NED Holdings
# Holdings: Graphics

(Latest Revision: 23 February 2021)

### Sky Density

The distribution of NED objects on the sky as of October 2019, after the integration of the AllWISE Source Catalog in NED. The data are
plotted in Galactic coordinates. The color bar shows the number density
ranging from 0 (black) to 7.06 million (red) objects per square
degree. The densest region in the current NED sky (centered at l =
223.59 deg and b = -54.39 deg) is within the Hubble Ultra Deep Field.

### Redshift Distribution

The spectroscopic and photometric redshift (look-back time)
distributions of NED objects as of February 2021.

#### Large Data Sets in NED

More information and graphics are available for some of the
largest data sets in NED.

#### Comparison to Model Predictions

We present here preliminary results (NED 2015, in prep.), comparing the
NED spectroscopic redshift distribution at z < 0.3
with a simple cosmological model. This gives an idea of the relative
completeness of NED as a function of redshift.

- Top left
- Differential counts of galaxies with spectroscopic redshifts (~ 1
million with z < 0.3) in NED. These are compared to model predictions of
the galaxy redshift distribution, obtained by multiplying the comoving
volume element by an empirical estimate of the galaxy number density. We
assume that galaxy number density is redshift-independent at these low
redshifts, and estimate its value by integrating over published
Schechter function fits to the observed galaxy luminosity function (e.g.,
Blanton et al. 2003;
Zucca et al. 2012 ), with a lower luminosity
cutoff of L/*L=0.0001. We assume a cosmology with H
_{0} = 68
km/s/Mpc, Ω(matter) = 0.3, Ω(Lambda) = 0.7, and Ω(k) =
0.0.
The three model curves show the median and 1-sigma variation in
published estimates of the average galaxy density in this redshift
range. The size of the redshift bins is dz = 0.0025.
- Top right
- Ratio of differential counts to model counts.
- Bottom left
- Cumulative counts of galaxies with spectroscopic redshifts in NED.
- Bottom right
- Ratio of cumulative counts to model counts.
**Note:** The low fractional completeness (< 0.1 at z < 0.03) is
characteristic of modern spectroscopic surveys which either cover a
small solid area on the sky, or target a selected subsample of galaxies
within their survey area. NED does its best to incorporate all available
published redshifts, and we will update these figures as new data are added.