Dense Knots Research Articles

On the relationship between the H2 emission and the physical structure of planetary nebulae

Mid-IR observations of planetary nebulae (PNe) have revealed diffuse emission associated to their main nebular shells and outer envelopes or haloes. The interpretation of this emission is uncertain because the broad-band mid-IR images may include contributions of different components. In particular, the Spitzer IRAC 8 {\mu}m images, that best reveal these nebular features, can include contributions not only of H2 lines, but also those of ionic species, PAH features, and thermal dust continuum emission. To investigate the nature of the emission detected in mid-IR observations of a sample of 10 PNe, we have obtained narrow-band near-IR H2 {\lambda}2.122 {\mu}m and optical [N II] {\lambda}6584 ?A images. The comparison between these images confrm that a significant fraction of the emission detected in the IRAC 8 {\mu}m images can be attributed to molecular hydrogen, thus confirming the utility of these mid-IR images to investigate the molecular component of PNe. We have also detected H2 emission from PNe whose physical structure cannot be described as bipolar, but rather as ellipsoidal or barrel-like. These detections suggest that, as more sensitive observations of PNe in the H2 {\lambda}2.122 line are acquired, the detection of H2 emission is not exclusive of bipolar PNe, although objects with this morphology are still the brightest H2 emitters. Finally, we remark that the bright H2 emission from the equatorial ring of a bipolar PN does not arise from a photodissociation region shielded from the UV stellar radiation by the ring itself, but from dense knots and clumps embedded within the ionized material of the ring.

THE ANATOMY OF THE YOUNG PROTOSTELLAR OUTFLOW HH 211

We present Spitzer Space Telescope 5-36 {mu}m mapping observations toward the southeastern lobe of the young protostellar outflow HH 211. The southeastern terminal shock of the outflow shows a rich mid-infrared spectrum including molecular emission lines from OH, H{sub 2}O, HCO{sup +}, CO{sub 2}, H{sub 2}, and HD. The spectrum also shows a rising infrared continuum toward 5 {mu}m, which we interpret as unresolved emission lines from highly excited rotational levels of the CO v = 1-0 fundamental band. This interpretation is supported by a strong excess flux observed in the Spitzer/IRAC 4-5 {mu}m channel 2 image compared to the other IRAC channels. The extremely high critical densities of the CO v = 1-0 ro-vibrational lines and a comparison to H{sub 2} and CO excitation models suggest jet densities larger than 10{sup 6} cm{sup -3} in the terminal shock. We also observed the southeastern terminal outflow shock with the Submillimeter Array and detected pure rotational emission from CO 2-1, HCO{sup +} 3-2, and HCN 3-2. The rotationally excited CO traces the collimated outflow backbone as well as the terminal shock. HCN traces individual dense knots along the outflow and in the terminal shock, whereas HCO{sup +} solely appears in themore » terminal shock. The unique combination of our mid-infrared and submillimeter observations with previously published near-infrared observations allow us to study the interaction of one of the youngest known protostellar outflows with its surrounding molecular cloud. Our results help us to understand the nature of some of the so-called green fuzzies (Extended Green Objects), and elucidate the physical conditions that cause high OH excitation and affect the chemical OH/H{sub 2}O balance in protostellar outflows and young stellar objects. In an appendix to this paper, we summarize our Spitzer follow-up survey of protostellar outflow shocks to find further examples of highly excited OH occurring together with H{sub 2}O and H{sub 2}.« less

SURFACE DETONATIONS IN DOUBLE DEGENERATE BINARY SYSTEMS TRIGGERED BY ACCRETION STREAM INSTABILITIES

We present three-dimensional simulations on a new mechanism for the detonation of a sub-Chandrasekhar CO white dwarf in a dynamically unstable system where the secondary is either a pure He white dwarf or a He/CO hybrid. For dynamically unstable systems where the accretion stream directly impacts the surface of the primary, the final tens of orbits can have mass accretion rates that range from $10^{-5}$ to $10^{-3} M_{\odot}$ s$^{-1}$, leading to the rapid accumulation of helium on the surface of the primary. After $\sim 10^{-2}$ $M_{\odot}$ of helium has been accreted, the ram pressure of the hot helium torus can deflect the accretion stream such that the stream no longer directly impacts the surface. The velocity difference between the stream and the torus produces shearing which seeds large-scale Kelvin-Helmholtz instabilities along the interface between the two regions. These instabilities eventually grow into dense knots of material that periodically strike the surface of the primary, adiabatically compressing the underlying helium torus. If the temperature of the compressed material is raised above a critical temperature, the timescale for triple-$\alpha$ reactions becomes comparable to the dynamical timescale, leading to the detonation of the primary's helium envelope. This detonation drives shockwaves into the primary which tend to concentrate at one or more focal points within the primary's CO core. If a relatively small amount of mass is raised above a critical temperature and density at these focal points, the CO core may itself be detonated.

