Photometric Errors Research Articles

Galaxy clustering, photometric redshifts and diagnosis of systematics in the DES Science Verification data

We study the clustering of galaxies detected at $i<22.5$ in the Science Verification observations of the Dark Energy Survey (DES). Two-point correlation functions are measured using $2.3\times 10^6$ galaxies over a contiguous 116 deg$^2$ region in five bins of photometric redshift width $\Delta z = 0.2$ in the range $0.2 < z < 1.2.$ The impact of photometric redshift errors are assessed by comparing results using a template-based photo-$z$ algorithm (BPZ) to a machine-learning algorithm (TPZ). A companion paper (Leistedt et al 2015) presents maps of several observational variables (e.g. seeing, sky brightness) which could modulate the galaxy density. Here we characterize and mitigate systematic errors on the measured clustering which arise from these observational variables, in addition to others such as Galactic dust and stellar contamination. After correcting for systematic effects we measure galaxy bias over a broad range of linear scales relative to mass clustering predicted from the Planck $\Lambda$CDM model, finding agreement with CFHTLS measurements with $\chi^2$ of 4.0 (8.7) with 5 degrees of freedom for the TPZ (BPZ) redshifts. We test a "linear bias" model, in which the galaxy clustering is a fixed multiple of the predicted non-linear dark-matter clustering. The precision of the data allow us to determine that the linear bias model describes the observed galaxy clustering to $2.5\%$ accuracy down to scales at least $4$ to $10$ times smaller than those on which linear theory is expected to be sufficient.

THE FRACTIONS OF INNER- AND OUTER-HALO STARS IN THE LOCAL VOLUME

We obtain a new determination of the metallicity distribution function (MDF) of stars within $\sim5$-$10$ kpc of the Sun, based on recently improved co-adds of $ugriz$ photometry for Stripe 82 from the Sloan Digital Sky Survey. Our new estimate uses the methodology developed previously by An et al. to study in situ halo stars, but is based on a factor of two larger sample than available before, with much-improved photometric errors and zero-points. The newly obtained MDF can be divided into multiple populations of halo stars, with peak metallicities at [Fe/H] $\approx -1.4$ and $-1.9$, which we associate with the inner-halo and outer-halo populations of the Milky Way, respectively. We find that the kinematics of these stars (based on proper-motion measurements at high Galactic latitude) supports the proposed dichotomy of the halo, as stars with retrograde motions in the rest frame of the Galaxy are generally more metal-poor than stars with prograde motions, consistent with previous claims. In addition, we generate mock catalogs of stars from a simulated Milk Way halo system, and demonstrate for the first time that the chemically- and kinematically-distinct properties of the inner- and outer-halo populations are qualitatively in agreement with our observations. The decomposition of the observed MDF and our comparison with the mock catalog results suggest that the outer-halo population contributes on the order of $\sim35\%$-$55\%$ of halo stars in the local volume.

Dense RGB-D visual odometry using inverse depth

In this paper we present a dense visual odometry system for RGB-D cameras performing both photometric and geometric error minimisation to estimate the camera motion between frames. Contrary to most works in the literature, we parametrise the geometric error by the inverse depth instead of the depth, which translates into a better fit of the distribution of the geometric error to the used robust cost functions. To improve the accuracy we propose to use a keyframe switching strategy based on a visibility criteria between frames. For the comparison of our approach with state-of-the-art approaches we use the popular datasets from the TUM for RGB-D benchmarking as well as two synthetic datasets. Our approach shows to be competitive with state-of-the-art methods in terms of drift in metres per second, even compared to methods performing loop closure too. When comparing to approaches performing pure odometry like ours, our method outperforms them in the majority of the tested datasets. Additionally we show that our approach is able to work in real time and we provide a qualitative evaluation on our own sequences showing a low drift in the 3D reconstructions. We have implemented this method within the scope of PCL (Point Cloud Library) as a branch of the code for large scale KinectFusion, where the original ICP system for odometry estimation has been completely substituted by our method. A PCL fork including the modified method is available for download.

