59 research outputs found
Dust Abundance and Properties in the Nearby Dwarf Galaxies NGC 147 and NGC 185
We present new mid- to far-infrared images of the two dwarf compact elliptical galaxies that are satellites of M31, NGC 185, and NGC 147, obtained with the Spitzer Space Telescope. Spitzer's high sensitivity and spatial resolution enable us for the first time to look directly into the detailed spatial structure and properties of the dust in these systems. The images of NGC 185 at 8 and 24 μm display a mixed morphology characterized by a shell-like diffuse emission region surrounding a central concentration of more intense infrared emission. The lower resolution images at longer wavelengths show the same spatial distribution within the central 50" but beyond this radius, the 160 μm emission is more extended than that at 24 and 70 μm. On the other hand, the dwarf galaxy NGC 147, located only a small distance away from NGC 185, shows no significant infrared emission beyond 24 μm and therefore its diffuse infrared emission is mainly stellar in origin. For NGC 185, the derived dust mass based on the best fit to the spectral energy distribution is 1.9 × 10^3 M_⊙, implying a gas mass of 3.0 × 10^5 M_⊙. These values are in agreement with those previously estimated from infrared as well as CO and H I observations and are consistent with the predicted mass return from dying stars based on the last burst of star formation 1 × 10^9 yr ago. Based on the 70-160 μm flux density ratio, we estimate a temperature for the dust of ~17 K. For NGC 147, we obtain an upper limit for the dust mass of 4.5 × 10^2 M_⊙ at 160 μm (assuming a temperature of ~20 K), a value consistent with the previous upper limit derived using Infrared Space Observatory observations of this galaxy. In the case of NGC 185, we also present full 5-38 μm low-resolution (R ~ 100) spectra of the main emission regions. The Infrared Spectrograph spectra of NGC 185 show strong polycyclic aromatic hydrocarbons emission, deep silicate absorption features and H_2 pure rotational line ratios consistent with having the dust and molecular gas inside the dust cloud being impinged by the far-ultraviolet radiation field of a relatively young stellar population. Therefore, based on its infrared spectral properties, NGC 185 shows signatures of recent star formation (a few ×10^8 yr ago), although its current star formation rate is quite low
Infrared signature of active massive black holes in nearby dwarf galaxies
We investigate the possible presence of active galactic nuclei (AGN) in dwarf
galaxies and other nearby galaxies to identify candidates for follow-up
confirmation and dynamical mass measurements. We use the Wide-field Infrared
Survey Explorer (WISE) All-Sky Release Source Catalog and examine the infrared
colours of a sample of dwarf galaxies and other nearby galaxies in order to
identify both unobscured and obscured candidate AGN by applying the infrared
colour diagnostic. Stellar masses of galaxies are obtained using a combination
of three independent methods. Black hole masses are estimated using the
bolometric luminosity of the AGN candidates and computed for three cases of the
bolometric-to-Eddington luminosity ratio. We identify 303 candidate AGN, of
which 276 were subsequently found to have been independently identified as AGN
via other methods. The remaining 9% require follow-up observations for
confirmation. The activity is detected in galaxies with stellar masses from ~
10^6 to 10^9 solar masses; assuming the candidates are AGN, the black hole
masses are estimated to be ~ 10^3 - 10^6 solar masses, adopting L_bol = 0.1
L_Edd. The black hole masses probed are several orders of magnitude smaller
than previously reported for centrally located massive black holes. We examine
the stellar mass versus black hole mass relationship in this low galaxy mass
regime. We find that it is consistent with the existing relation extending
linearly (in log-log space) into the lower mass regime. These findings suggest
that CMBH are present in low-mass galaxies and in the Local Universe, and
provide new impetus for follow-up dynamical studies of quiescent black holes in
local dwarf galaxies.Comment: 14 pages, 9 figures, 4 tables, accepted for publication in Astronomy
& Astrophysics. The first version of this paper appeared on astro-ph in 2014
(arXiv:1411.3844). The new version includes 3 new tables, 1 new figure and
updated discussio
Galaxy Evolution in the Cluster Abell 85: New Insights from the Dwarf Population
We present the first results of a new spectroscopic survey of the cluster
Abell 85 targeting 1466 candidate cluster members within the central 1
deg of the cluster and having magnitudes using VIMOS/VLT and
HYDRA/WIYN. A total of 520 galaxies are confirmed as either relaxed cluster
members or part of an infalling population. A significant fraction are low
mass; the median stellar mass of the sample is , and 25%
have stellar masses below (i.e. 133 dwarf galaxies). We also
identify seven active galactic nuclei (AGN), four of which reside in dwarf host
galaxies. We probe the evolution of star formation rates, based on H
emission and continuum modeling, as a function of both mass and environment. We
find that more star forming galaxies are observed at larger clustercentric
distances, while infalling galaxies show evidence for recently enhanced star
forming activity. Main sequence galaxies, defined by their continuum star
formation rates, show different evolutionary behavior based on their mass. At
the low mass end, the galaxies have had their star formation recently quenched,
while more massive galaxies show no significant change. The timescales probed
here favor fast quenching mechanisms, such as ram-pressure stripping. Galaxies
within the green valley, defined similarly, do not show evidence of quenching.
