News and Views

News and Views on “Quasars in the Galactic Anti-Center Area from the LAMOST DR3

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News and Views

Richard F. Green

(Steward Observatory, University of Arizona, Tucson, AZ 85721-0065, USA)

For most large systematic optical quasar surveys, like the Sloan Digital Sky Survey, low Galactic latitude has been avoided forfor the color selection of quasars. The classical ‘Zone of Avoidance’ has a very low and irregular surface density of galaxies, because of the patchy extinction from the dust in the Galactic plane. The associated variable reddening broadens the intrinsic distribution of quasar colors into substantial overlap with the stellar (and compact galaxy) multi-color locus. The isolation of a statistically valid sample is therefore challenging because of the high and directionally variable contamination rate as well as extinction and reddening.

In this issue, Huo et al. show that the color selection of quasars in some low latitude directions is possible, and that such a compilation has value for Galactic science. Their investigation of spectra taken by LAMOST primarily targeting unusually blue stars toward the Galactic anti-center has shown that some sightlines, even down to Galactic latitudes of ~10 deg, have sufficiently low extinction and reddening that relatively bright quasars can also be distinguished for unusually blue colors. They confirmed the pipeline classifications and redshifts of 143 quasars, and also made a conscientious effort to examine spectra not automatically classified. Their rate of return was only 8/80,000 for additional quasars from the predominantly low signal/noise ratio unclassified list. These are bright objects, peaking in their distribution at i~17.5, distributed over an area of at least 900 sq deg. Given that the surface density of such objects at high Galactic latitudes is ~1 / sq. deg. (e.g., Palanque-Delabrouille et al 2016), the patchy extinction and reddening are clearly significant this close to the Galactic plane.

The authors properly identify the utility of the new catalog – these quasars provide an extension of the accurate astrometric reference frame to lower latitudes toward the Galactic anti-center and probe full complex sightlines through the interstellar medium. Both hold great promise for future discoveries. As demonstrated by HST ultraviolet spectroscopy of high latitude quasars (Savage et al. 2000), sightlines through the lower halo and thick disk show a strong mix of ionization states, with complex velocity structures, some matching HI emission, and some not. Quasar sightlines near the anti-center are clearly rich in absorption velocity structure, traversing the Perseus and Orion arms. The vertical distribution of reddening shows complex spatial structure between absolute latitudes 10 and 30 degrees (Green et al. 2015) consistent with a history of supernova explosions seen in the significant surface density of old supernova remnants (Gao 2012). This catalog provides a valuable list of bright probes of the full outer disk and low halo, inviting vigorous follow-up.


  • Gao, X. 2012, PASP, 124, 1369 ADS

  • Green G. M., Schlafly E. F., Finkbeiner D. P. et al  2015 ApJ 810 25ADS

  • Palanque-Delabrouille, N. et al, 2016 A&A, 587, A41ADS

  • Savage et al. 2000, ApJS, 129, 563ADS