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

TL;DR

This work addresses the key uncertainty in forecasting near-infrared space missions by constructing three unified empirical H$\alpha$ luminosity function evolution models, calibrated with an expanded dataset up to $z\sim2.3$ and incorporating multiple LF shapes and error treatments. The authors provide detailed LF parameters, redshift and flux distributions, and number counts for Euclid and WFIRST-AFTA flux limits, and contrast their predictions with semi-analytic mock catalogs. They find that the bright end of the H$\alpha$ LF evolves strongly with redshift, but overall predicted emitter densities at $z>0.9$ are lower than earlier forecasts, yielding tens of millions of detectable H$\alpha$ emitters for upcoming surveys. The results feed instrument simulations and survey optimization, highlighting the need to account for completeness, contamination, and the detailed H$\alpha$ vs [NII] blending in planning cosmological analyses. These H$\alpha$ LFs also illuminate the cosmic star formation history encoded in emission-line galaxies across $0<z<2.5$.

Abstract

The future space missions Euclid and WFIRST-AFTA will use the Halpha emission line to measure the redshifts of tens of millions of galaxies. The Halpha luminosity function at z>0.7 is one of the major sources of uncertainty in forecasting cosmological constraints from these missions. We construct unified empirical models of the Halpha luminosity function spanning the range of redshifts and line luminosities relevant to the redshift surveys proposed with Euclid and WFIRST-AFTA. By fitting to observed luminosity functions from Halpha 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. 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_Halpha>0.5 - 3 x 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--13300 and 2000--4800 deg^-2 respectively at fluxes above F_Halpha>1 and 2 x 10^-16 erg cm^-2 s^-1, and 32000--48000 for F_Halpha>0.5 x 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 Halpha luminosity density of the Universe, and the closely related cosmic star formation history.

Figures (7)

• Figure 1: H$\alpha$ LFs at various redshifts. The dotted lines mark the nominal flux limit of Euclid ($3\times 10^{-16}$ erg cm$^{-2}$ s$^{-1}$) in the lower bound of each redshift range. Observed Schechter LFs are shown as thin lines and squares in the observed luminosity range and listed in the labels. For comparison, the LFs from Empirical Models 1, 2, and 3 are shown (in yellow, cyan, and pink, respectively) as thick lines in the same redshift range (shown in the two extremes of each redshift bin).
• Figure 2: H$\alpha$ LFs empirical Schechter parameters (using the same colours as Figure \ref{['fig:haLF']}) as a function of redshift (at the center redshift of each surveys), along with the evolution of parameters in the models.
• Figure 3: Residuals to the H$\alpha$ luminosity function fits for Model 3, plotted as a function of observed-frame H$\alpha$ flux at the bin centre (horizontal axis). All redshifts are plotted together. The green lines show the fit line and factors of 2 above and below. The error bars shown do not include the cosmic variance, which is included in the fit but is highly correlated across luminosity bins.
• Figure 4: Left panel: Cumulative H$\alpha$ number counts, integrated over the redshift ranges $0.7<z<1.5$ (WISP range). The observed counts from the WISP survey 2013ApJ...779...34C are shown (blue circles) and from new WISP analysis by Mehta15 (cyan circles), and compared to the empirical Model 1, 2, and 3, (blue, black and red lines, respectively). Also shown (as dotted lines and empty squares) are the counts obtained integrating the observed LFs (see legend) in the same redshift range. Right panel: The same cumulative H$\alpha$ number counts compared to the predictions from L12 mocks (green dashed and solid lines using intrinsic and extincted H$\alpha$ fluxes, respectively) and GP14 mocks (dark and light grey for $H<27$ and $H<24$ mocks, respectively).
• Figure 5: H$\alpha$ redshift distribution above various flux thresholds (from $0.5\times 10^{-16}$ erg cm$^{-2}$ s$^{-1}$ to $3\times 10^{-16}$ erg cm$^{-2}$ s$^{-1}$, from top to bottom panels). Observed redshift distributions are indicated with open circles, while data obtained integrating LFs are shown with squares. HaLF predictions from Model 1, 2, 3 are shown as thick solid lines. The predictions from L12 mocks (green dashed and solid lines using intrinsic and extincted H$\alpha$ fluxes, respectively) and GP14 mocks (dark and light grey for $H<27$ and $H<24$ mocks, respectively) are also shown.