arXiv subjects

Lennox L. Cowie

Publications and source records attributed to Lennox L. Cowie.

68 records · Page 4Linked to original sources

arXiv1999

Evidence for a Gradual Decline in the Universal Rest-Frame UV Luminosity Density for z < 1

We have utilized various magnitude-limited samples drawn from an extremely deep and highly complete spectroscopic redshift survey of galaxies observed in seven colors in the Hawaii Survey Fields and the Hubble Deep Field to investigate the evolution of the universal rest-frame ultraviolet luminosity density from z = 1 to the present. The multi-color data (U', B, V, R, I, J, HK') enable the sample selection to be made in the rest-frame ultraviolet for the entire redshift range. Due to the large sample size and depth (U_{AB} = 24.75, B_{AB} = 24.75, I_{AB} = 23.5), we are able to accurately determine the luminosity density to z = 1. We do not confirm the very steep evolution reported by Lilly et al. (1996) but instead find a shallower slope, approximately (1+z)^{1.5} for q0 = 0.5, which would imply that galaxy formation is continuing smoothly to the present time rather than peaking at z = 1. Much of the present formation is taking place in smaller galaxies. Detailed comparisons with other recent determinations of the evolution are presented.

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arXiv1998

The Density of Lyman-alpha Emitters at Very High Redshift

We describe narrowband and spectroscopic searches for emission-line star forming galaxies in the redshift range 3 to 6 with the 10 m Keck II Telescope. These searches yield a substantial population of objects with only a single strong (equivalent width >> 100 Angstrom) emission line, lying in the 4000 - 10,000 Angstrom range. Spectra of the objects found in narrowband-selected samples at lambda ~5390 Angstroms and ~6741 Angstroms show that these very high equivalent width emission lines are generally redshifted Lyman alpha 1216 Angstrom at z~3.4 and 4.5. The density of these emitters above the 5 sigma detection limit of 1.5 e-17 ergs/cm^2/s is roughly 15,000 per square degree per unit redshift interval at both z~3.4 and 4.5. A complementary deeper (1 sigma \~1.0 e-18 ergs/cm^2/s) slit spectroscopic search covering a wide redshift range but a more limited spatial area (200 square arcminutes) shows such objects can be found over the redshift range 3 to 6, with the currently highest redshift detected being at z=5.64. The Lyman alpha flux distribution can be used to estimate a minimum star formation rate in the absence of reddening of roughly 0.01 solar masses/Mpc^3/year (H_0 = 65 km/s/Mpc and q_0 = 0.5). Corrections for reddening are likely to be no larger than a factor of two, since observed equivalent widths are close to the maximum values obtainable from ionization by a massive star population. Within the still significant uncertainties, the star formation rate from the Lyman alpha-selected sample is comparable to that of the color-break-selected samples at z~3, but may represent an increasing fraction of the total rates at higher redshifts. This higher-z population can be readily studied with large ground-based telescopes.

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arXiv1998

High-z Lyman-alpha Emitters. I. A Blank-Field Search for Objects Near Redshift z=3.4 in and around the Hubble Deep Field and the Hawaii Deep Field SSA22

We present deep narrow-band (lambda = 5390 Angstroms, delta lambda = 77 Angstroms) and multi-color observations of the Hubble Deep Field and the Hawaii Deep Field SSA22 obtained with the LRIS instrument at the Keck II 10-m telescope. It is shown that there is a substantial population of galaxies at z~3.4 which can be selected by Ly alpha emission. Comparison with color-selected samples shows that the samples selected with these different criteria have substantial, but not complete overlap, and that there is a comparable surface density in the two selected populations. The emission-line selected samples include objects with strong Ly alpha, and which are significant contributers to the integrated star formation at these epochs. For a Salpeter IMF we estimate a minimum star formation rate of 0.01 solar masses per cubic Mpc per yr at z=3.4 for H_0 = 65 km/s/Mpc and q_0 = 0.5 in the Ly alpha-selected objects, though the value could be substantially higher if there is significant extinction.

