High Redshift Candidates and the Nature of Small Galaxies in the Hubble Deep Field

We present results on two related topics: 1. A discussion of high redshift candidates (z>4.5), and 2. A study of very small galaxies at intermediate redshifts, both sets being detected in the region of the northern Hubble Deep Field covered by deep NICMOS observations at 1.6 and 1.1 microns. The high redshift candidates are just those with redshift z>4.5 as given in the recent catalog of Thompson, Weymann and Storrie-Lombardi, while the ``small galaxy'' sample is defined to be those objects with isophotal area <= 0.2 squ. arcsec and with photometric redshifts 1<z<4.5. Of the 19 possible high redshift candidates listed in the Thompson et al. catalog, 11 have (nominal) photometric redshifts less than 5.0. Of these, however, only 4 are ``robust'' in the sense of yielding high redshifts when the fluxes are randomly perturbed with errors comparable to the estimated measuring error in each wave band. For the 8 other objects with nominal photometric redshifts greater than 5.0, one (WFPC2 4--473) has a published spectroscopic redshift. Of the remaining 7, 4 are robust in the sense indicated above. Two of these form a close pair (NIC 586 and NIC 107). The redshift of the object having formally the highest redshift, at 6.56 (NIC118 = WFPC2 4--601), is problematic, since F606W and F814W flux are clearly present, and the nature of this object poses a dilemma. (abridged)Comment: 44 pages, 12 figures, to appear in ApJ v591, July 10, 200

Deep H-band Galaxy Counts and Half-light Radii from HST/NICMOS Parallel Observations

We present deep galaxy counts and half-light radii from F160W ($\lambda_c=1.6\mu$) images obtained with NICMOS on HST. Nearly 9 arcmin$^2$ have been imaged with camera 3, with $3\sigma$ depths ranging from H = 24.3 to 25.5 in a 0.6$''$ diameter aperture. The slope of the counts fainter than H~$= 20$ is 0.31, and the integrated surface density to H$\leq 24.75$ is $4 \times 10^5$ galaxies per square degree. The half-light radii of the galaxies declines steeply with apparent magnitude. At H~$=24$ we are limited by both the delivered FWHM and the detection threshold of the images.Comment: 8 pages. Accepted for publication in ApJ Letter

NICMOS observations of the HDF

This paper presents initial results and performance levels from the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) observations of the Hubble Deep Field (HDF). These observations represent the deepest view of individual objects yet obtained with photometric colors of some objects indicating redshift values greater than 6. These observations add significant value to the previous optical observations of the HDF with the Wide Field and Planetary Camera II (WFPC II)

The HDF, The Arizona View

We present an initial analysis of the star formation rates in the deep NICMOS field of the Hubble Deep Field. The analysis utilizes template photometric redshift and extinction techniques on the combination of six optical and near infrared fluxes available for this region from observations with both NICMOS and WFPC2. Our results are consistent with a constant star formation rate for a redshift range of 1 to 6

Star Formation History in the NICMOS Northern Hubble Deep Field

We present the results of an extensive analysis of the star formation rates determined from the NICMOS deep images of the northern Hubble Deep Field. We use SED template fitting photometric techniques to determine both the redshift and the extinction for each galaxy in our field. Measurement of the individual extinctions provides a correction for star formation hidden by dust obscuration. We determine star formation rates for each galaxy based on the 1500 Å UV flux and add the rates in redshift bins of width 1.0 centered on integer redshift values. We find a rise in the star formation rate from a redshift of 1 to 2 then a falloff from a redshift of 2 to 3. However, within the formal limits of the error bars this could also be interpreted as a constant star formation rate from a redshift of 1 to 3. The star formation rate from a redshift of 3 to 5 is roughly constant followed by a possible drop in the rate at a redshift of 6. The measured star formation rate at a redshift of 6 is approximately equal to the present day star formation rate determined in other work. The high star formation rate measured at a redshift of 2 is due to the presence of two possible ULIRGs in the field. If real, this represents a much higher density of ULIRGs than measured locally. We also develop a new method to correct for faint galaxies or faint parts of galaxies missed by our sensitivity limit, based on the assumption that the star formation intensity distribution function is independent of redshift. We measure the 1.6 μm surface brightness due to discrete sources and predict the 850 μm brightness of all of our galaxies based on the determined extinction. We find that the far-infrared fluxes predicted in this manner are consistent with the lack of detections of 850 μm sources in the deep NICMOS HDF, the measured 850 μm sky brightness due to discrete sources and the ratio of optical-UV sky brightness to far-infrared sky brightness. From this we infer that we are observing a population of sources that contributes significantly to the total star formation rate and these sources are not overwhelmed by the contribution from sources such as the extremely superluminous galaxies represented by the SCUBA detections. We have estimated the errors in the star formation rate due to a variety of sources including photometric errors, the near-degeneracy between reddening and intrinsic spectral energy distribution as well as the effects of sampling errors and large-scale structure. We have tried throughout to give as realistic and conservative an estimate of the errors in our analysis as possible