Variable stars in Local Group Galaxies – I. Tracing the early chemical enrichment and radial gradients in the Sculptor dSph with RR Lyrae stars

We identified and characterized the largest (536) RR Lyrae (RRL) sample in a Milky Way dSph satellite (Sculptor) based on optical photometry data collected over $\sim$24 years. The RRLs display a spread in V-magnitude ($\sim$0.35 mag) which appears larger than photometric errors and the horizontal branch (HB) luminosity evolution of a mono-metallic population. Using several calibrations of two different reddening free and metal independent Period-Wesenheit relations we provide a new distance estimate $\mu$=19.62 mag ($\sigma_{\mu}$=0.04 mag) that agrees well with literature estimates. We constrained the metallicity distribution of the old population, using the $M_I$ Period-Luminosity relation, and we found that it ranges from -2.3 to -1.5 dex. The current estimate is narrower than suggested by low and intermediate spectroscopy of RGBs ($\Delta$[Fe/H] $\le$ 1.5). We also investigated the HB morphology as a function of the galactocentric distance. The HB in the innermost regions is dominated by red HB stars and by RRLs, consistent with a more metal-rich population, while in the outermost regions it is dominated by blue HB stars and RRLs typical of a metal-poor population. Our results suggest that fast chemical evolution occurred in Sculptor, and that the radial gradients were in place at an early epoch.

Strömgrenuvbyphotometry of the peculiar globular cluster NGC 2419

NGC 2419 is a peculiar Galactic globular cluster in terms of size/luminosity, and chemical abundance anomalies. Here, we present Str\"omgren $uvby$ photometry of the cluster. Using the gravity- and metallicity-sensitive $c_1$ and $m_1$ indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius with an estimated contamination by non-members of only 1%. We derive photometric [Fe/H] of red giants, and depending on which literature metallicity relation we use, find reasonable to excellent agreement with spectroscopic [Fe/H]. We demonstrate explicitly that the photometric errors are not Gaussian, and using a realistic model for the photometric uncertainties, find a formal internal [Fe/H] spread of $\sigma=0.11^{+0.02}_{-0.01}$ dex. This is an upper limit to the cluster's true [Fe/H] spread and may partially/entirely reflect the limited precision of the photometric metallicity estimation and systematic effects. The lack of correlation between spectroscopic and photometric [Fe/H] of individual stars is further evidence against a [Fe/H] spread on the 0.1 dex level. Finally, the CN-sensitive $\delta_4$ anti-correlates strongly with Mg abundance, indicating that the 2nd generation stars are N-enriched. Absence of similar correlations in some other CN-sensitive indices supports the second generation being He-rich, which in these indices approximately compensates the shift due to CN. Compared to a single continuous distribution with finite dispersion, the observed $\delta_4$ distribution is slightly better fit by two discrete populations, with the N-enhanced stars accounting for 53$\pm$5%. NGC 2419 appears to be very similar to other metal-poor Galactic globular clusters with a similarly N-enhanced second generation and little or no variation in [Fe/H], which sets it apart from other suspected accreted nuclei such as {\omega}Cen. (abridged)

Absolute magnitudes and slope parameters for 250,000 asteroids observed by Pan-STARRS PS1 – Preliminary results

We present the results of a Monte Carlo technique to calculate the absolute magnitudes (H) and slope parameters (G) of ∼240,000 asteroids observed by the Pan-STARRS1 telescope during the first 15months of its 3-year all-sky survey mission. The system’s exquisite photometry with photometric errors ≲0.04mag, and well-defined filter and photometric system, allowed us to derive accurate H and G even with a limited number of observations and restricted range in phase angles. Our Monte Carlo method simulates each asteroid’s rotation period, amplitude and color to derive the most-likely H and G, but its major advantage is in estimating realistic statistical+systematic uncertainties and errors on each parameter. The method was tested by comparison with the well-established and accurate results for about 500 asteroids provided by Pravec et al. (Pravec, P. et al. [2012]. Icarus 221, 365–387) and then applied to determining H and G for the Pan-STARRS1 asteroids using both the Muinonen et al. (Muinonen, K. et al. [2010]. Icarus 209, 542–555) and Bowell et al. (Bowell, E. et al. [1989]. Asteroids III, Chapter Application of Photometric Models to Asteroids. University of Arizona Press, pp. 524–555) phase functions. Our results confirm the bias in MPC photometry discovered by Jurić et al. (Jurić, M. et al. [2002]. Astrophys. J. 124, 1776–1787).

