Telescope Archive Research Articles

Detection of Radial Surface Brightness Fluctuations and Color Gradients in Elliptical Galaxies with the Advanced Camera for Surveys

We study surface brightness fluctuations (SBFs) in a sample of eight elliptical galaxies using the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) data drawn from the Hubble Space Telescope archive. SBF magnitudes in the F814W bandpass and galaxy colors from F814W, F435W, and F606W images, when available, are presented. Galaxy surface brightness profiles are determined as well. We present the first SBF-broadband color calibration for the ACS WFC F814W bandpass and (relative) distance moduli estimates for seven of our galaxies. We detect and study in detail the SBF variations within individual galaxies as a probe of possible changes in the underlying stellar populations. Inspecting both the SBF and color gradients in comparison to model predictions, we argue that SBFs and SBF gradients can in principle be used for unraveling the different evolutionary paths taken by galaxies, although a more comprehensive study of this issue would be required. We confirm that the radial variation of galaxy stellar population properties is mainly connected to the presence of radial chemical abundance gradients, with the outer galaxy regions being more metal-poor than the inner ones.

Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres

We search the Hubble Space Telescope (HST) archive for previously unanalyzed observations of stellar H I Lyα emission lines, our primary purpose being to look for new detections of Lyα absorption from the outer heliosphere and to also search for analogous absorption from the astrospheres surrounding the observed stars. The astrospheric absorption is of particular interest because it can be used to study solar-like stellar winds that are otherwise undetectable. We find and analyze 33 HST Lyα spectra in the archive. All the spectra were taken with the E140M grating of the Space Telescope Imaging Spectrograph (STIS) instrument on board HST. The HST STIS spectra yield four new detections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD 165185) and seven new detections of astrospheric absorption (EV Lac, 70 Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling the previous number of heliospheric and astrospheric detections. When combined with previous results, 10 of 17 lines of sight within 10 pc yield detections of astrospheric absorption. This high detection fraction implies that most of the ISM within 10 pc must be at least partially neutral, since the presence of H I within the ISM surrounding the observed star is necessary for an astrospheric detection. In contrast, the detection percentage is only 9.7% (3 out of 31) for stars beyond 10 pc. Our Lyα analyses provide measurements of ISM H I and D I column densities for all 33 lines of sight, and we discuss some implications of these results. Finally, we measure chromospheric Lyα fluxes from the observed stars. We use these fluxes to determine how Lyα flux correlates with coronal X-ray and chromospheric Mg II emission, and we also study how Lyα emission depends on stellar rotation.

Space Telescope Imaging Spectrograph Ultraviolet Spectra of Large Magellanic Cloud Planetary Nebulae: A Study of Carbon Abundances and Stellar Evolution

We acquired spectra of 24 LMC planetary nebulae (PNs) in the 1150-3000 A range in order to determine carbon and other ionic abundances. The sample more than doubles the number of LMC PNs with high-quality UV spectra in this wavelength range and whose optical images are available in the Hubble Space Telescope archive. The Space Telescope Imaging Spectrograph was used with a very large aperture to obtain virtually slitless spectra; thus, the monochromatic images in the major nebula emission lines are also available. The analysis of the data shows extremely high quality spectra. This paper presents the emission lines identified and measured and the calculation of the ionic abundances of the emitting carbon and other ions, as well as total carbon abundance. P Cygni profiles have been found in a fraction of the nebulae, and the limiting velocities of the stellar winds estimated. The total carbon abundance can be inferred reliably in most nebulae. We found that the average carbon abundance in round and elliptical PNs is one order of magnitude larger than that of the bipolar PNs, while elliptical and round PNs with a bipolar core have a bimodal behavior. This results confirm that bipolarity in LMC PNs is tightly correlated with high-mass progenitors. When compared with predicted yields, we found that the observed abundance ratio shows a shift toward higher carbon abundances, which may be due to initial conditions assumed in the models not appropriate for LMC PNs.

