Top-Cited Papers of 2003

by Scott Dodelson

As a new feature of the topcites reviews this year, we include a special review of the topcited articles in the astro-ph arXiv. As many of the topics (and people) involved in HEP and Astrophysics become mingled, SPIRES has begun tracking papers from the astro-ph arXiv, including their citations. We have also gone back and retrieved the citations of most astro-ph papers from the beginning of that archive. This is a review and listing of the 43 articles (from any source) over the years that have accumulated the most citations from astro-ph articles.

The list of top-cited articles in astrophysics is dominated by those pertaining to cosmology. One reason for this is that theory and observations have made tremendous strides over the past decade working hand-in-hand in that marvelous way that makes science so stimulating. New physics has been proposed to account for observations (e.g. dark matter for rotation curves; dark energy for supernovae brightnesses; and inflation for structure). And new observations promise to test these hypotheses and discriminate among various models. Another reason for the success of cosmology on this list is that -- as it has matured -- cosmology has broadened. Whereas it was once dubbed the "Search for Two Numbers" (the Hubble constant and the average density of the universe), the now-standard cosmology (Big Bang with cold dark matter, inflation, and late-time dark energy) is now able to explain a wide variety of astrophysical phenomena. Thirty years ago, an article like "Primordial Nucleosynthesis Redux," [43] would have surely been placed in the cosmology bin, but a title like "A Universal Density Profile from Hierarchical Clustering" [7] would have fallen in the realm of extragalactic astronomy. The fact that cosmology, the science of the Gigaparsec horizon, now purports to make predictions for the mass distribution inside the inner kiloparsec of galaxies is extraordinary.

It is useful to sub-divide the cosmology articles into those that deal with the zero-order, smooth, homogeneous universe and those that tackle the perturbations around this smooth universe. In the first category falls the recent spectacular discovery that the universe is accelerating. Perlmutter et al. [2] and Riess et al. [3] are the discovery papers: recording the experiments which found high redshift Type Ia supernovae and measured their apparent brightnesses to trace out the luminosity distance as a function of redshift. Caldwell, Dave and Steinhardt [38] and Ratra and Peebles [40] represent two attempts to explain acceleration with alternatives other than a cosmological constant. Freedman et al. [29] presents the most up-to-date measurement of the Hubble constant, one of the most important parameters in the homogeneous cosmology. Walker et al. [43], which constrains the baryon abundance in the universe by analyzing Big Bang Nucleosynthesis, exploits another prediction of the standard, homogeneous cosmology.

A bridge between these "zero-order" papers and the remaining cosmology papers which focus on structure in the universe is Alan Guth's initial proposal for cosmological inflation [33]. Although Guth proposed inflation as a solution to long-standing problems in the standard Big Bang cosmology (smoothness and flatness), physicists quickly realized that the seeds for structure may have been planted via quantum mechanical fluctuations in the rapidly expanding inflationary era. These seeds have been observed experimentally by observations of anisotropies in the cosmic microwave background (CMB). No fewer than six experimental papers focusing on observations of anisotropies in the CMB -- COBE [11,36], Boomerang [16,31], WMAP [17], and Maxima [35] -- make the top-cited list. Theorists are also represented in the CMB with Paper [14], an innovative method of calculating CMB anisotropies without solving the whole hierarchy of Boltzmann equations. CMBFAST, the computer code introduced by Seljak and Zaldarriaga in [14], has been used in hundreds of papers to make predictions for a wide variety of cosmological models.

