NICMOS Research Articles

Resolving the Stellar Population of the Standard Elliptical Galaxy NGC 3379

Using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on board the Hubble Space Telescope, we have obtained F110W (J-band) and F160W (H-band) images of three fields in NGC3379, a nearby normal giant elliptical galaxy. These images resolve individual red giant stars, yielding the first accurate color-magnitude diagrams for a normal luminous elliptical. The photometry reaches 1 magnitude below the red giant branch tip with errors of < 0.2 mags in F110W-F160W. A strong break in the luminosity function at F160W = 23.68 +/- 0.06 is identified as the tip of the red giant branch (RGB); comparison with theoretical isochrones implies a distance of 10.8 +/- 0.6 Mpc, in good agreement with a number of previous estimates using various techniques. The mean metallicity is close to solar, but there is an appreciable spread in abundance, from at least as metal poor as [Fe/H] = -1.5 to as high as +0.8. There is a significant population of stars brighter than the RGB tip by up to about 1 magnitude. The observations of each field were split over two epochs, separated by 2-3 months, allowing the identification of candidate long period variables; at least 40% of the stars brighter than the RGB tip are variable. Lacking period determinations, the exact nature of these variables remains uncertain, but the bright AGB stars and variables are similar to those found in metal rich globular clusters and are not luminous enough to imply an intermediate age population. All of the evidence points to a stellar population in NGC3379 which is very similar to the bulge of the Milky Way, or an assortment of Galactic globular clusters covering a large metallicity spread.

Barred Galaxies at z &gt; 0.7: NICMOS Hubble Deep Field-North Observations

Previous optical studies found an unexpected deficit of bars at z > 0.7. To investigate the effects of band shifting, we have studied the fraction of barred spiral galaxies in the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) Hubble Deep Field-North. At z > 0.7, we find at least four barred spiral galaxies, doubling the number previously detected. The number of barred galaxies is small because these (and previous) data lack adequate spatial resolution. A typical 5 kpc bar at z > 0.7 is only marginally detectable for the Wide Field Planetary Camera 2 (WFPC2) at 0.8 μm; the NICMOS data even have lower resolution and can only find the largest bars. The average size of the four bars seen at z > 0.7 is 12 kpc. The fraction of such large bars (4/95) is higher than that seen in nearby spiral galaxies (1/44); all known selection effects suggest that the observed fraction is a lower limit. However, important caveats such as small numbers and difficulties in defining comparable samples at high and low redshifts should be noted. We conclude that there is no significant evidence of a decrease in the fraction of barred spiral galaxies beyond z ~ 0.7.

NICMOS Coronagraphic Observations of the GM Aurigae Circumstellar Disk

We present Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) coronagraphic observations of the environment in the region of the young star-disk system GM Aurigae. Scattered near-infrared light in two spectral bands (F110W = 1.1 lm and F160W = 1.6 lm) trace the morphology of circumstellar dust to a distance of � 700 AU from the star. An � 300 AU radius outwardly flared disk inclined 50 � –55 � to the plane of the sky surrounded by a tenuous envelope is seen in the NICMOS images, confirming the size and suspected flared nature of the disk suggested by earlier CO and optical observations. The NICMOS images probe the disk region with spatial resolutions of � 0>1 at radial distances of 0>35 to � 4>5–5>0 from the largely unobscured (Av 3 from GM Aurigae is revealed in silhouette against the previously unseen ‘‘ lower ’’ portion of the illuminated disk along its minor axis (as projected onto the sky). We comment on surface brightness profiles along the disk major and minor axes, as well as isophotal maps of the disk. From these photometric data we have measured the integrated flux density of the disk, beyond the instrumental inner radius of 0>35, as 8.0 and 9.3 mJy (� 20%) at F110W and F160W, respectively, corresponding to disk scattering fractions of Ldisk/L * = 0.025 (� 20%) in both bands. By fitting the photometric properties of the disk to a scattered-light model, we estimate the disk mass to be � 0.04 M� . Additionally, we find two diffuse red polar lobes along the disk minor axis, likely the result of a bipolar outflow, at distances of � 3>8 from GM Aurigae (� 900 AU with our inferred inclination) with peak H-band surface brightnesses of � 14 lJy arcsec � 2 . We also note the existence of a broad (� 3 00 wide) band of blue material within the NICMOS field of view, spatially coincident with and extending at least 12 00 from the northeast outer region of the disk major axis.