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arXiv1997

The Redshift Evolution of the Lyman Alpha Forest

We have analyzed the properties of low column density Lyman alpha forest clouds (12.8 < log N_HI < 16.0) toward 5 QSOs at different redshifts, using high signal-to-noise data from the HIRES spectrograph on the Keck I 10-m telescope. The results are used to study the evolution of these clouds in the interval 2.1 < z < 3.5. We find: 1) The differential density distribution of forest clouds, f(N_HI), fits an empirical power law with a typical slope of -1.35 to -1.55 for clouds with N_HI < 10^14.3 cm^{-2} and changes extremely slowly with redshift over this N_HI range. 2) The deviation of f(N_HI) from this power-law distribution at higher column densities depends on redshift: for higher redshifts, the departure from a power-law distribution occurs at higher N_HI, which means that higher column density clouds rapidly disappear with decreasing redshift. This may be consistent with an invariant shape for the different column density distributions, but with a shift to a lower column density normalization for systems at different redshifts driven by the overall expansion of the universe. 3) The line widths of the forest clouds increase as redshift decreases. 4) The correlation strength of the forest clouds seems to increase as redshift decreases. We discuss these results in terms of the evolution of the IGM comparing the results to models and analytic descriptions of the evolution of structure in the gas.

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arXiv1997

The Evolution of the Distribution of Star Formation Rates in Galaxies

A large deep and nearly complete B<24.5 redshift sample is used to measure the change in distribution function of the stellar mass production rate in individual galaxies with redshift. The evolution of the star formation rate distribution with redshift is interpreted in terms of the history of spiral galaxy formation, with the disk component modelled as a single evolving entity, and the characteristic timescales, luminosities, and epochs varying according to galaxy type. The more massive forming galaxies seen at z=1 to 3 are identified as earlier type spirals, whose star formation rates are initially high and then decline rapidly at z<1, while for later type spirals and smaller mass irregulars the mass formation rates at z<1 are lower, and the formation process persists to redshifts much closer to the present epoch. We find that these models can be consistent with the data and fit well into a broad picture of other recent results if qnought=0.02 and many of the disks begin their growth at z<<3, but that they predict too many bright star formers at high z in flat universes.

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arXiv1996

A High Deuterium Abundance in the Early Universe

The amount of deuterium relative to hydrogen (D/H) in clouds with close to primordial abundance seen at high redshift in the spectra of distant quasars currently provides the best estimate of the baryonic density of the Universe. The first measurements have yielded discrepant values of D/H both high [around 2 x 10^(-4)] and an order of magnitude lower. The low values of D/H imply a high Omega(Baryon) that is difficult to reconcile with determinations of the primordial abundances of other light elements, notably He^4, and with the known number of light neutrinos. We report an independent measurement of the neutral hydrogen (HI) column density in the cloud toward Q1937-1009 where one of the low D/H values was obtained. Our measurement excludes the reported value and we give a lower limit of D/H > 4 x 10^(-5) in this system, which implies Omega(Baryon) < 0.016 for a Hubble constant of 100 km/sec/Mpc. This new upper limit on Omega(Baryon) relieves the conflict with standard Big Bang nucleosynthesis.

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arXiv1996

Constraints on Galaxy Formation from Deep Galaxy Redshift Surveys and Quasar Absorption Line Studies

Magnitude-limited galaxy redshift surveys are now providing large samples of galaxies to beyond z=2, while color-selected and emission-line-selected samples are finding galaxies to z=4.7. A broad picture is emerging of galaxy formation peaking in the z=1 range, which ties in with the metallicity and density evolution seen in the quasar absorption lines. We still have no direct information beyond z=5, but the ionization of the IGM at this redshift argues for activity prior to this time. The metallicities of around 0.01 solar which appear to be relatively ubiquitous in quasar absorption lines beyond z=2, even in very low column density clouds, could be a relict of this period.

