Cross-correlating Carbon Monoxide Line-intensity Maps with Spectroscopic and Photometric Galaxy Surveys

Collaboration
Sep 12, 2018
19 pages
Published in:
  • Astrophys.J. 872 (2019) 2, 186
  • Published: Feb 25, 2019
e-Print:
DOI:

Citations per year

2018202020222024202402468
Abstract: (IOP)
Line-intensity mapping is an emerging field of observational work, with strong potential to fit into a larger effort to probe large-scale structure and small-scale astrophysical phenomena using multiple complementary tracers. Taking full advantage of such complementarity means, in part, undertaking line-intensity surveys with galaxy surveys in mind. We consider the potential for detection of a cross-correlation signal between COMAP and blind surveys based on photometric redshifts (as in COSMOS) or based on spectroscopic data (as with the HETDEX survey of Lyα emitters). We find that obtaining accuracy in redshifts and 10−4 sources per Mpc3 with spectroscopic redshift determination should enable a CO-galaxy cross spectrum detection significance at least twice that of the CO auto spectrum. Either a future targeted spectroscopic survey or a blind survey like HETDEX may be able to meet both of these requirements.
Note:
  • 19 pages + appendix (31 pages total), 16 figures, 6 tables; accepted for publication in ApJ
  • galaxies: high-redshift
  • galaxies: statistics
  • radio lines: galaxies
  • cosmology: theory
  • [1]
    What number of sources do we need for significant cross-correlation, in the case of a hypothetical spectroscopic follow-up to complement COMAP?
    • [2]
      What redshift accuracy must the reference galaxy catalog achieve to enable significant cross-correlation?
      • [3]
        What would be the detection significance of the various cross-power spectra under consideration?
        • [1]
          HETDEX targets a redshift range of z = 1.9-3.5,, 10: #apjab0027fn1 well beyond the typical redshifts of z ~ 1 of other dark-energy-centric optical and NIR surveys. (eBOSS and DESI will target quasars, and thus the Ly-α forest at z 2, but emission line galaxies only up to z ~
          • [2]
            • [2]
              HETDEX does not target specific points on the sky based on prior imaging, but rather samples its survey footprint with integral field spectroscopy, integrating for ~20 minutes at each spot in the sky, and picks sources out from the noisy spectra
              • [1]
                Bin the halo luminosities into resolution elements in frequency and angular position, resulting in a certain luminosity Lline,vox for each voxel that is simply the cumulative line luminosity of all halos in that voxel
                • [2]
                  Convert these luminosities into surface brightness (apparent spectral intensity, in units of luminosity per unit area, per unit frequency, per unit solid angle):\begin{eqnarray}&&{I}_{\nu,\mathrm{obs}}=\displaystyle \frac{{L}_{\mathrm{line},\mathrm{vox}}}{4\pi {D}_{L}^{2}}\displaystyle \frac{1}{{\delta }_{x}{\delta }_{y}{\delta }_{\nu }},\end{eqnarray} \tag{ 6 } where DL is the luminosity distance to that voxel
                  • [3]
                    Convert to the expected brightness temperature contribution from each voxel. The Rayleigh-Jeans brightness temperature for a given surface brightness is\begin{eqnarray}&&T=\displaystyle \frac{{c}^{2}{I}_{\nu,\mathrm{obs}}}{2{k}_{B}{\nu }_{\mathrm{obs}}^{2}},\end{eqnarray} \tag{ 7 } from which we obtain our temperature TCO(x) at each voxel position x in the data cube
                    • [1]
                      With perfect or at least very precise redshift knowledge, the exercise could be done with as few as several thousand sources covering the COMAP survey volume, corresponding to a source abundance of 10-4Mpc-3 or 102 per square degree per {\rm{\Delta }}z=0.1
                      • [2]
                        However, to provide a significant advantage in cross-correlation alone over auto-correlation alone in signal-to-noise ratio, the galaxy catalog must achieve a redshift accuracy of {\sigma }_{z}/(1+z)\lesssim 0.003, which is best obtained with low- to medium-resolution spectroscopy and will be challenging (at best) with photometry at high redshift
                        • [3]
                          If the redshift accuracy and source density satisfy the above, cross-correlations could result in a cross spectrum detection at an S/N of up to 15-30, compared to {\rm{S}}/{\rm{N}}\lesssim 5 for a CO auto spectrum detection (in a single patch). We expect this to be true in the case of cross-correlation with HETDEX, although this (and cross-correlation with Lyα surveys in general) requires further investigation with more faithful treatment of radiative processes
                          • [1]
                            Pengelly (,: #apjab0027bib82) (communicated by Seaton), the first of three papers including Pengelly & Seaton (,: #apjab0027bib83) and Seaton (,: #apjab0027bib91)
                          • [2]
                            Brocklehurst (,: #apjab0027bib13) (again communicated by Seaton, and in fact the content is very similar to Pengelly (, 1964: #apjab0027bib82))
                          • [3]
                            Hummer & Storey (,: #apjab0027bib56)
                          • [4]
                            and Hu et al. (,: #apjab0027bib55) (who cite Brocklehurst (, 1971: #apjab0027bib13), but are sometimes cited in isolation—for example, in Hayes (, 2015: #apjab0027bib49) and Bridge et al. (, 2018: #apjab0027bib12)
                          • [1]
                            It increases monotonically with redshift (converging to 1 as z\to \infty)
                            • [2]
                              It decreases with higher SFR
                              • [1]
                                Results in
                                • Yajima
                              • [2]
                                Results in
                                • Garel
                              • [1]
                                We add log-normal scatter to SFR while preserving the linear mean \mathrm{SFR}({M}_{\mathrm{vir}},z). In practice, this means that, for each halo, we calculate the expected mean SFR, then multiply this by a sample value from a log-normal distribution with a log-space standard deviation of {\sigma }_{\mathrm{SFR}}=0.3 (in units of dex) and a mean logarithm of -{\sigma }_{\mathrm{SFR}}^{2}\mathrm{ln}10/2. Thus, the mean logarithm is not equal to the logarithm of the linear mean SFR value, but rather \mathrm{log}[\left\langle \mathrm{SFR}\right\rangle /({M}_{\odot }\,{\mathrm{yr}}^{-1})]-{\sigma }_{\mathrm{SFR}}^{2}\mathrm{ln}10/2, which is necessary for the linear mean of the distribution to be the desired \left\langle \mathrm{SFR}\right\rangle
                                • [2]
                                  We then add log-normal scatter to {L}_{\mathrm{CO}} and {L}_{{\rm{L}}y\alpha } in the same manner, multiplying the mean line luminosity from the {L}_{\mathrm{IR}}-{L}_{\mathrm{line}} relation by a sample value from a log-normal distribution with a log-space standard deviation of {\sigma }_{{L}_{\mathrm{line}}} (again in units of dex) but a mean logarithm of -{\sigma }_{{L}_{\mathrm{line}}}^{2}\mathrm{ln}10/2
                                  • [1]
                                    Scattering in the CGM results in diffuse Ly-α halos or blobs, significantly increasing the total flux over radii of ~10''
                                    • Steidel
                                  • [2]
                                    Scattering in the IGM may result in anisotropic clustering observed in the Lyα intensity cube, as demonstrated in a simulation study from
                                    • Zheng
                                  • [2]
                                    see Visbal & McQuinn (,: #apjab0027bib104), showing this at z\sim 7