92 research outputs found
Mid--IR emission of galaxies in the Virgo cluster: II. Integrated properties
We analyse the integrated properties of the Mid-IR emission of a complete,
optically selected sample of galaxies in the Virgo cluster observed with the
ISOCAM instrument on board the ISO satellite. The analysis shows that the
Mid-IR emission up to 15 mic of optically-selected, normal early-type galaxies
(E, S0 and S0a) is dominated by the Rayleigh-Jeans tail of the cold stellar
component. The Mid-IR emission of late-type galaxies is instead dominated by
the thermal emission from dust. The small dust grains emitting in the Mid-IR
have an excess of emission if compared to big grains emitting in the Far-IR.
While the Far-IR emission increases with the intensity of the interstellar
radiation field, their Mid-IR emission is non--linearly related to the UV
radiation field. The spectral energy distributions of the target galaxies
indicate that there is a linear relationship between the UV radiation field and
the Mid-IR emission of galaxies for low or intermediate activities of star
formation, while the emission from the hot dust seems to drop for strong UV
fields. The Mid-IR colour of late-type galaxies is not related to their
activity of star formation. The properties of the dust emission in the Mid-IR
seem more related to the mass than to the morphological type of the target
galaxy. Since the activity of star formation is anticorrelated to the mass of
galaxies, this reflects a relationship between the emission of dust in the
Mid-IR and the UV radiation field: galaxies with the lowest Mid-IR emission for
a given UV field are low mass, dwarf galaxies. These observational evidences
are easily explained if the carriers of the Unidentified Infrared Bands that
dominate the 6.75 mic emission are destroyed by the intense UV radiation field
of dwarf galaxies, although abundance effects can also play a role.Comment: 17 pages, Latex, 7 figures; to be published in Astronomy &
Astrophysics, Main Journal; Figure legend should be corrected in: 1 - 1a; 2 -
1b; 3 - 2; 4 - 3a; 5 - 3b; 6 - 3c; 7 - 3d; 8 - 3e; 9 - 4; 10 - 5; 11 - 6; 12
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Dissociation of sensitivity to spatial frequency in word and face preferential areas of the fusiform gyrus
Different cortical regions within the ventral occipitotemporal junction have been reported to show preferential responses to particular objects. Thus, it is argued that there is evidence for a left-lateralized visual word form area and a right-lateralized fusiform face area, but the unique specialization of these areas remains controversial. Words are characterized by greater power in the high spatial frequency (SF) range, whereas faces comprise a broader range of high and low frequencies. We investigated how these high-order visual association areas respond to simple sine-wave gratings that varied in SF. Using functional magnetic resonance imaging, we demonstrated lateralization of activity that was concordant with the low-level visual property of words and faces; left occipitotemporal cortex is more strongly activated by high than by low SF gratings, whereas the right occipitotemporal cortex responded more to low than high spatial frequencies. Therefore, the SF of a visual stimulus may bias the lateralization of processing irrespective of its higher order properties
[CII] at 158 mic as a star formation tracer in late-type galaxies
We present a calibration of the massive star formation rate vs. [CII]
luminosity relation based on a sample of nearby, late-type galaxies observed
with ISO-LWS and imaged in the Halpha+[NII] line. The relation holds for far-IR
luminosities 10^8 < L_{FIR} < 10^{10.5} L (solar units). The derived star
formation rates have an uncertainty of about a factor of 10. Part of this
uncertainty is due to the different mix of contributions to the [CII] emission
from the different components of the interstellar medium in individual
galaxies, as discussed in an appendix.Comment: Accepted for publication on Astronomy & Astrophysic
Obscured Activity: AGN, Quasars, Starbursts and ULIGs observed by the Infrared Space Observatory
Some of the most active galaxies in the Universe are obscured by large
quantities of dust and emit a substantial fraction of their bolometric
luminosity in the infrared. Observations of these infrared luminous galaxies
with the Infrared Space Observatory (ISO) have provided a relatively unabsorbed
view to the sources fuelling this active emission. The improved sensitivity,
spatial resolution and spectroscopic capability of ISO over its predecessor
Infrared Astronomical Satellite (IRAS), has enabled significant advances in the
understanding of the infrared properties of active galaxies. ISO surveyed a
wide range of active galaxies which, in the context of this review, includes
those powered by intense bursts of star-formation as well as those containing a
dominant active galactic nucleus (AGN). Mid infrared imaging resolved for the
first time the dust enshrouded nuclei in many nearby galaxies, while a new era
in infrared spectroscopy was opened by probing a wealth of atomic, ionic and
molecular lines as well as broad band features in the mid and far infrared.
