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Journal ArticleDOI

A three-dimensional map of the hot Local Bubble using diffuse interstellar bands

TL;DR: In this article, the authors report the three-dimensional structure of the Local Bubble using two different Diffuse Interstellar Bands (DIB) tracers and reveal that DIB carriers are present within the Bubble.
Abstract: The Solar System is located within a low-density cavity, known as the Local Bubble, which appears to be filled with an X-ray emitting gas at a temperature of 10$^6$ K. Such conditions are too harsh for typical interstellar atoms and molecules to survive. There exists an enigmatic tracer of interstellar gas, known as Diffuse Interstellar Bands (DIB), which often appears as absorption features in stellar spectra. The carriers of these bands remain largely unidentified. Here we report the three-dimensional structure of the Local Bubble using two different DIB tracers ($\lambda$5780 and $\lambda$5797), which reveals that DIB carriers are present within the Bubble. The map shows low ratios of $\lambda$5797/$\lambda$5780 inside the Bubble compared to the outside. This finding proves that the carrier of the $\lambda$5780 DIB can withstand X-ray photo-dissociation and sputtering by fast ions, where the carrier of the $\lambda$5797 DIB succumbs. This would mean that DIB carriers can be more stable than hitherto thought and that the carrier of the $\lambda$5780 DIB must be larger than that of the $\lambda$5797 DIB. Alternatively, small-scale denser (and cooler) structures that shield some of the DIB carriers must be prevalent within the Bubble, implying that such structures may be an intrinsic feature of supernova-driven bubbles.

Summary (1 min read)

A three-dimensional map of the hot Local Bubble using

  • Such conditions are too harsh for typical interstellar atoms and molecules to survive2, 3.
  • In the 2nd quadrant (upper-left sector) DIB carriers are abundant in the direction of the Taurus dark clouds and molecular.
  • H. Gh. Khosroshahi - Present address: Institude in research in fundamental science, Tehran, Iran.

Methods

  • To map DIB absorption in and around the LB, the authors conducted a high signal-to-noise (S/N) survey of 637 nearby early-type stars in both hemispheres (see Supplementary Fig. 4).
  • This error was estimated based on fitting three different continuum lines in ±2 Å range around the peak (linear fit, quadratic fit, and simultaneous fit to the DIB and a linear continuum33).
  • All distances to the target stars are measured from parallaxes from the second Gaia data re- lease (GDR2)34, 35.
  • The lack of constraints as a result of empty voxels renders the inversion method unable to accurately determine the shape of the clouds especially at the cloud edges (See Supplementary Fig. 6).
  • Λ5780 DIB vertical slices, also known as Supplementary Figure 2.

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A three-dimensional map of the hot Local Bubble using
diffuse interstellar bands
Amin Farhang
1,2
, Jacco Th. van Loon
3
, Habib G. Khosroshahi
1
, Atefeh Javadi
1
& Mandy Bailey
4
1
School of Astronomy, Institute for Research in Fundamental Sciences, 19395–5531 Tehran, Iran
2
Department of Physics and Astronomy, The University of Western Ontario, N6A 3K7, Canada
3
Lennard-Jones Laboratories, Keele University, ST5 5BG, UK
4
The Open University, Associate Lecturer Services (STEM), 351, Altrincham Road, Sharston,
Manchester, M22 4UN, UK
The Solar System is located within a low-density cavity, known as the Local Bubble
1, 2, 3
,
which appears to be filled with an X-ray emitting gas at a temperature of 10
6
K
4
. Such con-
ditions are too harsh for typical interstellar atoms and molecules to survive
2, 3
. There exists
an enigmatic tracer of interstellar gas, known as Diffuse Interstellar Bands (DIB), which of-
ten appears as absorption features in stellar spectra
5, 6, 7
. The carriers of these bands remain
largely unidentified
8
. Here we report the three-dimensional structure of the Local Bubble us-
ing two different DIB tracers (λ5780 and λ5797), which reveals that DIB carriers are present
within the Bubble
9, 10, 11
. The map shows low ratios of λ5797/λ5780 inside the Bubble com-
pared to the outside. This finding proves that the carrier of the λ5780 DIB can withstand
X-ray photo-dissociation and sputtering by fast ions, where the carrier of the λ5797 DIB
succumbs. This would mean that DIB carriers can be more stable than hitherto thought,
and that the carrier of the λ5780 DIB must be larger than that of the λ5797 DIB
12
. Alterna-
1
arXiv:1907.07429v1 [astro-ph.GA] 17 Jul 2019

