High-resolution Hubble Space Telescope Research Articles

A census of young stellar populations in the warm ULIRG PKS 1345+12

We present a detailed investigation of the young stellar populations (YSP) in the radio-loud ultraluminous infrared galaxy (ULIRG), PKS 1345+12 (z = 0.12), based on high-resolution Hubble Space Telescope (HST) imaging and long-slit spectra taken with the William Herschel Telescope (WHT) at La Palma. While the images clearly show bright knots suggestive of super star clusters (SSCs), the spectra reveal the presence of YSP in the diffuse light across the full extent of the halo of the merging double nucleus system. Spectral synthesis modelling has been used to estimate the ages of the YSP for both the SSC and the diffuse light sampled by the spectra. For the SSC, we find ages tSSC < 6 Myr with reddenings 0.2 < E(B − V) < 0.5 and masses 10 6 < M YSP < 10 7 M� . In the region to the south of the western nucleus that contains the SSC our modelling of the spectrum of the diffuse light is also consistent with a relatively young age for the YSP (∼5 Myr), although older YSP ages cannot be ruled out. However, in other regions of the galaxy we find that the spectra of the diffuse light component can only be modelled with a relatively old post-starburst YSP (0.04‐1.0 Gyr) or with a disc galaxy template spectrum. The results demonstrate the importance of accounting for reddening in photometric studies of SSC and highlight the dangers of focusing on the highest surface brightness regions when trying to obtain a general impression of the star formation activity in the host galaxies of ULIRGs. The case of PKS 1345+12 provides clear evidence that the star formation histories of the YSP in ULIRGs are complex. While the SSC represent the vigorous phase of star formation associated with the final stages of the merger, the YSP in the diffuse light are likely to represent star formation in one or more of the merging galaxies at an earlier stage or prior to the start of the merger. Intriguingly, our long-slit spectra show line splitting at the locations of the SSC, indicating that they are moving at up to 450 km s −1 with respect to the local ambient gas. Given their kinematics, it is plausible that the SSCs have been formed either in fast moving gas streams/tidal tails that are falling back into the nuclear regions as part of the merger process or as a consequence of jet-induced star formation linked to the extended, diffuse radio emission detected in the halo of the galaxy.

Arp 116: interacting system or chance alignment?

Using high spatial resolution Hubble Space Telescope /Advanced Camera for Surveys archival imaging observations of Arp 116, centred on the elliptical galaxy NGC 4649, we explore the novel technique of pixel-by-pixel analysis of the galaxy's colour–magnitude diagram to search for any evidence of recent enhanced star formation due to the apparent tidal interaction with its spiral companion, NGC 4647. From a detailed analysis of the system's geometry, and based on additional circumstantial evidence from extant multi-wavelength observations, we conclude that, while there may be grounds for the tidal-interaction assumption for this system, any interaction has thus far been of insufficient strength to trigger an enhanced level of recent star formation in the elliptical component, although close inspection of our colour images shows a faint excess of bluer pixels (a ~0.20 mag bluer “loop”) in the elliptical galaxy on the side of the spiral companion. Given that there appears to be a moderate reservoir of available gas for ongoing star formation (although at low column density), this suggests that we are currently witnessing the onset of the tidal interaction between NGC 4647 and NGC 4649. In addition, the triggering of new star formation in NGC 4649 may be significantly impeded due to the much lower mass of the spiral component.

Measurements of winds on Mars with Hubble Space Telescope images in 2003 opposition

The winds of Mars at the water cloud level are measured by tracking cloud features in sequential high-resolution Hubble Space Telescope UV images for 5 days during the perihelion season in 2003. Global circulation model (GCM) predictions are in general agreement with the observations for the zonal wind component, but reveal significant differences for the meridional component.

Color, Structure, and Star Formation History of Dwarf Galaxies over the Last ∼3 Gyr with GEMS and SDSS

