We describe an object in the Hubble Deep Field North with very unusual near–infrared properties .
It is readily visible in Hubble Space Telescope NICMOS images at 1.6 \mu m and from the ground at 2.2 \mu m , but is undetected ( with signal–to–noise \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 316 $ } } 2 ) in very deep WFPC2 and NICMOS data from 0.3 to 1.1 \mu m. The f _ { \nu } flux density drops by a factor \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 318 $ } } 8.3 ( 97.7 % confidence ) from 1.6 to 1.1 \mu m. The object is compact but may be slightly resolved in the NICMOS 1.6 \mu m image .
In a low–resolution , near–infrared spectrogram , we find a possible emission line at 1.643 \mu m , but a reobservation at higher spectral resolution failed to confirm the line , leaving its reality in doubt .
We consider various hypotheses for the nature of this object .
Its colors are unlike those of known galactic stars , except perhaps the most extreme carbon stars or Mira variables with thick circumstellar dust shells .
It does not appear to be possible to explain its spectral energy distribution as that of a normal galaxy at any redshift without additional opacity from either dust or intergalactic neutral hydrogen .
The colors can be matched by those of a dusty galaxy at z \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 318 $ } } 2 , by a maximally old elliptical galaxy at z \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 318 $ } } 3 ( perhaps with some additional reddening ) , or by an object at z \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 318 $ } } 10 whose optical and 1.1 \mu m light have been suppressed by the intergalactic medium .
Under the latter hypothesis , if the luminosity results from stars and not an AGN , the object would resemble a classical , unobscured protogalaxy , with a star formation rate \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 318 $ } } 100 M _ { \odot } yr ^ { -1 } .
Such UV–bright objects are evidently rare at 2 < z < 12.5 , however , with a space density several hundred times lower than that of present–day L ^ { \ast } galaxies .