Question

The “bottom-up” theory of galaxy formation postulates that galaxies are built up from smaller bodies, knots of gas that were a bit more dense than the surrounding gas that then collapsed under their own gravity and began to accrete additional material.

A. Calculate how many “building blocks” are required to form a galaxy with the mass of the Milky Way. Assume that each “block” has a mass of 1 million solar masses. Show your calculation.

B. If this galaxy formed over a time span of 5 billion years, how many years must have elapsed between mergers of these building blocks during its “construction phase”?

C. What evidence do astronomers know of that supports this theory for galaxy formation? Give at least four pieces of observational evidence and explain how they support the theory.

Answer 1

A). Calculate how many “building blocks” are required to form a galaxy with the mass of the Milky Way. Assume that each “block” has a mass of 1 million solar masses. Show your calculation.

Mass of the Milky Way = 1.5 trillion slolar mass unit = 1.5 x 10¹² solar mass.

mass of each block =  1 million solar mass = 10⁶ solar mass.

Hence

  “building blocks” required to form a galaxy with the mass of the Milky

= ( 1.5 x 10¹² solar mass ) / ( 10⁶ solar mass ) = 1.5 x10⁶ building blocks

Or 1.5 million building blocks.

1.5 million building blocks are required to form a galaxy with the mass of the Milky Way.

B). If this galaxy formed over a time span of 5 billion years, how many years must have elapsed between mergers of these building blocks during its “construction phase”?

1 billion= 10⁹

Time span to form galaxy =5 billion years = 5 x 10⁹  years

Our Galaxy is a spiral galaxy that formed approximately 14 billion years ago.

If time span to form galaxy is 5 billion years, then

years must have elapsed between mergers of these building blocks during its “construction phase” is

=14 billio n years - 5 billion years = 9 billion years.

C). What evidence do astronomers know of that supports this theory for galaxy formation? Give at least four pieces of observational evidence and explain how they support the theory.

New Evidence About the Formation of Galaxies

* There are two leading theories to explain how the first galaxies formed. The truth may involve a bit of both ideas.

One says that galaxies were born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form.

The other, which has gained strength in recent years, says the young universe contained many small "lumps" of matter, which clumped together to form galaxies. Hubble Space Telescope has photographed many such lumps, which may be the precursors to modern galaxies. According to this theory, most of the early large galaxies were spirals. But over time, many spirals merged to form ellipticals.T

* Astronomers have long believed that galaxy formation in the early Universe was a spectacular event, with smaller groups smashing together to form larger elliptical galaxies, and star formation would have been everywhere.

* New data gathered by the SCUBA ( sensitive submillimetre camera, SCUBA) telescope help support this theory.

* The SCUBA images support a popular current model of galaxy formation in which today’s massive elliptical galaxies were assembled in the early Universe in dense regions of space through the rapid merging of smaller building blocks.

Give at least four pieces of observational evidence and explain how they support the theory.

* According to Prof. Jim Dunlop, Head of the University of Edinburgh’s Institute for Astronomy “For a long time astronomers have anticipated that the formation of the most massive galaxies should have been a spectacular event, but failed to find any observational evidence of massive galaxy formation from optical images. Now we have discovered that it is indeed spectacular, but because of the effects of interstellar dust, the spectaclular phenomenon  is only revealed at submillimetre wavelengths.”

* The dust absorbs the bright blue light emitted by young stars. The energy from the light heats the dust and makes it glow. It is this glow that is detected by the SCUBA camera.

* Dr Jason Stevens, astronomer at the UK ATC in Edinburgh explained why understanding the evolution of these galaxies is so important. “The distant, youthful Universe was a very different place to the one we inhabit today. Billions of years ago, massive galaxies are thought to have formed in spectacular bursts of star formation. These massive elliptical galaxies have relatively simple properties. We hope that by understanding how simple galaxies form we will be one step closer to understanding how our own, spiral, Milky Way galaxy formed”.

* The camera, built in Edinburgh, is operated on the James Clerk Maxwell Telescope in Hawaii. The images, published in Nature tomorrow , reveal prodigious amounts of dust-enshrouded star formation which could ultimately tell scientists more about the formation of our own galaxy.