Extracting Hαflux from photometric data in the J-PLUS survey

We present the main steps that will be taken to extract H$\alpha$ emission flux from Javalambre Photometric Local Universe Survey (J-PLUS) photometric data. For galaxies with $z\lesssim0.015$, the H$\alpha$+[NII] emission is covered by the J-PLUS narrow-band filter $F660$. We explore three different methods to extract the H$\alpha$ + [NII] flux from J-PLUS photometric data: a combination of a broad-band and a narrow-band filter ($r'$ and $F660$), two broad-band and a narrow-band one ($r'$, $i'$ and $F660$), and a SED-fitting based method using 8 photometric points. To test these methodologies, we simulated J-PLUS data from a sample of 7511 SDSS spectra with measured H$\alpha$ flux. Based on the same sample, we derive two empirical relations to correct the derived H$\alpha$+[NII] flux from dust extinction and [NII] contamination. We find that the only unbiased method is the SED fitting based one. The combination of two filters underestimates the measurements of the H$\alpha$ + [NII] flux by a 28%, while the three filters method by a 9%. We study the error budget of the SED-fitting based method and find that, in addition to the photometric error, our measurements have a systematic uncertainty of a 4.3%. Several sources contribute to this uncertainty: differences between our measurement procedure and the one used to derive the spectroscopic values, the use of simple stellar populations as templates, and the intrinsic errors of the spectra, which were not taken into account. Apart from that, the empirical corrections for dust extinction and [NII] contamination add an extra uncertainty of 14%. Given the J-PLUS photometric system, the best methodology to extract H$\alpha$ + [NII] flux is the SED-fitting based one. Using this method, we are able to recover reliable H$\alpha$ fluxes for thousands of nearby galaxies in a robust and homogeneous way.

Combining spectroscopic and photometric surveys: Same or different sky?

This paper looks at the combined constraints from a photometric and spectroscopic survey. These surveys will measure cosmology using weak lensing (WL), galaxy clustering, baryon acoustic oscillations (BAO) and redshift space distortions (RSD). We find, contrary to some findings in the recent literature, that overlapping surveys can give important benefits when measuring dark energy. We therefore try to clarify the status of this issue with a full forecast of two stage-IV surveys using a new approach to properly account for covariance between the different probes in the overlapping samples. The benefit of the overlapping survey can be traced back to two factors: additional observables and sample variance cancellation. Both needs to be taken into account and contribute equally when combining 3D power spectrum and 2D correlations for lensing. With an analytic example we also illustrate that for optimal constraints, one should minimize the (Pearson) correlation coefficient between cosmological and nuisance parameters and maximize the one among nuisance parameters (e.g. galaxy bias) in the two samples. This can be achieved by increasing the overlap between the spectroscopic and photometric surveys. We show how BAO, WL and RSD contribute to this benefit and also look at some other survey designs, such as photometric redshift errors and spectroscopic density.

POPULATION EFFECTS ON THE METALLICITY DISTRIBUTION FUNCTION DERIVED FROM THE RED GIANT BRANCH

We have tested the reliability of the red giant branch (RGB) as a metallicity indicator accounting for observational errors as well as the complexity of star formation histories (SFHs) and chemical evolution histories observed in various stellar systems. We generate model color-magnitude diagrams (CMDs) produced with a variety of evolutionary histories and compare the resultant metallicity estimates from the colors and magnitudes of RGB stars to the true input metallicities. We include realistic models for photometric errors and completeness in our synthetic CMDs. As expected, for simple simple stellar populations dominated by old stars, the RGB provides a very accurate estimate of the modular metallicity value for a population. An error in the age of a system targeted for this type of study may produce metallicity errors of a few tenths of a dex. The size of this metallicity error depends linearly on the age error, and we find this dependence to be stronger with more precise photometry. If the population has experienced any significant star formation within the last $\sim$6 Gyr, the metallicity estimates, [M/H], derived from the RGB may be in error by up to $\sim$0.5 dex. Perhaps the most important consideration for this technique is an accurate, independent estimate of the average age for the target stellar system, especially if it is probable that a significant fraction of the population formed less than $\sim$6 Gyr ago.

Difference image analysis: The interplay between the photometric scale factor and systematic photometric errors

Context: Understanding the source of systematic errors in photometry is essential for their calibration. Aims: We investigate how photometry performed on difference images can be influenced by errors in the photometric scale factor. Methods: We explore the equations for difference image analysis (DIA) and we derive an expression describing how errors in the difference flux, the photometric scale factor and the reference flux are propagated to the object photometry. Results: We find that the error in the photometric scale factor is important, and while a few studies have shown that it can be at a significant level, it is currently neglected by the vast majority of photometric surveys employing DIA. Conclusions: Minimising the error in the photometric scale factor, or compensating for it in a post-calibration model, is crucial for reducing the systematic errors in DIA photometry.

