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Department of Physics

# Staff profile

## Publication details for Prof David Alexander

Civano, F., Marchesi, S., Comastri, A., Urry, M.C., Elvis, M., Cappelluti, N., Puccetti, S., Brusa, M., Zamorani, G., Hasinger, G., Aldcroft, T., Alexander, D.M., Allevato, V., Brunner, H., Capak, P., Finoguenov, A., Fiore, F., Fruscione, A., Gilli, R., Glotfelty, K., Griffiths, R.E., Hao, H., Harrison, F.A., Jahnke, K., Kartaltepe, J., Karim, A., LaMassa, S.M., Lanzuisi, G., Miyaji, T., Ranalli, P., Salvato, M., Sargent, M., Scoville, N.J., Schawinski, K., Schinnerer, E., Silverman, J., Smolcic, V., Stern, D., Toft, S., Trakhtenbrot, B., Treister, E. & Vignali, C. (2016). The Chandra Cosmos Legacy Survey: Overview and Point Source Catalog. The Astrophysical Journal 819(1): 62.

### Abstract

The COSMOS-Legacy survey is a 4.6 Ms Chandra program that has imaged 2.2 deg2 of the COSMOS field with an effective exposure of $\simeq 160$ ks over the central 1.5 deg2 and of $\simeq 80$ ks in the remaining area. The survey is the combination of 56 new observations obtained as an X-ray Visionary Project with the previous C-COSMOS survey. We describe the reduction and analysis of the new observations and the properties of 2273 point sources detected above a spurious probability of 2 × 10−5. We also present the updated properties of the C-COSMOS sources detected in the new data. The whole survey includes 4016 point sources (3814, 2920 and 2440 in the full, soft, and hard band). The limiting depths are 2.2 × 10−16, 1.5 × 10−15, and 8.9 × 10−16 ${\text{erg cm}}^{-2}\;{{\rm{s}}}^{-1}$ in the 0.5–2, 2–10, and 0.5–10 keV bands, respectively. The observed fraction of obscured active galactic nuclei with a column density >1022 cm−2 from the hardness ratio (HR) is ~50${}_{-16}^{+17}$%. Given the large sample we compute source number counts in the hard and soft bands, significantly reducing the uncertainties of 5%–10%. For the first time we compute number counts for obscured (HR > −0.2) and unobscured (HR < −0.2) sources and find significant differences between the two populations in the soft band. Due to the unprecedent large exposure, COSMOS-Legacy area is three times larger than surveys at similar depths and its depth is three times fainter than surveys covering similar areas. The area-flux region occupied by COSMOS-Legacy is likely to remain unsurpassed for years to come.