A flux-limited sample of dusty star-forming galaxies from the Atacama Cosmology Telescope: physical properties and the case for multiplicity

Abstract

We report the modeling of the millimeter and far-infrared spectral energy distributions of 71 dusty star-forming galaxies (DSFGs) selected by the Atacama Cosmology Telescope (ACT) with a lower flux-density limit of 8 mJy at 220 GHz (1.4 mm). All sources were cross-identified with Herschel surveys at 500, 350, and 250 μm, and nineteen of our sources were observed at with the Submillimeter Array. A probabilistic cataloging algorithm, PCAT, favors multiple unresolved flux components in the Herschel data for the majority of ACT-selected DSFGs. We compare the derived physical properties of the DSFGs obtained from modeling the flux densities with those from similar studies of both lensed and unlensed DSFG populations. We find the median, 16th and 84th percentiles for the following model parameters: redshift zphot=3.3(+0.7)(-0.6), apparent size μd=5.2(+0.9)(-2.4) kpc, apparent dust mass log10(μMd/Msun)=9.14(+0.12)(-0.04) and cutoff temperature Tc=35.6(+4.8)(-1.6) K, and the corresponding apparent far-infrared luminosity log10(μLIR/Lsun)=13.6(+0.2)(-0.3), where μ is lensing magnification. While many of the properties broadly agree with those of samples of primarily lensed DSFGs, we exercise caution in interpreting them. ACT's lower flux limit, the PCAT decomposition, and the higher-resolution SMA observations all suggest that some fraction of these DSFGs are likely to be unlensed and possibly multiples. The SMA data indicate that at least fourteen out of nineteen sources are such, either via "missing" flux in comparison to the ensemble model or detection of additional sources in the fields. Additional high-resolution follow-up and targeted redshift observations are needed to better understand this flux-limited sample of DSFGs.

Figures (12)

• Figure 1: The SED of ACT-S J004532-000127 with imaging at different wavelengths (right). Left: Black points are the ACT de-boosted fluxes, blue points are the Herschel ensemble fluxes, magenta triangles are the PCAT brightest flux component, and the green star is the summed flux from the SMA detections. The blue curve is the best-fitting SED derived from the medians of the posterior distributions of the parameters. Right: Imaging data from the microwave to optical centered on the ACT source location. Green stars on the ACT and Herschel thumbnails indicate the location of the SMA source closest to the ACT source location, and we plot the 3, 5 and 7$\sigma$ ($\sigma$=0.8 mJy) SMA contours in green on the Spitzer, VHS, and PS1 images. The green ellipse at the bottom right of the high-resolution data shows the shape of the synthesized SMA beam, which for these data is 2.5"x4.1". The same figure for the other sources that were observed by the SMA are placed in a Figure set (19 images) and available in the online journal.
• Figure 1: ACT-selected DSFGs observed with the SMA. Left: SED using the ACT (black), Herschel ensemble (blue), SMA (green), and brightest PCAT-predicted counterpart (magenta) flux densities, and the best-fitting model SED (blue curve). Right: ACT and Herschel thumbnails, as well as near-IR and optical cutouts from Spitzer, Apache Point Observatory 1.5 m telescope, VHA, and PS1, as labeled. Green star is the location of the SMA source detected nearest to the ACT center location. Black dashed curve on the Herschel thumbnails is the 30" radius used to quantify PCAT counterparts, and the pink magenta circles on the Herschel, near-IR and optical images are the 90% flux radius of the PCAT posterior images, centered on the location of the brightest counterpart.
• Figure 2: SEDs for all 71 DSFGs in our sample. The ACT flux densities are shown as black points, the ensemble Herschel flux densities are blue points, and the brightest PCAT component flux densities are plotted as magenta triangles. For many sources, the brightest flux density is equal to the ensemble flux within uncertainty, indicating that the PCAT posteriors indicate only one point source flux component in the Herschel data. SMA flux densities are plotted as green stars. The blue curves are the best-fitting SEDs, including photometric redshift as a fit parameter, and for our five sources with spectroscopic redshifts, we plot the best-fitting fixed-z SED as a black dashed line. The median photometric redshift (or the spectroscopic redshift) and the median cutoff temperature are reported at the bottom of each figure, and the number of PCAT flux components within 30" is reported on the top left below the source name. Uniquely, the fluxes for J0209+0015 are all divided by 10.
• Figure 2: Continued
• Figure 3: Median values of the fit parameters when our model is fit to the flux densities of the Reuter20 sources. The red histograms and y-axis of the scatter plots use only the uniform priors on apparent size, apparent dust mass and cutoff temperature, and fixed spectroscopic redshift. The blue histograms and x-axis of the scatter plots correspond to using the newly defined priors (shown as black curves on the dust mass and temperature histograms) and fitting for redshift. The points are colored by their spectroscopic redshift.