The Formation of Filamentary Structures in Radiative Cluster Winds

We explore the dynamics of a “cluster wind” flow in the regime in which the shocks resulting from the interaction of winds from nearby stars are radiative. We show that for a cluster with low-intermedia mass stars, the wind interactions are indeed likely to be radiative. We then compute three dimensional, radiative simulations of a cluster of 75 young stars, exploring the effects of varying the wind parameters and the density of the initial ISM that permeates the volume of the cluster. These simulations show that the ISM is compressed by the action of the winds into a structure of dense knots and filaments.

Star Formation in Disks: Spiral Arms, Turbulence, and Triggering Mechanisms

AbstractStar formation is enhanced in spiral arms because of a combination of orbit crowding, cloud collisions, and gravitational instabilities. The characteristic mass for the instability is 107M⊙in gas and 105M⊙in stars, and the morphology is the familiar beads on a string with 1-2 kpc separation. Similar instabilities occur in resonance rings and tidal tails. Sequential triggering from stellar pressure occurs in two ways. For short times and near distances, it occurs in the bright rims and dense knots that lag behind during cloud dispersal. For long times, it occurs in swept-up shells and along the periphery of cleared regions. The first case should be common but difficult to disentangle from independent star formation in the same cloud. The second case has a causality condition and a collapse condition and is often easy to recognize. Turbulent triggering produces a hierarchy of dense cloudy structure and an associated hierarchy of young star positions. There should also be a correlation between the duration of star formation and the size of the region that is analogous to the size-linewidth relation in the gas. The cosmological context is provided by observations of star formation in high redshift galaxies. Sequential and turbulent triggering is not yet observable, but gravitational instabilities are, and they show a scale up from local instabilities by a factor of ~3 in size and ~100 in mass. This is most easily explained as the result of an increase in the ISM turbulent speed by a factor of ~5. In the clumpiest galaxies at high redshift, the clumps are so large that they should interact with each other and merge in the center, where they form or contribute to the bulge.

Probing the Ionization Structure of the Narrow‐Line Region in the Seyfert 1 Galaxy NGC 4151

We present a study of the distribution of [O III] λ5007 and [O II] λ3727 emission in the narrow-line region (NLR) of the Seyfert 1 galaxy NGC 4151. While the NLR of NGC 4151 exhibits an overall structure consistent with the unified model of Seyfert galaxies, narrowband [O III] and [O II] images obtained with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope reveal significant emission from outside the emission-line bicone. The [O III]/[O II] ratios are lower in these regions, consistent with a weaker ionizing flux. We performed a photoionization modeling analysis of the emission-line gas within a series of annuli, centered on the central continuum source, with inner radii from 13 to 90 pc. The gas is ionized by radiation that has been attenuated by a relatively highly ionized absorber (HABS), which completely covers the central source, and a lower ionization absorber (LABS), which has a covering factor ranging from 0 to 1. We found that the [O III]/[O II] ratios are well fit by assuming that, within each segment of an annulus, some fraction of the NLR gas is completely within the shadow of LABS, while the rest is irradiated by the continuum filtered only by HABS. This suggests that the structure of the NLR is due to filtering of the ionizing radiation by ionized gas, consistent with disk-wind models. One possible scenario is that the low-ionization absorbers are dense knots of gas swept up by a wind.

Monitoring the Variable Interstellar Absorption toward HD 219188 withHubble Space TelescopeSTIS

