671 research outputs found
Enhanced dust heating in the bulges of early-type spiral galaxies
Stellar density and bar strength should affect the temperatures of the cool (T ~ 20–30 K) dust component in the inner regions of galaxies, which implies that the ratio of temperatures in the circumnuclear regions to the disk should depend on Hubble type. We investigate the differences between cool dust temperatures in the central 3 kpc and disk of 13 nearby galaxies by fitting models to measurements between 70 and 500 μm. We attempt to quantify temperature trends in nearby disk galaxies, with archival data from Spitzer/MIPS and new observations with Herschel/SPIRE, which were acquired during the first phases of the Herschel observations for the KINGFISH (Key Insights on Nearby Galaxies: a Far-Infrared Survey with Herschel) sample. We fit single-temperature modified blackbodies to far-infrared and submillimeter measurements of the central and disk regions of galaxies to determine the temperature of the component(s) emitting at those wavelengths. We present the ratio of central-region-to-disk-temperatures of the cool dust component of 13 nearby galaxies as a function of morphological type. We find a significant temperature gradient in the cool dust component in all galaxies, with a mean center-to-disk temperature ratio of 1.15 ± 0.03. The cool dust temperatures in the central ~3 kpc of nearby galaxies are 23 (±3)% hotter for morphological types earlier than Sc, and only 9 (±3)% hotter for later types. The temperature ratio is also correlated with bar strength, with only strongly barred galaxies having a ratio over 1.2. The strong radiation field in the high stellar density of a galactic bulge tends to heat the cool dust component to higher temperatures, at least in early-type spirals with relatively large bulges, especially when paired with a strong bar
The Identification of Extreme Asymptotic Giant Branch Stars and Red Supergiants in M33 by 24 {\mu}m Variability
We present the first detection of 24 {\mu}m variability in 24 sources in the
Local Group galaxy M33. These results are based on 4 epochs of MIPS
observations, which are irregularly spaced over ~750 days. We find that these
sources are constrained exclusively to the Holmberg radius of the galaxy, which
increases their chances of being members of M33. We have constructed spectral
energy distributions (SEDs) ranging from the optical to the sub-mm to
investigate the nature of these objects. We find that 23 of our objects are
most likely heavily self-obscured, evolved stars; while the remaining source is
the Giant HII region, NGC 604. We believe that the observed variability is the
intrinsic variability of the central star reprocessed through their
circumstellar dust shells. Radiative transfer modeling was carried out to
determine their likely chemical composition, luminosity, and dust production
rate (DPR). As a sample, our modeling has determined an average luminosity of
(3.8 0.9) x 10 L and a total DPR of (2.3 0.1) x
10 M yr. Most of the sources, given the high DPRs and
short wavelength obscuration, are likely "extreme" AGB (XAGB) stars. Five of
the sources are found to have luminosities above the classical AGB limit
(M 54,000 L), which classifies them as probably
red supergiants (RSGs). Almost all of the sources are classified as oxygen
rich. As also seen in the LMC, a significant fraction of the dust in M33 is
produced by a handful of XAGB and RSG stars.Comment: 36 pages, 14 figures, 4 tables, Accepted for publication in A
Radio Continuum and HI study of Blue Compact Dwarf Galaxies
The multifrequency radio continuum and 21cm HI observations of five blue
compact dwarf (BCD) galaxies, Mrk 104, Mrk 108, Mrk 1039, Mrk 1069 and I Zw 97
using the Giant Meterwave Radio Telescope (GMRT) are presented here. Radio
continuum emission at 610 MHz and 325 MHz is detected from all the observed
galaxies whereas only a few are detected at 240 MHz. In our sample, three
galaxies are members of groups and two galaxies (Mrk 1069 and I Zw 97) are
isolated galaxies. The radio emission from Mrk 104 and Mrk 108 is seen to
encompass the entire optical galaxy whereas the radio emission from Mrk 1039,
Mrk 1069, I Zw 97 is confined to massive HII regions. This, we suggest,
indicates that the star formation in the latter group of galaxies has recently
been triggered and that the environment in which the galaxy is evolving plays a
role. Star formation rates (SFR) calculated from 610 MHz emission is in the
range 0.01-0.1 M_sun/yr; this is similar to the SFR obtained for individual
star forming regions in BCDs. The integrated radio spectra of four galaxies are
modelled over the frequency range where data is available. We find that two of
the galaxies Mrk 1069 and Mrk 1039, show a turnover at low frequencies which is
well fitted by free-free absorption whereas the other two galaxies, Mrk 104 and
Mrk 108, show a power law at the lowest GMRT frequencies. The flatter spectrum,