We present a study of the colors, structural properties, and star formation histories for a sample of ~1600 dwarfs over look-back times of ~3 Gyr (z=0.002-0.25). The sample consists of 401 distant dwarfs drawn from the Galaxy Evolution from Morphologies and SEDs (GEMS) survey, which provides high resolution Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images and accurate redshifts, and of 1291 dwarfs at 10-90 Mpc compiled from the Sloan Digitized Sky Survey (SDSS). The sample is complete down to an effective surface brightness of 22 mag arcsec^-2 in z and includes dwarfs with M_g=-18.5 to -14 mag. Rest-frame luminosities in Johnson UBV and SDSS ugr filters are provided by the COMBO-17 survey and structural parameters have been determined by S\'ersic fits. We find that the GEMS dwarfs are bluer than the SDSS dwarfs by ~0.13 mag in g-r, which is consistent with the color evolution over ~2 Gyr of star formation histories involving moderate starbursts and long periods of continuous star formation. The full color range of the samples cannot be reproduced by single starbursts of different masses or long periods of continuous star formation alone. Furthermore, an estimate of the mechanical luminosities needed for the gas in the GEMS dwarfs to be completely removed from the galaxies shows that a significant number of low luminosity dwarfs are susceptible to such a complete gas loss, if they would experience a starburst. On the other hand, a large fraction of more luminous dwarfs is likely to retain their gas. We also estimate the star formation rates per unit area for the GEMS dwarfs and find good agreement with the values for local dwarfs.

Infrared Imaging of Planetary Nebulae from the Ground Up

New ground-based telescopes and instruments, the return of the NICMOS instrument on the Hubble Space Telescope (HST), and the recent launch of the Spitzer Space Telescope have provided new tools that are being utilized in the study of planetary nebulae. Multiwavelength, high spatial resolution ground-based and HST imaging have been used to probe the inner regions of young PNe to determine their structure and evaluate formation mechanisms. Spitzer/IRAC and MIPS have been used to image more evolved PNe to determine the spatial distribution of molecular hydrogen, ionized gas, and dust in the nebulae and halos.

The Circumstellar Structure of the Class I Protostar TMC‐1 (IRAS 04381+2540) fromHubble Space TelescopeNICMOS Data

The class I protostar TMC-1 (IRAS 04381+2540) is oriented favorably for determining the properties of its circumstellar envelope and outflow cavity. Deep, high spatial resolution Hubble Space Telescope (HST) NICMOS images at 1.6 μm exhibit both a narrow jet and a wide-angle conical outflow cavity. Model images of the scattered-light distribution fit the data well, reproducing the intensity level, cavity width, and observed limb brightening. The best-fit geometry for TMC-1 has a 45° ± 5° source inclination and an 80° ± 5° deprojected wind opening angle (full width). The age, normally a poorly known quantity, is well constrained; the protostar age, i.e., time since the onset of cloud collapse, is 1 × 105 yr to within a factor of 2. We offer a possible resolution to the well-known luminosity problem. By considering the efficiency of infall onto the protostar, we find that plausible parameters can give an efficiency, and hence accretion luminosity, as low as 10% of the value derived from the collapsing cloud core. The efficiency, together with a luminosity constraint, leads to a mass estimate that ranges from about 0.1 M☉ for high efficiency to 0.2 M☉ for low accretion efficiency onto the protostar. Similarly, the estimated mass accretion rate onto the protostar ranges over roughly (0.9-1.4) × 10-6 M☉ yr-1, which is smaller than the (1.6-3.5) × 10-6 M☉ yr-1 infall rate of the cloud. If low efficiency rates are prevalent for protostars, one important consequence is that it will take longer to assemble the central star than the time t = Min/in, a time that assumes all of the infalling material lands on the protostar.

The Dynamical State and Blue Straggler Population of the Globular Cluster NGC 6266 (M62)

We have used a proper combination of multiband high-resolution Hubble Space Telescope WFPC2 and wide-field ground-based observations to image the Galactic globular cluster NGC 6266 (M62). The extensive photometric data set allows us to determine the center of gravity and to construct the most extended radial profile ever published for this cluster including, for the first time, detailed star counts in the very inner region. The star density profile is well reproduced by a standard King model with an extended core (∼19'') and a modest value of the concentration parameter (c = 1.5), indicating that the cluster has not yet experienced core collapse. The millisecond pulsar population (whose members are all in binary systems) and the X-ray-emitting population (more than 50 sources within the cluster half-mass radius) suggest that NGC 6266 is in a dynamical phase particularly active in generating binaries through dynamical encounters. UV observations of the central region have been used to probe the population of blue straggler stars, whose origin might be also affected by dynamical interactions. The comparison with other globular clusters observed with a similar strategy shows that the blue straggler content in NGC 6266 is relatively low, suggesting that the formation channel that produces binary systems hosting neutron stars or white dwarfs is not effective in significantly increasing the blue straggler population. Moreover, an anticorrelation between millisecond pulsar content and blue straggler specific frequency in globular clusters seems to be emerging with increasing evidence.