We discuss the results of continued optical and UV spectroscopic monitoring of the variable intermediate-velocity (IV) absorption at v☉ = -38 km s-1 toward HD 219188. After reaching maxima in mid-2000, the column densities of both Na I and Ca II in that IV component declined by factors ≳2 by the end of 2006. Comparisons between echelle spectra from HST STIS (2001, 2003, 2004) and HST GHRS (1994-1995) indicate the following: (1) The absorption from the dominant species S II, O I, Si II, and Fe II is roughly constant for all four epochs—suggesting that the total N(H) (~6 × 1017 cm-2) and the (mild) depletions did not change significantly over a period of nearly 10 years. (2) The column densities of the trace species C I (both ground and excited fine-structure states) and of the excited state C II* all increased by factors of 2-5 between 1995 and 2001—implying increases in the hydrogen density nH (from about 20 cm-3 to about 45 cm-3) and in the electron density ne (by a factor ≳3) over that 6 yr period. (3) The column densities of C I and C II*—and the corresponding inferred nH and ne—then decreased slightly between 2001 and 2004. (4) The changes in C I and C II* are very similar to those seen for Na I and Ca II. The relatively low total N(H) and the modest nH suggest that the -38 km s-1 cloud toward HD 219188 is not a very dense knot or filament. Partial ionization of hydrogen appears to be responsible for the enhanced abundances of Na I, C I, Ca II, and C II*. In this case, the variations in those species reflect differences in density and ionization [and not N(H)] over scales of tens of AU.

The Three-Dimensional Ionization Structure and Evolution of NGC 6720, The Ring Nebula

We have determined the gas kinematics, diagnostic and ionic radial profiles, spatial structure, and evolutionary phase of NGC 6720 (the Ring Nebula) by means of tomography and a three-dimensional recovery technique applied to long-slit high-resolution spectra. The main shell of the Ring Nebula is a triaxial ellipsoid (radii of 0.10, 0.13, and 0.20 pc) seen nearly pole-on and expanding in an approximately ballistic fashion (Vexp = 0.65 km s-1 arcsec-1). The central star characteristics [log(L*/L?) 2.3, T* 120,000 K], combined with the nebular age of 7000 yr, indicate that the M* 0.61-0.62 M? post-AGB star is approaching the white dwarf cooling sequence. The equator of the Ring Nebula is optically thick and much denser than the optically thin poles. The inner halo surrounding NGC 6720 represents the pole-on projection of the AGB wind at high latitudes (circumpolar) directly ionized by the central star, whereas the outer, fainter, and circular halo is the projection of the recombining AGB wind at mean to low latitudes, shadowed by the main nebula. The spatio-kinematical properties of the Ring Nebula and the origin of the dense knots commonly observed in late-stage planetary nebulae are critically compared with the predictions of radiation-hydrodynamic and wind interaction models.

The nuclear regions of NGC 7582 from [Ne II] spectroscopy at 12.8μm – an estimate of the black hole mass

We present a high-resolution (R ≈ 16 000) spectrum and a narrow-band image centered on the [Ne ii]12.8 μm line of the central kpc region of the starburst/Seyfert 2 galaxy NGC 7582. The galaxy has a rotating circum-nuclear starburst disk, shown at great detail at a diffraction-limited resolution of 0.4 (≈40 pc). The high spatial resolution allows us to probe the dynamics of the [Ne ii] gas in the nuclear regions, and to estimate the mass of the central black hole. We construct models of gas disks rotating in the combined gravitational potential from the stellar bulge and a central black hole, and derive a black hole mass of 5.5 × 10^7 M_⊙ with a 95% confidence interval of [3.6, 8.1] × 10^7 M_⊙. The black hole mass combined with stellar velocity dispersion measurements from the literature shows that the galaxy is consistent with the local M_(BH-σ∗) relation. This is the first time that a black hole mass in a galaxy except our own Milky Way system has been estimated from gas dynamics in the mid-infrared. We show that spatially resolved mid-infrared spectroscopy can be competitive with similar techniques in the optical and near-infrared, and may prove to be important for estimating black hole masses in galaxies with strong nuclear dust obscuration. The high spectral resolution allows us to determine the heliocentric systemic velocity of the galaxy to between 1614 and 1634 km s^(−1). The mid-infrared image reveals several dense knots of dust-embedded star formation in the circum-nuclear disk, and we briefly discuss its morphology.

