87 research outputs found
The nature of the ISM in galaxies during the star-formation activity peak of the Universe
We combine a semi-analytic model of galaxy formation, tracking atomic and
molecular phases of cold gas, with a three-dimensional radiative-transfer and
line tracing code to study the sub-mm emission from atomic and molecular
species (CO, HCN, [CI], [CII], [OI]) in galaxies. We compare the physics that
drives the formation of stars at the epoch of peak star formation (SF) in the
Universe (z = 2.0) with that in local galaxies. We find that normal
star-forming galaxies at high redshift have much higher CO-excitation peaks
than their local counterparts and that CO cooling takes place at higher
excitation levels. CO line ratios increase with redshift as a function of
galaxy star-formation rate, but are well correlated with H2 surface density
independent of redshift. We find an increase in the [OI]/[CII] line ratio in
typical star-forming galaxies at z = 1.2 and z = 2.0 with respect to
counterparts at z = 0. Our model results suggest that typical star-forming
galaxies at high redshift consist of much denser and warmer star-forming clouds
than their local counterparts. Galaxies belonging to the tail of the SF
activity peak at z = 1.2 are already less dense and cooler than counterparts
during the actual peak of SF activity (z = 2.0). We use our results to discuss
how future ALMA surveys can best confront our predictions and constrain models
of galaxy formation.Comment: 19 pages, 14 figures, accepted for publication in MNRA
A survey of HC_3N in extragalactic sources: Is HC_3N a tracer of activity in ULIRGs?
Context. HC_3N is a molecule that is mainly associated with Galactic star-forming regions, but it has also been detected in extragalactic environments.
Aims. To present the first extragalactic survey of HC_3N, when combining earlier data from the literature with six new single-dish detections, and to compare HC_3N with other molecular tracers (HCN, HNC), as well as other properties (silicate absorption strength, IR flux density ratios, C_(II) flux, and megamaser activity).
Methods. We present mm IRAM 30 m, OSO 20 m, and SEST observations of HC_3N rotational lines (mainly the J = 10–9 transition) and of the J = 1–0 transitions of HCN and HNC. Our combined HC_3N data account for 13 galaxies (excluding the upper limits reported for the non-detections), while we have HCN and HNC data for more than 20 galaxies.
Results. A preliminary definition “HC_3N-luminous galaxy” is made based upon the HC_3N/HCN ratio. Most (~80%) HC_3N-luminous galaxies seem to be deeply obscured galaxies and (U)LIRGs. A majority (~60% or more) of the HC3N-luminous galaxies in the sample present OH mega- or strong kilomaser activity. A possible explanation is that both HC_3N and OH megamasers need warm dust for their excitation. Alternatively, the dust that excites the OH megamaser offers protection against UV destruction of HC_3N. A high silicate absorption strength is also found in several of the HC_3N-luminous objects, which may help the HC3N to survive. Finally, we find that a high HC_3N/HCN ratio is related to a high dust temperature and a low C_(II) flux
Observational study of hydrocarbons in the bright photodissociation region of Messier 8
Hydrocarbons are ubiquitous in the interstellar medium, but their formation
is still not well understood, depending on the physical environment they are
found in. M8 is host to one of the brightest HII regions and PDRs in our
galaxy. Using the APEX, and the IRAM 30 m telescopes, we performed a line
survey toward Herschel 36 (Her 36), which is the main ionizing stellar system
in M8, and an imaging survey within 1.3 1.3 pc around Her 36 of
various transitions of CH and c-CH. We used both LTE and
non-LTE methods to determine the physical conditions of the emitting gas along
with the column densities and abundances of the observed species, which we
compared with (updated) gas phase photochemical PDR models. In order to examine
the role of PAHs in the formation of small hydrocarbons and to investigate
their association with M8, we compared archival GLIMPSE 8 m and the SPIRE
250 m continuum images with the CH emission maps. We observed a
total of three rotational transitions of CH with their hyperfine
structure components and four rotational transitions of c-CH with
ortho and para symmetries toward M8. Fragmentation of PAHs seems less likely to
contribute to the formation of small hydrocarbons as the 8 m emission does
not follow the distribution of CH emission, which is more associated with
the molecular cloud. From the quantitative analysis, we obtained abundances of
10 and 10 for CH and c-CH respectively,
and volume densities of the hydrocarbon emitting gas in the range
5 10--5 10 cm. The observed column
densities of CH and c-CH are reproduced reasonably well by our