Embedded Star Formation in the Eagle Nebula

M16=NGC 6611, the Eagle Nebula, is a well studied region of star formation and the source of a widely recognized Hubble Space Telescope (HST) image. High spatial resolution infrared observations with the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on HST reveal the detailed morphology of two embedded star formation regions that are heavily obscured at optical wavelengths. It is striking that only limited portions of the visually obscured areas are opaque at 2.2 microns. Although the optical images imply substantial columns of material, the infrared images show only isolated clumps of dense gas and dust. Rather than being an active factory of star production, only a few regions are capable of sustaining current star formation. Most of the volume in the columns may be molecular gas and dust, protected by capstones of dense dust. Two active regions of star formation are located at the tips of the optical northern and central large ``elephant trunk'' features shown in the WFPC2 images. They are embedded in two capstones of infrared opaque material that contains and trails behind the sources. Although the presence of these sources was evident in previous observations at the same and longer wavelengths, the NICMOS images provide a high resolution picture of their morphology. Two bright stars appear at the tip of the southern column and may be the result of recent star formation at the top of that column. These observations suggest that the epoch of star formation in M16 may be near its endpoint.

Ultraluminous Infrared Galaxies: Mergers of Sub‐L* Galaxies?

A sample of 27 low-redshift, mostly cool, ultraluminous infrared galaxies (ULIRGs) has been imaged at 1.6 μm with the Hubble Space Telescope (HST) Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). The majority (67%) of the sample's galaxies are multiple-nucleus galaxies with projected separations of up to 17 kpc, and the rest of the sample (33%) are single-nucleus galaxies, as determined by the NICMOS angular resolution limit. The average observed, integrated (host+nucleus) H magnitude of our HST H sample ULIRGs is -24.3, slightly above that of an L* galaxy (MH = -24.2), and 52% of the sample's galaxies have sub-L* luminosities. The ULIRGs in the HST H sample are not generated as a result of the merging of two luminous (i.e., ≥L*) spiral galaxies. Instead, the interactions and mergers occur in general between two, or in some cases more, less massive sub-L* (0.3-0.5L*) galaxies. Only one out of the 49 nuclei identified in the entire HST H sample has the properties of a bright quasar-like nucleus. On average, the brightest nuclei in the HST H sample galaxies (i.e., cool ULIRGs) are 1.2 mag fainter than warm ULIRGs and low-luminosity Bright Quasar Survey quasars (BQS QSOs) and 2.6 mag fainter than high-luminosity BQS QSOs. Since the progenitor galaxies involved in the merger are sub-L* galaxies, the mass of the central black hole in these ULIRGs would be only about (1-2) × 107 M☉, if the bulge-to-black hole mass ratio of nearby galaxies holds for ULIRGs. The estimated mass of the central black hole is similar to that of nearby Seyfert 2 galaxies but at least 1 order of magnitude lower than the massive black holes thought to be located at the center of high-luminosity QSOs. Massive nuclear starbursts with constant star formation rates of 10-40 M☉ yr-1 could contribute significantly to the nuclear H-band flux and are consistent with the observed nuclear H-band magnitudes of the ULIRGs in the HST H sample. An evolutionary merging scenario is proposed for the generation of the different types of ULIRGs and QSOs on the basis of the masses of the progenitors involved in the merging process. According to this scenario, cool ULIRGs would be the end product of the merging of two or more low-mass (0.3L*-0.5L*) disk galaxies. Warm ULIRGs and low-luminosity QSOs would be generated by a merger involving intermediate-mass (0.5 L*) disk galaxies. Under this scenario, warm ULIRGs could still be the dust-enshrouded phases of UV-bright low-luminosity QSOs, but cool ULIRGs, which are most ULIRGs, would not evolve into QSOs.