The seeds of structure planted during inflation which remain in the CMB have grown to fruition in the large scale structure of the universe. Unlike perturbations to the radiation, perturbations to matter grow with time due to gravitational instability, amplifying the small fluctuations at the time of last scattering into the galaxies and clusters we see today. The remaining cosmology papers deal with this transition from small, linear perturbations to large, nonlinear objects such as galaxies and clusters. Press and Schechter [6] performed a bit of magic: the Press-Schechter formalism alchemizes the variance in the linear perturbations into predictions for the abundance of large collapsed objects. It is appropriate that the Press-Schechter paper is the top-cited structure formation paper, because so much of the ensuing literature is based on it. For example, Eke, Cole, & Frenk [22] used numerical simulations to determine the abundance of galaxy clusters, finds that the Press-Schechter prediction is accurate, and then shows that these predictions can be used to constrain the mass density in the universe. Lacey and Cole [23] dealt with the question of how often galaxies merge over the course of their history, re-deriving the Press-Schechter formula in a more rigorous framework. And the standard picture of how galaxies form is set forth by White and Rees [42], which again makes use of Press-Schechter. Finally, the Press-Schechter formalism inspired the Schechter luminosity function [39], a more phenomenological approach to the distribution of light emanating from galaxies. Two other papers -- Bardeen et al. [8] and Peacock & Dodds [30] -- also start with linear theory and derive information about nonlinear structures. The brute force approach of N-Body simulations was exploited in Papers [7] and [12], where Navarro, Frenk, and White noticed that the dark matter halos in these simulations have a universal profile, the so-called NFW profile, falling off as r^{-3} as large distance and r^{-1} close to the center. Comparing this prediction of the standard cosmology to observations has occupied hundreds of researchers over the past half-decade. Navarro, Frenk, and White also teamed up with Evrard in Paper [34] to argue against a universe with matter density equal to the critical density. More precisely, they argued that clusters of galaxies appear to have about 7 times more dark matter than baryons. Assuming that clusters represent a fair sample of the universe, this should be the universal ratio. Since Big Bang Nucleosynthesis (recall Paper [43]) argues that baryons make up only ~4-5% of the critical density, the total matter density must hover around 30%. Remarkably, both of these predictions have recently been confirmed by the detailed CMB observations (most recently WMAP, Paper [17]): the peaks in the CMB spectrum are sensitive to both the total matter density and the baryon density. In fact, the CMB constraints on the baryon density now are tighter than those from nucleosynthesis.

The remaining cosmology papers are observational, focusing on structure in the high redshift universe. The Hubble Space Telescope has played a prominent role in these investigations, as evidenced by Madau et al. [15] and Williams et al. [32]. A relatively new technique of finding distant galaxies by looking for signs of Lyman alpha emission in different bands -- the so-called Lyman Break galaxies -- was applied to find z ~ 4 galaxies in Steidel et al. [20]. Even the Sloan Digital Sky Survey, introduced in York et al. [28], has made its mark on the field of high redshift structure by detecting the earliest known quasars. Segueing out of cosmology, the radio counterpart of the Sloan Survey is the VLA Sky Survey publicized in Condon et al. [27]. On a smaller scale, Harris [25] presented a catalogue of globular clusters in the Milky Way.

After cosmology, another important category consists of papers which enable astronomy to get done. This category includes Landolt [5] which offers the magnitudes of many stars in different standard bands, thereby allowing astronomers to calibrate their observations. Similarly, many astronomers use the solar element abundances reported in Anders & Grevesse [10] to normalize their observations of elements in other systems. Stetson [18] introduces a code which performs stellar photometry in crowded fields. Another important correction in astronomy is contamination along the line of sight, either by absorption or emission. Dust in particular must be corrected for, and this partially accounts for the success of Schlegel, Finkbeiner, & Davis [1], which produced maps of dust and Cardelli, Clayton, & Mathis [4] which related absorption at different wavelengths. Moving to higher energies, Morrison and McCammon [26] presented a model for interstellar absorption in the X-Ray regime.

A number of papers made the list because they produced particularly useful sets of quantities that can be used in making predictions and interpreting observations. For example, Bruzual and Charlot [9] presented simulated spectra of galaxies as a function of age and metallicity, a tool which has proved invaluable for interpreting broad-band spectra. Similarly, Worthey [24] outputs broadband magnitudes, spectra, and magnitudes for galaxies with varying metallicities, ages, and IMF's. Woosley and Weaver [19] computed the elements emerging from simulated supernovae with a wide range of masses. The oldest paper on the top-cited list is the 1955 work of Edwin Salpeter [13] which postulates the Salpeter initial mass function. Scalo's IMF from 1986 [41] is a highly popular alternative.

Finally, two of the top-cited papers dealt with aspects of black holes. Magorrian et al. [21] showed that the mass of the black hole in the center of a galaxy correlates with the mass of the bulge while Shakura and Sunyaev [37] developed the standard theory of thin accretion flows around black holes.