Photometric Redshifts Applied to WFPC2 and NICMOS HDF Data

A photometric redshift analysis of optical and infrared images of the Northern Hubble Deep Field indicate a constant star formation rate for redshifts between 1 and 6. The small size of the field and small number of high redshift objects limits this finding to just the area of the NICMOS image. The photometric redshift technique is a modified version of chi square fits to observed and calculated galaxy spectral energy distribution templates

The Nature of the Small Galaxies in the Hubble Deep Field

We present results from a study of very small galaxies (isophotal area ≤ 0.2°”) with photometric redshifts 1 ≤ z ≤ 4.5 detected in the region of the northern Hubble Deep Field covered by NICMOS observations at 1.6 and 1.1 microns. We estimate that ~50 percent of these sources are star-forming galaxies at redshifts 2 < z < 3.5 and ~45 percent at are 3.5 < z < 4.5, with the remaining 5% at 1 < z < 2. We have examined averaged images of these faint (V_(606) ~ 27-29), compact objects to search for extended, surrounding flux from older, fainter populations of stars. We find no evidence that the small objects in the Hubble Deep Field are embedded in fainter, more extended galaxies. The majority are indeed isolated and compact. We estimate the 5σ depth of the averaged images to be H_(160) ≈29.8 AB magnitudes per square arcsecond

Deep H-Band Galaxy Counts and Half-Light Radii from Hubble Space Telescope/NICMOS Parallel Observations

We present deep galaxy counts and half-light radii from F160W (λ_c=1.6 μm) images obtained with the Near-Infrared Camera and Multiobject Spectrograph on the Hubble Space Telescope. Nearly 9 arcmin^2 have been imaged with camera 3, with 3 σ depths ranging from H=24.3 to 25.5 in a 0."6 diameter aperture. The slope of the counts fainter than H = 20 is 0.31, and the integrated surface density to H ≤ 24.75 is 4×10^5 galaxies deg^(-2). The half-light radii of the galaxies decline steeply with apparent magnitude. At H = 24, we are limited by both the delivered FWHM and the detection threshold of the images

A Search for the Damped Ly-alpha Absorber at z=1.86 toward QSO 1244+3443 with NICMOS

We have carried out a high-resolution imaging search for the galaxy associated with the damped Lyman-alpha (DLA) absorber at z=1.859 toward the z_{em}=2.48 quasar QSO 1244+3443, using the HST and the NICMOS. Images were obtained in the broad filter F160W and the narrow filter F187N with camera 2 on NICMOS with the goal of detecting the rest-frame optical continuum and the H-alpha line emission from the DLA. After PSF subtraction, two weak features are seen at projected separations of 0.16-0.24" from the quasar. Parts of these features may be associated with the DLA absorber, although we cannot completely rule out that they could be artifacts of the point spread function (PSF). If associated with the DLA, the objects would be ~1-2 h_{70}^{-1} kpc in size with integrated flux densities of 2.5 and 3.3 mu Jy in the F160W filter, implying luminosities at lambda_{central}=5600 A in the DLA rest frame of 4.4-5.9 x 10^{9} h_{70}^{-2} L_{solar} at z=1.86, for q0=0.5. However, no significant H-alpha line emission is seen from these objects, suggesting low star formation rates (SFRs). Our 3 sigma upper limit on the SFR in the DLA is 1.3 h_{70}^{-2} M_{solar}/yr for q0 = 0.5 (2.4 h_{70}^{-2} M_{solar} yr^{-1} for q0 = 0.1). This together with our earlier result for LBQS 1210+1731 mark a significant improvement over previous constraints on the star formation rates of DLAs. A combination of low SFR and some dust extinction is likely to be responsible for the lack of H-alpha emission. Alternatively, the objects, may be associated with the quasar host galaxy. In any case, our observations suggest that the DLA is not a large bright proto-disk, but a compact object or a low-surface brightness galaxy. If the two features are PSF artifacts then the constraints on DLA properties are even more severe.Comment: 28 pages, 9 figures. Accepted for publication in The Astrophysical Journal. Figures are given at a slightly lower resolution here, to decrease file sizes. The higher resolution versions can be found in the Ap