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arXiv1996

Redshift clustering in the Hubble Deep Field

We present initial results from a redshift survey carried out with the Low Resolution Imaging Spectrograph on the 10~m W. M. Keck Telescope in the Hubble Deep Field. In the redshift distribution of the 140 extragalactic objects in this sample we find 6 strong peaks, with velocity dispersions of ${\sim}400${\kms}. The areal density of objects within a particular peak, while it may be non-uniform, does not show evidence for strong central concentration. These peaks have characteristics (velocity dispersions, density enhancements, spacing, and spatial extent) similar to those seen in a comparable redshift survey in a different high galactic latitude field (Cohen et al 1996), confirming that the structures are generic. They are probably the high redshift counterparts of huge galaxy structures (``walls'') observed locally.

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arXiv1996

New Insight on Galaxy Formation and Evolution from Keck Spectroscopy of the Hawaii Deep Fields

We present the results of spectroscopic studies with the LRIS spectrograph on Keck of two of the Hawaii deep survey fields. The 393 objects observed cover an area of 26.2 square arcmin and constitute a nearly complete sample down to K = 20, I = 23, and B = 24.5. The rest-frame K-band luminosity function and its evolution with redshift are described. Comparisons are made with other optically selected (B and I) samples in the literature, and the corresponding rest-frame B-band luminosity function evolution is presented. The B-band counts near B = 24 are shown to be a mixture of normal galaxies at modest redshifts and galaxies undergoing rapid star formation, which have a wide range of masses and which are spread over the redshift interval from z = 0.2 to beyond z = 1.7. The luminosity functions, number counts, and color distributions at optical and IR wavelengths are discussed in terms of a consistent picture of the star-forming history of the galaxy sample. [OII] emission-line diagnostics or rest-frame ultra-violet--infrared color information are used in combination with rest-frame absolute K magnitudes to construct a ``fundamental plane'' in which the evolution of the global star-formation rate with redshift can be shown, and we find that the maximum rest-frame K luminosity of galaxies undergoing rapid star formation has been declining smoothly with decreasing redshift from a value near L* at z > 1. This smooth decrease in the characteristic luminosity of galaxies dominated by star formation can simultaneously account for the high B-band galaxy counts at faint magnitudes and the redshift distribution at z < 1 in both the B- and K-selected samples. Finally, the overall K-band light density evolution is discussed as a tracer of the baryonic mass in stars and compared with the rate of star formation.

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arXiv1996

Metal Enrichment and Ionization Balance in the Lyman $α$ Forest at $z = 3$

The recent discovery of carbon in close to half of the low neutral hydrogen column density [$N({\rm H~I}) > 3\ten{14}\cm2$] Lyman forest clouds toward $z \sim 3$ quasars has challenged the widely held view of this forest as a chemically pristine population uniformly distributed in the intergalactic medium, but has not eliminated the possibility that a primordial population might be present as well. Using extremely high signal-to-noise observations of a sample of quasars we now show that \ion{C}{4} can be found in 75% of clouds with $N({\rm H~I}) > 3\ten{14}\cm2$ and more than 90% of those with $N({\rm H~I}) > 1.6\ten{15}\cm2$. Clouds with $N({\rm H~I}) > 10^{15}\cm2$ show a narrow range of ionization ratios, spanning less than an order of magnitude in \ion{C}{4}/\ion{H}{1}, \ion{C}{2}/\ion{C}{4}, \ion{Si}{4}/\ion{C}{4} and \ion{N}{5}/\ion{C}{4}, and their line widths require that they be photoionized rather than collisionally ionized. This in turn implies that the systems have a spread of less than an order of magnitude in both volume density and metallicity. Carbon is seen to have a typical abundance of very approximately $10^{-2}$ of solar and Si/C about three times solar, so that the chemical abundances of these clouds are very similar to those of Galactic halo stars. \ion{Si}{4}/\ion{C}{4} decreases rapidly with redshift from high values ($> 0.1$) at $z > 3.1$, a circumstance which we interpret as a change in the ionizing spectrum as the intergalactic medium becomes optically thin to He$^+$\ ionizing photons. Weak clustering is seen in the \ion{C}{4} systems for $Δv < 250\kms$, which we argue provides an upper limit to the clustering of \ion{H}{1} clouds. If the clouds are associated with galaxies, this requires a rapid evolution in galaxy clustering between $z = 3$ and $z = 0$.

