We have combined deep starcount data with Galaxy model predictions to investigate how effectively such measurements probe the faint end of the halo luminosity function, We have tested a number of star/galaxy classification techniques using images taken in 0.5 arcsec seeing with LRIS on the Keek telescope, and we iind that different combinations of these techniques can produce variations of 10% in the inferred starcounts at R = 22.5 and 30% at R = 24.5 mag. The decreasing average angular size of galaxies with fainter magnitude effectively limits ground-based work to R < 25.5 mag. The higher angular resolution provided by HST allows one to probe at least 2 mag fainter, but the small field size is a significant limitation, In either case, our models show that the contribution from halo subdwarfs is effectively limited to colours of (R-1)<1.0. with the redder stars being members of the Galactic disk, The apparent increase in number density for M(V) > 10 in the derived luminosity function is a result of contributions from disk stars at fainter absolute magnitudes and does not provide evidence for an upturn in the halo subdwarf mass function. Indeed, starcount data alone are not an effective method of probing the shape of the halo luminosity function close to the hydrogen-burning limit, Finally, we examine how the Hubble Deep Field observations can be used to constrain the contribution of various stellar components to die dark-matter halo. (C) 1996 American Astronomical Society.

We present deep galaxy counts and half-light radii from F160W (lambda(c) = 1.6 mu m) 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 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 less than or equal to 24.75 is 4 x 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.

We present the results of a survey which utilizes the NICMOS Camera 3 Parallel grism and imaging observations of random fields. We have identified 33 H alpha emission-line galaxies at 0.75 < z < 1.9. The inferred co-moving number, density of these objects is 3.3 x 10(-4) h(50)(3), Mpc(-3), very similar to that of the bright Lyman break objects at z similar to 3. The mean star formation rate of these galaxies 21 M. yr(-1) for H-0 = 50km/s/Mpc. Using this sample, we derived the H alpha luminosity function (LF) at z = 1.3. The integrated H alpha luminosity density at z similar to 1.3 is 1.64 x 10(40) h(50) erg s(-1) Mpc(-3), approximately 14 times greater than the local value reported by Gallego et al. (1995). The volume averaged star formation rate at 1.3 +/- 0.5 is 0.13 M. yr(-1) Mpc(-3) without correction for extinction. The SFR derived at similar to 6500 Angstrom is a factor of 3 higher than that deduced from 2800 Angstrom continua. We believe that this difference is largely due to dust extinction. The implied total extinction at 2800 Angstrom is in the range of 2 - 4 magnitude. However, the precise determination of the total extinction is sensitive to the model assumptions.

We present results from our analysis of F160W NICMOS Parallel images. These data cover similar to 9 so. arcminutes and reach 3 sigma depths of H= 24.3 - 25.5 in a 0.6 " diameter aperture with integration times of 2,000 to 13,000 seconds. We derive the first deep H band galaxy counts. The slope of the counts for H< 20 is 0.31, consistent with various K-band measurements from the Keck telescopes. The measured number counts vs, magnitude relation is reasonably well fitted with no-evolution models with a low Omega value. The half-light radii of the galaxies declines steeply with apparent magnitude and reaches the NIC3 resolution limit at H=23.5. Deep ground-based VRI imaging of one NICMOS held has revealed an extremely red galaxy with R-H = 6 and H of 18.8. Our analyses of the grism data show that we can reach 3 sigma flux limits of of 1 x 10(-16) to 2 x 10(-17) ergs/sec/cm(2) for integration times of 2,000 to 21,000 seconds. We have detected a total of 33 emission line galaxies. The comoving number density is similar to 2 x 10(-4)Mpc(-3) The detected emission lines are probably H-alpha 6563 Angstrom. Thus, the derived star formation rates, without extinction correction, are 10 - 163M. per year for galaxies at redshifts between 0.7 and 1.9.

