Rafael Guzman

Keck Spectroscopy of Redshift z ~ 3 Galaxies in the Hubble Deep Field

We have obtained spectra with the 10 m Keck telescope of a sample of 24 galaxies having colors consistent with star-forming galaxies at redshifts 2 z 4.5 in the Hubble deep field (HDF). Eleven of these galaxies are confirmed to be at high redshift (zmed = 3.0), one is at z = 0.5, and the other 12 have uncertain redshifts but have spectra consistent with their being at z > 2. The spectra of the confirmed high-redshift galaxies show a diversity of features, including weak Ly? emission, strong Ly? breaks or damped Ly? absorption profiles, and the stellar and interstellar rest-UV absorption lines common to local starburst galaxies and high-redshift star-forming galaxies reported recently by others. The narrow profiles and low equivalent widths of C IV, Si IV, and N V absorption lines may imply low stellar metallicities. Combined with the five high-redshift galaxies in the HDF previously confirmed with Keck spectra by Steidel et al. (1996a), the 16 confirmed sources yield a comoving volume density of n ? 2.4 ? 10-4 h -->503 Mpc-3 for q0 = 0.05, or n ? 1.1 ? 10-3 h -->503 Mpc-3 for q0 = 0.5. These densities are 3-4 times higher than the recent estimates of Steidel et al. (1996b) based on ground-based photometry with slightly brighter limits and are comparable to estimates of the local volume density of galaxies brighter than L*. The high-redshift density measurement is only a lower limit and could be almost 3 times higher still if all 29 of the unconfirmed candidates in our original sample, including those not observed, are indeed also at high redshift. The galaxies are small but luminous, with half-light radii 1.8 50?1 kpc and absolute magnitudes -21.5 > MB > -23. The HST images show a wide range of morphologies, including several with very close, small knots of emission embedded in wispy extended structures. Using rest-frame UV continuum fluxes with no dust correction, we calculate star formation rates in the range 7-24 or 3-9 h -->50?2 M? yr-1 for q0 = 0.05 and q0 = 0.5, respectively. These rates overlap those for local spiral and H II galaxies today, although they could be more than twice as high if dust extinction in the UV is significant. If the objects at z = 3 were simply to fade by 5 mag (assuming a 107 yr burst and passive evolution) without mergers in the 14 Gyr between then and now (for q0 = 0.05, h50 = 1.0), they would resemble average dwarf elliptical/spheroidal galaxies in both luminosity and size. However, the variety of morphologies and the high number density of z = 3 galaxies in the HDF suggest that they represent a range of physical processes and stages of galaxy formation and evolution, rather than any one class of object, such as massive ellipticals. A key issue remains the measurement of masses. These high-redshift objects are likely to be the low-mass, starbursting building blocks of more massive galaxies seen today.

The Nature of Compact Galaxies in the Hubble Deep Field. II. Spectroscopic Properties and Implications for the Evolution of the Star Formation Rate Density of the Universe

We present a spectroscopic study of 51 compact field galaxies with redshifts z 0.7 have similar SFRs per unit mass to those at z < 0.7, they are on average ~10 times more massive. Our sample implies a lower limit for the global comoving SFR density of ~0.004 M? yr-1 Mpc-3 at z = 0.55, and ~0.008 M? yr-1 Mpc-3 at z = 0.85 (assuming Salpeter IMF, H0 = 50 km s-1 Mpc-1, and q0 = 0.5). These values, when compared to estimates for a sample of local compact galaxies selected in a similar fashion, support a history of the universe in which the SFR density declines by a factor ~10 from z = 1 to today. From the comparison with the SFR densities derived for magnitude-limited samples of field galaxies, we conclude that compact emission-line galaxies, though only ~20% of the general field population, may contribute as much as ~45% to the global SFR of the universe at 0.4 < z < 1.

