NGSS proposal: Clusters and Normal Galaxies
mod/revision to proposal section A.1.3.7
Adam Stanford & Tom Jarrett
revised Aug 9, 2001

Clusters and Normal Galaxies

NGSS will provide a census of star formation in exceedingly large samples of normal galaxies and clusters out to cosmological distances. NGSS will detect over 200 million galaxies in at least one band, and approximately 50 million galaxies in all four of its bands. The 23/12 micron flux ratio (Fig. A) will provide an estimate of the redshift for these 50 million galaxies from NGSS data alone. The 3.5 and 4.7 micron measurements will estimate the total stellar mass of all detected galaxies at 0.5 < z < 2 by measuring the rest frame 1.6-2.2 micron emission (Gavazzi et al. 1996), thus providing a history of star formation in the most massive field and cluster galaxies.

Since cluster galaxies generally contain stellar populations that are old relative to those in surrounding field galaxies, the contrast of galaxy clusters relative to the field increases with redshift when viewed in the IR. By virtue of covering the entire sky, NGSS will compile a complete sample of the *most* massive galaxy clusters (M_vir > 10^15 M_sun) up to z = 1, significantly beyond the redshifts where current large area surveys die. The ability of SDSS and RASS to find clusters becomes seriously incomplete at z ~ 0.5. For a standard flat cosmology, Holder et al. estimate that there are ~1000 clusters at >10^15 M_sun over the whole sky in the range 0.5 < z < 1.0. By combining the redshift estimates from NGSS with the all-sky Planck measurements (which should reach a cluster mass limit of 10^15 M_sun, and become available on the timescale of NGSS) of the S-Z effect, it will be possible to measure Lambda to a precision of ~4%.

The NGSS cluster sample, complete by nature and with a high redshift extent, will create an unparalleled resource for studying the formation and evolution of structures on the largest scales. The construction of the cluster catalog will be carried out by Stanford using the NGSS point source catalog. Clusters will be found using techniques similar to those employed by Postman et al. (1996) on multiband images in the construction of the Palomar Distant Cluster Survey; the redshifts of the brightest cluster galaxy in each identified cluster will be estimated from the 23/12 micron flux ratios.

Figures (choose one fig for proposal)

Note: per discussions during the Aug 9 telecon, we should combine Fig 1 (above) with the figure used in the ULIRG section (Blain's plot).
Plots from Carol Lonsdale
revised Aug 22, 2001

L12-z plots

24/12 Color vs. f12 plots Color-z plots 24/12 micron flux ratio as a function of f12 and of z for the models of Xu et al. 2001, for all galaxies detected by NGSS in both the 12 and 24um bands (191uJy at 12um, 603uJy at 24). This model is appropriate only for dusty galaxies with thermal IR emission. The large dispersion in the colors at all redshifts and flux densities is due to the large dispersion in the SEDs of nearby IR-bright galaxies. This stresses the fact that one cannot assume a small number of fixed SEDs and hope to match what's really out there. No SED evolution has been explicity assumed, but SED shape is known to be a function of both galaxy type and of luminosity, and this is followed as luminosity increases with lookback time, so there is some *effective* SED evolution. There is no assumption of lensing in the model: all galaxies are unlensed.

Color-color plots

24/12 vs 12/4s color for 4 band NGSS detections from the Xu et al. model. Whilst typing and photometric redshifts are clearly very difficult with the NGSS data set, there's some indication that a color selection of 12/4<3 and 24/12>2 might be quite successful at isolating z>0.5 objects. The z>2 ULIRGs tend to appear in the extreme part of this color space, with 12/4<2 and 24/12>4.

The plots are for 10 sq deg.

Note that Xu et al.'s model uses f24/f12 as a classification parameter for local IR-bright galaxies, and thus the dependence of 24/12 with galaxy class at low z is in essence an ingoing assumption.

The strong features in the plots are the red-shifted PAH peaks and troughs.

The appearance of "tracks" in the figures is the effect of using a finite number of SEDs in the simulation.

Caution should be used in interpreting these models, because the current treatment of stellar populations is very simplified. In particular, if an SED from the local library for ULIRGs or AGN contains a significant contribution from old stellar populations in the NIR, then the predicted 12/4 color at the higher zs could be too blue for that SED, because the model evolves the entire SED in luminosity by (1+z)^~4, which is obviously incorrect for an old stellar population.