Puccini and the Girl: History and Reception of The Girl of the Golden West (review)

NINETEENTH- AND TWENTIETH-CENTURY MUSIC Puccini and the Girl: History and Reception of The Girl of the Golden West. By Annie J. Randall and Rosalind Gray Davis. Chicago: University of Chicago Press, 2005. [xv, 241 p. ISBN 0226703894. $35.] Music examples, bibliography, discography, filmography, illustrations, index. Annie Randall and Rosalind Gray Davis open their monograph Puccini and the Girl: History and Reception of The Girl of the Golden West by tracing their paths to their topic: in the early 1960s, Davis's father, journalist Marvin Gray purchased a set of twenty-nine letters from Puccini to Carlo Zangarini, La fcmciutta del West's librettist, written during the opera's composition. Gray never realized his plans to publish a small volume featuring the letters in their original Italian and English translation, and a history of the opera's creation, however, and it is here that Davis and Randall enter the scene. Inheriting the letters after her father's death, Davis, a former journalist turned independent scholar, also inherited his dreams; in 1996, she met Randall, a musicologist, and revived his project, generating a of the events surrounding La fanciullas, conception, composition, and popular and critical conception (p. 8). Though the scope of Puccini and the Girl is far wider than the twenty-nine letters Davis's father purchased some fifty years ago, at the center of Randall and Davis's account are a wide array of original documents: newspaper articles, items from the Metropolitan Opera archives concerning La fanciulla's 1910 premiere, programs, and related ephemera, as well as the Puccini-Zangarini correspondence. After a comprehensive of La fanriulla which simultaneously offers blow-by-blow descriptions of musical and narrative themes and their interplay, and positions Puccini's compositional strategies in relation to his contemporaries within the genre more broadly (chapter 2), the authors devote the middle section of the book (curiously split into chapters 3 and 4 despite their snared material and narrative trajectory) to these painstakingly gathered, translated, annotated, and-perhaps most importantly-decoded documents. The narrative that emerges, a compelling miseen-scene of Puccini's character and personal life during the composition of Ijifantiulla, is of interest to a broad audience of Puccini scholars and fans alike. Not only do the letters-featured in their entirely even when short and seemingly inconsequential -offer valuable insights into Puccini's relationship with Zangarini and his opera, but we gel a glimpse of his personality and sense of humor: his occasional letterpoems'' to Zangarini are simultaneously charming, manipulative, and wacky. 'I am on the sea / dancing a lot, / and what are you up to? / Do you think of The Girl?' he writes, and later in closing, Many greetings / From the high seas-/ If you want to play, / Cover up well / Ciao (pp. 69-70). The book's appendices, too, are a treasure trove, packed with well-organized and above all, readily accessible materials including the original Italian texts of the letters and a variety of other archival materials, a list of Zangarini's libretti and other works, the Metropolitan Opera's performance history of La fanciulla, and a filmography and discography of the opera. In their introduction, Randall and Davis promise a book that weaves together the letters with other material into a detailed of the opera's creation. (p. 8) Yet the resultant narrative is less a smoothly woven piece of cloth than a series of dense knots of information loosely strung together. Throughout, it is difficult to gain a sense of how each chapter connects to those surrounding it; with the exception of their reappearance in a chapter devoted to La fanciulla's redemption theme, for example, the Puccini letters disappear in the chapters that follow the middle section of the book. Moreover, it is difficult to gain a sense of the authors' agenda, critical perspectives, or personal conneclion to the opera. …

The UV Scattering Halo of the Central Source Associated with η Carinae

Eta Carinae is one of the most massive and luminous stars within our galaxy. It consists of a compact central source which suffers circumstellar and interstellar extinction, local dense knots which emit strong narrow nebular-like emission lines, and an outer dusty nebula called the Homunculus. The optical spectrum of the central star, first observed directly and without obvious nebular contamination by the HST, can be modeled successfully using a hot star with a radius (at the wind sonic point) of 60\,R\odot. The central star is losing mass, via a dense stellar wind, at the prodigious rate of 10(exp -3)\,M\odot/yr. Its effective temperature is low (< 10,000\,K), and is determined entirely by the wind properties. Until now the UV spectrum has not been explained. We show that HST UV spectrum, and the FUSE FUV spectrum, can both be understood using the same underlying model that explains the optical spectrum. To do so, however, it is necessary to take into account the occultation of the central source by dust. It is also important to realize that in the UV, the HST is partially resolving the central source. Due to strong mass loss, the wind is optically thick in UV resonance lines even at large radii. The UV resonance lines are responsible for the UV halo seen around Eta Carinae, and provide a partial explanation of why Eta Carinae can even be seen at UV wavelengths.

The [Oiii] Veil: Astropause of η Carinae's Wind?