Near-Infrared Photometry of Globular Clusters NGC 6287 and NGC 6341 (M92): The Formation of the Galactic Halo

We present Hubble Space Telescope (HST) Near Infrared Camera and Multi-Object Spectrometer (NICMOS) camera 3 photometry of the inner halo globular cluster NGC 6287 and the intermediate halo globular cluster NGC 6341 (M92). Our photometric measurements range from the lower red giant branch to ≈3 mag below the main-sequence turnoff, allowing us to define accurate mean loci of the two clusters. Our relative age estimate between NGC 6287 and M92 using the color difference between the main-sequence turnoff and the base of the red giant branch shows that they essentially have the same ages within ±2 Gyr. The slightly different chemical compositions between the two clusters do not significantly influence our result. Our simulations using synthetic spectra show that the temperature dependence of the interstellar reddening law for the HST NICMOS photometric system could be a more serious problem in the relative age estimate using the color difference between the main-sequence turnoff and the base of the red giant branch. However, this effect adds no more than ±0.5–1 Gyr to the uncertainty in our relative age estimate. Our color-magnitude diagram of NGC 6287 shows a well-defined blue horizontal branch population. This is consistent with its age and metallicity. We also present the relative interstellar reddening toward NGC 6287 and distance modulus with respect to those of M92. We find E(B-V) = 0.62 and (m-M)0 = 14.35 for NGC 6287, which are generally in good agreement with previous estimates. Our photometry suggests that there exists a differential reddening across NGC 6287 with an amount of ΔE(B-V) = 0.07–0.09 mag, consistent with the previous study of Stetson & West. The Galactocentric distance of NGC 6287 becomes RGC = 1.6 kpc if R0 = 8.0 kpc. Based on our new radial velocity measurement for NGC 6287, the cluster's (minimum) apogalacticon distance is Ra ≈ 5.5 kpc, suggesting that NGC 6287 is an inner halo cluster. Within this framework, our results are consistent with the idea that the globular cluster formation must have been triggered everywhere at the same time in our Galaxy. In the appendices, we discuss that the interstellar extinction law of the HST NICMOS photometric system, particularly in the F110W passband, depends on the temperature of the stars in the sense that hotter stars suffer more interstellar absorption than cooler stars.

Homogenization of the Stellar Population along Late-Type Spiral Galaxies

We present a study of the broadband UBV color profiles for 257 Sbc barred and nonbarred galaxies, using photoelectric aperture photometry data from the literature. Using robust statistical methods, we have estimated the color gradients of the galaxies, as well as the total and bulge mean colors. A comparative photometric study using CCD images was done. In our sample, the color gradients are negative (reddish inward) in approximately 59% of the objects, are almost null in 27%, and are positive in 14%, considering only the face-on galaxies, which represent approximately 51% of the sample. The results do not change, essentially, when we include the edge-on galaxies. As a consequence of this study we have also found that barred galaxies are overrepresented among the objects having null or positive gradients, indicating that bars act as a mechanism of homogenization of the stellar population. This effect is more evident in the U-B color index, although it can also be detected in the B-V color. A correlation between the total and bulge colors was found that is a consequence of an underlying correlation between the colors of bulges and disks found by other authors. Moreover, the mean total color is the same irrespective of the gradient regime, while bulges are bluer in galaxies with null or positive gradients, which indicates an increase of the star formation rate in the central regions of these objects. We have also made a quantitative evaluation of the amount of extinction in the center of these galaxies. This was done using the Wide Field Planetary Camera 2 (WFPC2) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) Hubble Space Telescope (HST) archival data, as well as CCD B, V, and I images. We show that although the extinction in the V-band can reach values up to 2 mag in the central region, it is unlikely that dust plays a fundamental role in global color gradients. We found no correlation between color and O/H abundance gradients. This result could suggest that the color gradients are more sensitive to the age rather than to the metallicity of the stellar population. However, the absence of this correlation may be caused by dust extinction. We discuss this result by considering a picture in which bars are a relatively fast, recurrent phenomenon. These results are not compatible with a pure classical monolithic scenario for bulge and disk formation. On the contrary, they favor a scenario in which both these components are evolving in a correlated process in which stellar bars play a crucial role.

