78 research outputs found
Are spiral galaxies optically thin or thick?
The opacity of spiral galaxies is examined by modelling the dust and stellar
content of individual galaxies. The model is applied to five late-type spiral
galaxies (NGC 4013, IC 2531, UGC 1082, NGC 5529 and NGC 5907). Having analyzed
a total of seven galaxies thus far, the five galaxies mentioned above plus UGC
2048 and NGC 891 presented in (Xilouris et al. 1997, 1998), we are able to draw
some general conclusions, the most significant of which are: 1) The face-on
central optical depth is less than one in all optical bands indicating that
typical spiral galaxies like the ones that we have modelled would be completely
transparent if they were to be seen face-on. 2) The dust scaleheight is about
half that of the stars, which means that the dust is more concentrated near the
plane of the disk. 3) The dust scalelength is about 1.4 times larger than that
of the stars and the dust is more radially extended than the stars. 4) The dust
mass is found to be about an order of a magnitude more than previously measured
using the IRAS fluxes, indicating the existence of a cold dust component. The
gas-to-dust mass ratio calculated is close to the value derived for our Galaxy.
5) The derived extinction law matches quite well the Galactic extinction law,
indicating a universal dust behaviour.Comment: 13 pages. Accepted for publication in A&
The peculiar supernova remnant CTB 80
Deep CCD exposures of the peculiar supernova remnant CTB 80 in the light of
major optical lines have been obtained. These images reveal significant shock
heated emission in the area of the remnant. The sulfur line image shows
emission in the north along the outer boundary of the IRAS and HI shells. The
comparison between the [OIII] and [OII] line images further suggest the
presence of significant inhomogeneities in the interstellar medium. The flux
calibrated images do not indicate the presence of incomplete recombination
zones, and we estimate that the densities of the preshock clouds should not
exceed a few atoms per cm^3. The area covered by the optical radiation along
with the radio emission at 1410 MHz suggest that CTB 80 occupies a larger
angular extent than was previously known.Comment: 19 pages, 6 png figures. Submitted revised version to A &
Multi-band optical micro-variability observations of the BL Lac object S4 0954+658
We have observed S4 0954+658 in the BVRI bands in 2001, and in the BI bands
in 2002. The observations resulted in almost evenly sampled light curves, 3-8
hours long, with an average sampling interval of ~5-15 min. Because of the
dense sampling and the availability of light curves in more than one optical
bands we are able to study the intra-night flux and spectral variability of the
source in detail. Significant observations were observed in all but one cases.
On average, the flux variability amplitude, on time scales of minutes/hours,
increases from ~2-3% in the I, to ~3.5-5.5% in the B band light curves. We do
not detect any flares within the individual light curves. However, there is a
possibility that the April 2001 and late May 2002 observations sample two
flares which lasted longer than ~1-3 days. The evidence is only suggestive
though, due to the limited length of the present light curves with regard to
the duration of the assumed flares. No spectral variations are detected during
the April 2001 observations. The source flux rises and decays with the same
rate, in all bands. This variability behaviour is typical of S4 0954+658, and
is attributed to geometrical effects. However, significant spectral variations
are observed in May 2002. We find that the spectrum hardens/softens as the flux
increases/decreases, respectively. Furthermore, the "hardening" rate of the
energy spectrum is faster than the rate with which the spectrum becomes
"redder" as the flux decays. We also find evidence (although of low statistical
significance) that the I band variations are delayed with respect to the B band
variations. If the May 2002 observations sample a flaring event, these results
suggest that the variations are caused by energetic processes which are
associated with the particle cooling and the source light travel time scales.Comment: 7 pages, 7 figures, accepted for publication in A&
The distribution of the ISM in the Milky Way A three-dimensional large-scale model
We use the COBE/DIRBE (1.2, 2.2, 60, 100, 140, and 240 m) maps and the
COBE/FIRAS spectra (for the wavelength range 100 - 1000 m) to constrain a
model for the spatial distribution of the dust, the stars, and the gas in the
Milky Way. By assuming exponential axisymmetric distributions for the dust and
the stars and by performing the corresponding radiative transfer calculations
we closely (given the simple geometry of the model) reproduce the FIR and NIR
maps of the Milky Way. Similar distributions for the atomic and molecular
hydrogen in the disk are used (with an inner cut-off radius for the atomic
hydrogen) to fit the gas data. The star formation rate as a function of the
Galactic radius is derived from the FIR emission and is well in agreement with
existing estimates from various star formation tracers. The gas surface density
is plotted against the star formation rate density and an ``intrinsic''
Galactic Schmidt law is derived with excellent agreement with the ``external''
Schmidt law found for spiral galaxies. The Milky Way is found to consume and of its gas in the outer and inner regions respectively (for
a period of 0.1 Gyr) to make stars. The dust-induced B-V color excess observed
in various directions and distances (up to kpc) with well-studied
Cepheid stars is compared with the model predictions showing a good agreement.
