Using the NASA/ESA Hubble Space Telescope, astronomers have uncovered for the first time a population of ancient white dwarf stars embedded in the Milky Way Galaxy’s crowded central hub of stars.
Left: a ground-based view of the Milky Way’s central bulge; giant dust clouds block most of the starlight coming from the galactic center. The Hubble Space Telescope, however, peered through a region (marked by the arrow) called the Sagittarius Window, which offers a keyhole view into the galaxy’s hub. Upper right: this is a small section of Hubble’s view of the dense collection of stars crammed together in the galactic bulge. Lower right: a sample of 4 out of the 70 brightest white dwarfs spied by Hubble in the Milky Way Galaxy’s central bulge. The numbers in the inset images correspond to the stars’ location in the larger Hubble view. Image credit: NASA / ESA / A. Calamida & K. Sahu, STScI / SWEEPS Science Team / A. Fujii.
About 13 billion years ago, the construction of Milky Way Galaxy was just beginning. Infant stars in the central bulge provided the building blocks for the galaxy’s foundation.
Many of these stars have long since burned out, and are now just dying embers, known as white dwarfs. These stellar relics are about the size of our planet but 200,000 times denser; a teaspoon of white dwarf material would weigh about 15 tons.
But contained within these small and extremely dense objects is the early history of Milky Way, providing clues on how it came to be.
An analysis of the new Hubble data, published in the Astrophysical Journal (arXiv.org preprint), supports the idea that the Milky Way’s bulge formed first and that its stellar inhabitants were born very quickly – in less then two billion years.
The rest of the galaxy’s sprawling disk of 2nd- and 3rd-generation stars grew more slowly in the suburbs, encircling the central bulge like the brim of a giant sombrero.
Lead author Dr Annalisa Calamida of the Space Telescope Science Institute and his colleagues based their results on an analysis of 70 of the hottest white dwarfs detectable by Hubble in a tiny region of Milky Way’s central bulge among tens of thousands of stars. The region surveyed is part of the Sagittarius Window Eclipsing Extrasolar Planet Search field and is located 26,000 light-years away.
“These 70 white dwarfs represent the peak of the iceberg. We estimate that the total number of white dwarfs is about 100,000 in this tiny Hubble view of the bulge,” said co-author Dr Kailash Sahu, also of the Space Telescope Science Institute.
The team used deep Hubble images of the same field of 240,000 stars, taken a decade apart, to separate the bulge stars from the myriad stars in the foreground of Milky Way’s disk.
The long timespan allowed the astronomers to make measurements of the stars’ motion and pick out 70,000 bulge stars. They identified the white dwarfs by analyzing the colors of the bulge stars and comparing them with theoretical models.
“It is important to observe the Milky Way’s bulge because it is the only bulge we can study in detail,” Dr Calamida said.
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A. Calamida et al. 2015. New insights on the Galactic Bulge Initial Mass Function. ApJ 810, 8; doi: 10.1088/0004-637X/810/1/8