Ultraluminous Infrared Galaxies: Atlas of Near‐Infrared Images

A sample of 27 ultraluminous infrared galaxy (ULIRG) systems has been imaged at 1.6 μm using the Hubble Space Telescope (HST) Near Infrared Camera and Multi-Object Spectrometer (NICMOS). These ULIRGs are from a larger sample also imaged with HST in the I band. Images and catalog information for the NICMOS subsample, as well as brief morphological descriptions of each system are presented. Inspection of the infrared images and a comparison with optical images of these systems shows that at least 85% are obviously composed of two or more galaxies involved in a close interaction or merger event, with as many as 93% showing some signs of interaction history. Approximately 37% of the systems show either spectroscopic or morphological characteristics of an active galactic nucleus. The infrared morphologies of these systems are generally less complicated or disturbed than their optical morphologies, indicating that some of the small-scale features seen in optical images are likely due to complicated patterns of dust obscuration, as well as widely distributed star formation activity. In some systems the high-resolution HST infrared images have revealed nuclear remnants that are obscured or unidentified in ground-based imaging, which has led to changes in previously determined interaction stage classifications or system content. In general, however, the NICMOS images support previous conclusions from previous HST optical imaging.

The Stellar Content of the Super–Star Clusters in NGC 1569

We discuss HST Faint Object Spectrograph (FOS) ultraviolet spectroscopy and Near-Infrared Camera and Multiobject Spectrometer (NICMOS) photometry of four young super–star clusters in the central region of the irregular starburst galaxy NGC 1569. The new observations, coupled with previous HST WFPC2 photometry and ground-based optical spectroscopy, allow us to isolate and age-date the hot and cool stellar components of these massive clusters. We analyze the two components, A1 and A2, of the brightest super–star cluster NGC 1569-A. This cluster received previous attention owing to the simultaneous presence of Wolf-Rayet stars and red supergiants. The FOS spectra provide the first evidence for O stars in NGC 1569-A, indicating a young (≤5 Myr) stellar component in A1 and/or A2. Comparison with other high-mass star-forming regions suggests that the O and Wolf-Rayet stars are spatially coincident. If so, cluster A2 could be the host of the very young O and Wolf-Rayet population, and the somewhat older red supergiants could be predominantly located in A1. The mass-to-light ratio of NGC 1569-A1 is analyzed in five optical and infrared photometric bands and compared with evolutionary synthesis models. No indications for an anomalous initial mass function are found, consistent with a scenario where this cluster is the progenitor of present-day globular clusters. The clusters A1 and A2 are compared with clusters B and No. 30. The latter two clusters are older and fully dominated by red supergiants. All four super–star clusters provide a significant fraction (20%–25%) of the total optical and near-infrared light in the central region of the galaxy. The photometric properties of the super–star cluster population in NGC 1569 resemble those of the populous clusters in the Magellanic Clouds.

Properties of PACE‐I HgCdTe Detectors in Space: The NICMOS Warm‐Up Monitoring Program

ABSTRACTWe summarize the results of a monitoring program which was executed following the cryogen exhaustion of the Near Infrared Camera and Multi‐Object Spectrometer (NICMOS) on board the Hubble Space Telescope. During the subsequent warm‐up, detector parameters such as detective quantum efficiency, dark current, bias offsets, and saturation levels have been measured over the temperature range 62 K to about 100 K. The measurements provide a unique database of the characteristics of PACE‐I HgCdTe detector arrays in the space environment. A surprising result of the analysis is the fact that all three NICMOS detectors showed an enhanced dark current in the temperature range between 77 and 85 K. However, a subsequent laboratory experiment designed to replicate the on‐orbit warm‐up did not reproduce the anomaly, despite the fact that it employed a flight‐spare detector of the same pedigree. The mechanism behind the on‐orbit dark current anomaly is therefore believed to be unique to the space environment. We discuss possible explanations for these unexpected observational results, as well as their implications for future NICMOS operations.

The New Two-Image Gravitational Lens System CLASS B2319+051

We report the discovery of a new two-image gravitational lens system from the Cosmic Lens All-Sky Survey, CLASS B2319+051. Radio imaging with the Very Large Array (VLA) and Multi-Element Radio-Linked Interferometer Network (MERLIN) shows two compact components with a flux density ratio of 5:1, separated by 1.36 arcsec. Observations with the Very Long Baseline Array (VLBA) resolve each of the radio components into a pair of parity-reversed subcomponents. Hubble Space Telescope (HST) observations with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) show a bright elliptical galaxy (G1) coincident with the radio position, and a second irregular galaxy (G2) 3.4 arcsec to the northwest. Previous spectroscopic studies have indicated that these galaxies are at different redshifts: z(G1) = 0.624, z(G2) = 0.588. Infrared counterparts to the lensed radio components are not detected in the NICMOS image, and the source redshift has not yet been determined. Preliminary mass modeling based on the VLBA subcomponent data indicates that the lensing potential includes a strong external shear contribution. A VLA monitoring program is currently being undertaken to measure the differential time delay.