Star Cluster Formation in Extreme Starburst Environments

AbstractThe currently available empirical evidence on the star formation processes in the extreme, high-pressure environments induced by galaxy encounters, mostly based on high-resolution Hubble Space Telescope imaging observations, strongly suggests that star cluster formation is an important and perhaps even the dominant mode of star formation in the starburst events associated with galaxy interactions.

The Small Blue Straggler Star Population in the Dense Galactic Globular Cluster NGC 6752

We have used high resolution WFPC2-HST and wide field ground-based observations to construct a catalog of blue straggler stars (BSS) which spans the entire radial extent of the globular cluster NGC 6752. The BSS sample is the most extensive ever obtained for this cluster. Though NGC 6752 is a high density cluster with a large binary population, we found that its BSS content is surprisingly low: the specific number of BSS is among the lowest ever measured in a cluster. The BSS distribution is highly peaked in the cluster center, shows a rapid decrease at intermediate radii, and finally rises again at larger distances. This distribution closely resembles those observed in M3 and 47Tuc by Ferraro et al. (1993), Ferraro et al. (2003c). To date, BSS surveys covering the central regions with HST and the outer regions with wide field CCD ground-based observations have been performed for only these three clusters. Despite the different dynamical properties, a bimodal radial distribution has been found in each. A detailed comparison of observed BSS luminosity and temperature distributions with theoretical models reveals a population of luminous, hot BSS which is not easily interpreted.

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.

Hubble Space TelescopeImaging of HD 44179, The Red Rectangle

We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 and deep ground-based images of the Red Rectangle (RR), a bipolar proto–planetary nebula associated with the post-AGB binary system HD 44179. The high-resolution HST images reveal complex new structures, many of them unique to this object. The RR nebula is dominated by a discontinuous whose bright, sharp linear edges give the nebula an overall X-shaped appearance. The edges of the bicone are connected by a remarkable series of linear features elongated perpendicular to the radius vector, giving the object a ladder-like structure. The rungs of the ladder structure show a quasi-periodic spacing, suggesting that they have arisen from discrete episodes of mass loss from the central star, separated by a few hundred years. The total timescale over which mass has been shed into the visible nebula is of order 14,000 yr. Outside the X-shaped bicone, parabolas curl inward, resembling wineglasses, which terminate on the bicone edges in large, limb-brightened vortices. The central object is bisected by a dark band, indicating that the star is not seen directly but is instead obscured by a surrounding opaque dust disk.

A high-resolution study of the interstellar medium in the luminousIRASgalaxy Arp 193

We present high-resolution multi-element radio-linked interferometer network (MERLIN) and Very Large Array (VLA) observations of the H I absorption towards the starburst nucleus of Arp 193 (IC 883, UGC 08387), a luminous IRAS merger. We compare the distribution of atomic gas with that of radio continuum emission, high-resolution CO data and near-infrared Hubble Space Telescope (HST) Near-Infrared Camera and Multi-object Spectrometer (NICMOS) data. We find, that for a constant spin temperature and CO flux to H 2 mass conversion factor, X CO , the interstellar medium (ISM) becomes progressively more H 2 rich towards the centre of the source. We also find that the projected velocity distribution of the atomic and molecular gas components are not the same. We discuss possible reasons for the observed differences and find that the principle difference is probably in the spatial distribution of the molecular and atomic ISM, although the addition of real dynamical differences provides a better explanation of the data. Evidence for free-free absorption towards the centre of the starburst nucleus is found, implying a large mass of ionized gas, and therefore young stars. Arp 193 is one of the very few IRAS luminous objects in which such a detailed study is possible.

Infrared mergers and infrared quasi-stellar objects with galactic winds - I. NGC 2623: nuclear outflow in a proto-elliptical candidate

