72 research outputs found
Mapping atomic and diffuse interstellar band absorption across the Magellanic Clouds and the Milky Way
Diffuse interstellar bands (DIBs) trace warm neutral and weakly-ionized
diffuse interstellar medium (ISM). Here we present a dedicated, high
signal-to-noise spectroscopic study of two of the strongest DIBs, at 5780 and
5797 \AA, in optical spectra of 666 early-type stars in the Small and Large
Magellanic Clouds, along with measurements of the atomic Na\,{\sc i}\,D and
Ca\,{\sc ii}\,K lines. The resulting maps show for the first time the
distribution of DIB carriers across large swathes of galaxies, as well as the
foreground Milky Way ISM. We confirm the association of the 5797 \AA\ DIB with
neutral gas, and the 5780 \AA\ DIB with more translucent gas, generally tracing
the star-forming regions within the Magellanic Clouds. Likewise, the Na\,{\sc
i}\,D line traces the denser ISM whereas the Ca\,{\sc ii}\,K line traces the
more diffuse, warmer gas. The Ca\,{\sc ii}\,K line has an additional component
at --220 km s seen towards both Magellanic Clouds; this may be
associated with a pan-Magellanic halo. Both the atomic lines and DIBs show
sub-pc-scale structure in the Galactic foreground absorption; the 5780 and 5797
\AA\ DIBs show very little correlation on these small scales, as do the
Ca\,{\sc ii}\,K and Na\,{\sc i}\,D lines. This suggests that good correlations
between the 5780 and 5797 \AA\ DIBs, or between Ca\,{\sc ii}\,K and Na\,{\sc
i}\,D, arise from the superposition of multiple interstellar structures.
Similarity in behaviour between DIBs and Na\,{\sc i} in the SMC, LMC and Milky
Way suggests the abundance of DIB carriers scales in proportion to metallicity.Comment: Accepted for publication in MNRA
Diffuse Interstellar Bands in z < 0.6 CaII Absorbers
The diffuse interstellar bands (DIBs) probably arise from complex organic
molecules whose strength in local galaxies correlates with neutral hydrogen
column density, N(HI), and dust reddening, E(B-V). Since CaII absorbers in
quasar (QSO) spectra are posited to have high N(HI) and significant E(B-V),
they represent promising sites for the detection of DIBs at cosmological
distances. Here we present the results from the first search for DIBs in 9
CaII-selected absorbers at 0.07 < z_abs < 0.55. We detect the 5780Ang DIB in
one line of sight at z_abs = 0.1556; this is only the second QSO absorber in
which a DIB has been detected. Unlike the majority of local DIB sight-lines,
both QSO absorbers with detected DIBs show weak 6284Ang absorption compared
with the 5780Ang band. This may be indicative of different physical conditions
in intermediate redshift QSO absorbers compared with local galaxies. Assuming
that local relations between the 5780Ang DIB strength and N(HI) and E(B-V)
apply in QSO absorbers, DIB detections and limits can be used to derive N(HI)
and E(B-V). For the one absorber in this study with a detected DIB, we derive
E(B-V) = 0.23mag and log[N(HI)] >= 20.9, consistent with previous conclusions
that CaII systems have high HI column densities and significant reddening. For
the remaining 8 CaII-selected absorbers with 5780Ang DIB non-detections, we
derive E(B-V) upper limits of 0.1-0.3mag.Comment: 5 pages, 2 figures. Accepted to MNRAS Letter
A survey of diffuse interstellar bands in the Andromeda galaxy: optical spectroscopy of M31 OB stars
We present the largest sample to-date of intermediate-resolution blue-to-red
optical spectra of B-type supergiants in M31 and undertake the first survey of
diffuse interstellar bands (DIBs) in this galaxy. Spectral classifications,
radial velocities and interstellar reddenings are presented for 34 stars in
three regions of M31. Radial velocities and equivalent widths are given for the
5780 and 6283 DIBs towards 11 stars. Equivalent widths are also presented for
the following DIBs detected in three sightlines in M31: 4428, 5705, 5780, 5797,
6203, 6269, 6283, 6379, 6613, 6660, and 6993. All of these M31 DIB carriers
reside in clouds at radial velocities matching those of interstellar Na I
and/or H I. The relationships between DIB equivalent widths and reddening
(E(B-V)) are consistent with those observed in the local ISM of the Milky Way.
Many of the observed sightlines show DIB strengths (per unit reddening) which
lie at the upper end of the range of Galactic values. DIB strengths per unit
reddening are found (with 68% confidence), to correlate with the interstellar
UV radiation field strength. The strongest DIBs are observed where the
interstellar UV flux is lowest. The mean Spitzer 8/24 micron emission ratio in
our three fields is slightly lower than that measured in the Milky Way, but we
identify no correlation between this ratio and the DIB strengths in M31.
