23,758 research outputs found
Evidence of ongoing radial migration in NGC 6754: Azimuthal variations of the gas properties
Understanding the nature of spiral structure in disk galaxies is one of the
main, and still unsolved questions in galactic astronomy. However, theoretical
works are proposing new testable predictions whose detection is becoming
feasible with recent development in instrumentation. In particular, streaming
motions along spiral arms are expected to induce azimuthal variations in the
chemical composition of a galaxy at a given galactic radius. In this letter we
analyse the gas content in NGC 6754 with VLT/MUSE data to characterise its 2D
chemical composition and H line-of-sight velocity distribution. We find
that the trailing (leading) edge of the NGC 6754 spiral arms show signatures of
tangentially-slower, radially-outward (tangentially-faster, radially-inward)
streaming motions of metal-rich (poor) gas over a large range of radii. These
results show direct evidence of gas radial migration for the first time. We
compare our results with the gas behaviour in a -body disk simulation
showing spiral morphological features rotating with a similar speed as the gas
at every radius, in good agreement with the observed trend. This indicates that
the spiral arm features in NGC 6754 may be transient and rotate similarly as
the gas does at a large range of radii.Comment: 8 pages, 4 figures, accepted for publication in ApJL 2016 September
2
The properties of the extended warm ionised gas around low-redshift QSOs and the lack of extended high-velocity outflows
(Abridged) We present a detailed analysis of a large sample of 31
low-redshift, mostly radio-quiet type 1 QSOs observed with integral field
spectroscopy to study their extended emission-line regions (EELRs). We focus on
the ionisation state of the gas, size and luminosity of extended narrow line
regions (ENLRs), which corresponds to those parts of the EELR dominated by
ionisation from the QSO, as well as the kinematics of the ionised gas. We
detect EELRs around 19 of our 31 QSOs (61%) after deblending the unresolved QSO
emission and the extended host galaxy light in the integral field data. We
identify 13 EELRs to be entirely ionised by the QSO radiation, 3 EELRs are
composed of HII regions and 3 EELRs display signatures of both ionisation
mechanisms at different locations. The typical size of the ENLR is 10kpc at a
median nuclear [OIII] luminosity of log(L([OIII])/[erg/s])=42.7+-0.15. We show
that the ENLR sizes are least a factor of 2 larger than determined with HST,
but are consistent with those of recently reported type 2 QSOs at matching
[OIII] luminosities. The ENLR of type 1 and type 2 QSOs appear to follow the
same size-luminosity relation. Furthermore, we show for the first time that the
ENLR size is much better correlated with the QSO continuum luminosity than with
the total/nuclear [OIII] luminosity. We show that ENLR luminosity and radio
luminosity are correlated, and argue that radio jets even in radio-quiet QSOs
are important for shaping the properties of the ENLR. Strikingly, the
kinematics of the ionised gas is quiescent and likely gravitationally driven in
the majority of cases and we find only 3 objects with radial gas velocities
exceeding 400km/s in specific regions of the EELR that can be associate with
radio jets. In general, these are significantly lower outflow velocities and
detection rates compared to starburst galaxies or radio-loud QSOs.Comment: 34 page, 22 figures (slightly degraded in resolution), 10 tables,
accepted for publication in A&A, minor corrections to match with the
publisher versio
The low-metallicity QSO HE 2158-0107: A massive galaxy growing by the accretion of nearly pristine gas from its environment?
[abridged] The metallicities of AGN are usually well above solar in their
NLR, often reaching up to several times solar in their broad-line regions.
Low-metallicity AGN are rare objects which have so far always been associated
with low-mass galaxies hosting low-mass BHs (M_BH<10^6Msun). In this paper we
present IFS data of the low-redshift QSO HE 2158-0107 for which we find strong
evidence for sub-solar NLR metallicities associated with a massive BH
(M_BH~3x10^8Msun). The QSO is surrounded by a large extended emission-line
region reaching out to 30kpc from the QSO in a tail-like geometry. We present
optical and near-IR images and investigate the properties of the host galaxy.