A Search for the Damped Lyα Absorber atz = 1.86 toward QSO 1244+3443 with NICMOS

We have carried out a high-resolution imaging search for the galaxy associated with the damped Lyα absorber (DLA) at z = 1.859 toward the zem = 2.48 quasar QSO 1244+3443, using the Hubble Space Telescope (HST) and the near-infrared camera and multiobject spectrometer (NICMOS). Images were obtained in the broad filter F160W and the narrow filter F187N with camera 2 on NICMOS with the goal of detecting the rest-frame optical continuum and the Hα line emission from the DLA. After point-spread function (PSF) subtraction, two weak features are seen at projected separations of 016-024 from the quasar. Parts of these features may be associated with the DLA, although we cannot completely rule out that they could be artifacts of the PSF. If associated with the DLA, the objects would be ≈1-2 h kpc in size with integrated flux densities of 2.5 and 3.3 μJy in the F160W filter, implying luminosities at λcentral = 5600 A in the DLA rest frame of 4.4-5.9 × 109 h L☉ at z = 1.86, for q0 = 0.5. However, no significant Hα line emission is seen from these objects, suggesting low star formation rates (SFRs). Our 3 σ upper limit on the SFR in the DLA is 1.3 h-270 M☉ yr-1 for q0 = 0.5 (2.4 h-270 M☉ yr-1 for q0 = 0.1). This together with our earlier result for LBQS 1210 + 1731 mark a significant improvement over previous constraints on the star formation rates of DLAs. Dust within the DLA could extinguish Hα emission, but this would require the dust content in the DLA to be much higher than that inferred from previous DLA observations. A combination of low star formation rate and some dust extinction is likely to be responsible for the lack of Hα emission. Alternatively, the objects, if real, may be associated with the host galaxy of the quasar rather than with the DLA. In any case, our observations suggest that the DLA is not a large bright protodisk, but a compact object or a low surface brightness galaxy. If the two features are PSF artifacts or associated with the quasar host, then the constraints on the size and luminosity of the DLA are even more severe.

The Cuspy Liner Nucleus of the S0/a Galaxy NGC 2681

The nucleus of the bulge-dominated, multiply barred S0/a galaxy NGC 2681 is studied in detail using the high-resolution Hubble Space Telescope Faint Object Camera (FOC), Near-Infrared Camera and Multiobject Spectrometer (NICMOS) imaging, and the Faint Object Spectrograph (FOS). The ionized gas central velocity dispersion is found to increase by a factor ≈2 when narrowing the aperture from R ≈ 15 (ground) to R ≈ 01 (FOS). Dynamical modeling of these velocity dispersions suggests that NGC 2681 does host a supermassive black hole (BH) for which one can estimate a firm mass upper limit MBH ≲ 6 × 107 M☉. This upper limit is consistent with the relation between the central BH mass and velocity dispersion MBH - σ known for other galaxies. The emission-line ratios place the nucleus of NGC 2681 among LINERs. It is likely that the emission-line region comes from a rather mild, but steady, feeding of gas to the central BH in this galaxy. The inner stellar population lacks any measurable color gradient (to a radius of 0.6 kpc) from the infrared to the ultraviolet, consistently with FOC, FOS, and IUE data, all indicating that this system underwent a starburst ≈1 Gyr ago that encompassed its whole interior, down to its very center. The most likely source of such a widely distributed starburst is the dumping of tidally extruded gas from a galaxy neighbor. If so, then NGC 2681 can be considered as the older brother of M82, seen face-on as opposed to the edge-on view we have for M82.