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arXiv1995

Detection of Massive Forming Galaxies at Redshifts Greater than One

The complex problem of when and how galaxies formed has not until recently been susceptible of direct attack. It has been known for some time that the excessive number of blue galaxies counted at faint magnitudes implies that a considerable fraction of the massive star formation in the universe occurred at z < 3, but, surprisingly, spectroscopic studies of galaxies down to a B magnitude of 24 found little sign of the expected high-z progenitors of current massive galaxies, but rather, in large part, small blue galaxies at modest redshifts z \sim 0.3. This unexpected population has diverted attention from the possibility that early massive star-forming galaxies might also be found in the faint blue excess. From KECK spectroscopic observations deep enough to encompass a large population of z > 1 field galaxies, we can now show directly that in fact these forming galaxies are present in substantial numbers at B \sim 24, and that the era from redshifts 1 to 2 was clearly a major period of galaxy formation. These z > 1 galaxies have very unusual morphologies as seen in deep HST WFPC2 images.

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arXiv1995

Astrophysical Limits on the Evolution of Dimensionless Physical Constants over Cosmological Time

We report new upper limits on any possible long-term time variation of the ratio of the electron to proton masses, $(m_e/m_p)$, the fine-structure constant, ($α$), and the quantity $α^2 g_p(m_e/m_p)$, where $g_p$ is the proton gyromagnetic ratio. These limits are based on extremely high precision observations of H$_2$, Si$^{3+}$, C$^0$ and H$^0$ in high-redshift quasar absorption lines. They amount to 95\% confidence ranges of $(-7.6\to 9.7) \times 10^{-14}$ yr$^{-1}$ for ($m_e/m_p$), $(-4.6\to 4.2)\times 10^{-14}$ yr$^{-1}$ for $α$\ and $(-2.2\to 4.2) \times 10^{-15}$ yr$^{-1}$ for $α^2 g_p(m_e/m_p)$, where the elapsed time has been computed for a cosmology with \hnought = 75 km s$^{-1}$ Mpc$^{-1}$ and \qnought = 0.5.

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arXiv1995

Faintest Galaxy Morphologies from $HST$ WFPC2 Imaging of the Hawaii Survey Fields

We present very deep $HST$ WFPC2 images in the F814W filter of two Hawaii Survey fields, SSA13 and SSA22. Using these data with previous ground-based imaging and spectroscopy, we compare the colors, star-forming properties and morphologies of the faintest galaxies with a reference sample of bright nearby galaxies and analyze the changes in field galaxy morphology with magnitude. Our principal result is the identification of a new morphological class of ``chain'' galaxies at the faintest magnitudes. Based on limited spectroscopy, we tentatively conclude that these are linearly organized giant star-forming regions at $z = 0.5-3$ and, if this is correct, that these are large galaxies in the process of formation.

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arXiv1995

The Distribution of Column Densities and b Values in the Lyman-Alpha Forest

We describe the properties of the Ly$α$ forest in the column density range $\nhi \geq 2\times10^{12}$ cm$^{-2}$ based on 1056 lines in the wavelength range 4300--5100Åmeasured in extremely high S/N, $R=36,000$ spectra of four quasars. The column density distribution is well described by a -1.5 power law to $2\times10^{12}$ cm$^{-2}$, below which limit confusion becomes too severe to measure a spectrum of individual clouds. The distribution of $b$ values shows a well-defined lower envelope with a cutoff at $b=20\kms$ corresponding to a cloud temperature of 24,000 K. There is only a very small fraction (less than 1%) of narrow line clouds which cannot be identified with metal-lines. From modeling the Ly$α$ absorption lines as complexes of clouds each with thermal broadening corresponding to $b_c$ we find the $b$ distribution can be understood if there is a mean of 3.25 clouds per absorption line with a spread in velocity centroids characterized by a dispersion of $10.75\kms$.

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