We present the first results of a study of the morphology, kinematics and ionization structure of infrared (IR) mergers/quasi-stellar objects (QSOs) with galactic winds. This study is based mainly on integral two-dimensional (2D) fibre spectroscopy [obtained on the 4.2-m William Herschel Telescope (WHT), La Palma] combined with high-resolution Hubble Space Telescope (HST) observations. Clear evidence of outflow (OF) from the nucleus of the luminous infrared merger NGC 2623 is reported. Specifically: (i) the integral 2D Hα, [N ii] and [S ii] emission line maps depict a cone-shaped extended nebula that emerges from the nucleus, with an aperture angle θ= 100°± 5° and reaching a distance of ∼3.2 kpc from the nucleus; (ii) inside the nebula and in the central region, all the emission-line WHT spectra show low velocity blue/OF components, with 〈VOF〉= (−405 ± 35) km s−1; and (iii) in the OF nebula, the emission line ratios are consistent with ionization by a dusty nuclear starburst plus shock heating. These results are consistent with a galactic wind process powered mainly by a nuclear starburst. The integral 2D Hα and [N ii]λ6583 velocity field (VF) maps for the main body of NGC 2623 (16.4 × 12.3 arcsec2; ∼5.9 × 4.4 kpc2) show outflow motion in the nuclear and the Hα+[N ii] nebular regions superposed on a general circular motion. This circular motion prevails inside r∼ 1.5 kpc, and for larger radii we detected non-circular motions. In the central region, the average observed rotation curve was fitted with a model corresponding to a single-component Plummer spherical potential. After the subtraction of the Plummer and an axisymmetric polynomial model, the residues of the VF in both cases indicate ejection as the origin of the cone nebula. The fitted Plummer model implies a total mass of MT= 1.5 × 1010 M⊙ and a spherical distribution of matter in the central region. The high-resolution HST WFPC2 F555W (∼V) and F814W (∼I) broad-band images display a strongly obscured nucleus in the apex of a small nuclear cone, an asymmetrical clumpy spiral arm located to the east of the nucleus, a ring plus an arc to the west and several large-scale filaments of dust. A good r1/4-law fit to the HST WFPC2 I band luminosity profile was found. In 85 per cent of the integral 2D field we measure very high values (>1) of the [N ii]λ6583/Hα and [S ii]λ6717 + 31/Hα ratios, suggesting that shocks are important on large scales (in almost all the main body). Furthermore, the 2D full width at half maximum FWHM–[N ii] and VF residual maps show a good spatial correlation, suggesting that the OF shocks ionize the gas and broaden the emission lines. However, close to the nucleus, the OF nebula shows low values of the [N ii]λ6583/Hα ratio (in the range 0.1–0.4), indicating that photoionization by a dusty nuclear starburst also plays a significant role in the excitation of the nebula. The characteristics of the nucleus of NGC 2623 could be associated with a starburst-related LINER. The properties found in IR mergers/QSOs with galactic winds mainly underline the importance of studying the possible link between IR mergers with starburst + galactic wind IR QSOs with composite nature + galactic wind, and elliptical galaxies.

The Puzzling Dynamical Status of the Core of the Globular Cluster NGC 6752

We have used high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and ground-based wide-field images to determine the center of gravity and construct an extended radial density and brightness profile of the cluster NGC 6752 including, for the first time, detailed star counts in the very inner region. The barycenter of the nine innermost X-ray sources detected by Chandra is located only 19 off the new center of gravity. Both the density and the brightness profile of the central region are best fitted by a double King model, suggesting that NGC 6752 is experiencing a post-core-collapse bounce. Taking advantage of our new optical data, we discuss the puzzling nature of the accelerations displayed by the innermost millisecond pulsars detected in this cluster. We discuss two possible origins to the accelerations: (1) the overall cluster gravitational potential, which would require a central projected mass-to-light ratio of the order of 6-7 and the existence of a few thousand solar masses of low-luminosity matter within the inner 0.08 pc of NGC 6752, and (2) the existence of a local perturber(s) of the pulsar dynamics, such as a recently proposed binary black hole of intermediate (100-200 M☉) mass.

High-Resolution Near-Infrared Imaging and Polarimetry of Four Proto-Planetary Nebulae

High-resolution near-infrared Hubble Space Telescope (HST) NICMOS (F160W, F222M) images and polarization (2 ?m) observations were made of four bipolar proto?planetary nebulae (PPNs): IRAS 17150-3224, 17441-2411, 17245-3951, and 16594-4656. The first three of these are viewed nearly edge-on, and for the first time the central stars in them are seen. Color maps reveal a reddened torus between the bipolr lobes in the edge-on cases, with bluer lobes. The polarization values are high, with maximum values ranging from 40% to 80%. The polarization patterns are basically centrosymmetric, with some deviations in the low-polarization equatorial regions. For IRAS 17150-3224, circumstellar arcs are seen at 1.6 ?m, along with a newly discovered loop in the equatorial region. Bright caps are seen at the end of the lobes, indicating that they are not open-ended. A distinct point-symmetric pattern is seen in the strengths of the polarization vectors, especially in IRAS 17150-3224. HST NICMOS observations provide a valuable complement to the WFPC2 visible images in deriving the basic structure of bipolar PPNs.