Stellar Variability in the Metal‐rich, Obscured Globular Cluster Terzan 5

We present the results of a search for variability in and near the core of the metal-rich, obscured globular cluster Terzan 5, using the near-infrared camera and multiobject spectrometer (NICMOS) on board the Hubble Space Telescope. This extreme cluster has approximately solar metallicity and a central density that places it in the upper few percent of all clusters. It is estimated to have the highest interaction rate of any galactic globular cluster. The large extinction toward Terzan 5 and the severe stellar crowding near the cluster center present a substantial observational challenge. Using time-series analysis, we discovered two variable stars in this cluster. The first is an RRab Lyrae variable with a period of ~0.61 days, a longer period than that of field stars with similar high metallicities. This period is shorter, however, than the average periods of RR Lyraes found in the metal-rich globular clusters NGC 6441, NGC 6388, and 47 Tuc. The second variable is a blue star with a 7 hr period sinusoidal variation and a likely orbital period of 14 hr. This star is probably an eclipsing blue straggler or (less likely) the infrared counterpart to the low-mass X-ray binary known in Terzan 5. Because of the extreme crowding and overlapping Airy profile of the IR point-spread function, we fall short of our original goal of detecting cataclysmic variables via Paα emission and detecting variable infrared emission from the location of the binary millisecond pulsar in Terzan 5.

Hubble Space TelescopeInfrared Imaging Polarimetry of Centaurus A: Implications for the Unified Scheme and the Existence of a Misdirected BL Lacertae Nucleus

We report results from the Hubble Space Telescope (HST)/near-infrared camera and multiobject spectrometer (NICMOS) 2 μm imaging polarimetry of the central region of Centaurus A. In the vicinity of the nucleus we observe a complex polarization structure, which we explain by a combination of scattering of nuclear light and dichroic polarization associated with the dust lane. The scattered nuclear radiation is found in an angular region that extends over 70°, and thus it does not originate from a highly collimated beam but is associated with more omnidirectional nuclear illumination. These observations also show the presence of an unresolved, highly polarized (P = 11.1%) nuclear source whose polarization angle θ = 1482 is perpendicular to the jet axis. We set an upper limit of 004 (~0.8 pc) to its extent. The observed nuclear polarization is naturally accounted for if we are observing scattered light from an otherwise obscured nucleus provided that both the scattering region and the occulting torus are extremely compact, with an outer radius of less than ~1 pc. Alternatively, we might be directly seeing the infrared counterpart of the radio core, similar to those found in other low-luminosity radio galaxies observed with HST. We discuss these results in the framework of the FR I/BL Lac unifying model.

The Very Low Mass Component of the Gliese 105 System

Multiple-epoch, multicolor images of the astrometric binary Gliese 105A and its very low mass companion Gliese 105C have been obtained using the Hubble Space Telescope's Wide Field Planetary Camera 2 (WFPC2) and near-infrared camera and multiobject spectrometer (NICMOS). The optical and near-infrared colors of Gl 105C strongly suggest a spectral type of M7 V for that star. Relative astrometric measurements spanning 3 yr reveal the first evidence of Gl 105C's orbital motion. Previous long-term astrometric studies at Sproul and McCormick Observatories have shown that the period of Gl 105A's perturbation is ~60 yr. To satisfy both the observed orbital motion and Gl 105A's astrometric period, Gl 105C's orbit must have an eccentricity of ~0.75 and a semimajor axis of ~15 AU. Measurements of Gl 105A's radial velocity over 12 yr show a linear trend with a slope of 11.3 m s-1 yr-1, which is consistent with these orbital constraints and a nearly face-on orbit. As no other faint companions to Gl 105A have been detected, we conclude that Gl 105C is probably the source of the 60 yr astrometric perturbation.