tively, small-scale denser (and cooler) structures that shield some of the DIB carriers must
be prevalent within the Bubble, implying that such structures may be an intrinsic feature of
supernova-driven bubbles.
The origin of the Local Bubble (LB) is unknown, but measurements of
60
Fe column densi-
ties can be explained by successive explosions of massive stars (supernovae) within the Scorpius–
Centaurus stellar group
13
. The high temperature within the cavity is hostile to atoms, molecules
and dust grains
2, 3, 4
. Observations aimed at detecting highly ionized gas, which could be present at
million-degree temperatures, reveal no such plasma within the LB
14
. The DIB carriers offer an al-
ternative tracer of the wall and interior of the LB. Although their nature is unknown, recent studies
indicate that DIB carriers are likely carbon-based organic molecules
8
. They are universal and have
been detected in different environments of Milky Way (MW) and within different galaxies
5, 6, 7
and
in earlier partial results from our survey
9, 10, 11
. Our 3D map of the distribution of DIB carriers
within the vicinity of the Sun opens a new window on a substantial fraction of interstellar carbon
that may be locked up in the molecular carriers and that may play an important role in interstellar
chemistry; this may include the C
+
60
(“buckyballs”) anion which may be responsible for the DIBs
at 9577
˚
A and 9632
˚
A wavelengths
15
. Therefore, we employed two of the strongest DIB tracers at
5780
˚
A and 5797
˚
A wavelengths, in direction of 359 different sightlines, to mapped the LB out to
a distance of 200 pc. While the λ5780 DIB could possibly persist under harsh conditions of the
LB
16, 17
, the λ5797 DIB cannot survive in such an environment and must be shielded within the
inner regions of clouds
10
.
2

The outlines of the LB have been mapped with absorption lines of singly ionized calcium
(Ca II) and neutral sodium (Na I) in the spectra of background stars
2
, as well as by the attenuation
of stellar light by interstellar dust based on E(B V ) measurements
3, 18, 19
, revealing that the LB
cavity extends out to 80 pc in the Galactic plane (GP) and up to several hundred pc perpendiculars
to the GP. Some dust maps have been produced by translating the strength of λ15273 DIB into
E(B V ) and merging it with other E(B V ) measurements as priors; thus not strictly speaking
a DIB map, but really a dust map
18
. Also, a pseudo-3D map of the λ8620 DIB within 3 kpc from
the Sun have been produced recently
20
. They compared the DIB map with the dust distribution
and found the λ8620 DIB and dust to have a similar distribution, but the scale height of the DIB
exceeds that of the dust. This is a clear indication that DIBs and dust do not necessarily trace the
same interstellar material and/or conditions
9
. Their pseudo-3D map traces large-scale structure,
with the whole LB being confined to one voxel.
Because of the high temperature of the LB, it is difficult to detect ordinary atoms within
the LB. However, our observations show that DIB carriers are present within the LB
9, 10, 11
. An
example of different DIBs within and around the LB is shown in Fig. 1. To ensure that the observed
DIBs are physical features not noise, only those absorptions with a confidence level exceeding
3σ are accepted in the mapping. In the following, we present three principal slices of the 3D
distribution of the DIB pseudo volume density within 200 pc from the Sun. The maps are in
Galactic coordinates, i.e. the Sun is located at the origin and the primary direction is towards the
Galactic Center (GC). The slices show the maps in imaginary planes of the Galactic, meridian and
rotational planes. The GP slices the MW disk, the meridian plane is perpendicular to the GP with
3

