Probing dark energy with future redshift surveys: A comparison of emission line and broad band selection in the near infrared

TL;DR

This study compares two near-infrared galaxy selection strategies—Hα emission-line and H-band continuum—for future redshift surveys aimed at constraining dark energy in the range $0.5<z<2$. Using two GALFORM semi-analytic models (Bau05 and Bow06), the authors predict Hα luminosity functions, EW distributions, and clustering properties, and build mock catalogs to evaluate the effective survey volume $V_{ m eff}$ for different survey configurations. They find that Hα emitters preferentially occupy lower-mass halos and filamentary structures, while H-band galaxies trace the most massive halos, leading to distinct redshift-space distortions and growth-rate measurements. The results show that, for typical Euclid-like surveys, H-band selections yield larger $V_{ m eff}$ at given depth, but sufficiently deep Hα surveys can match or exceed this performance; these findings inform optimal survey design and demonstrate that both selection methods can robustly constrain the dark energy equation of state through large-scale structure observations, including measurements of the growth rate via redshift-space distortions.

Abstract

Future galaxy surveys will map the galaxy distribution in the redshift interval $0.5<z<2$ using near-infrared cameras and spectrographs. The primary science goal of such surveys is to constrain the nature of the dark energy by measuring the large-scale structure of the Universe. This requires a tracer of the underlying dark matter which maximizes the useful volume of the survey. We investigate two potential survey selection methods: an emission line sample based on the \ha line and a sample selected in the H-band. We present predictions for the abundance and clustering of such galaxies, using two published versions of the \galform galaxy formation model. Our models predict that \ha selected galaxies tend to avoid massive dark matter haloes and instead trace the surrounding filamentary structure; H-band selected galaxies, on the other hand, are found in the highest mass haloes. This has implications for the measurement of the rate at which fluctuations grow due to gravitational instability. We use mock catalogues to compare the effective volumes sampled by a range of survey configurations. To give just two examples: a redshift survey down to $H_{\rm AB}=22$ samples an effective volume that is $\sim 5-10$ times larger than that probed by an \ha survey with $\logfha > -15.4$; a flux limit of at least $\logfha = -16$ is required for an \ha sample to become competitive in effective volume.

Figures (12)

• Figure 1: The $\rmn{H}\alpha$ luminosity function, including attenuation by dust, at different redshifts. The blue curves show the predictions of the Bau05 model, whereas red curves show the Bow06 model. The observational estimates are represented by the symbols (see text for details). The redshift displayed in the bottom-right corner of each panel gives the redshift at which the GALFORM models were run. The vertical black dashed line shows the $\rmn{H}\alpha$ luminosity corresponding to the flux $\hbox{${\rm log}(F_{H\alpha}\hbox{$[\rmn{erg} \ \rmn{s}^{-1} \ \rmn{cm}^{-2}]$})$} = -15.4$ for $z>0$, displayed to show the expected luminosity limit of current planned space missions.
• Figure 2: The distribution of $\rmn{H}\alpha$ equivalent width in the observer frame as a function of $\rmn{H}\alpha$ flux, over the redshift interval $0.7<z<1.9$. The top panel shows the predictions of the Bau05 model and the bottom panel shows the Bow06 model, calculated as described in the text. The black line shows the median EW at each flux. The shaded regions enclose $68\%$ (dark grey) and $95\%$ (light grey) respectively of the GALFORM predictions around the median (black circles). The blue circles show observational data from hopkins00, green asterisks show data from shim09 and red diamonds show data from mccarthy99, as indicated by the key. The magenta dashed lines show the GALFORM predictions for the median equivalent width after applying the empirically derived continuum flux and line luminosity rescalings described in Section \ref{['sec.DE']}.
• Figure 3: The effective bias parameter as a function of $\rmn{H}\alpha$ luminosity for redshifts spanning the range $0<z<2$. The Bau05 model results are shown using circles connected with solid lines and the Bow06 model results are shown with asterisks connected by dashed lines. Each colour corresponds to a different redshift, as indicated by the key.
• Figure 4: Number counts in the H band. The upper panel shows the differential counts on a log scale. The lower panel shows the counts after dividing by a power law $N_{\rm ref} \propto H_{\rm AB}^{0.32}$ to expand the dynamic range on the y-axis. The symbols show the observational data, as shown by the key in the upper panel. The lines show the model predictions. The dotted lines show the original GALFORM predictions for the Bau05 model (blue) and the Bow06 model (red). The solid curves show the rescaled GALFORM predictions after rescaling the model galaxy luminosities to match the observed number counts at $H_{\rm AB}=22$.
• Figure 5: The redshift distribution of galaxies with $H_{\rm AB}=22$ (left column) and $H_{\rm AB}<23$ (right column). The top panels show the predictions after rescaling the model luminosities to better match the number counts as explained in the text. Red and blue lines show the model predictions for $H_{\rm AB}<22$ and $H_{\rm AB}<23$ respectively. Solid lines show the Bau05(r) model and the dashed lines show the Bow06(r) model. The lower panel shows the redshift distribution obtained from the Bow06 model by diluting the galaxies, randomly selecting 0.63 of the sample, the Bow06(d) model (recall this is a purely illustrative case with no physical basis; see § 4.1.1). In both panels, the histogram shows an estimate of the redshift distribution derived from spectroscopic observations in the COSMOS and UDF fields cirasuolo08.