PDR models. This supports the idea that in high-UV flux PDRs, gas phase
chemistry is sufficient to explain hydrocarbon abundances.Comment: 14 pages, 10 figure
The ionized and hot gas in M17 SW: SOFIA/GREAT THz observations of [C II] and 12CO J=13-12
With new THz maps that cover an area of ~3.3x2.1 pc^2 we probe the spatial
distribution and association of the ionized, neutral and molecular gas
components in the M17 SW nebula. We used the dual band receiver GREAT on board
the SOFIA airborne telescope to obtain a 5'.7x3'.7 map of the 12CO J=13-12
transition and the [C II] 158 um fine-structure line in M17 SW and compare the
spectroscopically resolved maps with corresponding ground-based data for low-
and mid-J CO and [C I] emission. For the first time SOFIA/GREAT allow us to
compare velocity-resolved [C II] emission maps with molecular tracers. We see a
large part of the [C II] emission, both spatially and in velocity, that is
completely non-associated with the other tracers of photon-dominated regions
(PDR). Only particular narrow channel maps of the velocity-resolved [C II]
spectra show a correlation between the different gas components, which is not
seen at all in the integrated intensity maps. These show different morphology
in all lines but give hardly any information on the origin of the emission. The
[C II] 158 um emission extends for more than 2 pc into the M17 SW molecular
cloud and its line profile covers a broader velocity range than the 12CO
J=13-12 and [C I] emissions, which we interpret as several clumps and layers of
ionized carbon gas within the telescope beam. The high-J CO emission emerges
from a dense region between the ionized and neutral carbon emissions,
indicating the presence of high-density clumps that allow the fast formation of
hot CO in the irradiated complex structure of M17 SW. The [C II] observations
in the southern PDR cannot be explained with stratified nor clumpy PDR models.Comment: 4 pages, 4 figures, letter accepted for the SOFIA/GREAT A&A 2012
special issu
Disentangling the excitation conditions of the dense gas in M17 SW
We probe the chemical and energetic conditions in dense gas created by
radiative feedback through observations of multiple CO, HCN and HCO
transitions toward the dense core of M17 SW. We used the dual band receiver
GREAT on board the SOFIA airborne telescope to obtain maps of the ,
, and transitions of CO. We compare these maps with
corresponding APEX and IRAM 30m telescope data for low- and mid- CO, HCN and
HCO emission lines, including maps of the HCN and HCO
transitions. The excitation conditions of CO, HCO and HCN are
estimated with a two-phase non-LTE radiative transfer model of the line
spectral energy distributions (LSEDs) at four selected positions. The energy
balance at these positions is also studied. We obtained extensive LSEDs for the
CO, HCN and HCO molecules toward M17 SW. The LSED shape, particularly the
high- tail of the CO lines observed with SOFIA/GREAT, is distinctive for the
underlying excitation conditions. The critical magnetic field criterion implies
that the cold cloudlets at two positions are partially controlled by processes
that create and dissipate internal motions. Supersonic but sub-Alfv\'enic
velocities in the cold component at most selected positions indicates that
internal motions are likely MHD waves. Magnetic pressure dominates thermal
pressure in both gas components at all selected positions, assuming random
orientation of the magnetic field. The magnetic pressure of a constant magnetic
field throughout all the gas phases can support the total internal pressure of
the cold components, but it cannot support the internal pressure of the warm
components. If the magnetic field scales as , then the
evolution of the cold cloudlets at two selected positions, and the warm
cloudlets at all selected positions, will be determined by ambipolar diffusion.Comment: 26 pages, 13 figures, A&A accepte
Probing X-ray irradiation in the nucleus of NGC 1068 with observations of high-J lines of dense gas tracers
With the incorporation of high-J molecular lines, we aim to constrain the
physical conditions of the dense gas in the central region of the Seyfert 2
galaxy NGC 1068 and to determine signatures of the AGN or the starburst
contribution.
We used the James Clerk Maxwell Telescope to observe the J=4-3 transition of
HCN, HNC, and HCO+, as well as the CN N_J=2_{5/2}-1_{3/2} and
N_J=3_{5/2}-2_{5/2}, in NGC 1068.
We estimate the excitation conditions of HCN, HNC, and CN, based on the line
intensity ratios and radiative transfer models.
We find that the bulk emission of HCN, HNC, CN, and the high-J HCO+ emerge
from dense gas n(H_2)>=10^5 cm^-3). However, the low-J HCO+ lines (dominating
the HCO+ column density) trace less dense (n(H_2)<10^5 cm^-3) and colder
(T_K30 K) gas than
the other molecules.
