Hintze Centre for Astrophysical Surveys

Abstract:

We present Galaxy Zoo DECaLS: detailed visual morphological classifications for Dark Energy Camera Legacy Survey images of galaxies within the SDSS DR8 footprint. Deeper DECaLS images (r = 23.6 versus r = 22.2 from SDSS) reveal spiral arms, weak bars, and tidal features not previously visible in SDSS imaging. To best exploit the greater depth of DECaLS images, volunteers select from a new set of answers designed to improve our sensitivity to mergers and bars. Galaxy Zoo volunteers provide 7.5 million individual classifications over 314 000 galaxies. 140 000 galaxies receive at least 30 classifications, sufficient to accurately measure detailed morphology like bars, and the remainder receive approximately 5. All classifications are used to train an ensemble of Bayesian convolutional neural networks (a state-of-the-art deep learning method) to predict posteriors for the detailed morphology of all 314 000 galaxies. We use active learning to focus our volunteer effort on the galaxies which, if labelled, would be most informative for training our ensemble. When measured against confident volunteer classifications, the trained networks are approximately 99 per cent accurate on every question. Morphology is a fundamental feature of every galaxy; our human and machine classifications are an accurate and detailed resource for understanding how galaxies evolve.

Abstract:

ABSTRACT Active galactic nuclei (AGN) are typically identified through radio, mid-infrared, or X-ray emission or through the presence of broad and/or narrow emission lines. AGN can also leave an imprint on a galaxy’s spectral energy distribution (SED) through the re-processing of photons by the dusty torus. Using the SED fitting code ProSpect with an incorporated AGN component, we fit the far-ultraviolet to far-infrared SEDs of ∼494 000 galaxies in the D10-COSMOS field and ∼230 000 galaxies from the GAMA survey. By combining an AGN component with a flexible star formation and metallicity implementation, we obtain estimates for the AGN luminosities, stellar masses, star formation histories, and metallicity histories for each of our galaxies. We find that ProSpect can identify AGN components in 91 per cent of galaxies pre-selected as containing AGN through narrow-emission line ratios and the presence of broad lines. Our ProSpect-derived AGN luminosities show close agreement with luminosities derived for X-ray selected AGN using both the X-ray flux and previous SED fitting results. We show that incorporating the flexibility of an AGN component when fitting the SEDs of galaxies with no AGN has no significant impact on the derived galaxy properties. However, in order to obtain accurate estimates of the stellar properties of AGN host galaxies, it is crucial to include an AGN component in the SED fitting process. We use our derived AGN luminosities to map the evolution of the AGN luminosity function for 0 < z < 2 and find good agreement with previous measurements and predictions from theoretical models.

Abstract:

ABSTRACT We present the 1.4 GHz radio luminosity functions (RLFs) of galaxies in the Cosmic Evolution Survey (COSMOS) field, measured above and below the 5σ detection threshold, using a Bayesian model-fitting technique. The radio flux densities from Very Large Array (VLA)-COSMOS 3-GHz data are extracted at the position of stellar-mass-selected galaxies. We fit a local RLF model, which is a combination of active galactic nuclei and star-forming galaxies (SFGs), in 10 redshift bins with a pure luminosity evolution model. Our RLF exceeds previous determinations at low radio luminosities at z < 1.6 with the same radio data, due to our ability to directly constrain the knee and faint-end slope of the RLF. Beyond z ∼ 2, we find that the SFG part of the RLF exhibits a negative evolution (L* moves to lower luminosities) due to the decrease in low stellar-mass galaxies in our sample at high redshifts. From the RLF for SFGs, we determine the evolution in the cosmic star formation rate density (SFRD), which we find to be consistent with the established behaviour up to z ∼ 1 using far-infrared data, but exceeds that from the previous radio-based work for the reasons highlighted above. Beyond z ∼ 1.5 the cosmic SFRD declines. We note that the relation between radio luminosity and star formation rate is crucial in measuring the cosmic SFRD from radio data at z > 1.5. We investigate the effects of stellar mass on the total RLF by splitting our sample into low (108.5 ≤ M/M⊙ ≤ 1010) and high ($M\gt 10^{10}\, \mathrm{M}_{\odot }$) stellar-mass subsets. We find that the SFRD is dominated by sources in the high stellar masses bin, at all redshifts.