The Initial Mass Function of Low‐Mass Stars and Brown Dwarfs in Young Clusters

We have obtained images of the Trapezium Cluster (140'' × 140''; 0.3 pc × 0.3 pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Combining these data with new ground-based K-band spectra (R = 800) and existing spectral types and photometry, we have constructed an H-R diagram and used it and other arguments to infer masses and ages. To allow comparison with the results of our previous studies of IC 348 and ρ Oph, we first use the models of D'Antona & Mazzitelli. With these models, the distributions of ages of comparable samples of stars in the Trapezium, ρ Oph, and IC 348 indicate median ages of ~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass initial mass functions (IMFs) in these sites of clustered star formation are similar over a wide range of stellar densities (ρ Oph, n = 0.2-1 × 103 pc-3; IC 348, n = 1 × 103 pc-3; Trapezium, n = 1-5 × 104 pc-3) and other environmental conditions (e.g., presence or absence of OB stars). With current data, we cannot rule out modest variations in the substellar mass functions among these clusters. We then make the best estimate of the true form of the IMF in the Trapezium by using the evolutionary models of Baraffe et al. and an empirically adjusted temperature scale and compare this mass function to recent results for the Pleiades and the field. All of these data are consistent with an IMF that is flat or rises slowly from the substellar regime to about 0.6 M☉ and then rolls over into a power law that continues from about 1 M☉ to higher masses with a slope similar to or somewhat larger than the Salpeter value of 1.35. For the Trapezium, this behavior holds from our completeness limit of ~0.02 M☉ and probably, after a modest completeness correction, even from 0.01-0.02 M☉. These data include ~50 likely brown dwarfs. We test the predictions of theories of the IMF against (1) the shape of the IMF, which is not log-normal, in clusters and the field, (2) the similarity of the IMFs among young clusters, (3) the lowest mass observed for brown dwarfs, and (4) the suggested connection between the stellar IMF and the mass function of prestellar clumps. In particular, most models do not predict the formation of the moderately large numbers of isolated objects down to 0.01 M☉ that we find in the Trapezium.

Revealing the Photodissociation Region:HST/NICMOS Imaging of NGC 7027

We report results from a Hubble Space Telescope (HST) and Near-Infrared Camera and Multiobject Spectrometer (NICMOS) program to study the distribution of hot neutral (molecular hydrogen) and ionized circumstellar material in the young planetary nebulae NGC 7027. HST/NICMOS provided very high spatial resolution imaging in line and continuum emission, and the stability and large dynamic range needed for investigating detailed structures in the circumstellar material. We present dramatic new images of NGC 7027 that have led to a new understanding of the structure in this important planetary nebula. The central star is clearly revealed, providing near-infrared fluxes that are used to directly determine the stellar temperature very accurately (T = 198,000 K). It is found that the photodissociation layer as revealed by near-infrared molecular hydrogen emission is very thin (ΔR ~ 6 × 1015 cm) and is biconical in shape. The interface region is structured and filamentary, suggesting the existence of hydrodynamic instabilities. We discuss evidence for the presence of one or more highly collimated, off-axis jets that might be present in NGC 7027. The NICMOS data are combined with earlier Hubble Space Telescope data to provide a complete picture of NGC 7027 using the highest spatial resolution data to date. The evolutionary future of NGC 7027 is discussed.

CLASS B0739+366: A New Two-Image Gravitational Lens System

We present the discovery of CLASS B0739 + 366, a new gravitational lens system from the Cosmic Lens All-Sky Survey. Radio imaging of the source with the Very Large Array shows two compact components separated by with a flux density ratio of similar to6:1. High-resolution follow-up observations using the Very Long Baseline Array (VLBA) at 1.7 GHz detect weak, parity-reversed jet emission from each of the radio components. Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) F160W observations detect infrared counterparts to the lensed images, as well as an extended object between them, which we identify as the lensing galaxy. Redshifts for the galaxy and lensed source have not yet been obtained. For typical lens and source redshifts of z = 0.5 and z = 1.5, respectively, preliminary mass modeling predicts a time delay of similar to7 h(-1) days in a flat Omega (M) = 1.0 verse. The small predicted time delay and weak radio components will make CLASS B0739 + 366 a challenging target for Hubble constant determination.

Extremely Red Objects from the [ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] NICMOS Parallel Imaging Survey

We present a catalog of extremely red objects (EROs) discovered using the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) parallel imaging database and ground-based optical follow-up observations. Within an area of 16 arcmin^2, we detect 15 objects with R-F160W > 5 and F160W 6. Our objects have F110W-F160W colors in the range 1.3–2.1, redder than the cluster elliptical galaxies at z ~ 0.8 and nearly 1 mag redder than the average population selected from the F160W images at the same depth. In addition, among only 22 NICMOS pointings, we detected two groups or clusters in two fields; each contains three or more EROs, suggesting that extremely red galaxies may be strongly clustered. At bright magnitudes with F160W < 19.5, the ERO surface density is similar to what has been measured by other surveys. At the limit of our sample, F160W = 21.5, our measured surface density is 0.94 ± 0.24 arcmin^(-2). Excluding the two possible groups or clusters and the one apparently stellar object reduces the surface density to 0.38 ± 0.15 arcmin^(-2).