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Register a new userGalactic bulges from Hubble Space Telescope NICMOS observations: the lack of r^{1/4} bulges
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We use HST near-infrared imaging to explore the shapes of the surface brightness profiles of bulges of S0-Sbc galaxies at high resolution. Modeling extends to the outer bulge via bulge-disk decompositions of combined HST - ground based profiles. Compact, central unresolved components similar to those reported by others are found in ~84% of the sample. We also detect a moderate frequency (~34%) of nuclear components with exponential profiles which may be disks or bars. Adopting the Sersic r^{1/n} functional form for the bulge, none of the bulges have an r^{1/4} behaviour; derived Sersic shape-indices are <n> = 1.7 pm 0.7. For the same sample, fits to NIR ground-based profiles yield Sersic indices up to n = 4-6. The high-$n$ of ground-based profiles are a result of nuclear point sources blending with the bulge extended light due to seeing. The low Sersic indices are not expected from merger violent relaxation, and argue against significant merger growth for most bulges.
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We investigate bulge and disk scaling relations using a volume-corrected sample of early- to intermediate-type disk galaxies in which, importantly, the biasing flux from additional nuclear components has been modeled and removed. Structural parameters are obtained from a seeing-convolved, bulge+disk+nuclear-component decomposition applied to near-infrared surface brightness profiles spanning ~10 pc to the outer disk. Bulge and disk parameters, and bulge-to-disk ratios, are analyzed as a function of bulge luminosity, disk luminosity, galaxy central velocity dispersion, and galaxy Hubble type. Mathematical expressions are given for the stronger relations, which can be used to test and constrain galaxy formation models. Photometric parameters of both bulges and disks are observed to correlate with bulge luminosity and with central velocity dispersion. In contrast, for the unbarred, early to intermediate types covered by the sample, Hubble type does not correlate with bulge and disk components, nor their various ratios. In this sense, the early-to-intermediate spiral Hubble sequence is scale-free. However, galaxies themselves are not scale-free, the critical scale being the luminosity of the bulge. Bulge luminosity is shown to affect the disk parameters, such that central surface brightness becomes fainter, and scale-length bigger, with bulge luminosity. The lack of significant correlations between bulge pararmeters (size, luminosity or density) on disk luminosity, remains a challenge for secular evolution models of bulge growth.
We have measured the central structural properties for a sample of S0-Sbc galaxies down to scales of ~10 pc using HST NICMOS images. We find that the photometric masses of the central star clusters, which occur in 58% of our sample, are related to their host bulge masses such that MassPt = 10^{7.75pm0.15}(MassBul/10^{10}MassSun)^{0.76pm 0.13}. Put together with recent data on bulges hosting supermassive black holes, we infer a non-linear dependency of the `Central Massive Object mass on the host bulge mass such that MassCMO = 10^{7.51pm 0.06} (MassBul/10^{10}MassSun)^{0.84 pm 0.06}. We argue that the linear relation presented by Ferrarese et al. is biased at the low-mass end by the inclusion of the disc light from lenticular galaxies in their sample. Matching our NICMOS data with wider-field, ground-based K-band images enabled us to sample from the nucleus to the disk-dominated region of each galaxy, and thus to perform a proper bulge-disk decomposition. We found that the majority of our galaxies (~90%) possess central light excesses which can be modeled with an inner exponential and/or an unresolved point source in the case of the nuclear star clusters. All the extended nuclear components, with sizes of a few hundred pc, have disky isophotes, which suggest that they may be inner disks, rings, or bars; their colors are redder than those of the underlying bulge, arguing against a recent origin for their stellar populations. Surface brightness profiles rise inward to the resolution limit of the data, with a continuous distribution of logarithmic slopes from the low values typical of dwarf ellipticals (0.1 leq gamma leq 0.3) to the high values (gamma ~ 1) typical of intermediate luminosity ellipticals; the nuclear slope bi-modality reported by others is not present in our sample.
We present 2micron polarization measurements of positions in the BN region of the Orion Molecular Cloud (OMC-1) made with NICMOS Camera 2 (0.2 resolution) on HST. Our results are as follows: BN is sim 29% polarized by dichroic absorption and appears to be the illuminating source for most of the nebulosity to its north and up to sim 5 to its south. Although the stars are probably all polarized by dichroic absorption, there are a number of compact, but non-point-source, objects that could be polarized by a combination of both dichroic absorption and local scattering of star light. We identify several candidate YSOs, including an approximately edge-on bipolar YSO 8.7 east of BN, and a deeply-embedded variable star. Additional strongly polarized sources are IRc2-B, IRc2-D, and IRc7, all of which are obviously self-luminous at mid-infrared wavelengths and may be YSOs. None of these is a reflection nebula illuminated by a star located near radio source I, as was previously suggested. Other IRc sources are clearly reflection nebulae: IRc3 appears to be illuminated by IRc2-B or a combination of the IRc2 sources, and IRc4 and IRc5 appear to be illuminated by an unseen star in the vicinity of radio source I, or by Star n or IRc2-A. Trends in the magnetic field direction are inferred from the polarization of the 26 stars that are bright enough to be seen as NICMOS point sources. The most polarized star has a polarization position angle different from its neighbors by sim 40^o, but in agreement with the grain alignment inferred from millimeter polarization measurements of the cold dust cloud in the southern part of OMC-1.
Massive young stellar objects (YSOs), like low-mass YSOs, appear to be surrounded by optically thick envelopes and/or disks and have regions, often bipolar, that are seen in polarized scattered light at near-infrared wavelengths. We are using the 0.2 spatial resolution of NICMOS on Hubble Space Telescope to examine the structure of the disks and outflow regions of massive YSOs in star-forming regions within a few kpc of the Sun. Here we report on 2 micron polarimetry of NGC 6334 V and S255 IRS1. NGC 6334 V consists of a double-lobed bright reflection nebula seen against a dark region, probably an optically thick molecular cloud. Our polarization measurements show that the illuminating star lies ~ 2 south of the line connecting the two lobes; we do not detect this star at 2 micron, but there are a small radio source and a mid-infrared source at this location. S255 IRS1 consists of two YSOs (NIRS1 and NIRS3) with overlapping scattered light lobes and luminosities corresponding to early B stars. Included in IRS1 is a cluster of stars from whose polarization we determine the local magnetic field direction. Neither YSO has its scattered light lobes aligned with this magnetic field. The line connecting the scattered light lobes of NIRS1 is twisted symmetrically around the star; the best explanation is that the star is part of a close binary and the outflow axis of NIRS1 is precessing as a result of non-coplanar disk and orbit. The star NIRS3 is also offset from the line connecting its two scattered light lobes. We suggest that all three YSOs show evidence of episodic ejection of material as they accrete from dense, optically thick envelopes.
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