A Young Brown Dwarf Companion to DH Tauri

We present the detection of a young brown dwarf companion DH Tau B associated with the classical T Tauri star DH Tau. Near-infrared coronagraphic observations with CIAO on the Subaru Telescope have revealed DH Tau B with H = \~15 mag located at 2.3 (330 AU) away from the primary DH Tau A. Comparing its position with a Hubble Space Telescope archive image, we confirmed that DH Tau A and B share the common proper motion, suggesting that they are physically associated with each other. The near-infrared color of DH Tau B is consistent with those of young stellar objects. The near-infrared spectra of DH Tau B show deep water absorption bands, a strong K I absorption line, and a moderate Na I absorption line. We derived its effective temperature and surface gravity of Teff = 2700 -- 2800 K and log g = 4.0--4.5, respectively, by comparing the observed spectra with synthesized spectra of low-mass objects. The location of DH Tau B on the HR diagram gives its mass of 30 -- 50 M_Jupiter.

Clarification of the Nature of the Galaxy [CFC97] Cen 0511Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5‐26555.

ABSTRACTThe galaxy [CFC97] Cen 05 has in the past been considered an H i–rich dwarf galaxy in the nearby Centaurus A group. We have used Australia Telescope Compact Array observations to show that the H i associated with [CFC97] Cen 05 by Côté et al. is most likely a Galactic high‐velocity cloud that is centered ∼17′ from the optical image of the galaxy. At the optical location of the galaxy, which is not the one tabulated by Banks et al., there is no indication of the presence of H i for velocities less than the upper limit of the H i Parkes All‐Sky Survey at ∼12,500 km s−1. In addition, WFPC2 images of [CFC97] Cen 05 obtained from the Hubble Space Telescope archive reveal that this galaxy is in fact not a dwarf at all, but rather is a distant background spiral not associated with the Centaurus A group.

Physical Structure of Planetary Nebulae. II. NGC 7662

We present a study of the structure and kinematics of the triple-shell planetary nebula NGC 7662 based on long-slit echelle spectroscopic observations and Hubble Space Telescope archival narrowband images. The structure of the main nebula consists of a central cavity surrounded by two concentric shells. The bright inner shell has a nonnegligible thickness and the highest density in the nebula. The outer shell is filled with nebular material whose density profile shows a ∝r-1 drop-off in the inner regions of the shell and flattens in the outermost regions. Both the inner and the outer shells can be described as prolate ellipsoids: the inner shell is more elongated, while the outermost layer of the outer shell expands faster. The physical structure of NGC 7662 is qualitatively consistent with the predictions of hydrodynamical simulations based on the interacting stellar wind models for planetary nebulae with a 0.605 M⊙ central star. Models with a higher asymptotic giant branch wind velocity are needed for detailed comparisons.

The Stellar Populations of Low‐Luminosity Active Galactic Nuclei. II. Space Telescope Imaging Spectrograph Observations

We present a study of the stellar populations of low-luminosity active galactic nuclei (LLAGNs). Our goal is to search for spectroscopic signatures of young and intermediate-age stars and to investigate their relationship with the ionization mechanism in LLAGNs. The method used is based on the stellar population synthesis of the optical continuum of the innermost (20-100 pc) regions in these galaxies. For this purpose, we have collected high spatial resolution optical (2900-5700 ?) STIS spectra of 28 nearby LLAGNs that are available in the Hubble Space Telescope archive. The analysis of these data is compared with a similar analysis also presented here for 51 ground-based spectra of LLAGNs. Our main findings are as follows: (1) No features due to Wolf-Rayet stars were convincingly detected in the STIS spectra. (2) Young stars contribute very little to the optical continuum in the ground-based aperture. However, the fraction of light provided by these stars is higher than 10% in most of the weak-[O I] ([O ]/H? ? 0.25) LLAGN STIS spectra. (3) Intermediate-age stars contribute significantly to the optical continuum of these nuclei. This population is more frequent in objects with weak than with strong [O I]. Weak-[O I] LLAGNs that have young stars stand out for their intermediate-age population. (4) Most of the strong-[O I] LLAGNs have predominantly old stellar population. A few of these objects also show a featureless continuum that contributes significantly to the optical continuum. These results suggest that young and intermediate-age stars do not play a significant role in the ionization of LLAGNs with strong [O I]. However, the ionization in weak-[O I] LLAGNs with young and/or intermediate-age populations could be due to stellar processes. A comparison of the properties of these objects with Seyfert 2 galaxies that harbor a nuclear starburst suggests that weak-[O I] LLAGNs are the lower luminosity counterparts of the Seyfert 2 composite nuclei.