We present a luminosity function for H alpha emission from galaxies at redshifts between 0.7 and 1.9 based on slitless spectroscopy with the near-infrared camera and multiobject spectrometer on the Hubble Space Telescope. The luminosity function is well fit by a Schechter function over the range 6 x 10(41) < L (H alpha) < 2 x 10(43) ergs s(-1) with L* = 7 x 10(42) ergs s(-1) and phi* = 1.7 x 10(-3) Mpc(-3) for H-0 = 50 km s(-1) Mpc(-1) and q(0) = 0.5. We derive a volume-averaged star formation rate at z = 1.3 +/- 0.5 of 0.13 M. yr(-1) Mpc(-3) without correction for extinction. The star formation rate that we derive at similar to 6500 Angstrom is a factor of 3 higher than that deduced from 2800 Angstrom continua. If this difference is caused entirely by reddening, the extinction correction at 2800 Angstrom is quite significant. The precise magnitude of the total extinction correction at rest-frame UV wavelengths (e.g., 2800 and 1500 Angstrom) is sensitive to the relative spatial distribution of the stars, gas, and dust, as well as to the extinction law. In the extreme case of a homogeneous foreground dust screen and a Milky Way or LMC extinction law, we derive a total extinction at 2800 Angstrom of 2.1 mag, or a factor of 7 correction to the UV luminosity density. If we use the Calzetti reddening curve, which was derived for the model in which stars, gas, and dust are well mixed and nebular gas suffers more extinction than stars, our estimate of A(2800) is increased by more than 1 mag.

We present the first results of a survey of random fields with the slitless G141 (lambda(c) = 1.5 mu m, Delta lambda = 0.8 mu m) grism on the near-IR camera and multiobject spectrometer (NICMOS) on board the Hubble Space Telescope (HST). Approximately 64 arcmin(2) have been observed at intermediate and high Galactic latitudes. The 3 sigma limiting line and continuum fluxes in each field vary from 7.5 x 10(-17) to 1 x 10-17 ergs cm(-2) s(-1) and from H = 20 to 22, respectively. Our median and area-weighted 3 sigma limiting line fluxes within a 4 pixel aperture are nearly identical at 4.1 x 10(-17) ergs cm(-2) s(-1) and are 60% deeper than the deepest narrowband imaging surveys from the ground. We have identified 33 emission-line objects and derive their observed wavelengths, fluxes, and equivalent widths. We argue that the most likely line identification is Ho! and that the redshift range probed is from 0.75 to 1.9. The 2 sigma rest-frame equivalent width Limits range from 9 to 130 Angstrom, with an average of 40 Angstrom. The survey probes an effective comoving volume of 10(5) h(50)(-3) Mpc(3) for q(0) = 0.5. Our derived comoving number density of emission-line galaxies in the range 0.7 < z < 1.9 is 3.3 x 10(-4) h(50)(3) Mpc(-3), very similar to that of the bright Lyman break objects at z similar to 3. The objects with detected emission lines have a median F160W magnitude of 20.4 (Vega scale) and a median Ka luminosity of 2.7 x 10(42) ergs s(-1). The implied star formation rates range from 1 to 324 M-circle dot yr(-1), with an average [N II] lambda lambda 6583, 6548 corrected rate of 21 M-circle dot yr(-1) for H-0 = 50 km s(-1) Mpc and q(0) = 0.5 (34 M-circle dot yr(-1) for q(0) = 0.1).

We report here optical observations during the September/October 1998 outburst of the black hole candidate XTE J1550-564. CCD photometry was obtained for the optical counterpart with the 0.9m Dutch telescope at La Silla since the onset of this event. We analysed 211 U, V and i frames, from Sep 10 to Oct 23. Stochastic flaring activity was observed on Sep 11-16, but no evidence for a superhump period, as seen in other soft X-ray transients, was found. An optical flare nearly simultaneous to an X-ray flare that occurred on Sep 21 was observed in the V-band. A reddened optical spectrum showed the typical emission lines corresponding to X-ray transients in outburst. From the interstellar absorption lines, we derive E(B-V) = 0.70 +/- 0.10 and suggest D approximate to 2.5 kpc. In such case, M-B approximate to +7 mag. for the progenitor, which is consistent with the spectral type of a low-mass similar to K0-K5 main-sequence companion.