Instead, the low mass galaxies maintain their levels of star forming activity,
while the more massive galaxies have experienced a recent burst.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Starburst Galaxies in Cluster-feeding Filaments Unveiled by Spitzer
We report the first direct detection with Spitzer of galaxy filaments. Using Spitzer and ancillary optical data, we have discovered two filamentary structures in the outskirts of the cluster Abell 1763. Both filaments point toward Abell 1770, which lies at the same redshift as Abell 1763 (z = 0.23), at a projected distance of ~13 Mpc. The X-ray cluster emission is elongated along the same direction. Most of the far-infrared emission is powered by star formation. According to the optical spectra, only one of the cluster members is classified as an active galactic nucleus. Star formation is clearly enhanced in galaxies along the filaments: the fraction of starburst galaxies in the filaments is more than twice than that in other cluster regions. We speculate that these filaments are feeding the cluster Abell 1763 by the infall of galaxies and galaxy groups. Evidence for one of these groups is provided by the analysis of galaxy kinematics in the central cluster region
Point Source Extraction with MOPEX
MOPEX (MOsaicking and Point source EXtraction) is a package developed at the
Spitzer Science Center for astronomical image processing. We report on the
point source extraction capabilities of MOPEX. Point source extraction is
implemented as a two step process: point source detection and profile fitting.
Non-linear matched filtering of input images can be performed optionally to
increase the signal-to-noise ratio and improve detection of faint point
sources. Point Response Function (PRF) fitting of point sources produces the
final point source list which includes the fluxes and improved positions of the
point sources, along with other parameters characterizing the fit. Passive and
active deblending allows for successful fitting of confused point sources.
Aperture photometry can also be computed for every extracted point source for
an unlimited number of aperture sizes. PRF is estimated directly from the input
images. Implementation of efficient methods of background and noise estimation,
and modified Simplex algorithm contribute to the computational efficiency of
MOPEX. The package is implemented as a loosely connected set of perl scripts,
where each script runs a number of modules written in C/C++. Input parameter
setting is done through namelists, ASCII configuration files. We present
applications of point source extraction to the mosaic images taken at 24 and 70
micron with the Multiband Imaging Photometer (MIPS) as part of the Spitzer
extragalactic First Look Survey and to a Digital Sky Survey image. Completeness
and reliability of point source extraction is computed using simulated data.Comment: 20 pages, 13 Postscript figures, accepted for publication in PAS
Rethinking the Model of Moving Cosmic Strings Producing Anisotropy in the Microwave Background
We re-analyze the issue of redshifts induced by a moving cosmic string by
looking at moving sources and observers on a conical spacetime in a fully
relativistic approach. By replacing the concept of a moving spacetime with the
more clearly defined concept of moving sources and observers in the string
spacetime, we show that there is no effect: the only redshift is a Doppler
shift due to the motion of the source or observer.Comment: 11 pages, LaTex file, 1 figure available in PostScript format at
ftp://manitou.astro.utoronto.ca/pub/string/ (get fig1_mdp.ps
Seeking the Ultraviolet Ionizing Background at z~3 with the Keck Telescope
We describe the initial results of a deep long-slit emission line search for
redshifted (2.7<z<4.1) Lyman-alpha. These observations are used to constrain
the fluorescent Ly-alpha emission from the population of clouds whose
absorption produces the higher-column-density component of the Ly-alpha forest
in quasar spectra. We use the results to set an upper limit on the ultraviolet
ionizing background. Our spectroscopic data obtained with the Keck II telescope
at lambda/(Delta lambda FWHM)~2000 reveals no candidate Ly-alpha emission over