The host galaxies of narrow-line Seyfert-1s: Evidence for bar-driven fueling

We have studied the host-galaxy morphologies of narrow- and broad-line Seyfert 1 galaxies (NLS1s and BLS1s) based on broad-band optical images from the Hubble Space Telescope archives. We find that large-scale stellar bars, spanning 1–10 kpc from the nucleus, are much more common in NLS1s than BLS1s. Furthermore, the fraction of NLS1 spirals that have bars increases with decreasing full-width at half-maximum (FWHM) of the broad component of Hβ. This suggests a link between the large-scale bars, which can support high fueling rates to the inner kpc, and the high mass-accretion rates associated with the supermassive black holes in NLS1s. A preliminary examination of the inner (<1 kpc) dust morphologies has revealed no significant differences between NLS1s and BLS1s, except that we confirm the previous finding that galaxies with large-scale bars have a much higher frequency of “grand design” nuclear dust spirals.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A rotational light curve for the Kuiper belt object 1997 CV 29

We present R- and V-band rotational light curves for classical Kuiper belt object 1997 CV 29. The imaging data was obtained from the Canada–France–Hawaii Telescope (CFHT) archive located at the Canadian Astronomical Data Center (CADC). The data consist of one nights observations of a series of 42, 8 minute exposures in R band followed by 33, 8 minute exposures in V band acquired on the following night. Using relative photometry we determined a highly significant variation in the brightness of 1997 CV 29. Using phase-dispersion-minimization we find 8.0, 11.2, and 15.8 hrs to be the most likely, periods of rotation and we argue that the ∼16 hr period is the most likely based on our limited observing window. The phased light curve has a peak-to-peak range of Δ m≳0.4±0.1 mag suggesting an axial ratio of a/ b≳1.45.

Host Galaxies of Low‐zRadio‐loud Quasars: A Search ofHubble Space TelescopeArchives

We searched the Hubble Space Telescope (HST) archives for unpublished Wide Field Planetary Camera 2 images of low-redshift (z < 0.5) radio-loud quasars (RLQs). This led to the identification of 11 objects. Here we present the results of the analysis of these images from which we derive the properties of their host galaxies. All objects are clearly resolved, and their surrounding nebulosity is consistent with an elliptical galaxy model. These new data, together with previous published HST observations, form a sample of 34 sources that significantly expand all previous studies of low-redshift RLQs based on HST data. For this full sample we derive the average absolute magnitude of the host galaxies = -24.01 ± 0.48 and the effective radius = 10.5 ± 3.7 kpc. No significant correlation is found between the nucleus and the host galaxy luminosity. Using the relationship between black hole mass (BH) and bulge luminosity we investigate the relation between BH and total radio power for RLQs and compare it with other classes of radio sources. The overall distribution of active galactic nuclei in the BH-Pradio plane exhibits a trend for increasing BH with increasing Pradio but with a substantial spread. RLQs occupy the region of the most powerful sources and the most massive black hole. The quasars appear to emit over a wide range of power with respect to their Eddington luminosity, as deduced by the estimated BH.

The Host Galaxies of Narrow-Line Seyfert 1 Galaxies: Evidence for Bar-Driven Fueling

We present a study of the host galaxy morphologies of narrow- and broad-line Seyfert 1 galaxies (NLS1's and BLS1's) based on broadband optical images from the Hubble Space Telescope archives. We find that large-scale stellar bars, starting at ~1 kpc from the nucleus, are much more common in NLS1's than BLS1's. Furthermore, the fraction of NLS1 spirals that have bars increases with decreasing full width at half-maximum of the broad component of Hβ. These results suggest a link between the large-scale bars, which can support high fueling rates to the inner kiloparsecs, and the high mass accretion rates associated with the supermassive black holes in NLS1's.

New detections of optical emission from kiloparsec-scale quasar jets

We report initial results from the detection of optical emission in the arcsecond-scale radio jets of two quasars utilizing images from the Hubble Space Telescope archive. The optical emission has a very knotty appearance and is consistent with synchrotron emission from highly relativistic electrons in the jet. Combining these observations with those of previously reported features in other quasars, an emerging trend appears to be that their radio-to-optical spectral indices are steeper than those of similar features in jets of lower power radio sources.

