NICMOS IMAGES OF THE UDF Rodger I. Thompson Steward Observatory University of Arizona

The NICMOS UDF Team Rodger I. ThompsonU of Arizona Garth IllingworthUC Santa Cruz Mark DickinsonSTScI Rychard BouwensUC Santa Cruz Daniel EisensteinU of Arizona Xiaohui FanU of Arizona Marijn FranxU Leiden Marcia RiekeU of Arizona Adam RiessSTScI Pieter van DokkumYale

NICMOS NICMOS is the Near Infrared Camera and Multi-Object Spectrometer It is HST’s infrared instrument Operates at wavelengths between 0.8 and 2.5 microns Restored to operation in 2002 by installation of the NICMOS Cooling System (NCS)

The Images Covers the inner region of the ACS UDF image Images at 2 wavelengths 1.1 and 1.6 microns. The 1.6 micron band is the lowest background band available to HST. The image is a 3 by 3 mosaic due to the much smaller NICMOS field of view.

Improvement due to the NCS The NICMOS UDF image is at the same sensitivity as the previous NICMOS NHDF image that was 9 times smaller in area. This improvement in efficiency is due to the improved sensitivity after the installation of the NCS. Many thanks to all the people who made that happen, including the crew of STS 109 who installed the cooler.

Location Relative to the ACS Image NICMOS Image Outlined in Red. The ACS Image is the blue outline.

What makes this interesting? Redshift makes this interesting! Redshift moves the light from the blue to the red. The earlier in time we observe, the higher the redshift. At times earlier than 750 Million years all of the light is shifted to wavelengths that HST can only observe with NICMOS.

Sensitivity to High Redshift Objects NICMOS ACS Curves for a standard (L*) bright galaxy

Implications The NICMOS UDF image has the sensitivity to see standard bright galaxies out to a redshift of greater than 12. The universe was only 300 million years old at that time, 2.4% of its present age. This image represents the furthest look back time for objects ever achieved.

What do we do next? We combine the just released ACS data with the NICMOS data to measure the properties of the galaxies in the image. We will measure: The redshift via photometric redshifts The type of galaxies The dust extinction in the galaxies The shape and structure of the galaxies

Photometric Redshifts We match the flux in the NICMOS and ACS bands against the predicted flux calculated from galaxy spectrum templates that have been numerically redshifted. The galaxy templates are also numerically altered for dust extinction. The procedure produces a redshift, galaxy type and dust extinction for each object.

Redshift 0 f435w f606w f775w f850lp f110w f160w

Redshift 1

Redshift 2

Redshift 3

Redshift 6

Redshift 9

Redshift 12

A Fundamental Question Are the number and nature of galaxies at redshifts greater than 6 the same as the number and nature at lower redshifts? There are some hints that either the number of galaxies or the brightness of galaxies is far less at these high redshifts. Have we seen back to a time when there were far fewer galaxies than there are now? Have we seen past the end of the beginning?

Another part of the question When and how did the universe get reionized? The beautiful microwave background images from WMAP showed us the universe as it became cool enough to go from ionized gas to neutral gas. At the present time and at least back to a redshift of 6 we know that the universe is ionized again. When and how did this happen.

Reionization WMAP has an indication that the reionization may have started around redshift 17. Quasar spectra indicate that the universe was still partially neutral at redshifts greater than 6. We think that the light from young galaxies reionized the universe. Can we see them?

NICMOS Probe of the Epoch of Reionization Epoch of Reionization

What caused the reionization? Bright galaxies? Quasars? A large number of lower luminosity galaxies? Galaxies with low mass but a large number of very massive stars?

What is the star formation history of the universe? NICMOS directly observes the UV light of very young stars at high redshift. This is a direct measurement of the star formation rate. The Spitzer observations in the UDF will spot any star formation hidden by dust. Previous work with NICMOS and WFPC2 indicates a constant star formation rate from z = 1 – 6. Is there a time when the star formation rate becomes significantly lower?

Impact on JWST JWST is designed to study the universe at a far distance and an early time. If galaxies at this time are significantly fewer or significantly fainter it will alter the observing strategy and instrument design. In either case we need JWST to explore the early universe.

Conclusion It is an amazing time to be an astrophysicist!