The SED of the host is rather blue, indicative of a significant young age
stellar population formed within the last 1Gyr. A 3sigma upper limit of
L_bulge<4.5x10^10Lsun for the H band luminosity and a corresponding stellar
mass upper limit of M_bulge<3.4x10^10Msun show that the host is offset from the
local BH-bulge relations. This is independently supported by the kinematics of
the gas. Although the stellar mass of the host galaxy is lower than expected,
it cannot explain the exceptionally low metallicity of the gas. We suggest that
the extended emission-line region and the galaxy growth are caused by the
infall of nearly pristine gas from the environment of the QSO host. Minor
mergers of dwarf galaxies or the theoretically predicted smooth accretion of
cold gas are both potential drivers behind that process. Since the metallicity
of the gas in the NLR is much lower than expected, we suspect that the external
gas has already reached the galaxy centre and may even contribute to the
current feeding of the BH. HE 2158-0107 appears to represent a particular phase
of substantial BH and galaxy growth that can be observationally linked with the
accretion of external material from its environment.Comment: 14 pages, 12 figures, accepted for publication in A&
Comment on "Theory of tailoring sonic devices: Diffraction dominates over refraction"
Recently N. Garcia et al. (Phys. Rev. E 67, 046606 (2003)) theoretically
studied several acoustic devices with dimensions on de order of several
wavelenghts. The authors discussed on experimental results previously reported
by several of us (F. Cervera et al., Phys. Rev. Lett. 88, 023902 (2002)). They
concluded that diffraction and not refraction is the ominating mechanism that
explain the focusing effects observed in those experiments. In this Comment we
reexamined their calculations and discussed why some of their interpretations
of our results are misleading.Comment: 2 pages, 2 figures, a comment on an articl
Deep into the Water Fountains: The case of IRAS 18043-2116
(Abridged) The formation of large-scale (hundreds to few thousands of AU)
bipolar structures in the circumstellar envelopes (CSEs) of post-Asymptotic
Giant Branch (post-AGB) stars is poorly understood. The shape of these
structures, traced by emission from fast molecular outflows, suggests that the
dynamics at the innermost regions of these CSEs does not depend only on the
energy of the radiation field of the central star. Deep into the Water
Fountains is an observational project based on the results of programs carried
out with three telescope facilities: The Karl G. Jansky Very Large Array
(JVLA), The Australia Telescope Compact Array (ATCA), and the Very Large
Telescope (SINFONI-VLT). Here we report the results of the observations towards
the WF nebula IRAS 180432116: Detection of radio continuum emission in the
frequency range 1.5GHz - 8.0GHz; HO maser spectral features and radio
continuum emission detected at 22GHz, and H ro-vibrational emission lines
detected at the near infrared. The high-velocity HO maser spectral
features, and the shock-excited H emission detected could be produced in
molecular layers which are swept up as a consequence of the propagation of a
jet-driven wind. Using the derived H column density, we estimated a
molecular mass-loss rate of the order of Myr. On the
other hand, if the radio continuum flux detected is generated as a consequence
of the propagation of a thermal radio jet, the mass-loss rate associated to the
outflowing ionized material is of the order of 10Myr.
The presence of a rotating disk could be a plausible explanation for the
mass-loss rates estimated.Comment: 10 pages, 5 figures. Accepted for publication in A&
Census of HII regions in NGC 6754 derived with MUSE: Constraints on the metal mixing scale
We present a study of the HII regions in the galaxy NGC 6754 from a two
pointing mosaic comprising 197,637 individual spectra, using Integral Field
Spectrocopy (IFS) recently acquired with the MUSE instrument during its Science
Verification program. The data cover the entire galaxy out to ~2 effective
radii (re ), sampling its morphological structures with unprecedented spatial
resolution for a wide-field IFU. A complete census of the H ii regions limited
by the atmospheric seeing conditions was derived, comprising 396 individual
ionized sources. This is one of the largest and most complete catalogue of H ii
regions with spectroscopic information in a single galaxy. We use this
catalogue to derive the radial abundance gradient in this SBb galaxy, finding a
negative gradient with a slope consistent with the characteristic value for
disk galaxies recently reported. The large number of H ii regions allow us to
estimate the typical mixing scale-length (rmix ~0.4 re ), which sets strong
constraints on the proposed mechanisms for metal mixing in disk galaxies, like
radial movements associated with bars and spiral arms, when comparing with
simulations. We found evidence for an azimuthal variation of the oxygen
abundance, that may be related with the radial migration. These results
illustrate the unique capabilities of MUSE for the study of the enrichment
mechanisms in Local Universe galaxies.Comment: 13 pages, 7 Figurs, accepted for publishing in A&
Frenkel Excitons in Random Systems With Correlated Gaussian Disorder
Optical absorption spectra of Frenkel excitons in random one-dimensional
systems are presented. Two models of inhomogeneous broadening, arising from a
Gaussian distribution of on-site energies, are considered. In one case the
on-site energies are uncorrelated variables whereas in the second model the
on-site energies are pairwise correlated (dimers). We observe a red shift and a
broadening of the absorption line on increasing the width of the Gaussian
distribution. In the two cases we find that the shift is the same, within our
numerical accuracy, whereas the broadening is larger when dimers are
introduced. The increase of the width of the Gaussian distribution leads to
larger differences between uncorrelated and correlated disordered models. We
suggest that this higher broadening is due to stronger scattering effects from
dimers.Comment: 9 pages, REVTeX 3.0, 3 ps figures. To appear in Physical Review
Hierarchically-structured metalloprotein composite coatings biofabricated from co-existing condensed liquid phases
Complex hierarchical structure governs emergent properties in biopolymeric materials; yet, the material processing involved remains poorly understood. Here, we investigated the multi-scale structure and composition of the mussel byssus cuticle before, during and after formation to gain insight into the processing of this hard, yet extensible metal cross-linked protein composite. Our findings reveal that the granular substructure crucial to the cuticle’s function as a wear-resistant coating of an extensible polymer fiber is pre-organized in condensed liquid phase secretory vesicles. These are phase-separated into DOPA-rich proto-granules enveloped in a sulfur-rich proto-matrix which fuses during secretion, forming the sub-structure of the cuticle. Metal ions are added subsequently in a site-specific way, with iron contained in the sulfur-rich matrix and vanadium coordinated by DOPA-catechol in the granule. We posit that this hierarchical structure self-organizes via phase separation of specific amphiphilic proteins within secretory vesicles, resulting in a meso-scale structuring that governs cuticle function
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