Searching for Embedded Super-Star Clusters in IC 4662, NGC 1705, and NGC 5398

Radio observations of IC 4662, NGC 5398, and NGC 1705 are used in combination with optical images to search for extremely young star clusters that are still embedded in their birth material. Radio data were obtained at 3 and 6 cm using the Australia Telescope Compact Array, and optical data were retrieved from the Hubble Space Telescope (HST) archive. In IC 4662, three inverted spectrum thermal radio sources and one nonthermal source were detected. The thermal sources have Lyman continuum fluxes ranging from ~20–200 × 1049 s-1 (the equivalent of ~20–200 O7.5-type stars), modeled core radii of r 1 pc, and densities ne 103–104 cm-3. The HST images of this galaxy indicate a number of optical star clusters that are 10 Myr and have masses of ~103–104 M⊙. The young optical clusters and radio clusters in IC 4662 are spatially exclusive—supporting either large amounts of extinction in the vicinity of the radio clusters or sequential star formation. No thermal sources are detected in NGC 1705 down to a 3 σ level of ~5 × 1024 ergs s-1 Hz-1 (0.17 mJy). At the distance of NGC 1705, the Galactic ultracompact H II region complex W49A would have been a ~5 σ detection; therefore, the lack of detections of any thermal sources down to this detection threshold implies that star formation in NGC 1705 has very nearly ceased. NGC 5398 hosts an impressive thermal radio source that is associated with the giant H II region Tol 89. This source has a Lyman continuum flux of ~4500 × 1049 s-1 (the equivalent of ~4500 O7.5-type stars). At the distance to Tol 89 and the resolution of these radio data, we are only able to set an upper limit on its radius of r 90 pc and a lower limit on its mean density of ne > 50 cm-3.

The Ultraviolet Continuum Emission of FR I and FR II Radio Galaxies and a Proposal for a Unified AGN Model for FR I sources

This paper is the second in a series of two on the UV continuum emission (in the range from 1400 to 3700 A) of radio galaxies that were extracted from the Hubble Space Telescope (HST) Archives. The sample consists of 31 3C and Parkes radio galaxies that have redshifts below 0.2 (the majority have redshifts of ~0.03) and radio powers of ~1025–1027 W Hz-1 (using H0 = 50 km-1 s-1 Mpc and q0 = 0.0). Paper I describes the sample selection and the properties of individual sources; this paper deals with the analysis. We find that only about half the radio galaxies display any UV flux at wavelengths shorter than 2300 A. More specifically, those galaxies that are dominated by a nuclear UV component are either BL Lac objects or radio galaxies with broad emission lines. We detect a nuclear and an extended UV component only among half the radio galaxies with narrow emission lines. Although we do not find a correlation of the UV luminosity with emission-line luminosity or radio power, there does appear to be a dependence on radio morphology. While (narrow line) FR II sources do not show a nuclear UV component, FR I's do, however, only if they also have an optical jet (this is the case for seven of 21 FR I's). These results are broadly consistent with orientation-dependent unification models. In radio galaxies in which the torus does not obscure our view of the engine, the observed UV radiation appears to be point source–like. This is the case for broad-line radio galaxies and BL Lac objects. In other radio galaxies that are oriented at an angle to us, the torus presumably blocks the nuclear UV component. The narrow-line FR I galaxies with optical jets can then be interpreted as objects at a critical angle at which some, but not all, nuclear UV emission is blocked. The UVλ luminosities (with λ ranging from 1400 to 3700 A) and the UVλ-V colors of radio galaxies show a larger scatter than those of radio-quiet elliptical galaxies. At wavelengths shorter than 2300 A, some radio galaxies have on average bluer colors, but beyond 3000 A, their colors are on average slightly redder. This picture is also consistent with unification models—the galaxies with bluer colors are either BL Lac objects or broad-line radio galaxies. All other radio galaxies (including the jetted FR I's) have somewhat redder colors than radio-quiet elliptical galaxies. We suspect that this is primarily due to reddening by dust, which we know is present in some of the radio galaxies in the sample. At longer wavelengths (>3000 A), all radio galaxies (14 of 14) show an extended component. The morphology of the extended emission is in most, but not all, cases comparable to the morphology at optical wavelengths, implying that this is likely to be starlight. However, the polarization images of two of seven radio galaxies indicate that scattered light from the active galactic nucleus may also contribute toward the UV luminosity.

