A New Catalog of the Visible Milky Way

A density map of part of the Milky Way disk, constructed from IPHAS data. The axes show galactic latitude and longitude, coordinates that relate to the position of the center of the galaxy. The mapped data are the counts of stars detected in i, the longer (redder) wavelength broad band of the survey, down to a faint limit of 19th magnitude. Although this is just a small section of the full map, it portrays in exquisite detail the complex patterns of obscuration due to interstellar dust. Credit: Hywel Farnhill, University of Hertfordshire.

Astronomers used IPHAS data to create a new catalog of the visible part of the Milky Way, revealing no fewer than 219 million stars.

A new catalog of the visible part of the northern part of our home Galaxy, the Milky Way, includes no fewer than 219 million stars. Geert Barentsen of the University of Hertfordshire led a team who assembled the catalog in a ten-year program using the Isaac Newton Telescope (INT) on La Palma in the Canary Islands. Their work appears today in the journal Monthly Notices of the Royal Astronomical Society.

From dark sky sites on Earth, the Milky Way appears as a glowing band stretching across the sky. To astronomers, it is the disk of our own galaxy, a system stretching across 100,000 light-years, seen edge-on from our vantage point orbiting the Sun. The disk contains the majority of the stars in the galaxy, including the Sun, and the densest concentrations of dust and gas.

The unaided human eye struggles to distinguish individual objects in this crowded region of the sky, but the 2.5-meter mirror of the INT enabled the scientists to resolve and chart 219 million separate stars. The INT program charted all the stars brighter than 20th magnitude – or 1 million times fainter than can be seen with the human eye.

Using the catalog, the scientists have put together an extraordinarily detailed map of the disk of the Galaxy that shows how the density of stars varies, giving them a new and vivid insight into the structure of this vast system of stars, gas, and dust.

The image included here, a cut-out from a stellar density map mined directly from the released catalog, illustrates the new view obtained. The Turner-like brush strokes of dust shadows would grace the wall of any art gallery. Maps like these also stand as useful tests of new-generation models for the Milky Way.

The production of the catalog, IPHAS DR2 (the second data release from the survey program The INT Photometric H-alpha Survey of the Northern Galactic Plane, IPHAS), is an example of modern astronomy’s exploitation of ‘big data’. It contains information on 219 million detected objects, each of which is summarized in 99 different attributes.

With this catalog release, the team are offering the world community free access to measurements taken through two broadband filters capturing light at the red end of the visible spectrum, and in a narrow band capturing the brightest hydrogen emission line, H-alpha. The inclusion of H-alpha also enables exquisite imaging of the nebulae (glowing clouds of gas) found in the greatest number within the disk of the Milky Way. The stellar density map illustrated here is derived from the longest (reddest) wavelength band in which the darkening effect of the dust is moderated in a way that brings out more of its structural detail, compared to maps built at shorter (bluer) wavelengths.

Reference: “The second data release of the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS DR2)” by Geert Barentsen, H. J. Farnhill, J. E. Drew, E. A. González-Solares, R. Greimel, M. J. Irwin, B. Miszalski, C. Ruhland, P. Groot, A. Mampaso, S. E. Sale, A. A. Henden, A. Aungwerojwit, M. J. Barlow, P. J. Carter, R. L. M. Corradi, J. J. Drake, J. Eislöffel, J. Fabregat, B. T. Gänsicke, N. P. Gentile Fusillo, S. Greiss, A. S. Hales, S. Hodgkin, L. Huckvale, J. Irwin, R. King, C. Knigge, T. Kupfer, E. Lagadec, D. J. Lennon, J. R. Lewis, M. Mohr-Smith, R. A. H. Morris, T. Naylor, Q. A. Parker, S. Phillipps, S. Pyrzas, R. Raddi, G. H. A. Roelofs, P. Rodríguez-Gil, L. Sabin, S. Scaringi, D. Steeghs, J. Suso, R. Tata, Y. C. Unruh, J. van Roestel, K. Viironen, J. S. Vink, N. A. Walton, N. J. Wright and A. A. Zijlstra, 15 September 2014, MNRAS.
DOI: 10.1093/mnras/stu1651
arXiv: 1406.4862