Interstellar oxygen abundances derived from the spectra of three M31 H II
regions in one of the fields indicate that the average metallicity of the ISM
in that region is 12 + log[O/H] = 8.54 +- 0.18, which is approximately equal to
the value in the solar neighbourhood
The Diffuse Interstellar Bands: A Major Problem in Astronomical Spectroscopy
A critical review of the very long-standing problem of the diffuse
interstellar bands is presented with emphasis on spectroscopic aspects of
observational, modelling and laboratory-based research. Some research themes
and ideas that could be explored theoretically and experimentally are
discussed. The article is based on the Journal of Molecular Spectroscopy Review
Lecture presented at the 60th Ohio State University International Symposium on
Molecular Spectroscopy, June 2005
Small-scale structure in the interstellar medium: time-varying interstellar absorption towards κ Velorum
Ultra-high spectral resolution observations of time-varying interstellar absorption towards κ Vel are reported, using the Ultra-High Resolution Facility on the Anglo-Australian Telescope. Detections of interstellar Ca i, Ca ii, K i, Na i and CH are obtained, whilst an upper limit on the column density is reported for C_2. The results show continued increases in column densities of K i and Ca i since observations ∼4 yr earlier, as the transverse motion of the star carried it ∼10 au perpendicular to the line of sight. Line profile models are fitted to the spectra and two main narrow components (A and B) are identified for all species except CH. The column density N(K i) is found to have increased by 82^(+10)_(−9) per cent between 1994 and 2006, whilst N(Ca i) is found to have increased by 32 ± 5 per cent over the shorter period of 2002–2006. The line widths are used to constrain the kinetic temperature to T_(k,A) < 671^(+18)_(−17) K and T_(k,B) < 114^(+15)_(−14) K. Electron densities are determined from the Ca i/Ca ii ratio, which in turn place lower limits on the total number density of n_A ≳ 7 × 10^3 cm^(−3) and n_B ≳ 2 × 10^4 cm^(−3). Calcium depletions are estimated from the Ca i/K i ratio. Comparison with the chemical models of Bell et al. confirms the high number density, with n = 5 × 10^4 cm^(−3) for the best-fitting model. The first measurements of diffuse interstellar bands (DIBs) towards this star are made at two epochs, but only an upper limit of ≲ 40 per cent is placed on their variation over ∼9 yr. The DIBs are unusually weak for the measured E(B − V) and appear to exhibit similar behaviour to that seen in Orion. The ratio of equivalent widths of the λ5780 to λ5797 DIBs is amongst the highest known, which may indicate that the carrier of λ5797 is more sensitive to ultraviolet radiation than to local density
The EDIBLES Survey. VII. A survey of C2 and C3 in interstellar clouds
We carried out a sensitive survey of C and C using the EDIBLES data
set. We also expanded our searches to C, C, and CC
isotopologue in the most molecule-rich sightlines.
We fit synthetic spectra generated following a physical excitation model to
the C (2-0) Phillips band to obtain the C column density () as well
as the kinetic temperature () and number density () of the
host cloud. The C molecule was measured through its
(000-000) electronic origin band system. We simulated the excitation of this
band with a double-temperature Boltzmann distribution.
We present the largest combined survey of C and C to date in which
the individual transitions can be resolved. In total we detected C in 51
velocity components along 40 sightlines, and C in 31 velocity components
along 27 sightlines. The two molecules are detected in the same velocity
components. We find a very good correlation between (C) and (C)
with Pearson and an average (C)/(C) ratio of
15.51.4. A comparison with the behaviour of the C DIBs shows that
there are no clear differences among sightlines with and without detection of
C and C. This is in direct contrast to the better-studied non-C
DIBs who have reduced strengths in molecule-rich environments. We also identify
for the first time the (2), (3), and (4) transitions of the
CC (2-0) Phillips band in a stacked average spectrum, and
estimate the isotopic ratio of carbon C/C as 798. Our search
for the C and C optical bands was unsuccessful.Comment: 31 pages, 23 figures. To appear in A&
Detailed maps of interstellar clouds in front of omega Centauri: Small-scale structures in the Galactic Disc-Halo interface
We used the multiplex capabilities of the AAOmega spectrograph at the
Anglo-Australian Telescope to create a half-square-degree map of the neutral
and low-ionized ISM in front of the nearby (~5 kpc), most massive Galactic
globular cluster, omega Centauri. Its redshifted, metal-poor and hot horizontal
branch stars probe the medium-strong Ca II K and Na I D2 line absorption, and
weak absorption in the lambda5780 and lambda5797 Diffuse Interstellar Bands
(DIBs), on scales around a parsec. The kinematical and thermodynamical picture
emerging from these data is that we predominantly probe the warm neutral medium
and weakly-ionized medium of the Galactic Disc-Halo interface, ~0.3-1 kpc above
the mid-plane. A comparison with Spitzer Space Telescope 24-micron and
DIRBE/IRAS maps of the warm and cold dust emission confirms that both Na I and
Ca II trace the overall column density of the warm neutral and weakly-ionized
medium. Clear signatures are seen of the depletion of calcium atoms from the
gas phase into dust grains. Curiously, the coarse DIRBE/IRAS map is a more
reliable representation of the relative reddening between sightlines than the
Na I and Ca II absorption-line measurements, most likely because the latter are
sensitive to fluctuations in the local ionization conditions. The behaviour of
the DIBs is consistent with the lambda5780 band being stronger than the
lambda5797 band in regions where the ultraviolet radiation level is relatively
high, as in the Disc-Halo interface. This region corresponds to a sigma-type
cloud. In all, our maps and simple analytical model calculations show in
unprecedented detail that small-scale density and/or ionization structures
exist in the extra-planar gas of a spiral galaxy. (abridged)Comment: Accepted for publication in MNRA
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