We report results from deep NICMOS imaging of the fields

of two damped Lyman-α (DLA) quasar absorbers at Z_(DLA) = 1.89 and Z_(DLA) = 1.86. The images were obtained in the broad filter F160W and the narrow filter F190N or F187N with camera 2 on NICMOS, with the goal of detecting the rest-frame optical continuum and H-α emission from

the DLA absorbers. The broad band images put sensitive constraints on the sizes of the DLAs, while the narrow band images put the tightest existing limits on the star formation rates in DLAs. These results have important implications for the physical nature of DLA absorbers

Hill, John M.

Jannuzi, Buell

Kulkarni, Varsha P.

Rieke, Marcia J.

Schneider, Glenn

Storrie-Lombardi, Lisa J.

Thompson, Rodger I.

Weymann, Ray J.

Caltech Authors - Main

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The Hy Redshift Universe 
ASP Conference Series, Vol. 193, 1999 
A. J. Bunker & W. J. M. van Breugel, eds. 
NICMOS Imaging of Damped Ly-a Absorbers at z = 2 
Varsha P. Kulkarni1 , John M. Hill, Glenn Schneider, 
Steward Observatory, University of Arizona, Tucson, AZ 85721 
Ray J. Weymann, Lisa J. Storrie-Lombardi 
Carnegie Observatories, Pasadena, CA 91101 
Marcia J. Rieke, Rodger I. Thompson, 
Steward Observatory, University of Arizona, Tucson, AZ 85721 
Buell Jannuzi 
National Optical Astronomy Observatories, Tucson, AZ 85726 
Abstract. We report results from deep NICMOS imaging of the fields 
of two damped Lyman-a (DLA) quasar absorbers at ZDLA = 1.89 and 
ZDLA = 1.86. The images were obtained in the broad filter F160W and 
the narrow filter F190N or F187N with camera 2 on NICMOS, with the 
goal of detecting the rest-frame optical continuum and H-a emission from 
the DLA absorbers. The broad band images put sensitive constraints on 
the sizes of the DLAs, while the narrow band images put the tightest 
existing limits on the star formation rates in DLAs. These results have 
important implications for the physical nature of DLA absorbers. 
1. Introduction 
Damped Lyman-alpha (DLA) absorbers in spectra of high-redshift quasars are 
believed to trace the precursors of present-day galaxies. But the nature of ab-
sorbing galaxies is not yet understood. DLAs have been variously thought to 
arise in large rotating proto-disks (e.g., Wolfe et al. 1986, Prochaska & Wolfe 
1997) or dwarf galaxies (e.g., York et al. 1986, Matteucci et al. 1997). Un-
fortunately, most previous efforts to directly image the objects causing high-z 
DLAs have been unsuccessful. Most of these previous searches attempted to 
detect the redshifted Ly-a emission from the DLAs. However, these nondetec-
tions cannot directly constrain the star formation rate (SFR) in DLAs, since 
the Ly-a line can easily get extinguished by even small amounts of dust, ow-
ing to resonant scattering. Here we report HST NICMOS observations of the 
fields of the DLAs toward LBQS 1210+1731(znLA=1.892, ZQSO = 2.543) and 
1 Current Address: Clemson University, Dept. of Physics and Astronomy, Clemson, SC 29634 
603 
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604 Kulkarni et al. 
Q1244+3443 (znLA = 1.859, ZQSO = 2.48), aimed at detecting redshifted H-a 
and optical continuum emission from the DLAs. 
2. Observations, Data Reduction, PSF Subtraction 
Broad band images were obtained with NICMOS camera 2, using filter F160W 
(Acentral = 1.5940 µm, FWHM = 0.4030 µm), dithered in steps of 7.5 pixels. 
Narrow band images were also obtained using dither steps of 7 .5 pixels, in filter 
F190N (Acentral = 1.9005 µm, FWHM = 0.0174 µm) for LBQS 1210+1731 and 
in filter F187N (Acentral = 1.8740 µm, FWHM = 0.0192 µm) for Q1244+3443. 
These filters should contain the redshifted H-a lines at z = 1.892 and z = 1.859, 
respectively. Finally, coronagraphic F160W images were also obtained for each 
QSO (See Kulkarni et al. 1999a, 1999b ). Data were reduced using the IRAF 
package Nicred 1.8 (McLeod 1997). Reference PSFs for quasar PSF subtraction 
were obtained from observations of stars in the same filter / aperture combi-
nations. The PSF star images were chosen to match as closely as possible the 
telescope "breathing" focus values for our quasar observations. For the coro-
nagraphic images, more importance was given to getting close match between 
the exact positions in the coronagraphic hole where the quasar and the PSF 
star were located. The PSF star observations were analyzed in exactly the same 
manner as the quasar observations. The reduced PSF stars were registered and 
scaled to match the quasar and then subtracted from the quasar. 
3. Searching for the DLA near LBQS 1210+1731 
3.1. Non-coronagraphic F160W and F190N images 
