ALMA constraints on the faint millimetre source number counts and their contribution to the cosmic infrared background

¹ Dipartimento di Fisica e Astronomia, Università di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino ( Firenze), Italy
e-mail: carniani@arcetri.astro.it
² Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ SISSA, via Bonomea 265, 34136 Trieste, Italy
⁵ INAF-Osservatorio Astronomico di Padova, Vicolo dell’ Osservatorio 5, 35122 Padova, Italy
⁶ Spitzer Science Center, 314-6 Caltech, 1201 East California Boulevard, Pasadena, CA 91125, USA
⁷ Department of Astronomy, 249-17 Caltech, 1201 East California Boulevard, Pasadena, CA 91125, USA
⁸ National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
⁹ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
¹⁰ INAF–Trieste Astronomical Observatory, via Tiepolo 11, 34143 Trieste, Italy
¹¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹² Physics Department, New Mexico Tech, Socorro, NM 87801, USA
¹³ Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan
¹⁴ Dipartimento di Fisica e Astronomia, Universita’ di Bologna, Viale Berti Pichat 6/2, 30127, Bologna, Italy
¹⁵ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁶ Square Kilometre Array Organization, Jodrell Bank Observatory, Lower Withington, Macclesfield, Cheshire SK11 9DL, UK

Received: 1 February 2015
Accepted: 2 September 2015

Abstract

We have analysed 18 ALMA continuum maps in Bands 6 and 7, with rms down to 7.8 μJy, to derive differential number counts down to 60 μJy and 100 μJy at λ = 1.3 mm and λ = 1.1 mm, respectively. Furthermore, the non-detection of faint sources in the deepest ALMA field enabled us to set tight upper limits on the number counts down to 30 μJy. This is a factor of four deeper than the currently most stringent upper limit. The area covered by the combined fields is 9.5 × 10^-4 deg² at 1.1 mm and 6.6 × 10^-4 deg² at 1.3 mm. With respect to previous works, we improved the source extraction method by requiring that the dimension of the detected sources be consistent with the beam size. This method enabled us to remove spurious detections that have plagued the purity of the catalogues in previous studies. We detected 50 faint sources (at fluxes <1 mJy) with signal-to-noise (S/N) >3.5 down to 60 μJy, hence improving the statistics by a factor of four relative to previous studies. The inferred differential number counts are dN/ d(Log₁₀S) = 1 × 10⁵ deg² at a 1.1 mm flux S_{λ = 1.1 mm} = 130 μJy, and dN/ d(Log₁₀S) = 1.1 × 10⁵ deg² at a 1.3 mm flux S_{λ = 1.3 mm} = 60 μJy. At the faintest flux limits probed by our data, i.e. 30 μJy and 40 μJy, we obtain upper limits on the differential number counts of dN/ d(Log₁₀S) < 7 × 10⁵ deg² and dN/ d(Log₁₀S) < 3 × 10⁵ deg², respectively. Determining the fraction of cosmic infrared background (CIB) resolved by the ALMA observations was hampered by the large uncertainties plaguing the CIB measurements (a factor of four in flux). However, our results provide a new lower limit to CIB intensity of 17.2 Jy deg^-2 at 1.1 mm and of 12.9 Jy deg^-2 at 1.3 mm. Moreover, the flattening of the integrated number counts at faint fluxes strongly suggests that we are probably close to the CIB intensity. Our data imply that galaxies with star formation rate (SFR) < 40 M_⊙/yr certainly contribute less than 50% to the CIB (and probably a much lower percentage) while more than 50% of the CIB must be produced by galaxies with SFR> 40 M_⊙/yr. The differential number counts are in nice agreement with recent semi-analytical models of galaxy formation even as low as our faint fluxes. Consequently, this supports the galaxy evolutionary scenarios and assumptions made in these models.