Accretion and Outflow in Interacting Binary Systems:Far Ultraviolet Spectroscopic ExplorerObservations of the Nova‐like Cataclysmic Variable UX Ursae Majoris

We present far-ultraviolet (905-1182 A), time-series spectroscopy of the eclipsing, nova-like cataclysmic variable UX UMa acquired with Far Ultraviolet Spectroscopic Explorer (FUSE). The time-averaged spectrum is complex and is dominated by overlapping spectral features. The most prominent features are emission lines of C III, N III, N IV, and O VI. They are broad (FWHM ≥ 1800 km s-1) and double-peaked, with a central absorption at zero velocity. During eclipse, the spectrum is simpler: the emission lines remain bright, but the absorption components of the lines and the weaker features between the emission lines disappear entirely, leaving a flat continuum. This behavior is also evident in Goddard High-Resolution Spectrograph (1149-1660 A) spectra that we retrieved from the Hubble Space Telescope (HST) archive. The FUV spectra show flickering on timescales of several minutes. The flickering is seen primarily in the continuum and/or the weaker lines rather than in the prominent emission lines. The orbital light curve has a dip in the FUV flux between orbital phases 0.45-0.65, similar to a preeclipse dip detected in HST observations. The equivalent widths of the line absorption features decrease during the dip. We have detected a systematic wavelength shift of spectral features on the orbital period, but with a phase lag of 20°, a phenomenon that has been reported at optical wavelengths. We discuss the implications of our results in the context of models of an accretion disk with a chromosphere between the disk and the extended wind. Finally, we note that the observed FUV flux is too low to be consistent with the temperature and radius of the WD derived in 1995 by Baptista et al., suggesting that their remaining binary parameters, including a mass ratio of 1, ought to be viewed with skepticism.

The Gravitational Lens B1608+656. I.V‐,I‐, andH‐BandHubble Space TelescopeImaging

We present a multiwavelength analysis of high-resolution observations of the quadruple lens B1608+656 from the Hubble Space Telescope archive, acquired with WFPC2 through filters F606W (V band) and F814W (I band) and with NIC1 in filter F160W (H band). In the three bands, the observations show extended emission from the four images of the source in a ringlike configuration that surrounds the two resolved, ensing galaxies. B1608+656 was discovered as a double-lobed radio source and later identified as a poststarburst galaxy in the optical. Based on photometry and optical spectroscopy we estimate that the stellar population of the source has an age of ~500 Myr. This provides a model for the spectrum of the source that extends over spectral regions where no observations are available and is used to generate Tiny Tim point-spread functions (PSFs) for the filters. Deconvolutions performed with the Lucy-Richardson method are presented, and the limitations of these restorations are discussed. V-I and I-H color maps show evidence of extinction by dust associated with one of the lensing galaxies, a late-type galaxy presumably disrupted after its close encounter with the other lens, an elliptical galaxy. The extinction affects the two lens galaxies and two of the four multiple images. The diagnostic of wavelength-dependent effects in the images shows that corrections for contamination with light from the lenses, extinction, and PSF convolution need to be applied before using the extended structure in the images as a constraint on lens models. We will present the restoration of the images in a subsequent paper.

Surface brightness profiles and structural parameters for 53 rich stellar clusters in the Large Magellanic Cloud

We have compiled a pseudo-snapshot data set of two-colour observations from the Hubble Space Telescope archive for a sample of 53 rich LMC clusters with ages of 106–1010 yr. We present surface brightness profiles for the entire sample, and derive structural parameters for each cluster, including core radii, and luminosity and mass estimates. Because we expect the results presented here to form the basis for several further projects, we describe in detail the data reduction and surface brightness profile construction processes, and compare our results with those of previous ground-based studies. The surface brightness profiles show a large amount of detail, including irregularities in the profiles of young clusters (such as bumps, dips and sharp shoulders), and evidence for both double clusters and post-core-collapse (PCC) clusters. In particular, we find power-law profiles in the inner regions of several candidate PCC clusters, with slopes of approximately −0.7, but showing considerable variation. We estimate that 20 ± 7 per cent of the old cluster population of the Large Magellanic Cloud (LMC) has entered PCC evolution, a similar fraction to that for the Galactic globular cluster system. In addition, we examine the profile of R136 in detail and show that it is probably not a PCC cluster. We also observe a trend in core radius with age that has been discovered and discussed in several previous publications by different authors. Our diagram has better resolution, however, and appears to show a bifurcation at several hundred Myr. We argue that this observed relationship reflects true physical evolution in LMC clusters, with some experiencing small-scale core expansion owing to mass loss, and others large-scale expansion owing to some unidentified characteristic or physical process.