No evidence for significant age spreads in young massive LMC clusters

Recent discoveries have put the picture of stellar clusters being simple stellar populations into question. In particular, the color-magnitude diagrams of intermediate age (1-2 Gyr) massive clusters in the Large Magellanic Cloud (LMC) show features that could be interpreted as age spreads of 100-500 Myr. If multiple generations of stars are present in these clusters then, as a consequence, young (<1 Gyr) clusters with similar properties should have age spreads of the same order. In this paper we use archival Hubble Space Telescope (HST) data of eight young massive LMC clusters (NGC 1831, NGC 1847, NGC 1850, NGC 2004, NGC 2100, NGC 2136, NGC 2157 and NGC 2249) to test this hypothesis. We analyzed the color-magnitude diagrams of these clusters and fitted their star formation history to derive upper limits of potential age spreads. We find that none of the clusters analyzed in this work shows evidence for an extended star formation history that would be consistent with the age spreads proposed for intermediate age LMC clusters. Tests with artificial single age clusters show that the fitted age dispersion of the youngest clusters is consistent with spreads that are purely induced by photometric errors. As an additional result we determined a new age of NGC 1850 of ~100 Myr, significantly higher than the commonly used value of about 30 Myr, although consistent with early HST estimates.

Using large scale structure to measurefNL,gNLandτNL

Primordial non-Gaussianity of local type is known to produce a scale-dependent contribution to the galaxy bias. Several classes of multifield inflationary models predict non-Gaussian bias which is stochastic, in the sense that dark matter and halos do not trace each other perfectly on large scales. In this work, we forecast the ability of next-generation large-scale structure surveys to constrain common types of primordial non-Gaussianity like ${f}_{\mathrm{NL}}$, ${g}_{\mathrm{NL}}$ and ${\ensuremath{\tau}}_{\mathrm{NL}}$ using halo bias, including stochastic contributions. We provide fitting functions for statistical errors on these parameters which can be used for rapid forecasting or survey optimization. A next-generation survey with volume $V=25{h}^{\ensuremath{-}3}\text{ }\text{ }{\mathrm{Gpc}}^{3}$, median redshift $z=0.7$ and mean bias ${b}_{g}=2.5$ can achieve $\ensuremath{\sigma}({f}_{\mathrm{NL}})=6$, $\ensuremath{\sigma}({g}_{\mathrm{NL}})={10}^{5}$ and $\ensuremath{\sigma}({\ensuremath{\tau}}_{\mathrm{NL}})={10}^{3}$ if no mass information is available. If halo masses are available, we show that optimally weighting the halo field in order to reduce sample variance can achieve $\ensuremath{\sigma}({f}_{\mathrm{NL}})=1.5$, $\ensuremath{\sigma}({g}_{\mathrm{NL}})={10}^{4}$ and $\ensuremath{\sigma}({\ensuremath{\tau}}_{\mathrm{NL}})=100$ if halos with mass down to ${M}_{\mathrm{min}}={10}^{11}{h}^{\ensuremath{-}1}{M}_{\ensuremath{\bigodot}}$ are resolved, outperforming Planck by a factor of 4 on ${f}_{\mathrm{NL}}$ and nearly an order of magnitude on ${g}_{\mathrm{NL}}$ and ${\ensuremath{\tau}}_{\mathrm{NL}}$. Finally, we study the effect of photometric redshift errors and discuss degeneracies between different non-Gaussian parameters, as well as the impact of marginalizing Gaussian bias and shot noise.

STELLAR LOCI. I. METALLICITY DEPENDENCE AND INTRINSIC WIDTHS

Stellar loci are widely used for selection of interesting outliers, reddening determinations, and calibrations. However, hitherto the dependence of stellar loci on metallicity has not been fully explored and their intrinsic widths are unclear. In this paper, by combining the spectroscopic and re-calibrated imaging data of the SDSS Stripe 82, we have built a large, clean sample of dwarf stars with accurate colors and well determined metallicities to investigate the metallicity dependence and intrinsic widths of the SDSS stellar loci. Typically, one dex decrease in metallicity causes 0.20 and 0.02 mag decrease in colors u-g and g-r, and 0.02 and 0.02 mag increase in colors r-i and i-z, respectively. The variations are larger for metal-rich stars than for metal-poor ones, and for F/G/K stars than for A/M ones. Using the sample, we have performed two dimensional polynomial fitting to the u-g, g-r, r-i, and i-z colors as a function of color g-i and metallicity [Fe/H]. The residuals, at the level of 0.029, 0.008, 0.008 and 0.011 mag for the u-g, g-r, r-i, and i-z colors, respectively can be fully accounted for by the photometric errors and metallicity uncertainties, suggesting that the intrinsic widths of the loci are at maximum a few mmag. The residual distributions are asymmetric, revealing that a significant fraction of stars are binaries. In a companion paper, we will present an unbiased estimate of the binary fraction for field stars. Other potential applications of the metallicity dependent stellar loci are briefly discussed.