We present narrowband images of η Carinae in the light of [O III] λ5007 obtained with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2, as well as a ground-based image in the same emission line with a larger field of view. These images show a thin veil of [O III] emission around η Car and its ejecta, confirming the existence of an oxygen-bearing cocoon inferred from spectra. This [O III] veil may be the remnant of the preoutburst wind of η Car, and its outer edge probably marks the interface where η Car's ejecta meet the stellar wind of the nearby O4 V((f)) star HD 303308 or other ambient material in the Carina Nebula; i.e., it marks the astropause in η Car's wind. This veil is part of a more extensive [O III] shell that appears to be shaped and ionized by HD 303308. A pair of HST images with a 10 yr baseline shows no significant proper motion, limiting the expansion speed away from η Car to 12 ± 13 km s-1, or an expansion age of a few times 104 yr. Thus, this is probably the decelerated preoutburst luminous blue variable wind of η Car. The [O III] morphology is very different from that seen in [N II], which traces younger dense knots of CNO-processed material; this represents a dramatic shift in the chemical makeup of material recently ejected by η Car. This change in the chemical abundances of the ejecta may have resulted from the sudden removal of the star's outer envelope during η Car's 19th century outburst or an earlier but similar event.

Fame and Disability

Fame and Disability

Systematic Errors in Elemental Abundances Derived from Nebular Spectra

We examine the global accuracy of the commonly used multizone approach to inferring elemental abundances from nebular spectra. We use a large model grid of constant-density models to construct ionization correction factors (iCF's) for several ions with bright optical lines. The emission lines from the model suite were analyzed as coming from a two-zone nebula, with temperatures and densities based on diagnostic line ratios. These were used to infer the ionic abundances responsible for the emission lines, to which the iCF's were applied. The procedure was also applied to model grids simulating blister-type H II regions, as well as individual dense knots. Comparison of the model elemental abundances to those inferred from their line emission show large deviations with respect to the demonstrated accuracy of the analysis procedure; in addition, the inaccuracies of the three models follow significantly different trends, defeating any secondary correction appropriate for arbitrary nebulae. We thus strongly favor other methods for determining abundances from observations; e.g., direct modeling of photoionized nebulae or their small-scale structures.

Further Multiwavelength Observations of the SSA 22 Lyα‐Emitting Blob

We present new follow-up observations of the submillimeter luminous Lyα-emitting object in the SSA 22 z = 3.09 galaxy overdensity, referred to as blob 1 by Steidel and coworkers. In particular, we discuss high-resolution Hubble Space Telescope (HST) optical imaging, Owens Valley Radio Observatory (OVRO) spectral imaging, Keck spectroscopy, Very Large Array (VLA) 20 cm radio continuum imaging, and Chandra X-ray observations. We also present a more complete analysis of the existing James Clerk Maxwell Telescope (JCMT) submillimeter data. We detect several optical continuum components that may be associated with the core of the submillimeter-emitting region. A radio source at the position of one of the HST components [α = 22h17m2594, δ = +00°12'389] identifies it as the likely counterpart to the submillimeter source. We also tentatively detect the CO(4-3) molecular line centered on the radio position. We use the CO(4-3) intensity to estimate a limit on the gas mass for the system. The optical morphology of sources within the Lyα cloud appears to be filamentary, while the optical source identified with the radio source has a dense knot that may be an active galactic nucleus (AGN) or compact starburst. We obtain a Keck Low-Resolution Imaging Spectrograph (LRIS) spectrum of this object, despite its faintness (R = 26.8). The spectrum reveals weak Lyα emission but no other obvious features, suggesting that the source is not an energetic AGN (or that it is extremely obscured). We use nondetections in deep Chandra X-ray images to constrain the nature of the blob. Although conclusive evidence regarding the nature of the object remains hard to obtain at this redshift, the evidence presented here is at least consistent with a dust-obscured AGN surrounded by a starburst situated at the heart of this giant Lyα cloud.

NGC 6543. I. Understanding the Anatomy of the Cat's Eye

Hubble Space Telescope images of NGC 6543 taken in 1994.7 through a variety of narrowband filters yield a very detailed structure in emission lines of low and moderate ionization. On the whole, the core of NGC 6543 is optically thin to ionizing radiation. As expected, the general ionization state of the nebula decreases with radius; however, some of the fluctuations are unexpectedly large in scale and impressive in their complex geometries. Other sinewy or arclike features near the edge of the core, known as the caps, ansae, and jets, are extremely low in ionization. The images show that they are made up of closely packed complexes of very dense knots whose recombination times are 10 yr. Furthermore, we find thin and perfectly radial features that we interpret as ionization shadows behind many of the dense knots. Image ratios show no detectable microscopic or macroscopic changes in electron temperature Te seen in the [O III] lines even though the densities probably range between 103 and 105 cm-3 in various regions. Finally, no internal variations of extinction or the He+/H+ ratios are found within the core.