The HCO+ J=4-3 line intensity, compared with the lower transition lines and
with the HCN J=4-3 line, support the influence of a local XDR environment. The
estimated N(CN)/N(HCN)~1-4 column density ratios are indicative of an XDR/AGN
environment with a possible contribution of grain-surface chemistry induced by
X-rays or shocks.Comment: Main text: 8 pages, 5 tables, 1 figure. Appendix: 7 pages, 1 table, 8
figures. Accepted for publication in A&
The deeply obscured AGN of NGC4945 I. Spitzer-IRS maps of [Ne V], [Ne II], H2 0-0 S(1), S(2), and other tracers
The nearly edge-on galaxy NGC4945 is one of the closest galaxies where an AGN
and starburst coexist, and is one of the brightest sources at 100 keV. Near and
mid-infrared spectroscopy have shown very strong obscuration of its central
region, rivaled only in strength by some of the most deeply obscured ULIRGs. We
aim to determine the spatial distribution of ISM features in the central
426x426 pc^2 of NGC4945. We map the central region of NGC4945 in three
Spitzer-IRS modules (SH, SL and LL). We produce maps of the flux distribution
of the starburst tracers [Ne II], [Ne III], [S III] and [S IV] at 12.81, 15.56,
18.71 and 10.51 mum, respectively, and a map of the AGN narrow-line region
tracer [Ne V] at 14.32 mum. We also mapped the S(1), S(2) and S(3) pure
rotational lines of H2, which trace the distribution of warm molecular
hydrogen. We obtained an extinction map (A_V) based on the apparent strength of
the 9.7 mum silicate absorption feature. Our A_V map traces the contours of the
starburst ring but the highest extinction (A_V(9.85 mum)~60) is found at the
nucleus. Within the uncertainty of the astrometry all emission lines are found
to peak on the nucleus, except for the warm molecular hydrogen emission which
shows a maximum 60-100 pc NW of the nucleus. We favour a scenario in which the
lower H2 0-0 S(1) and S(2) rotational lines originate mainly from an unobscured
extra-nuclear component associated with the super-wind cone observed in the HST
NICMOS map of the H2 1-0 S(1) vibrational line. For the [Ne V] emission we
infer an attenuation of a factor 12-160 (A_V=55-112) based on a comparison of
the ratio of our [Ne V] flux and the absorption-corrected 14-195 keV Swift-BAT
flux to the average [Ne V]/BAT ratio for Seyfert 1 nuclei. The high attenuation
indicates that [Ne V] and [O IV] cannot be used as extinction-free tracers of
AGN power in galaxies with deeply buried nuclei.Comment: 19 pages, 16 figures, accepted for publication in A&
Studying the effects and cause of the massive star formation in Messier 8 East
Messier 8 (M8), one of the brightest HII regions in our Galaxy, is associated
with two prominent massive star-forming regions: M8-Main, the particularly
bright part of the large scale HII region (mainly) ionised by the stellar
system Herschel 36 (Her 36) and M8 East (M8 E), which is mainly powered by a
deeply embedded young stellar object (YSO), a bright infrared (IR) source,
M8E-IR. We aim to study the interaction of the massive star-forming region M8 E
with its surroundings and to compare the star-forming environments of M8-Main
and M8 E. We used the IRAM 30 m telescope to perform an imaging spectroscopy
survey of the molecular environment of M8E-IR. We imaged and analysed data for
the = 1 0 rotational transitions of CO, CO, NH,
HCN, HCN, HCO, HCO, HNC and HNC observed for the
first time toward M8~E. We used LTE and non-LTE techniques to determine column
densities of the observed species and to constrain the physical conditions of
the gas responsible for their emission. Examining the YSO population in M8~E
allows us to explore the observed ionization front (IF) as seen in GLIMPSE
8~m emission image. We find that CO probes the warm diffuse gas
also traced by the GLIMPSE 8~m emission, while NH and HNC
trace the cool and dense gas. We find that the star-formation in M8~E appears
to be triggered by the earlier formed stellar cluster NGC~6530, which powers an
HII region giving rise to an IF that is moving at a speed
0.26~km~s across M8~E. We derive temperatures of 80 K and 30 K for the
warm and cool gas components, respectively, and constrain H volume
densities to be in the range of 10--10~cm. Comparison of the
observed abundances of various species reflects the fact that M8~E is at an
earlier stage of massive star formation than M8-Main
Radiation-driven Fountain and Origin of Torus around Active Galactic Nuclei
We propose a plausible mechanism to explain the formation of the so-called
"obscuring tori" around active galactic nuclei (AGNs) based on
three-dimensional hydrodynamic simulations including radiative feedback from
the central source. The X-ray heating and radiation pressure on the gas are
explicitly calculated using a ray-tracing method. This radiation feedback
drives a "fountain", that is, a vertical circulation of gas in the central a
few to tens parsecs. Interaction between the non-steady outflows and inflows
causes the formation of a geometrically thick torus with internal turbulent
motion. As a result, the AGN is obscured for a wide range of solid angles. In a
quasi-steady state, the opening angles for the column density toward a black
hole < 10^23 cm^-2 are approximately +-30 deg and +-50 deg for AGNs with 10%
and 1% Eddington luminosity, respectively. Mass inflows through the torus
coexist with the outflow and internal turbulent motion, and the average mass
accretion rate to the central parsec region is 2x10^-4 ~ 10^-3, M_sun/yr this
is about ten times smaller than accretion rate required to maintain the AGN
luminosity. This implies that relatively luminous AGN activity is intrinsically
intermittent or that there are other mechanisms, such as stellar energy
feedback, that enhance the mass accretion to the center.Comment: 12 pages, 9 figures, accepted publication in Ap
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