its x-axis pointing towards the GC, and the rotational plane is perpendicular to the GP and faced
towards the GC.
In upper left panel of Fig. 2 (λ5780 in the GP) the bulk of the material lies in front of the
Scutum and Aquila dark nebulae (in the 1st quadrant in the upper-right sector) at distances of
30–120 pc from the Sun. The outer layer of this dense DIB structure coincides with a dense
concentration in the dust map, however, the inner region is stretched within the LB and continued
to interrupt the whole LB where there is not any dust
3
(see Supplementary Fig. 1 for an RGB map
of dust, DIB and Na I). A lower-density DIB filament lies in front of the Cygnus rift molecular
clouds. In this direction, X-rays with energies of order keV probably affect the DIB carriers beyond
a distance of approximately 100 pc
21
. In the 2nd quadrant (upper-left sector) DIB carriers are
abundant in the direction of the Taurus dark clouds and molecular. The 3rd quadrant (lower-left
sector) is characterized by a general paucity of the DIB carrier, in the direction of the β Canis
Majoris (CMa) interstellar tunnel and the GSH 238+00+09 supershell (l = 260
). However, a low-
density DIB trunk defies the odds; it may be associated with the photo-ionizing effect of βCMa
2
.
Finally, the 4th quadrant (lower-right sector) features the most famous cavity connected to the LB
(Loop I) in the direction of the GC (l = 345
) at a distance of 200 pc. A narrow tunnel connects
the LB with Loop I, as revealed in previous LB maps
2, 3
; our map reveals that it is filled with a
DIB filament and that Loop I itself is filled with DIB material as well (see Supplementary Fig. 2
for more slices).
The vertically extended structures are seen in the meridian plane (middle left panel of Fig. 2),
4

one towards the GC in front of the Ophiuchus and Lupus complexes and R Coronae Australis (CrA)
and another in the opposite direction in front of the Taurus star formation complex in between the
Hyades and Pleiades star clusters. The atomic maps of the LB in this view show a striking open-
ended tunnel known as the Local Chimney, tilted at an angle of 35
from the North Galactic Pole
(NGP)
22, 2
, but our map shows that the tunnel is not entirely devoid of DIB carriers. Interestingly,
there are some high latitude clouds (H I shells) in this direction
23
. The tilted tunnel can also be
discerned in the rotational plane, but most noticeable is the DIB complex in the 3rd quadrant and
material some 150 pc below the GP in the direction of the South Galactic Pole.
In the right column of Fig. 2, we present the λ5797 distributions. The carrier of the λ5797
DIB is highly susceptible to the destructive effect of energetic photons
9, 10
. The GP view shows
the largest concentrations lie between 30120 pc distance in the direction of Scutum and a filament
extends over 200 pc to meet Loop I, both akin to what was seen in the λ5780. Likewise, the Local
Chimney is visible in the meridian plane and a thin filament coincides with the densest λ5780 DIB
concentration in the 3rd quadrant of the rotational plane. Apart from the similarity between the
two DIB carriers there are also notable differences, with the λ5797 DIB distribution generally more
fragmented and more tenuous. The λ5780 DIB is known to trace relatively energetic environments
as compared to the λ5797 that traces more neutral, shielded regions (see Supplementary Fig. 3
where the strength of the λ5780 saturates at high Na I densities). Indeed, the ratio of the two DIB
carrier distributions shows that the interior of the LB is depleted in the λ5797 DIB carrier, more so
than the λ5780 DIB carrier (Fig. 3) though more neutral, shielded cloudlets seem to persist within
the LB.
5