We present a catalog of extremely red objects (EROs) discovered using the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) parallel imaging database and ground-based optical follow-up observations. Within an area of 16 arcmin(2), we detect 15 objects with R-F160W > 5 and F160W < 21.5. We have also obtained K-band photometry for a subset of the 15 EROs. All of the R-F160W selected EROs imaged at K-band have R-K > 6. Our objects have F110W-F160W colors in the range 1.3-2.1, redder than the cluster elliptical galaxies at z similar to 0.8 and nearly 1 mag redder than the average population selected from the F160W images at the same depth. In addition, among only 22 NICMOS pointings, we detected two groups or clusters in two fields; each contains three or more EROs, suggesting that extremely red galaxies may be strongly clustered. At bright magnitudes with F160W < 19.5, the ERO surface density is similar to what has been measured by other surveys. At the limit of our sample, F160W = 21.5, our measured surface density is 0.94 +/- 0.24 arcmin(-2). Excluding the two possible groups or clusters and the one apparently stellar object reduces the surface density to 0.38 +/- 0.15 arcmin(-2).

An increasing number of observations have shown that dusty starburst. galaxies are probably much more numerous at high redshifts than today, and optical surveys of the distant universe suffer from large extinction corrections. In this talk, I present the quantitative estimate on how much dust extinction correction needed to apply to the rest-frame UV luminosity density at z similar to 1.3. In addition, I will discuss the recent deep 1.4GHz radio observations of extremely red galaxies (EROs), covering an area of 26 x 26 with complimentary deep optical and near-IR images. We found that the fraction of bright EROs (H G 20) detected in deep 1.4GHz images is small, only 8-17%. The implication is that a large fraction of bright EROs are probably old ellipticals or systems with a small amount of star forming activities at z similar to 1 - 2. We found that similar to 20% of well detected micro-Jansky radio sources are very faint or even not detected in the optical and near-IR. These faint micro-Jansky sources have H > 20.5 and K > 19.5 - 20. The interpretation of these optically faint, micro-Jansky radio sources is that they are potentially candidates of dust enshrouded starburst galaxies beyond a redshift of 1. Our 1.4GHz detection threshold of 40muJy (5sigma) sets the minimum limit of star formation activities of - 200M(circle dot)/yr at z - 1.5, which can be probed by our deep VLA data. An increasing number of recent observations [2,4] support our speculation that optically faint, micro-Jansky radio sources are potential dusty starbursts at high redshift. This may be the most efficient way of detecting a large sample of sub-mm sources with the current available instruments.

Optical spectra of 14 emission-line galaxies representative of the 1999 NICMOS parallel grism Halpha survey of McCarthy et al. are presented. Of the 14, 9 have emission lines confirming the redshifts found in the grism survey. The higher resolution of our optical spectra improves the redshift accuracy by a factor of 5. The [O II]/Halpha values of our sample are found to be more than 2 times lower than expected from Jansen et al. This [O II]/Halpha ratio discrepancy is most likely explained by additional reddening in our Halpha-selected sample [on average, as much as an extra E(B-V)=0.6], as well as by a possible stronger dependence of the [O II]/Halpha ratio on galaxy luminosity than is found in local galaxies. The result is that star formation rates (SFRs) calculated from [O II]lambda3727 emission, uncorrected for extinction, are found to be on average 4+/-2 times lower than the SFRs calculated from Halpha emission. Classification of emission-line galaxies as starburst or Seyfert galaxies based on comparison of the ratios [O II]/Hbeta and [Ne III]lambda3869/Hbeta is discussed. New Seyfert 1 diagnostics using the Halpha line luminosity, H-band absolute magnitude, and Halpha equivalent widths are also presented. One galaxy is classified as a Seyfert 1 based on its broad emission lines, implying a comoving number density for Seyfert 1 galaxies of 2.5(-2.1)(+5.9)x10(-5) Mpc(-3). This comoving number density is a factor of 2.4(-2.0)(+5.5) times higher than estimated by other surveys.