Multiplicative errors in the galaxy power spectrum: self-calibration of unknown photometric systematics for precision cosmology

We develop a general method to ‘self-calibrate’ observations of galaxy clustering with respect to systematics associated with photometric calibration errors. We first point out the danger posed by the multiplicative effect of calibration errors, where large-angle error propagates to small scales and may be significant even if the large-scale information is cleaned or not used in the cosmological analysis. We then propose a method to measure the arbitrary large-scale calibration errors and use these measurements to correct the small-scale (high-multipole) power which is most useful for constraining the majority of cosmological parameters. We demonstrate the effectiveness of our approach on synthetic examples and briefly discuss how it may be applied to real data.

AN ACCURATE NEW METHOD OF CALCULATING ABSOLUTE MAGNITUDES ANDK-CORRECTIONS APPLIED TO THE SLOAN FILTER SET

We describe an accurate new method for determining absolute magnitudes, and hence also K-corrections, which is simpler than most previous methods, being based on a quadratic function of just one suitably chosen observed color. The method relies on the extensive and accurate new set of 129 empirical galaxy template SEDs from Brown et al. (2014). A key advantage of our method is that we can reliably estimate random errors in computed absolute magnitudes due to galaxy diversity, photometric error and redshift error. We derive K-corrections for the five Sloan Digital Sky Survey filters and provide parameter tables for use by the astronomical community. Using the New York Value-Added Galaxy Catalog we compare our K-corrections with those from kcorrect. Our K-corrections produce absolute magnitudes that are generally in good agreement with kcorrect. Absolute g, r, i, z-band magnitudes differ by less than 0.02 mag, and those in the u-band by ~0.04 mag. The evolution of rest-frame colors as a function of redshift is better behaved using our method, with relatively few galaxies being assigned anomalously red colors and a tight red sequence being observed across the whole 0.0 < z < 0.5 redshift range.

Histogram of gradient phases: a new local descriptor for face recognition

Gradient‐based local descriptors have received more attention these years and have been successfully used in many applications such as human detection and face recognition. The advantages of the local descriptors are the resistance to the local geometric and photometric errors and the robustness to the expression variations. In this paper, the authors propose a new local descriptor called the histogram of gradient phases (HGP), which has some intriguing properties compared with the existing local descriptors such as the histogram of orientated gradients and DAISY for face recognition under the unconstrained conditions. In contrast with the histogram of the oriented gradient descriptor, the orientation histogram is computed from the estimated gradient phase distributions instead of weighting the votes of the gradient magnitudes. In this paper, the phase distributions are estimated by means of the gradient phases and the variances are decided by the estimated gradient signal‐to‐noise ratios of the pixels in a local region. The HGP descriptor which takes the confidence of the gradient phase into account is more discriminative and less sensitive to the normalisation process than most local descriptors, which significantly degrade without a proper normalisation. The simulation results show that the proposed HGP descriptor achieves a better performance and is more robust than the existing local descriptors.