Citations
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TL;DR: In this paper, the structure of the Galactic magnetic field on the Local Bubble (LB) scale was derived from 3D dust extinction maps of the local interstellar medium (ISM) and applied to the Planck 353 GHz dust polarized emission maps over the Galactic polar caps.
Abstract: The Sun is embedded in the so-called Local Bubble (LB)-a cavity of hot plasma created by supernova explosions and surrounded by a shell of cold, dusty gas. Knowing the local distortion of the Galactic magnetic field associated with the LB is critical for the modeling of interstellar polarization data at high Galactic latitudes. In this his paper, we relate the structure of the Galactic magnetic field on the LB scale to three-dimensional (3D) maps of the local interstellar medium (ISM). First, we extracted the geometry of the LB shell, its inner surface, in particular from 3D dust extinction maps of the local ISM. We expanded the shell inner surface in spherical harmonics, up to a variable maximum multipole degree, which enabled us to control the level of complexity for the modeled surface. Next, we applied an analytical model for the ordered magnetic field in the shell to the modeled shell surface. This magnetic field model was successfully fitted to the Planck 353 GHz dust polarized emission maps over the Galactic polar caps. For each polar cap, the direction of the mean magnetic field derived from dust polarization (together with the prior that the field points toward longitude 90 • ± 90 •) is found to be consistent with the Faraday spectra of the nearby diffuse synchrotron emission. Our work presents a new approach to modeling the local structure of the Galactic magnetic field. We expect our methodology and our results to be useful both in modeling the local ISM as traced by its different components and in modeling the dust polarized emission, which is a long-awaited input for studies of the polarized foregrounds for cosmic microwave background.

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TL;DR: In this article, the interrelation between five diffuse interstellar bands (DIBs) was investigated for high-resolution spectra with high signal-to-noise ratio (S/N) of 54 hot O -- B stars with reddening 0.12 -1.45 mag were used.
Abstract: Considered here is the interrelation between five diffuse interstellar bands (DIBs): $\lambda\lambda$ 5545, 6113, 6196, 6445 and 6614 A. Two DIBs ($\lambda$ 6196 and $\lambda$ 6614 A) have already been known as well correlated with each other; their relation with three other, weaker bands, was investigated for the first time. To accomplish this task high-resolution spectra ($\lambda/\Delta\lambda$ $\approx$ 100,000) with high signal-to-noise ratio (S/N) of 54 hot O -- B stars with reddening 0.12 -- 1.45 mag were used. Analysis of measured equivalent widths has allowed to establish linear dependencies and evaluate linear correlation coefficients as high as 0.968 -- 0.988 between the intensities of these five DIBs. Such a degree of correlation may indicate their common origin. Several spacings in wavenumbers found between these DIBs correspond to the energies of vibrational transitions in some PAHs resulting in IR emissions at $\lambda\lambda$ 16.4, 11.3, 7.7, 6.2 and 3.3 $\mu$m.

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TL;DR: In this paper, the authors studied the correlations between the equivalent widths of four diffuse interstellar bands and properties of the target stars (colour excess values, distances and Galactic coordinates) and their maps were produced and further analysed.
Abstract: With the use of the data from archives, we studied the correlations between the equivalent widths of four diffuse interstellar bands (4430$A$, 5780$A$, 5797$A$, 6284$A$) and properties of the target stars (colour excess values, distances and Galactic coordinates). Many different plots of the diffuse interstellar bands and their maps were produced and further analysed. There appears to be a structure in the plot of equivalent widths of 5780$A$ DIB (and 6284$A$ DIB) against the Galactic $x$-coordinate. The structure is well defined below $\sim150$ m$A$ and within $|x|<250$ pc, peaking around $x=170$ pc. We argue that the origin of this structure is not a statistical fluctuation. Splitting the data in the Galactic longitude into several subregions improves or lowers the well known linear relation between the equivalent widths and the colour excess, which was expected. However, some of the lines of sight display drastically different behaviour. The region within $150^\circ

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Cites result from "A three-dimensional map of the hot ..."

  • ...6 the comparison with the data from Bailey et al. (2016) and the recent work by Farhang et al. (2019) who studied mostly objects within 200 pc. Data from this work complete our picture of the map within 100 pc radius area where data from Table 1 is lacking in the number of observations....

    [...]