The Nature of Compact Galaxies in the Hubble Deep Field. I. Global Properties

We present 10 m Keck spectroscopy and photometry for a sample of 61 small (r1/2 ≤ 05), faint (I814 ≤ 23.74), high-surface brightness (μI814 2). The Keck redshift and photometric data are combined with Hubble Space Telescope images to derive luminosities and physical sizes. We also use emission-line widths, where available, to estimate masses. About two-thirds of the emission-line galaxies, or roughly one-half the sample, are small, low-mass, relatively luminous systems with properties resembling those of local H II galaxies. We compare the properties and numbers of these galaxies to the bursting dwarf model of Babul & Ferguson. Our sample includes many galaxies similar to the model galaxies in the redshift range 0.4 z 0.7, but the majority of our compact galaxies are more luminous (by up to an order of magnitude) than those of the model. The number of galaxies fitting the model parameters are lower by a factor of 2-3 than predicted. An examination of samples used in analyses of disk surface brightness at redshifts z > 0.5 shows that compact galaxies are likely to contribute to the strong disk luminosity evolution found in some studies. Estimates of comoving volume densities indicate that the population of apparent H II galaxies evolves rapidly from redshifts of z ~ 1 to the present. It appears that not all of these galaxies can be progenitors of present-day spheroidal galaxies, although the numbers of them with sizes and masses comparable to spheroidals is not dissimilar to estimates of the local field spheroidal density. We also present 51 additional redshifts, acquired for other projects during the same observing period, for a total of 105 objects with identified redshifts z < 2 in the Hubble Deep Field and its flanking fields.

Optical rotation curves of distant field galaxies: Sub-L* systems

Moderate-resolution spectroscopic observations from the Keck 10 m telescope are used to derive internal kinematics for eight faint disk galaxies in the fields flanking the Hubble Deep Field. The spectroscopic data are combined with high-resolution F814W Wide Field Planetary Camera 2 images from the Hubble Space Telescope that provide morphologies, scale lengths, inclinations, and orientations. The eight galaxies have redshifts 0.15 z 0.75, magnitudes 18.6 ≤ I814 ≤ 22.1, and luminosities -21.8 ≤ MB ≤ -19.0 (H0 = 75 km s-1 Mpc-1 and q0 = 0.05). Terminal disk velocities are derived from the spatially resolved velocity profiles by modeling the effects of seeing, slit width, slit misalignment with galaxy major axis, and inclination for each source. These data are combined with the sample of Vogt et al. to provide a high-redshift Tully-Fisher relation that spans 3 magnitudes. This sample was selected primarily by morphology and magnitude, rather than color or spectral features. We find no obvious change in the shape or slope of the relation with respect to the local Tully-Fisher relation. The small offset of 0.4 mag in B with respect to the local relation is presumably caused by luminosity evolution in the field galaxy population and does not correlate with galaxy mass. A comparison of disk surface brightness between local and high-redshift samples yields a similar offset, ~0.6 mag. These results provide further evidence for only a modest increase in luminosity with look-back time.

On the Nature of the Faint Compact Narrow Emission-Line Galaxies: The Half-Light Radius-Velocity Width Diagram*

We present new measurements of emission-line velocity widths for five faint compact narrow emission-line galaxies (CNELGs) with 20.5 #B #21.5 and redshifts z10.19 to 0.35. The spectra were taken at the Keck telescope using the HIRES spectrograph with a resolution of 8 km s 21 (FWHM). Emission-line profiles are roughly Gaussian with velocity widths s130 to 50 km s 21 . The new sdata, in combination withHubble Space Telescope(HST) measurements of half-light radiiRe, indicate that these CNELGs are low-mass stellar systems (i.e.,M110 9 MJ),whiletheirunusuallylowmass-to-lightratios(typically ,0.2MJ/LJ)areconsistentwithbeing undergoing a major burst of star formation. SinceRe and sare roughly independent of the fading of the stellar population, theRe‐sdiagram is particularly useful for comparing the properties of these young galaxies to those of evolved stellar systems. Several physical processes that may modify the position of CNELGs in this diagram during their evolution, including dissipation, mergers, stripping, and winds, are discussed briefly. We conclude that the new data support a simple evolutionary scenario in which these low-mass, young galaxies will fade after the starburst, without major changes inRe or s, to become today’s spheroidal galaxies. Subject headings: cosmology: observations—galaxies: compact—galaxies: evolution—galaxies: formation— galaxies: fundamental parameters

On the Nature of the Strong Emission-Line Galaxies in Cluster Cl 0024+1654: Are Some the Progenitors of Low-mass Spheroidals?*

We present new size, line ratio, and velocity width measurements for six strong emission-line galaxies in the galaxy cluster, Cl 0024+1654, at redshift z ~ 0.4. The velocity widths from Keck spectra are all narrow (30 σ 120 km s-1), with three profiles showing double peaks. Four galaxies have low masses (M 1010 M☉). Whereas three galaxies were previously reported to be possible active galactic nuclei (AGNs), none exhibit AGN-like emission-line ratios or velocity widths. Two or three appear as very blue spirals with the remainder more akin to luminous H II galaxies undergoing a strong burst of star formation. We propose that after the burst subsides, these galaxies will transform into quiescent dwarfs, and that they are thus progenitors of some cluster spheroidals seen today.