This was particularly useful since it resulted in the understanding of the
power production, excitation and fuelling mechanisms in the nuclei of active
galaxies including the intriguing but so far elusive ultraluminous infrared
galaxies. Detailed studies of various classes of AGN and quasars greatly
improved our understanding of the unification scenario. Far-infrared imaging
and photometry also revealed the presence of a new very cold dust component in
galaxies and furthered our knowledge of the far-infrared properties of faint
starbursts, ULIGs and quasars. We summarise almost nine years of key results
based upon ISO data spanning the full range of luminosity and type of active
galaxies.Comment: Accepted for publication in 'ISO science legacy - a compact review of
ISO major achievements', Space Science Reviews - dedicated ISO issue. To be
published by Springer in 2005. 62 pages (low resolution figures version).
Higher resolution PDFs available from
http://users.physics.uoc.gr/~vassilis/papers/VermaA.pdf or
http://www.iso.vilspa.esa.es/science/SSR/Verma.pd
Global Physical Conditions of the Interstellar Medium in Nearby Galaxies
Far-infrared spectra (43-197um) of 34 nearby galaxies obtained by the Long
Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO) were
analyzed to investigate the general properties of interstellar matter in
galaxies. The line fluxes of [CII]158um and [NII]122um relative to the total
far-infrared flux (FIR) decrease as the far-infrared color becomes bluer, while
the ratio of the [OI]63um flux to FIR does not show a systematic trend with the
color. The [OIII]88um to FIR ratio shows a large scatter with a weak trend of
increase with the color. We estimate the physical conditions of
photodissociation regions (PDRs) in the sample galaxies, such as the
far-ultraviolet radiation field intensity Go and the gas density n by assuming
that all the observed [OI]63um and far-infrared continuum emissions come from
PDRs. The present analysis suggests that the decrease in [CII]158um/FIR with
the far-infrared color may not be accounted for by the decrease in the
photoelectric heating efficiency owing to the increase in positive charges of
dust grains because a measure of the efficiency, Go/n, is found to stay
constant with the far-infrared color. Instead the decrease can be interpreted
in terms of either the increase in the collisional de-excitation of the [CII]
transition due to the increase in the gas density or the decrease in the
ionized component relative to the far-infrared intensity suggested by the
decrease in [NII]122um/FIR. Based on the present analysis, we derive average
relations of the far-infrared color with Go and n in galaxies, which can be
applied to the investigation of interstellar matter in distant galaxies.Comment: to apear in A&
A large-scale CO survey of the Rosette Molecular Cloud: assessing the effects of O stars on surrounding molecular gas
We present a new large-scale survey of J=3-2 12CO emission covering 4.8
square degrees around the Rosette Nebula. Approximately 2000 compact clumps are
identified, with a spatially-invariant power law mass distribution index of
-1.8. Most of the inner clumps show velocity gradients of 1-3 km/s/pc, directed
away from the exciting nebula. The gradients decrease with distance from the
central O stars, and are consistent with a photoionised gas acceleration model,
assuming clump lifetimes of a few 10^5 yrs. However, in one clear case, the
observed near-constant velocity gradient is difficult to explain with simple
models. Most blue-shifted but very few of the red-shifted clumps are associated
with dark absorbing optical globules, confirming that the dominant molecular
gas motion is expansion away from the central nebula and O stars. Many clumps
also lie in a molecular ring, having an expansion velocity of 30 km/s, radius
11pc, and dynamical lifetime of ~1Myr. The J=3-2/1-0 12CO line ratios of the
clumps decrease with distance from the O stars, implying a gradient in their
surface temperatures; the results are consistent with a simple model of clump
surface heating due to the central stars.
Seven high-velocity molecular flows are found in the region, with a close
correspondence between these flows and embedded young clusters. These outflows
are sufficiently energetic to drive local gas turbulence within each cluster.
We find 14 clear examples of association between embedded young stars seen at
24um and CO clumps; these are thought to be photoevaporating molecular
envelopes. The CO clumps without evidence of embedded stars tend to have lower
velocity gradients, and it is suggested that the presence of the young star may
extend the lifespan of the photoevaporating envelope.Comment: 19 pages, 18 figures, 2 tables; to be published in MNRA
Infrared Emission of Normal Galaxies from 2.5 to 12 Microns: ISO Spectra, Near-Infrared Continuum and Mid-Infrared Emission Features
We present ISO-PHOT spectra of the regions 2.5-4.9um and 5.8-11.6um for a
sample of 45 disk galaxies from the U.S. ISO Key Project on Normal Galaxies.
