Modelling the number density of Hαemitters for future spectroscopic near-IR space missions

Abstract

Context. The future space missions Euclid and WFIRST-AFTA will use the Hα emission line to measure the redshifts of tens of millions of galaxies. The Hα luminosity function at z > 0.7 is one of the major sources of uncertainty in forecasting cosmological constraints from these missions. Aims. We construct unified empirical models of the Hα luminosity function spanning the range of redshifts and line luminosities relevant to the redshift surveys proposed with Euclid and WFIRST-AFTA. Methods. By fitting to observed luminosity functions from Hα surveys, we build three models for its evolution. Different fitting methodologies, functional forms for the luminosity function, subsets of the empirical input data, and treatment of systematic errors are considered to explore the robustness of the results. Results. Functional forms and model parameters are provided for all three models, along with the counts and redshift distributions up to z ∼ 2.5 for a range of limiting fluxes (F Hα > 0.5−3 × 10 −16 erg cm −2 s −1 ) that are relevant for future space missions. For instance, in the redshift range 0.90 < z < 1.8, our models predict an available galaxy density in the range 7700-130 300 and 2000-4800 deg −2 respectively at fluxes above F Hα > 1 and 2×10 −16 erg cm −2 s −1 , and 32 000-48 0000 for F Hα > 0.5×10 −16 erg cm −2 s −1 in the extended redshift range 0.40 < z < 1.8. We also consider the implications of our empirical models for the total Hα luminosity density of the Universe, and the closely related cosmic star formation history.

Predicting Hα emission-line galaxy counts for future galaxy redshift surveys

“…In both instances we assume a redshift limit of 0.9 z 1.8, however in the first instance we assume a flux limit of 3 × 10 −16 erg s −1 cm −2 and in the second instance we assume a deeper a flux limit of 2 × 10 −16 erg s −1 cm −2 . These instances are consistent with two of the Euclid-like strategies presented in Pozzetti et al (2016). For a WFIRST-like design we consider a redshift range of 1 z 2 and a flux limit of 1 × 10 −16 erg s −1 cm −2 .…”

“…At bright fluxes of 1 × 10 −15 erg s −1 cm −2 , the Ferrara et al (1999) method yields counts that are approximately a factor of 2 higher than the WISP counts. Lightcone catalogues built from other semianalytical models have previously under-predicted the number of galaxies and been unable to match the Mehta et al (2015) observations (we refer the reader to the discussion in Pozzetti et al 2016). One possible reason that GALACTICUS is able to reproduce the Mehta et al (2015) counts could be the use of CLOUDY, which has not been previously coupled to other semi-analytical models.…”

“…The cumulative blended flux counts as predicted by the three empirical models from Pozzetti et al (2016) Pozzetti et al (2016) model 3. This is not surprising, as the Pozzetti et al (2016) model 3 is also calibrated using WISP data.…”

“…Attempts to describe the Hα line luminosity functions (and hence the number counts) using empirically motivated models have recently been presented in Pozzetti et al (2016), who compared three different models that were fitted to the Hα luminosity functions measured from WISP, the ground-based narrow-band High-z Emission Line Survey (HiZELS, Geach et al 2008;Sobral et al 2009), as well as datasets obtained using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS, Yan et al 1999;Shim et al 2009). In this work we present the cumulative Hα count predictions from an open-source semi-analytical galaxy formation model.…”

“…In this work we present the cumulative Hα count predictions from an open-source semi-analytical galaxy formation model. Our work is complementary to that of Pozzetti et al (2016), though has more predictive power since we use simulations that incorporate a physical model of galaxy formation and are calibrated using the observational data. We compare the predicted counts to the WISP estimates, in particular the estimates from Mehta et al (2015).…”