The simple assumption of exponential distributions of stars and dust in the
Galaxy is found to be quite instructive and adequate in modeling all the
available data sets from 0.45 m (B-band) to 1000 m.Comment: 14 Pages, 10 figure
B and I-band optical micro-variability observations of the BL Lac objects S5 2007+777 and 3C371
We have observed S5 2007+777 and 3C371 in the B and I bands for 13 and 8
nights, respectively, during various observing runs in 2001, 2002 and 2004. The
observations resulted in almost evenly sampled light curves, 6-9 hours long. We
do not detect any flares within the observed light curves, but we do observe
small amplitude, significant variations, in both bands, on time scales of hours
and days. The average variability amplitude on time scales of minutes/hours is
2.5% and 1-1.5% in the case of S5 2007+777 and 3C371, respectively. The average
amplitudes increase to 5-12% and 4-6%, respectively, on time scales of days. We
find that the B and I band variations are highly correlated, on both short and
long time scales. During the 2004 observations, which resulted in the longest
light curves, we observe two well defined flux-decay and rising trends in the
light curves of both objects. When the flux decays, we observe significant
delays, with the B band flux decaying faster than the flux in the I band. As a
result, we also observe significant, flux related spectral variations as well.
The flux-spectral relation is rather complicated, with loop-like structures
forming during the flux evolution. The presence of spectral variations imply
that the observed variability is not caused by geometric effects. On the other
hand, our results are fully consistent with the hypothesis that the observed
variations are caused by perturbations which affect different regions in the
jet of the sources.Comment: Accepted for publication in Astronomy and Astrophysic
Multi-band optical micro-variability observations of BL Lacertae
We have observed BL Lacertae in the B, R and I bands for 2 nights in July,
1999, and 3 nights in July, 2001. The observations resulted in almost evenly
sampled light curves, with an average sampling interval of ~5 min. The source
is significantly variable in all bands. On average, the variability amplitude
increases from ~5% in the I band, to ~5.5% in the R and ~6.5% in the B band
light curves. The rising and decaying time scales are comparable within each
band, but they increase from the B, to R and I band light curves. The optical
power spectrum shows a red noise component with a slope of ~ -2.
Cross-correlation analysis shows that in most cases the delay between the
variations in the B and I band light curves is less than ~ 0.4 hrs. The
cross-correlation functions are asymmetric, implying complex delays of the I
band variations with respect to the B band variations. Furthermore, in one case
we find that the I band variations are significantly delayed (by ~0.2 hrs) with
respect to the B band variations. We also detect significant spectral
variations: the spectrum becomes steeper as the flux increases, and the
flattest spectral index corresponds to the maximum B band flux. Our results
imply that the fast, intra-night variations of the source correspond to
perturbations of different regions in the jet which cause localized injections
of relativistic particles on time scales much sorter that the average sampling
interval of the light curves. The variations are controlled by the cooling and
light crossing time scales, which are probably comparable.Comment: Accepted for publication in A&
RoboPol: Connection between optical polarization plane rotations and gamma-ray flares in blazars
We use results of our 3 year polarimetric monitoring program to investigate
the previously suggested connection between rotations of the polarization plane
in the optical emission of blazars and their gamma-ray flares in the GeV band.
The homogeneous set of 40 rotation events in 24 sources detected by {\em
RoboPol} is analysed together with the gamma-ray data provided by {\em
Fermi}-LAT. We confirm that polarization plane rotations are indeed related to
the closest gamma-ray flares in blazars and the time lags between these events
are consistent with zero. Amplitudes of the rotations are anticorrelated with
amplitudes of the gamma-ray flares. This is presumably caused by higher
relativistic boosting (higher Doppler factors) in blazars that exhibit smaller
amplitude polarization plane rotations. Moreover, the time scales of rotations
and flares are marginally correlated.Comment: 12 pages, 16 figures, accepted to MNRA
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