the wavelength range of 4500-6200 Ang along a 3 arcmin slit in a 5400 s
integration. Our 3 sigma upper bound on the mean intensity of the ionizing
background at the Lyman limit is J(nu 0) < 2E-21 erg/s/cm**2/Hz/sr for
2.7<z<3.1 (where we are most sensitive), assuming Lyman limit systems have
typical radii of 70 kpc (q_0=0.5, H_0=50 km/s/Mpc). This constraint is more
than an order of magnitude more stringent than any previously published direct
limit. However, it is still a factor of three above the ultraviolet background
level expected due to the integrated light of known quasars at z~3. This pilot
study confirms the conclusion of Gould \& Weinberg (1996) that integrations of
several hours on a 10-m class telescope should be capable of measuring J(nu 0)
at high redshift.Comment: 22 pages, 2 postscipt figures. Latex requires aaspp4.sty and epsf.sty
(included). Accepted for publication in the Astronomical Journal (Nov 1998
Quantitative Morphology of Galaxies in the Hubble Deep Field
We measure quantitative structural parameters of galaxies in the Hubble Deep
Field (HDF) on the drizzled F814W images. Our structural parameters are based
on a two-component surface brightness made up of a S\'ersic profile and an
exponential profile. We compare our results to the visual classification of van
den Bergh et al. (1996) and the classification of Abraham et al. (1996a).
Our morphological analysis of the galaxies in the HDF indicates that the
spheroidal galaxies, defined here as galaxies with a dominant bulge profile,
make up for only a small fraction, namely 8% of the galaxy population down to
m = 26.0. We show that the larger fraction of early-type systems
in the van den Bergh sample is primarily due to the difference in
classification of 40% of small round galaxies with half-light radii <
0\arcsecpoint 31. Although these objects are visually classified as elliptical
galaxies, we find that they are disk-dominated with bulge fractions < 0.5.
Given the existing large dataset of HDF galaxies with measured spectroscopic
redshifts, we are able to determine that the majority of distant galaxies
() from this sample are disk-dominated. Our analysis reveals a subset of
HDF galaxies which have profiles flatter than a pure exponential profile.Comment: 35 pages, LaTeX, 18 Postscript Figures, Tables available at
http://astro.berkeley.edu/~marleau/. Accepted for Publication in The
Astrophysical Journa
The Nature of the Halo Population of NGC 5128 Resolved with NICMOS on the Hubble Space Telescope
We present the first infrared color-magnitude diagram (CMD) for the halo of a
giant elliptical galaxy. The CMD for the stars in the halo of NGC 5128
(Centaurus A) was constructed from HST NICMOS observations of the WFPC2 CHIP-3
field of Soria et al. (1996) to a 50% completeness magnitude limit of
[F160W]=23.8. This field is located at a distance of 08'50" (~9 kpc) south of
the center of the galaxy. The luminosity function (LF) shows a marked
discontinuity at [F160W]=20.0. This is 1-2 mag above the tip of the red giant
branch (TRGB) expected for an old population (~12 Gyr) at the distance modulus
of NGC 5128. We propose that the majority of stars above the TRGB have
intermediate ages (~2 Gyr), in agreement with the WFPC2 observations of Soria
et al. (1996). Five stars with magnitudes brighter than the LF discontinuity
are most probably due to Galactic contamination. The weighted average of the
mean giant branch color above our 50% completeness limit is
[F110W]-[F160W]=1.22+-0.08 with a dispersion of 0.19 mag. From our
artificial-star experiments we determine that the observed spread in color is
real, suggesting a real spread in metallicity. We estimate the lower and upper
bounds of the stellar metallicity range by comparisons with observations of
Galactic star clusters and theoretical isochrones. Assuming an old population,
we find that, in the halo field of NGC 5128 we surveyed, stars have
metallicities ranging from roughly 1% of solar at the blue end of the color
spread to roughly solar at the red end, with a mean of [Fe/H]=-0.76 and a
dispersion of 0.44 dex.Comment: Accepted for publication in AJ, 23 pages of text, 13 figures, uses
aastex v5.
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