The Compact Nucleus of the Deep Silicate Absorption Galaxy NGC 4418

High-resolution Hubble Space Telescope (HST) near-infrared and Keck mid-infrared images of the heavily extinguished infrared-luminous galaxy NGC 4418 are presented. These data make it possible to observe the imbedded near-infrared structure on scales of 10–20 pc and to constrain the size of the mid-infrared–emitting region. The 1.1–2.2 μm data of NGC 4418 show no clear evidence of nuclear star clusters or of a reddened active galactic nucleus. Instead, the nucleus of the galaxy consists of a ~100–200 pc linear structure with fainter structures extending radially outward. The near-infrared colors of the linear feature are consistent with a 10–300 Myr starburst suffering moderate levels (a few magnitudes) of visual extinction. At 7.9–24.5 μm NGC 4418 has estimated size upper limits in the range of 30–80 pc. These dimensions are consistent with the highest-resolution radio observations obtained to date of NGC 4418, as well as the size of 50–70 pc expected for a blackbody with a temperature derived from the 25, 60, and 100 μm flux densities of the galaxy. Further, a spectral energy distribution constructed from the multiwavelength mid-infrared observations shows the strong silicate absorption feature at 10 μm, consistent with previous mid-infrared observations of NGC 4418. An infrared surface brightness of ~2.1 × 10^(13) L_⊙ kpc^(-2) is derived for NGC 4418. Such a value, though consistent with the surface brightness of warm ultraluminous infrared galaxies [L_(IR)(8–1000 μm) ≥ 10^(12) L_⊙], such as IRAS 05189-2524 and IRAS 08572+3915, is not large enough to distinguish NGC 4418 as a galaxy powered by an active galactic nucleus, as opposed to a lower surface brightness starburst.

Hubble Space TelescopeWFPC2 Morphologies ofK‐selected Extremely Red Galaxies

We selected 115 extremely red objects (EROs) from deep Hubble Space Telescope (HST) WFPC2 archive data combined with ground-based K-band images, with (F814W-Ks) ≥ 4, K-band signal-to-noise ratio ≥5, and a median limiting Ks magnitude of ~18.7, over a corresponding area of 228 arcmin2, for a morphological study of the ERO galaxy population. The survey covered a total of ~409 arcmin2 over 77 separate WFPC2 fields. This is the first complete sample of bright EROs with high-resolution HST morphologies. From a visual morphological classification, we find that 30% ± 5% of our (F814W-Ks) ≥ 4 selected sample have morphologies consistent with a pure bulge or bulge-dominated galaxy (equivalent to E/S0), while disks comprise 64% ± 7% of the sample. Only 6% of the EROs remained unclassifiable. Mergers or strongly interacting systems, which includes sources from both classes, make up 17% ± 4% of the full sample. The quantitative Medium Deep Survey profile fitting is consistent with these results. These results highlight the complex nature of optical/near-IR color selected EROs. The dominant component of our sample is composed of disks, not spheroids or strongly interacting systems like HR 10. Using Bruzual & Charlot spectral energy distribution models, we investigated population differences in EROs selected by their (I-K) versus (R-K) colors and found that I-band based surveys preferentially select systems with prolonged star formation. Real differences in the surface densities of EROs in R-band and I-band based survey may reflect this color selection effect, complicating the comparisons between and interpretations on the nature of the ERO population. We conclude that only a small fraction of EROs at z ~ 1-2 could be passively evolving elliptical galaxies formed at high redshift through a monolithic collapse mechanism. For the majority of EROs, even if most of their stellar mass is already in place at z ~ 1, interaction with the environment and accretion of gas still play important roles in their continuing evolution.

