Intermediate-mass black holes in dwarf galaxies out to redshift ∼2.4 in the Chandra COSMOS-Legacy Survey. Issue 2 (4th May 2018)

• Record Type: Journal Article
• Authors: Mezcua, M; Civano, F; Marchesi, S; Suh, H; Fabbiano, G; Volonteri, M
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 478:Issue 2(2018)
• Page Start: 2576
• Page End: 2591
• Publication Date: 2018-05-04
• Subjects: galaxies: active -- galaxies: dwarf -- galaxies: starburst -- X-rays: galaxies; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty1163
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We present a sample of 40 active galactic nucleus (AGN) in dwarf galaxies at redshifts z ≲ 2.4. The galaxies are drawn from the Chandra COSMOS-Legacy survey as having stellar masses 10 7 ≤ M * ≤ 3 × 10 9 M⊙ . Most of the dwarf galaxies are star forming. After removing the contribution from star formation to the X-ray emission, the AGN luminosities of the 40 dwarf galaxies are in the range L 0.5–10 keV ∼ 10 39 −10 44 erg s −1 . With 12 sources at z > 0.5, our sample constitutes the highest-redshift discovery of AGN in dwarf galaxies. The record-holder is cid_1192, at z = 2.39 and with L 0.5–10 keV ∼ 10 44 erg s −1 . One of the dwarf galaxies has M * = 6.6 × 10 7 M⊙ and is the least massive galaxy found so far to host an AGN. All the AGN are of type 2 and consistent with hosting intermediate-mass black holes (BHs) with masses ∼10 4 –10 5 M⊙ and typical Eddington ratios >1 per cent. We also study the evolution, corrected for completeness, of AGN fraction with stellar mass, X-ray luminosity, and redshift in dwarf galaxies out to z = 0.7. We find that the AGN fraction for 10 9 < M * ≤ 3 × 10 9 M⊙ and L X ∼ 10 41 –10 42 erg s −1 is ∼0.4 per cent for z ≤ 0.3 and that it decreases with X-ray luminosity and decreasing stellar mass. Unlike massive galaxies, the AGN fraction seems to decrease with redshift, suggesting that AGN in dwarf galaxies evolve differently than those in high-mass galaxies. Mindful of potential caveats, the results seem to favour a direct collapse formation mechanism for the seed BHs in the early Universe.