localized star formation and radio continuum in isolated galaxies lend support
to stochastic self-propagating star formation (SSPSF). The HI observations of
four galaxies Mrk 104, Mrk 108, Mrk 1039 and Mrk 1069 show extended disks as
large as ~1.1-6 times the optical size. All the observed BCDs (except Mrk 104)
show rotating disk with a half power width of ~50-124 km/s. Solid body rotation
is common in our sample. We note that the tidal dwarf (TD) origin is possible
for two of the BCDs in our sample.Comment: 39 pages, 8 figures, 38 sub-figures. Accepted for publication in Ap
Super Star Clusters in SBS0335-052E
As one of the lowest metallicity star forming galaxies, with a nucleus of
several super star clusters, SBS0335-052E is the subject of substantial current
study. We present new insights on this galaxy based on new and archival high
spatial resolution NICMOS and ACS images. We provide new measurements and
limits on the size of several of the SSCs. The images have sufficient
resolution to divide the star formation into compact regions and newly
discovered extended regions, indicating a bi-modal form of star formation. The
star formation regions are dated via the equivalent width of the Pa alpha
emission and we find that two of the extended regions of star formation are
less than 10 million years old. Our previous finding that stellar winds confine
the photo-ionizing flux to small regions around individual stars is consistent
with the new observations. This may allow planet formation in what would
traditionally be considered a harsh environment and has implications for the
number of planets around globular cluster stars. In addition the images
pinpoint the regions of H2 emission as located in, but not at the center of the
two star forming super star clusters, S1 and S2.Comment: Accepted by the Astrophysical Journa
The Spitzer Infrared Nearby Galaxies Survey: A High-Resolution Spectroscopy Anthology
High resolution mid-infrared spectra are presented for 155 nuclear and
extranuclear regions from the Spitzer Infrared Nearby Galaxies Survey (SINGS).
The fluxes for nine atomic forbidden and three molecular hydrogen mid-infrared
emission lines are also provided, along with upper limits in key lines for
infrared-faint targets. The SINGS sample shows a wide range in the ratio of
[SIII]18.71um/[SIII]33.48um, but the average ratio of the ensemble indicates a
typical interstellar electron density of 300-400 cm^{-3} on ~23"x15" scales and
500-600 cm^{-3} using ~11"x9" apertures, independent of whether the region
probed is a star-forming nuclear, a star-forming extranuclear, or an AGN
environment. Evidence is provided that variations in gas-phase metallicity play
an important role in driving variations in radiation field hardness, as
indicated by [NeIII]15.56um/[NeII]12.81um, for regions powered by star
formation. Conversely, the radiation hardness for galaxy nuclei powered by
accretion around a massive black hole is independent of metal abundance.
Furthermore, for metal-rich environments AGN are distinguishable from
star-forming regions by significantly larger [NeIII]15.56um/[NeII]12.81um
ratios. Finally, [FeII]25.99um/[NeII]12.81um versus [SiII]34.82um/[SIII]33.48um
also provides an empirical method for discerning AGN from normal star-forming
sources. However, similar to [NeIII]15.56um/[NeII]12.81um, these mid-infrared
line ratios lose their AGN/star-formation diagnostic powers for very low
metallicity star-forming systems with hard radiation fields.Comment: Accepted for publication in Ap
Spitzer 70~m Emission as a SFR Indicator for Sub--Galactic Regions
We use Spitzer 24 m, 70 m and ground based H data for a
sample of 40 SINGS galaxies to establish a star formation rate (SFR) indicator
using 70 m emission for sub--galactic ()
line-emitting regions and to investigate limits in application. A linear
correlation between 70 m and SFR is found and a star formation indicator
SFR(70) is proposed for line-emitting sub-galactic regions as $\rm \Sigma(SFR)\
({M_{\odot}\cdot yr^{-1}\cdot kpc^{-2}})=9.4\times10^{-44}\ \Sigma(70)\
\rm{(ergs\cdot s^{-1}\cdot kpc^{-2})}12+\rm{log(O/H)}\gtrsim8.4\rm \Sigma(SFR)\gtrsim10^{-3}\
(M_{\odot}\cdot yr^{-1}\cdot kpc^{-2})\sigma\sim0.16\mu\sim40%\mu$m emission in galaxies, which can be attributed to stellar populations not
involved in the current star formation activity.Comment: 36 pages, 1 table, 18 figures, accepted by Ap
Total Infrared Luminosity Estimation of Resolved and Unresolved Galaxies
The total infrared (TIR) luminosity from galaxies can be used to examine both
star formation and dust physics. We provide here new relations to estimate the
TIR luminosity from various Spitzer bands, in particular from the 8 micron and
24 micron bands. To do so, we use 45" subregions within a subsample of nearby
face-on spiral galaxies from the Spitzer Infrared Nearby Galaxies Survey
(SINGS) that have known oxygen abundances as well as integrated galaxy data
from the SINGS, the Local Volume Legacy Survey (LVL) and Engelbracht et al.