  • ...Finally, it is possible to compare our results from the wide-minimum region with the data from Bailey et al. (2016) and Farhang et al. (2019) which show basically the same result....

    [...]

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References
More filters
Journal ArticleDOI
TL;DR: In this article, a small survey of 19 DIBs' properties in 50 sightlines towards hot stars, distributed at a variety of galactic coordinates and interstellar reddening, is presented.
Abstract: Diffuse interstellar bands (DIBs) can trace different conditions of the ISM along the sightline toward the observed stars. A small survey was made in optical wavelengths, producing high resolution and high signal to noise spectra. We present measurements of 19 DIBs' properties in 50 sightlines towards hot stars, distributed at a variety of galactic coordinates and interstellar reddening. Equivalent widths were obtained by fitting asymmetric Gaussian and variable continuum to DIBs. Conditions of the ISM were calculated from 8 atomic and molecular interstellar lines. Two distinctively different types of DIBs were identified, by carefully comparing correlation coefficients between DIBs and reddening and by different behaviour in UV shielded ($\zeta$) and non-shielded ($\sigma$) sightlines. A ratio of DIBs at 5780 \AA\ and~5797 \AA\ proved to be reliable enough to distinguish between two different sightline types. Based on linear relations between DIB equivalent width and reddening for $\sigma$ and $\zeta$ sightlines, we divide DIBs into type {\sc i} (where both linear relations are similar) and type {\sc ii} (where they are significantly different). Linear relation for $\zeta$ type sightlines always show a higher slope and larger x-intercept parameter than the relation for $\sigma$ sightlines. Scatter around the linear relation is reduced after the separation, but it does not vanish completely. This means that UV shielding is the dominant factor of the DIB equivalent width vs.\ reddening relation shape for $\zeta$ sightlines, but in $\sigma$ sightlines other physical parameters play a major role. No similar dependency on gas density, electron density or turbulence was observed. A catalog of all observed interstellar lines is made public.

54 citations

Journal ArticleDOI
TL;DR: In this paper, the diffuse interstellar bands (DIBs) in the optical spectra of seven far-IR-selected starburst galaxies were detected with equivalent widths of 0.4-1 A and 0.1-0.6 A, respectively.
Abstract: We report the detection of the diffuse interstellar bands (DIBs) in the optical spectra of seven far-IR-selected starburst galaxies. The λλ6283.9 and 5780.5 features are detected with equivalent widths of ~0.4-1 A and 0.1-0.6 A, respectively. In the two starbursts with the highest quality spectra (M82 and NGC 2146), four other weaker DIBs at 5797.0, 6010.1, 6203.1, and 6613.6 A are detected with equivalent widths of ~0.1 A. The region over which the DIBs can be detected ranges from ~1 kpc in the less powerful starbursts to several kpc in the more powerful ones. The gas producing the DIBs is more kinematically quiescent on average than the gas producing the strongly blueshifted Na I λλ5890, 5896 absorption in the same starbursts. We show that the DIBs in these intense starbursts are remarkably similar to those in our Galaxy; the relative strengths of the features detected are similar, and the equivalent widths follow the same dependence as Galactic DIBs on E(B-V) and Na I column density. While the ISM in starbursts is heated by a photon and cosmic-ray bath that is ~103 times more intense than in the diffuse ISM of the Milky Way, the gas densities and pressures are also correspondingly larger in starbursts. This "homology" may help explain the strikingly similar DIB properties.