Dynamically knots setting in meshless method for solving time dependent propagations equation

In order to display a function with some finite discrete sampling data efficiently, we require more sampling data where the function is more oscillatory, and less sampling data where the function is more flat. If the function is happen to be a solution of partial differential equation and is solved by finite elements method, then we should construct finer element near the singularity. However, we do not know where the oscillation or even shocks will happen to a function, which is a solution of non-linear partial differential equation. Therefore we cannot preset the finer elements near such points. A trivial method is taking very dense knots or very fine elements everywhere to keep the accuracy. This strategy cost the computation time. Another idea is to move the position of the sampling points according to the varying of the function with the time. We observed that, this approach could be achieved by Lagrangian formulation for finite difference method. However the Lagrangian formulation does not possess shape preserving and variation diminishing properties. It is difficult to achieve the approach for finite elements methods too, because the moving knots will destroy the topology of the mesh. Recently the meshless method becomes to topic to solve partial differential equation numerically. The meshless method does not require any mesh or any structure of the knots (sampling points); therefore we can move the knots freely to simulate the problem. The only restriction is to keep the knots no overlapping. This paper is a test of our approach for the Burger's equation with radial basis quasi-interpolation.

Chandra observations of SNR 1987A

We report on the results of our monitoring program of the X-ray remnant of supernova 1987A with the Chandra X-ray Observatory. We have performed two new observations during the Chandra Cycle 3 period, bringing the total to six monitoring observations over the past three years. These six observations provide a detailed time history of the birth of a new supernova remnant in X-rays. The high angular resolution images indicate that soft X-ray bright knots are associated with the optical spots, while hard X-ray features are better correlated with radio images. We interpret this in terms of a model in which fast shocks propagating through the circumstellar HII region produce the hard X-ray and radio emission, while the soft X-ray and optical emission arise in slower shocks entering into dense knots in the circumstellar inner ring. New observations begin to show changes in the morphology that may herald a new stage in the development of this incipient supernova remnant. The observed X-ray fluxes increase by nearly a factor of three over the last 30 months. The X-ray remnant is expanding at a velocity of ∼5000 km s −1.

Light element nucleosynthesis from jet-cloud interactions in active galactic nuclei

The production of light nuclei via the interactions of jets from the central engines of active galactic nuclei (AGNs) and the surrounding medium are studied. Several environments ranging from hot, dense knots of gas near the central engine to the cold broad line region clouds are simulated by a nuclear reaction network that couples the thermonuclear processes in the cloud to the reactions between the jet particles and the cloud. Reaction products from the jet-cloud interactions are followed until they react or are thermalized, which may involve several subsequent reactions. Enhanced production of light nuclei well above their primordial abundances is possible even, under some conditions, of CNO nuclei. In these scenarios, the jets can enhance abundances of CNO nuclei by first producing excess amounts of nuclei with A<8, then by increasing the cloud density to the point at which the thermonuclear reaction rates become important. The comparison to observed abundances in quasars (QSOs) leads to the conclusion that the interactions of ejected matter from AGNs may be responsible for large observed abundances of light nuclei in addition to significant abundances of nuclei in the CNO region.

Gas cooling in simulations of the formation of the galaxy population

We compare two techniques for following the cooling of gas and its condensation into galaxies within high resolution simulations of cosmologically representative regions. Both techniques treat the dark matter using N-body methods. One follows the gas using smoothed particle hydrodynamics (SPH) while the other uses simplified recipes from semi-analytic (SA) models. We compare the masses and locations predicted for dense knots of cold gas (the `galaxies') when the two techniques are applied to evolution from the same initial conditions and when the additional complications of star formation and feedback are ignored. We find that above the effective resolution limit of the two techniques, they give very similar results both for global quantities such as the total amount of cooled gas and for the properties of individual `galaxies'. The SA technique has systematic uncertainties arising from the simplified cooling model adopted, while details of the SPH implementation can produce substantial systematic variations in the galaxy masses it predicts. Nevertheless, for the best current SPH methods and the standard assumptions of the SA model, systematic differences between the two techniques are remarkably small. The SA technique gives adequate predictions for the condensation of gas into `galaxies' at less than one percent of the computational cost of obtaining similar results at comparable resolution using SPH.