STIS Spectral Imagery of the OB Stars in NGC 604. II. The Most Luminous Stars

We present results using two-dimensional spectral imagery and photometry obtained with the Hubble Space Telescope (HST) for the starburst H II region NGC 604, in the nearby galaxy M33. The spectral imagery was acquired with the Space Telescope Imaging Spectrograph (STIS) using the MAMA-G140L configuration, which provided wavelength coverage spanning 1170–1730 A. From a single 1720 s STIS exposure, we have extracted spectra for 49 stars and derived individual UV spectral types for 40 stars in the crowded 25'' × 2'' stellar field sampled by the STIS aperture. These stars represent a significant fraction of the young, luminous O and B stars in NGC 604. Three objects have pronounced He II λ1640 emission, the signature of Wolf-Rayet (W-R) or luminous Of stars. By combining UV fluxes with HST WF/PC-1 and WFPC2 photometry at visible wavelengths, we derive the extinction curve for NGC 604. We use this extinction curve, together with the available accurate distance for M33, derived UV spectral types, and HST photometry, to determine positions of the luminous stars in the upper Hertzsprung-Russell diagram for NGC 604. The revision to the O star effective temperature scale by Martins et al., based on non-LTE, line-blanketed model atmospheres, is essential in obtaining reliable positions in the log L*–log Teff plane. These stars are quite young, with a characteristic age of ~3 Myr. The spectra and photometry indicate that three objects are exceedingly luminous. Their inferred locations in the H-R diagram relative to theoretical evolutionary tracks indicate stellar masses ≥120 M⊙. High spatial resolution HST imagery provides no evidence of multiple stars composing these objects. Still, we cannot eliminate the possibility that these objects are not unresolved multiple stars of lower mass, possibly W-R stars. Simple tests demonstrate that the 10 most luminous stars predominantly determine the UV spectral features seen in the total light of NGC 604. We conclude that the interpretation of spectral fitting of more distant starburst galaxies, where individual stars are not resolved, must be done with extreme care.

Searching for Bulges at the End of the Hubble Sequence

We investigate the stellar disk properties of a sample of 19 nearby spiral galaxies with low inclination and late Hubble type (Scd or later). We combine our high-resolution Hubble Space Telescope I-band observations with existing ground-based optical images to obtain surface brightness profiles that cover a high dynamic range of galactic radii. Most of these galaxies contain a nuclear star cluster, as discussed in a separate paper. The main goal of the present work is to constrain the properties of stellar bulges at these extremely late Hubble types. We find that the surface brightness profiles of the latest-type spiral galaxies are complex, with a wide range in shapes. We have sorted our sample in a sequence, starting with pure disk galaxies (approximately 30% of the sample). These galaxies have exponential stellar disks that extend inward to within a few tens of parsecs from the nucleus, where the light from the nuclear cluster starts to dominate. They appear to be truly bulgeless systems. Progressing along the sequence, the galaxies show increasingly prominent deviations from a simple exponential disk model on kiloparsec scales. Traditionally, such deviations have prompted bulge-disk decompositions. Indeed, the surface brightness profiles of these galaxies are generally well fitted by adding a second (exponential) bulge component. However, we find that most surface brightness profiles can be fitted equally well (or better) with a single Sersic-type R1/n profile over the entire radial range of the galaxy without requiring a separate bulge component. We warn in a general sense against identification of bulges solely on the basis of single-band surface brightness profiles. Despite the narrow range of Hubble types in our sample, the surface brightness profiles are far from uniform. The differences between the various galaxies appear unrelated to their Hubble types, thus questioning the usefulness of the Hubble sequence for the subcategorization of the latest-type spiral galaxies. A number of galaxies show central excess emission on spatial scales of a few hundred parsecs that cannot be attributed to the nuclear cluster, the Sersic-type description of the stellar disk, or what one would generally consider to be a bulge component. The origin of this light component remains unclear.

Passive Spiral Formation from Halo Gas Starvation: Gradual Transformation into S0s

Recent spectroscopic and high-resolution Hubble Space Telescope (HST) imaging observations have revealed significant numbers of spiral galaxies in distant clusters, with all the morphological hallmarks of a spiral galaxy (in particular, spiral arm structure), but with weak or absent star formation. Exactly how such spiral galaxies formed and whether they are the progenitors of present-day S0 galaxies is unclear. Based on analytic arguments and numerical simulations of the hydrodynamical evolution of a spiral galaxy's halo gas (which is a likely candidate for the source of gas replenishment for star formation in spirals), we show that the origin of passive spirals may well be associated with halo gas stripping. Such stripping results mainly from the hydrodynamical interaction between the halo gas and the hot intracluster gas. Our numerical simulations demonstrate that even if a spiral orbits a cluster with a pericenter distance ~3 times larger than the cluster core radius, ~80% of the halo gas is stripped within a few Gyr and, accordingly, cannot be accreted by the spiral. Furthermore, our study demonstrates that this dramatic decline in the gaseous infall rate leads to a steady increase in the Q parameter for the disk, with the spiral arm structure, although persisting, becoming less pronounced as the star formation rate gradually decreases. These results suggest that passive spirals formed in this way gradually evolve into red cluster S0s.