Performance of the VLT Planet Finder SPHERE

The new planet finder for the Very Large Telescope (VLT), the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE), just had its first light in Paranal. A dedicated instrument for the direct detection of planets, SPHERE, is composed of a polametric camera in visible light, the Zurich IMager POLarimeter (ZIMPOL), and two near-infrared sub-systems: the Infra-Red Dual-beam Imager and Spectrograph (IRDIS), a multi-purpose camera for imaging, polarimetry, and long-slit spectroscopy, and the integral field spectrograph (IFS), an integral field spectrograph. We present the results obtained from the analysis of data taken during the laboratory integration and validation phase, after the injection of synthetic planets. Since no continuous field rotation could be performed in the laboratory, this analysis presents results obtained using reduction techniques that do not use the angular differential imaging (ADI) technique. To perform the simulations, we used the instrumental point spread function (PSF) and model spectra of L and T-type objects scaled in contrast with respect to the host star. We evaluated the expected error in astrometry and photometry as a function of the signal to noise of companions, after spectral differential imaging (SDI) reduction for IRDIS and spectral deconvolution (SD) or principal component analysis (PCA) data reductions for IFS. We deduced from our analysis, for example, that $\beta$Picb, a 12~Myr old planet of $\sim$10~\MJ and semi-major axis of 9--10 AU, would be detected with IRDIS with a photometric error of 0.16~mag and with a relative astrometric position error of 1.1~mas. With IFS, we could retrieve a spectrum with error bars of about 0.15~mag on each channel and astrometric relative position error of 0.6~mas. For a fainter object such as HR8799d, a 13~\MJ planet at a distance of 27~AU, IRDIS could obtain a relative astrometric error of 3~mas.

Helium and multiple populations in the massive globular cluster NGC 6266 (M 62)

Recent studies suggest that the helium content of multiple stellar populations in globular clusters (GCs) is not uniform. The range of helium varies from cluster to cluster with more massive GCs having, preferentially, large helium spread. GCs with large helium variations also show extended-blue horizontal branch (HB). I exploit Hubble Space Telescope photometry to investigate multiple stellar populations in NGC6266 and infer their relative helium abundance. This cluster is an ideal target to investigate the possible connection between helium, cluster mass, and HB morphology, as it exhibits an extended HB and is among the ten more luminous GCs in the Milky Way. The analysis of color-magnitude diagrams from multi-wavelength photometry reveals that also NGC6266, similarly to other massive GCs, hosts a double main sequence (MS), with the red and the blue component made up of the 79+-1% and the 21+-1% of stars, respectively. The red MS is consistent with a stellar population with primordial helium while the blue MS is highly helium-enhanced by Delta Y=0.08+-0.01. Furthermore, the red MS exhibits an intrinsic broadening that can not be attributed to photometric errors only and is consistent with a spread in helium of ~0.025 dex. The comparison between NGC6266 and other GCs hosting helium-enriched stellar populations supports the presence of a correlation among helium variations, cluster mass, and HB extension.

A pipeline for digital restoration of deteriorating photographic negatives

Extending work presented at the second International Workshop on Historical Document Imaging and Processing, we demonstrate a digitization pipeline to capture and restore negatives in low-dynamic range file formats. The majority of early photographs were captured on acetate-based film. However, it has been determined that these negatives will deteriorate beyond repair even with proper conservation and no suitable restoration method is available without physically altering each negative. In this paper, we present an automatic method to remove various non-linear illumination distortions caused by deteriorating photographic support material. First, using a high-dynamic range structured-light scanning method, a 2D Gaussian model for light transmission is estimated for each pixel of the negative image. Estimated amplitude at each pixel provides an accurate model of light transmission, but also includes regions of lower transmission caused by damaged areas. Principal component analysis is then used to estimate the photometric error and effectively restore the original illumination information of the negative. A novel tone mapping approach is then used to produce the final restored image. Using both the shift in the Gaussian light stripes between pixels and their variations in standard deviation, a 3D surface estimate is calculated. Experiments of real historical negatives show promising results for widespread implementation in memory institutions.

Redshift-space distortions from the cross-correlation of photometric populations

Several papers have recently highlighted the possibility of measuring redshift-space distortions from angular auto-correlations of galaxies in photometric redshift bins. In this work, we extend this idea to include as observables the cross-correlations between redshift bins, as an additional way of measuring radial information. We show that this extra information allows us to reduce the recovered error in the growth rate index γ by a factor of ∼2. Although the final error in γ depends on the bias and the mean photometric accuracy of the galaxy sample, the improvement from adding cross-correlations is robust in different settings. Another factor of 2–3 improvement in the determination of γ can be achieved by considering two galaxy populations over the same photometric sky area but with different biases. This additional gain is shown to be much larger than the one from the same populations when observed over different areas of the sky (with twice the combined area). The total improvement of ∼5 implies that a photometric survey such as Dark Energy Survey should be able to recover γ at the 5–10 per cent from the angular clustering in linear scales of two different tracers. It can also constrain the evolution of f(z) × σ8(z) in few bins beyond z ∼ 0.8–0.9 at the 10–15 per cent level per bin, compatible with recent constrains from lower z spectroscopic surveys. We also show how further improvement can be achieved by reducing the photometric redshift error.