(2008) samples. Taking into account the oxygen abundances of the subregions,
the star formation rate intensity, and the relative emission of the polycyclic
aromatic hydrocarbons at 8 micron, the warm dust at 24 micron and the cold dust
at 70 micron and 160 micron we derive new relations to estimate the TIR
luminosity from just one or two of the Spitzer bands. We also show that the
metallicity and the star formation intensity must be taken into account when
estimating the TIR luminosity from two wave bands, especially when data
longward of 24 micron are not available.Comment: 11 pages, 10 figures, accepted for publication in Ap
The Structure of a Low-Metallicity Giant Molecular Cloud Complex
To understand the impact of low metallicities on giant molecular cloud (GMC)
structure, we compare far infrared dust emission, CO emission, and dynamics in
the star-forming complex N83 in the Wing of the Small Magellanic Cloud. Dust
emission (measured by Spitzer as part of the S3MC and SAGE-SMC surveys) probes
the total gas column independent of molecular line emission and traces
shielding from photodissociating radiation. We calibrate a method to estimate
the dust column using only the high-resolution Spitzer data and verify that
dust traces the ISM in the HI-dominated region around N83. This allows us to
resolve the relative structures of H2, dust, and CO within a giant molecular
cloud complex, one of the first times such a measurement has been made in a
low-metallicity galaxy. Our results support the hypothesis that CO is
photodissociated while H2 self-shields in the outer parts of low-metallicity
GMCs, so that dust/self shielding is the primary factor determining the
distribution of CO emission. Four pieces of evidence support this view. First,
the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11
\times 10^21 cm^-2/(K km/s), or 20-55 times the Galactic value. Second, the
CO-to-H2 conversion factor varies across the complex, with its lowest (most
nearly Galactic) values near the CO peaks. Third, bright CO emission is largely
confined to regions of relatively high line-of-sight extinction, A_V >~ 2 mag,
in agreement with PDR models and Galactic observations. Fourth, a simple model
in which CO emerges from a smaller sphere nested inside a larger cloud can
roughly relate the H2 masses measured from CO kinematics and dust.Comment: 17 pages, 10 figures (including appendix), accepted for publication
in the Astrophysical Journa
Spatially Resolved PAH Emission Features in Nearby, Low Metallicity, Star-Forming Galaxies
Low-resolution, mid-infrared Spitzer/IRS spectral maps are presented for
three nearby, low-metallicity dwarf galaxies (NGC 55, NGC 3109 and IC 5152) for
the purpose of examining the spatial distribution and variation of polycyclic
aromatic hydrocarbon (PAH) emission. The sample straddles a metallicity of
12+log(O/H)~8.0, a transition point below which PAH intensity empirically drops
and the character of the interstellar medium changes. We derive quantitative
radiances of PAH features and atomic lines on both global and
spatially-resolved scales. The Spitzer spectra, combined with extensive
ancillary data from the UV through the mid-infrared, allow us to examine
changes in the physical environments and in PAH feature radiances down to a
physical scale of 50 pc. We discuss correlations between various PAH emission
feature and atomic line radiances. The (6.2 micron)/(11.3 micron), (7.7
micron)/(11.3 micron), (8.6 micron)/(11.3 micron), (7.7 micron)/(6.2 micron),
and (8.6 micron)/(6.2 micron) PAH radiance ratios are found to be independent
of position across all three galaxies, although the ratios do vary from galaxy
to galaxy. As seen in other galaxies, we find no variation in the grain size
distribution as a function of local radiation field strength. Absolute PAH
feature intensities as measured by a ratio of PAH/(24 micron) radiances are
seen to vary both positionally within a given galaxy, and from one galaxy to
another when integrated over the full observed extent of each system. We
examine direct comparisons of CC mode PAH ratios (7.7 micron)/(6.2 micron) and
(8.6 micron)/(6.2 micron) to the mixed (CC/CH) mode PAH ratio (7.7
micron)/(11.3 micron). We find little variation in either mode, and no
difference in trends between modes. While the local conditions change markedly
over the observed regions of these galaxies, the properties of PAH emission
show a remarkable degree of uniformity.Comment: Astrophysical Journal, in pres
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