answer to that question requires a follow-up study with higher resolution simulations to explore how the galaxies are distributed within the Virgo cluster.

For the full press release, see the CFHT news page

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