An Application of Multi-band Forced Photometry to One Square Degree of SERVS: Accurate Photometric Redshifts and Implications for Future Science

Abstract

We apply The Tractor image modeling code to improve upon existing multi-band photometry for the Spitzer Extragalactic Representative Volume Survey (SERVS). SERVS consists of post-cryogenic Spitzer observations at 3.6 and 4.5 μm over five well-studied deep fields spanning 18 deg 2 . In concert with data from ground-based near-infrared (NIR) and optical surveys, SERVS aims to provide a census of the properties of massive galaxies out to z≈5. To accomplish this, we are using The Tractor to perform "forced photometry." This technique employs prior measurements of source positions and surface brightness profiles from a high-resolution fiducial band from the VISTA Deep Extragalactic Observations survey to model and fit the fluxes at lower-resolution bands. We discuss our implementation of The Tractor over a square-degree test region within the XMM Large Scale Structure field with deep imaging in 12 NIR/optical bands. Our new multi-band source catalogs offer a number of advantages over traditional position-matched catalogs, including (1) consistent source cross-identification between bands, (2) deblending of sources that are clearly resolved in the fiducial band but blended in the lowerresolution SERVS data, (3) a higher source detection fraction in each band, (4) a larger number of candidate galaxies in the redshift range 5<z<6, and (5) a statistically significant improvement in the photometric redshift accuracy as evidenced by the significant decrease in the fraction of outliers compared to spectroscopic redshifts. Thus, forced photometry using The Tractor offers a means of improving the accuracy of multi-band extragalactic surveys designed for galaxy evolution studies. We will extend our application of this technique to the full SERVS footprint in the future.

The ISM Properties and Gas Kinematics of a Redshift 3 Massive Dusty Star-forming Galaxy

“…HST limit is taken from Calanog et al (2014). Upper limits from Spitzer/IRAC and MIPS observations are the survey depths of SWIRE and SERVS (Lonsdale et al 2003;Nyland et al 2017). Herschel/PACS limit at 100 µm is obtained from Level 5 observations of the XMM-VIDEO3 field (Oliver et al 2012).…”

“…In the post-cryogenic period of Spitzer, more sensitive continuum images at 3.6 and 4.5 µm were obtained in the deeper Spitzer Extragalactic Representative Volume Survey (SERVS), which reaches 5σ limits of 1.25 µJy (Mauduit et al 2012;Nyland et al 2017). For the two SWIRE images observed at longer wavelengths (IRAC 5.8 and 8.0 µm), we perform aperture photometry to extract the fluxes of HXMM05 at the centroid position determined from the SMA 870 µm map.…”

A Subarcsecond Near-infrared View of Massive Galaxies at z > 1 with Gemini Multi-conjugate Adaptive Optics

“…The Tractor provides a convenient method of fitting modelbased galaxy photometry without the need to re-sample images with widely differing pixel scales (Nyland et al 2017). Sources in the Gemini data were fit either as point sources, or if resolved, with either exponential disk or de Vaucouleurs profiles (only a handful of the galaxies had high enough signal-to-noise ratios to reliably justify fitting a Sérsic profile using the Tractor, so we did not perform fits to Sérsic profiles).…”

“…The final photometry is given in Table 5. Further details on our implementation of the Tractor image modeling code may be found in Nyland et al (2017).…”

“…The noreddening option is useful to check how much the derived stellar mass is affected by age-dust degeneracies, which may lead to an underestimate of M * for star-forming galaxies (see Pforr et al 2012). This is known as the "iceberg effect" (Maraston et al 2010), which occurs when, by fitting the youngest stellar populations with reddening, the estimated stellar mass is closer to the mass involved in the burst (the tip 19 We note this differs from the procedure described in Nyland et al (2017), where the the shape and position parameters were held fixed to improve speed and stability, and the Tractor was used only to fit the fluxes of the objects. Figure 5.…”