Neil De Grasse Tyson

The Cosmic Evolution Survey (COSMOS): Overview

The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGNs), and dark matter (DM) with large-scale structure (LSS) over the redshift range z > 0.5-6. The survey includes multiwavelength imaging and spectroscopy from X-ray-to-radio wavelengths covering a 2 deg^2 area, including HST imaging. Given the very high sensitivity and resolution of these data sets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS data sets, and a summary the science goals.

On the possibility of gas-rich dwarf galaxies in the Lyman-alpha forest

It is argued here that the high-column density absorbers in the Lyman-alpha forest are low surface brightness, gas-rich, high space density dwarf galaxies that appear in the line of sight, thus lending the impression that spiral galaxies must have been enormous in early epochs. It is shown that the frequency distribution of dwarf galaxy radii inferred by Tyson and Scalo (1988) quite naturally accounts for the observed number of high column density absorbers in the Lyman-alpha forest without invoking extremely large H I parameters at early epochs and without interfering with the more securely established bright end of the luminosity function. 21 references.

Optical Light Curves of the Type I[CLC]a[/CLC] Supernovae SN 1990N and SN 1991T

We present UBVRI light curves for the bright Type Ia supernovae SN 1990N in NGC 4639 and SN 1991T in NGC 4527, based on photometry gathered in the course of the Calan/Tololo supernova program. Both objects have well-sampled light curves starting several days before maximum light and spanning well through the exponential tail. These data supersede the preliminary photometry published by Leibundgut et al. and Phillips et al. The host galaxies for these supernovae have (or will have) accurate distances based on the Cepheid period-luminosity relationship. The photometric data in this paper provide template curves for the study of the general population of Type Ia supernovae and accurate photometric indices, needed for the Cepheid-supernova distance scale.

The Faint-End Slopes of Galaxy Luminosity Functions in the COSMOS Field*

We examine the faint-end slope of the rest-frame V-band luminosity function (LF), with respect to galaxy spectral type,of fieldgalaxieswithredshiftz < 0:5,usingasampleof80,820galaxieswithphotometricredshiftsinthe2deg 2 Cosmic Evolution Survey (COSMOS) field. For all galaxy spectral types combined, the LF slope ranges from � 1.24 to � 1.12,fromthelowestredshiftbintothehighest.Inthelowestredshiftbin(0:02 < z < 0:1),wherethemagnitude limit isMV P � 13, the slope ranges from � �� 1:1 forgalaxies with early-type spectral energy distributions (SEDs) to � �� 1:9 forgalaxies with low-extinction starburst SEDs. In each galaxy SED category (early-type, Sbc, Scd+Irr, and starburst), the faint-end slopes grow shallower with increasing redshift; in the highest redshift bin (0:4 < z < 0:5),

An exposure guide for taking twilight flatfields with large format CCDS

Driven by the long readout times of todays large format CCDs, we present an efficient method to compute an ideal sequence of exposure times for twilight skyflats. The procedure will allow the observer to maximize the number of flats that can be obtained in the rapidly changing surface brightness of the dusk or dawn sky

Radial velocity distribution and line strengths of 33 carbon stars in the Galactic bulge

This paper presents optical spectra of 33 carbon stars discovered by Azzopardi et al. (1985) in low-extinction windows to the Galactic bulge. From these spectra, the radial velocity distribution for the stars was obtained; the dispersion was found to be 113 + or - 14 km/sec (consistent with that of the bulge K and M giants) and the mean to be -44 + or - 20 km/sec. From the velocity distribution, it was found that the carbon stars do not share the kinematics of the local neighborhood and are likely to be genuine bulge members, making it possible to assign distances and accurate luminosities. Nineteen of the stars showed enhanced C-13, classifying them as J-star candidates. Two stars showed weak C2 absorption, classifying them as CS stars, and thirteen carbon stars appeared to be hydrogen-deficient. 63 refs.

Bursting dwarf galaxies - implications for luminosity function, space density, and cosmological mass density

An attempt is made to estimate the true average space density of gas-rich dwarf galaxies by comparing the luminosity and size distributions of visible dwarf irregulars with a set of simulated observations of a bursting population of galaxies on which selection effects corresponding to the real observations have been imposed. The true size distribution is assumed to be a power law f(r) proportional to r exp y. A value of y = - 4.2 + or - 0.2 gives good agreement with the observed frequency distributions of luminosity, optical radius, and angular size. The same model accounts for the observed luminosity function in the Virgo cluster. The implications of this result for the true shape of the Galaxy luminosity function and the contribution to the cosmological density are discussed. 48 references.

Optical light curves of the type ia supernovae 1990n and 1991t

We present UBVRI light curves for the bright Type Ia supernovae SN 1990N in NGC 4639 and SN 1991T in NGC 4527, based on photometry gathered in the course of the Calan/Tololo supernova program. Both objects have well-sampled light curves starting several days before maximum light and spanning well through the exponential tail. These data supersede the preliminary photometry published by Leibundgut et al. and Phillips et al. The host galaxies for these supernovae have (or will have) accurate distances based on the Cepheid period-luminosity relationship. The photometric data in this paper provide template curves for the study of the general population of Type Ia supernovae and accurate photometric indices, needed for the Cepheid-supernova distance scale.

America's science legacy.

In 1863, U.S. President Abraham Lincoln delivered his now famous Gettysburg Address on the open field of that bloody Civil War battle. Not many speeches by political leaders get remembered beyond the moment. Even fewer ever get cut into stone. In this case, of course, both are true. We know the words. We recite passages from it. And, in its entirety, the speech graces a wall of the Lincoln Memorial in Washington, DC.