54 citations

Journal ArticleDOI
TL;DR: In this paper, the authors detect about 180 known DIBs, of which about 25 were listed as tentative and are now confirmed, and a dozen new DIB candidates longwards of 6900 A are identified.
Abstract: High-resolution VLT/UVES spectra of the strongly reddened O supergiant companion to the X-ray pulsar 4U 1907+09 provide a unique opportunity to study the nature of the diffuse interstellar bands (DIBs) at unprecedented strength. We detect about 180 known DIBs, of which about 25 were listed as tentative and are now confirmed. A dozen new DIB candidates longwards of 6900 A are identified. We show that the observed 5797 A DIB strength is in line with the Galactic correlation with reddening, whereas the 5780 A DIB strength is relatively weak. This indicates the contribution of denser regions, where the UV penetration is reduced. The presence of dense cloud cores is supported by the detection of C2 rotational transitions. Members of one DIB family (5797, 6379 A and 6196, 6613 A) behave coherently, although one can make a distinction between the two correlated pairs. The broadened profiles of narrow DIBs are shown to be consistent with the premise that each of the main clouds in the line of sight discerned in the interstellar K I profile is contributing proportionally to the DIB profile. We complement and extend the relation of DIB strength with reddening {E} (B-V) , as well as with neutral hydrogen column density N(H I), respectively, using strongly reddened sightlines towards another four distant HMXBs. The 5780 A DIB, and tentatively also the 5797 and 6613 A DIBs, are better correlated to the gas tracer H I than to the dust tracer {E} (B-V) . The resulting relationship can be applied to any line of sight to obtain an estimate of the H I column density. In the search for the nature of the DIB carrier, this strongly reddened line of sight is a complementary addition to single cloud line of sight studies.

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Journal ArticleDOI
15 Aug 2014-Science
TL;DR: This is the first pseudo–three-dimensional map of the strength of the DIB at 8620 angstroms covering the nearest 3 kiloparsecs from the Sun, and it follows the authors' independently constructed spatial distribution of extinction by interstellar dust along the Galactic plane.
Abstract: The diffuse interstellar bands (DIBs) are absorption lines observed in visual and near-infrared spectra of stars. Understanding their origin in the interstellar medium is one of the oldest problems in astronomical spectroscopy, as DIBs have been known since 1922. In a completely new approach to understanding DIBs, we combined information from nearly 500,000 stellar spectra obtained by the massive spectroscopic survey RAVE (Radial Velocity Experiment) to produce the first pseudo–three-dimensional map of the strength of the DIB at 8620 angstroms covering the nearest 3 kiloparsecs from the Sun, and show that it follows our independently constructed spatial distribution of extinction by interstellar dust along the Galactic plane. Despite having a similar distribution in the Galactic plane, the DIB 8620 carrier has a significantly larger vertical scale height than the dust. Even if one DIB may not represent the general DIB population, our observations outline the future direction of DIB research.

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TL;DR: In this article, the authors reported the detection of Diffuse Interstellar Bands (DIBs) in the optical spectra of seven far-infrared-selected starburst galaxies.
Abstract: We report the detection of the Diffuse Interstellar Bands (``$DIBs$'') in the optical spectra of seven far-infrared-selected starburst galaxies The $\lambda$62839 \AA and $\lambda$ 57805 \AA features are detected with equivalent widths of $\sim$ 04 to 1 \AA and 01 to 06 \AA respectively In the two starbursts with the highest quality spectra (M82 and NGC2146), four other weaker $DIBs$ at $\lambda$ 57970 \AA, 60101 \AA, 62031 \AA, and 66136 \AA are detected with equivalent widths of $\sim$ 01 \AA The region over which the $DIBs$ can be detected ranges from $\sim$ 1 kpc in the less powerful starbursts, to several kpc in the more powerful ones The gas producing the $DIBs$ is more kinematically quiescent on-average than the gas producing the strongly-blueshifted $NaI\lambda\lambda$5890,5896 absorption in the same starbursts We show that the $DIBs$ in these intense starbursts are remarkably similar to those in our Galaxy: the relative strengths of the features detected are similar, and the equivalent widths follow the same dependence as Galactic $DIBs$ on $E(B-V)$ and $NaI$ column density While the ISM in starbursts is heated by a photon and cosmic ray bath that is $\sim$ 10$^3$ times more intense than in the diffuse ISM of the Milky Way, the gas densities and pressures are also correspondingly larger in starbursts This ``homology'' may help explain the strikingly similar $DIB$ properties

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