Surface brightness profiles and structural parameters for 10 rich stellar clusters in the Small Magellanic Cloud

We have compiled a pseudo-snapshot data set of two-colour observations from the Hubble Space Telescope archive for a sample of 53 rich LMC clusters with ages 10^6-10^10 yr. We present surface brightness profiles for the entire sample, and derive structural parameters for each cluster, including core radii, and luminosity and mass estimates. Because we expect the results presented here to form the basis for several further projects, we describe in detail the data reduction and surface brightness profile construction processes, and compare our results with those of previous ground-based studies. The surface brightness profiles show a large amount of detail, including irregularities in the profiles of young clusters (such as bumps, dips, and sharp shoulders), and evidence for both double clusters and post core-collapse (PCC) clusters. In particular we find power-law profiles in the inner regions of several candidate PCC clusters, with slopes of approximately -0.7, but showing considerable variation. We estimate that 20 +/- 7 % of the old cluster population of the LMC has entered PCC evolution, a similar fraction to that for the Galactic globular cluster system. In addition, we examine the profile of R136 in detail and show that it is probably not a PCC cluster. We also observe a trend in core radius with age that has been discovered and discussed in several previous publications by different authors. Our diagram has better resolution however, and appears to show a bifurcation at several hundred Myr. We argue that this observed relationship reflects true physical evolution in LMC clusters, with some experiencing small scale core expansion due to mass loss, and others large scale expansion due to some unidentified characteristic or physical process.

Possible Recovery of SN 1961V in Hubble Space Telescope Archival Images11Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive of the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5‐26555.

ABSTRACTSN 1961V in NGC 1058 was originally classified by Fritz Zwicky as a “Type V” supernova. However, it has been argued that SN 1961V was not a genuine supernova, but instead the superoutburst of an η Carinae–like luminous blue variable star. In particular, Filippenko et al. used pre‐refurbishment Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC) images and the known radio position of SN 1961V to conclude that the star survived the eruption and is likely coincident with a V ≈ 25.6, V - I ≈ 1.9 mag object. Recently, Stockdale et al. recovered the fading SN 1961V at radio wavelengths and argue that its behavior is similar that of some Type II supernovae. We have analyzed post‐refurbishment archival HST WFPC2 data and find that the new radio position is still consistent with the Filippenko et al. object, which has not changed in brightness or color, but is also consistent with an adjacent, fainter (I ≈ 24.3 mag), and very red (V− I>1.0 mag) object. We suggest that this fainter object could be the survivor of SN 1961V. Forthcoming HST observations may settle this issue.

Compact Star Clusters in Nearby Dwarf Irregular Galaxies

Nearby dwarf irregular galaxies were searched for compact star clusters by using data from the Hubble Space Telescope archives. Eight of the 22 galaxies in our sample were found to host compact clusters of some type. Three of these have populous clusters, with MV < -9.5 at a fiducial age of 10 Myr, and the same three also have super–star clusters, with MV <-10.5 at 10 Myr. Four other dwarf galaxies, two of which contain populous and super–star clusters, are also considered by using data in the literature. The results suggest that galaxies fainter than MB = -16 or with star formation rates less than 0.003 M⊙ yr-1 kpc-2 do not form populous or super–star clusters and that even the brighter and more active dwarf galaxies rarely form them. Yet when they do form, the associated star formation activity is very high, with numerous compact clusters of similar age in the same complex and evidence for a galaxy-wide perturbation as the trigger. This tendency to concentrate star formation in localized regions of high column density is consistent with previous suggestions that self-gravity must be strong and the pressure must be high to allow a cool phase of gas to exist in equilibrium. Statistical considerations emphasize the peculiarity of super–star clusters in dwarf galaxies, which are too small to sample the cluster mass function to that extreme. We suggest that triggered large-scale flows and ambient gravitational instabilities in the absence of shear make the clouds that form super–star clusters in small galaxies. This is unlike the case in spiral galaxies in which density wave flows and scale-free compression from turbulence seem to dominate. Further comparisons with spiral galaxies give insight into Larsen & Richtler's relation between the star formation rate per unit area and the fraction of young stars in massive dense clusters. We suggest that this relation is the result of a physical connection between maximum cluster mass, interstellar pressure, interstellar column density, and star formation rate, combined with a size-of-sample effect.