The spectra can be decomposed into three spectral components: (1) continuum
emission from stellar photospheres, which dominates the near-infrared (2.5-
4.9um; NIR) spectral region; (2) a weak NIR excess continuum, which has a color
temperature of ~ 1000K, carries a luminosity of a few percent of the total
far-infrared luminosity L(FIR), and most likely arises from the ISM; and (3)
the well-known broad emission features at 6.2, 7.7, 8.6 and 11.3 um, which are
generally attributed to aromatic carbon particles. These aromatic features in
emission (AFEs) dominate the mid-infrared (5.8-11.6 um; MIR) part of the
spectrum, and resemble the so-called Type-A spectra observed in many
non-stellar sources and the diffuse ISM in our own Galaxy. The relative
strengths of the AFEs vary by 15-25% among the galaxies. However, little
correlation is seen between these variations and either IRAS 60um-to-100um flux
density ratio R(60/100) or the FIR-to-blue luminosity ratio L(FIR)/L(B),
suggesting that the observed variations are not a direct consequence of the
radiation field differences among the galaxies. We demonstrate that the NIR
excess continuum and AFE emission are correlated, suggesting that they are
produced by similar mechanisms and similar (or the same) material. On the other
hand, as the current star-formation activity increases, the overall strengths
of the AFEs and the NIR excess continuum drop significantly with respect to
that of the far-infrared emission from large dust grains. This is likely a
consequence of the preferential destruction in intense radiation fields of the
small carriers responsible for the NIR/AFE emission.Comment: With 8 tables and 12 figures; to appear in the Astrophysical Journa
Photon dominated regions in the spiral arms of M83 and M51
We present CI 3P1-3P0 spectra at four spiral arm positions and the nuclei of
the nearby galaxies M83 and M51 obtained at the JCMT. This data is complemented
with maps of CO 1-0, 2-1, and 3-2, and ISO/LWS far-infrared data of CII (158
micron), OI (63 micron), and NII (122 micron) allowing for the investigation of
a complete set of all major gas cooling lines. From the intensity of the NII
line, we estimate that between 15% and 30% of the observed CII emission
originate from the dense ionized phase of the ISM. The analysis indicates that
emission from the diffuse ionized medium is negligible. In combination with the
FIR dust continuum, we find gas heating efficiencies below ~0.21% in the
nuclei, and between 0.25 and 0.36% at the outer positions. Comparison with
models of photon-dominated regions (PDRs) of Kaufman et al. (1999) with the
standard ratios OI(63)/CII_PDR and (OI(63)+CII_PDR) vs. TIR, the total infrared
intensity, yields two solutions. The physically most plausible solution
exhibits slightly lower densities and higher FUV fields than found when using a
full set of line ratios, CII_PDR/CI(1-0), CI(1-0)/CO(3-2), CO(3-2)/CO(1-0),
CII/CO(3-2), and, OI(63)/CII_PDR. The best fits to the latter ratios yield
densities of 10^4 cm^-3 and FUV fields of ~G_0=20-30 times the average
interstellar field without much variation. At the outer positions, the observed
total infrared intensities are in perfect agreement with the derived best
fitting FUV intensities. The ratio of the two intensities lies at 4-5 at the
nuclei, indicating the presence of other mechanisms heating the dust
ISO observations of the interacting galaxy Markarian 297: with the powerful supernova remnant 1982aa
Markarian (Mkn) 297 is a complex system with two interacting galaxies.
Observations were made with ISO using ISOCAM, ISOPHOT and LWS. We present
ISOCAM maps at 6.7, 7.7, 12 and 14.3 microns which, with PHT-S spectrometry of
the central interacting region, probe the dust obscured star formation and dust
properties. ISOCAM reveals that the strongest emission region in the four MIR
bands is completely unremarkable at visible and near-IR (e.g. 2MASS)
wavelengths, and does not coincide with the nuclear region of either colliding
galaxy. It shares this striking characteristic with the overlap region of the
colliding galaxies in the Antennae (NGC 4038, 4039), the intragroup region of
Stephan's Quintet, and IC 694 in the interacting system Arp 299. At 15 microns,
the hidden source in Mkn 297 is, respectively, 14.6 and 3.8 times more luminous
than the hidden sources in the Antennae (NGC 4038/4039) and Stephan's Quintet.
Numerical simulations indicate that we see the Mkn 297 interaction about 1.5 x
10e8 years after the collision. ISOCAM shows knots and ridges of emission. The
14.3/7.7 micron ratio map implies widespread strong star formation. Strong
emission features were detected in the ISOPHOT spectrum, while [OI], [OIII] and
[CII] emission lines were seen with LWS. Using data from the three instruments,
luminosities and masses for two dust components were determined. The total
infrared luminosity is approximately 10e11 L_sol, marginally a LIRG. A 1979
supernova generated one of the most powerful known radio remnants (SN 1982aa)
close to the strongest MIR source and identified with star forming region 14 in
the optical. This exceptional supernova explosion may have been accompanied by
a GRB, and a search for a GRB in this direction in contemporaneous satellite
data is recommended.Comment: Accepted for publication in Astronomy and Astrophysics Updated to
better use recent SN/GRB work and tune terminology in Sec. 4.
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