About 99 3 of the flux in the non-coronagraphic F160W and F190N images 
comes from the quasar point source and disappears after PSF subtraction. A 
feature to the "lower right" of the center at about 0.26" from the quasar ("01") 
is the main asymmetric residual left behind in the images (Fig. 1 ). We have 
carried out several tests to determine whether 01 is real or an artifact arising 
from HST breathing, color differences between quasar and PSF star, differences 
in focus settings, asymmetries in the PSF core etc. (Kulkarni et al. 1999a). 
The best-fitting PSF and several others with reasonably close breathing values 
suggest that 01 is real, given the significant excess over a number of pixels. We 
cannot completely rule out that 01 is some artifact arising for reasons we have 
not been able to account for. If 01 is associated with the DLA, then it is 2.4 
hO.i kpc long for qo = 0.5 or 3.2 hO.i kpc long for q0 = 0.1. 01 has a flux of 
9.8 ± 2.4 µJy in the F160W filter and 10.6 ± 1.5 µJy in the F190N filter. On 
estimating the continuum contribution to the F190N flux, there is no evidence 
for statistically significant redshifted H-a line flux. Given the low dust-to-gas 
ratios inferred for DLAs (e.g. Pei et al. 1991; Pettini et al. 1997 and references 
therein), the nondetection of H-a emission is likely to be because of low SFR. 
Assuming no dust and using the H-a luminosity-SFR conversion of Kennicutt 
(1983), the 3a upper limit on the SFR is 4.0 h0.~ M0 yr-1 for q0 = 0.5. This is 
the tightest existing constraint on the SFR in DLAs. 
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-0.300 0.0 1.500 
-0.008 0.0 0.040 
-0.500 0.0 0.500 -0.025 0.0 0.025 
2.74" ~ 2.69" 
Figure 1. Left: Zoomed-in 2.74"x2.71" region of the NICMOS cam-
era 2 non-coronagraphic 1.6 µm broad-band image of Q1210+1731, 
(a) before (top) and (b) after (bottom) PSF subtraction (bottom). 
Right: Zoomed-in 2.69"x2.66" region of the NICMOS camera 2 non-
coronagraphic 1.9 µm narrow-band image of Q1210+1731, ( c) before 
(top) and ( d) after (bottom) PSF subtraction. 
© Astronomical Society of the Pacific • Provided by the NASA Astrophysics Data System 
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-0.030 0.0 0040 -0.030 00 0.040 
Figure 2. Zoomed-in 2.89" x 2.8111 regions of coronagraphic 1.6 µm 
image of Q1210+1731, (a) before (left) and (b) after (right) PSF sub-
traction 
3.2. Coronagraphic Image of LBQS1210+1731 
The NICMOS coronagraph reduces the scattered and diffracted energy from the 
occulted target's PSF core by factors of 4-6, compared to direct imaging (Schnei-
der et al. 1998; Lowrance et al. 1998). Most of the flux in the coronagraphic 
images comes from small amount of scattered light from the quasar and glints 
from the edge of the hole. These disappear almost completely after PSF sub-
traction. But a weak feature consisting of several knots (object "02") remains 
at about 0.7 "from the quasar center (Fig. 2). There are no known artifacts in 
the coronagraphic F160W image at the positions of the 02 knots. H 02 is at 
the redshift of the DLA, it has a total size of 4-5 h0.~ kpc for q0 = 0.5. 
4. The DLA toward Q1244+3443 
The results for this DLA are similar to those for the DLA toward Q1210+1731 
(Fig. 3). Note the possible presence of a compact object below the quasar center 
in the F160W image, at a separation of only 0.16 ". The narrow-band image 
reveals no strong extended emission from this or any other source, suggesting 
a low SFR. Further details are discussed by Kulkarni et al. (1999b ). Thus, 
our observations of both the DLAs suggest compact or low surface brightness 
objects with a few continuum emission knots, and low star formation rates. 
Acknowledgments. This work was supported by NASA grant NAG5-3042. 
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-0.250 0.0 0.250 -0.013 0.0 0.013 
Figure 3. Zoomed-in 2.8l"x2.79" regions of the NICMOS camera 2 
non-coronagraphic (a) 1.6 µm (left) and (b) 1.87 µm (right) images of 
the field of Q1244+3443, after PSF subtraction 
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Kennicutt, R. C. 1983, ApJ, 272, 54 
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Kulkarni, V. P., Hill, J. M., Schneider, G., Weymann, R. J., Storrie-Lombardi, 
L. J., Rieke, M. J., Thompson, R. I., & Jannuzi, B. 1999b, in preparation 
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© Astronomical Society of the Pacific • Provided by the NASA Astrophysics Data System 


NICMOS Imaging of Damped Ly-α Absorbers at z = 2

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NICMOS Imaging of Damped Ly-α Absorbers at z = 2

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