35 research outputs found
A search for CI J=2--1 emission in IRAS F10214+4724
Sensitive new observations of the fine structure line P
P (J=2--1) of the neutral atomic carbon CI ( GHz)
in the strongly lensed Ultra Luminous Infrared Galaxy (ULIRG) IRAS F10214+4724
at z=2.3 with the mm/sub-mm telescope James Clerk Maxwel (JCMT) are presented.
These do not confirm the presence of emission from this line at the flux levels
or angular extent previously reported in the literature. The new 2
upper limits are: \rm S_{CI}\la 7 Jy km s^{-1} (central position), and \rm <
S_{CI} > \la 8.5 Jy km s^{-1} (average over the two positions). A CI emission assumed fully concomitant with
the bulk of H and confined entirely within the strongly lensed object
yields an upper limit of \rm M_{CI}(H_2)\la 1.5 \times 10^{10} M_{\odot},
compatible with the reported CO-derived H gas mass, within the
uncertainties of the two methods. A comparison with the recent detection of the
P P (J=1--0) line in this galaxy by Weiss et al. (2004)
is made and the large discrepancy with the previous CI measurements is briefly
discussed.Comment: 8 pages, 3 figuers, accepted for publication in the Astrophysical
Journa
CI emission in Ultra Luminous Infrared Galaxies as a molecular gas mass tracer
We present new sensitive wide-band measurements of the fine structure line
3^P_1 -> 3^P_0 (J=1-0, 492GHz) of neutral atomic carbon (CI) in the two typical
Ultra Luminous Infrared Galaxies NGC6240 and Arp220. We then use them along
with several other CI measurements in similar objects found in the literature
to estimate their global molecular gas content under the assumption of a full
CI-H_2 concomitance. We find excellent agreement between the H_2 gas mass
estimated with this method and the standard methods using 12^CO. This may
provide a new way to measure H_2 gas mass in galaxies, and one which may be
very valuable in ULIRGs since in such systems the bright 12^CO emission is
known to systematically overestimate the gas mass while their 13^CO emission is
usually very weak. At redshifts z>=1 the CI J=1-0 line shifts to much more
favorable atmospheric windows and can become a viable alternative tracer of the
H_2 gas fueling starburst events in the distant Universe.Comment: 11 pages, 2 figures. Accepted for publication in ApJ Letter
First CO J=6-5, 4-3 detections in local ULIRGs: the dense gas in Mrk231, and its colling budget
We report on detections of the high-excitation CO J=6-5, J=4-3 lines in
Mrk231, a prototypical Ultra Luminous Infrared Galaxy (ULIRG) and Seyfert 1
QSO. These observations are combined with CO J=3-2, HCN J=4-3 (this work), and
CO J=2-1, J=1-0, 13CO J=2-1, HCN J=1-0 measurements taken from the literature
to provide better constraints on the properties of the molecular gas in an
extreme starburst/QSO in the local Universe. We find that the CO J=4-3 and
J=6-5 transitions trace a different gas phase from that dominating the lower
three CO transitions, with n(H_2) ~ (1-3)x10^4 cm-3 and Tk ~ (40-70) K. This
phase is responsible for the luminous HCN emission, and contains most of the H2
gas mass of this galaxy. The total CO line cooling emanating from this dense
phase is found similar to that of the [CII] line at 158 micron, suggesting a
very different thermal balance to that seen in lower IR-luminosity galaxies,
and one likely dominated by dense photon-dominated regions. Our dense
"sampling" of the CO rotational ladder and the HCN lines enables us to produce
well-constrained Spectral Line Energy Distributions (SLEDs) for the dense
molecular gas in Mrk231 and compare them to those of high redshift starbursts,
many of which have SLEDs that may be affected by strong lensing. Finally, we
use our local molecular line excitation template to assess the capabilities of
future cm and mm/sub-mm arrays in detecting CO and HCN transitions in similar
systems throughout the local and distant universe.Comment: accepted for publication in The Astrophysical Journal; 37 pages,
preprint format; 5 figures (2 in color
Extended dust emission and atomic hydrogen, a reservoir of diffuse H_2 in NGC 1068
We report on sensitive sub-mm imaging observations of the prototype
Seyfert~2/starburst galaxy NGC 1068 at 850 m and 450 m using the
Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell
Telescope (JCMT). We find clear evidence of dust emission associated with the
extended HI component which together with the very faint CO J=1--0
emission give a gas-to-dust ratio of . This contrasts with the larger ratio estimated within a galactocentric radius of kpc, where the
gas is mostly molecular and starburst activity occurs. The large gas-to-dust
ratio found for the starburst region is attributed to a systematic overestimate
of the molecular gas mass in starburst environments when the luminosity of the
CO J=1--0 line and a standard galactic conversion factor is used. On
the other hand sub-mm imaging proves to be a more powerful tool than
conventional CO imaging for revealing the properties of the diffuse
that coexists with HI. This molecular gas phase is characterized by low
densities ( cm), very faint emission from
sub-thermally excited CO, and contains more mass than HI, namely .Comment: Accepted for publication in the Astrophysical Journal Letter
HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies
It has been recently argued that the HCN J=1--0 line emission may not be an
unbiased tracer of dense molecular gas (\rm n\ga 10^4 cm^{-3}) in Luminous
Infrared Galaxies (LIRGs: ) and HCO J=1--0
may constitute a better tracer instead (Graci\'a-Carpio et al. 2006), casting
doubt into earlier claims supporting the former as a good tracer of such gas
(Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3
observations of four such galaxies are presented, revealing a surprisingly wide
excitation range for their dense gas phase that may render the J=1--0
transition from either species a poor proxy of its mass. Moreover the
well-known sensitivity of the HCO abundance on the ionization degree of the
molecular gas (an important issue omitted from the ongoing discussion about the
relative merits of HCN and HCO as dense gas tracers) may severely reduce
the HCO abundance in the star-forming and highly turbulent molecular gas
found in LIRGs, while HCN remains abundant. This may result to the decreasing
HCO/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and
casts doubts on the HCO rather than the HCN as a good dense molecular gas
tracer. Multi-transition observations of both molecules are needed to identify
the best such tracer, its relation to ongoing star formation, and constrain
what may be a considerable range of dense gas properties in such galaxies.Comment: 16 pages, 4 figures, Accepted for publication in the Astrophysical
Journa