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Register a new userGalactic Bulges from HST NICMOS Observations: central galaxian objects, and nuclear profile slopes
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M. Balcells
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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.
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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 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.
We present a catalog of extremely red objects discovered using the NICMOS/HST parallel imaging database and ground-based optical follow-up observations. Within an area of 16 square arc-minutes, we detect 15 objects with $rm R - F160W > 5$ and $rm F160W < 21.5$. We have also obtained K-band photometry for a subset of the 15 EROs. All of the $rm R - F160W$ selected EROs imaged at K-band have $rm R - K > 6$. Our objects have $rm F110W - F160W$ colors in the range of 1.3 - 2.1, redder than the cluster ellipticals at $z sim 0.8$ and nearly 1 magnitude 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 3 or more EROs, suggesting that extremely red galaxies may be strongly clustered. At bright magnitudes with $rm F160W < 19.5$, 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$pm 0.24$ arcmin^{-2}. Excluding the two possible groups/clusters and the one apparently stellar object, reduces the surface density to 0.38$pm 0.15$ arcmin^{-2}.
We present an analysis of NICMOS photometry and low-resolution grism spectroscopy of low-mass stars and sub-stellar objects in the young star-forming region NGC 1333. Our goal is to constrain the ratio of low-mass stars to sub- stellar objects down to 20 Mjup in the cluster as well as constrain the cluster IMF down to 30 Mjup in combination with a previous survey of NGC 1333 by Wilking et al. Our survey covers 4 fields of 51.2 x 51.2, centered on brown dwarf candidates previously identified in Wilking et al. We extend previous work based on the use of a water vapor index for spectral typing to wavelengths accessible with NICMOS on the HST. Spectral types were derived for the 14 brightest objects in our fields, ranging from <M0 - M8, which at the age of the cluster (0.3 Myr) corresponds to a range in mass of >0.25 - 0.02 Msun. In addition to the spectra, we present an analysis of the color-magnitude diagram using pre-main sequence evolutionary models of DAntona & Mazzitelli. Using an extinction-limited sample, we derive the ratio of low-mass stars to brown dwarfs. Comparisons of the observed ratio to that expected from the field IMF of Chabrier indicate that the two results are consistent. We combine our data with that of Wilking et al. to compute the ratio of intermediate-mass stars (0.1 - 1.0 Msun) to low-mass objects (0.03 - 0.1 Msun) in the cluster. We also report the discovery of a faint companion to the previously confirmed brown dwarf ASR 28, as well as a possible outflow surrounding ASR 16. If the faint companion is confirmed as a cluster member, it would have a mass of ~ 5 Mjup (mass ratio 0.15) at a projected distance of 350 AU, similar to 2MASS 1207-3923 B.
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