We study the relation between molecular gas and star formation in a volume-limited sample of 222 galaxies from the COLD GASS survey, with measurements of the CO(1-0) line from the IRAM 30-m ...telescope. The galaxies are at redshifts 0.025 < z < 0.05 and have stellar masses in the range 10.0 < log M
★/M⊙ < 11.5. The IRAM measurements are complemented by deep Arecibo H i observations and homogeneous Sloan Digital Sky Survey and GALEX photometry. A reference sample that includes both ultraviolet (UV) and far-infrared data is used to calibrate our estimates of star formation rates from the seven optical/UV bands. The mean molecular gas depletion time-scale
for all the galaxies in our sample is 1 Gyr; however,
increases by a factor of 6 from a value of ∼0.5 Gyr for galaxies with stellar masses of ∼1010 M⊙ to ∼3 Gyr for galaxies with masses of a few ×1011 M⊙. In contrast, the atomic gas depletion time-scale remains constant at a value of around 3 Gyr. This implies that in high-mass galaxies, molecular and atomic gas depletion time-scales are comparable, but in low-mass galaxies, the molecular gas is being consumed much more quickly than the atomic gas. The strongest dependences of
are on the stellar mass of the galaxy parametrized as
, and on the specific star formation rate (sSFR). A single
versus sSFR relation is able to fit both 'normal' star-forming galaxies in our COLD GASS sample and more extreme starburst galaxies (luminous infrared galaxies and ultraluminous infrared galaxies), which have
yr. Normal galaxies at z = 1-2 are displaced with respect to the local galaxy population in the
versus sSFR plane and have molecular gas depletion times that are a factor of 3-5 times longer at a given value of sSFR due to their significantly larger gas fractions.
Abstract
Atomic oxygen is a main component of the mesosphere and lower thermosphere of the Earth, where it governs photochemistry and energy balance and is a tracer for dynamical motions. However, ...its concentration is extremely difficult to measure with remote sensing techniques since atomic oxygen has few optically active transitions. Current indirect methods involve photochemical models and the results are not always in agreement, particularly when obtained with different instruments. Here we present direct measurements—independent of photochemical models—of the ground state
3
P
1
→
3
P
2
fine-structure transition of atomic oxygen at 4.7448 THz using the German Receiver for Astronomy at Terahertz Frequencies (GREAT) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). We find that our measurements of the concentration of atomic oxygen agree well with atmospheric models informed by satellite observations. We suggest that this direct observation method may be more accurate than existing indirect methods that rely on photochemical models.
Context. M 33 is a gas rich spiral galaxy of the Local Group. Its vicinity allows us to study its interstellar medium (ISM) on linear scales corresponding to the sizes of individual giant molecular ...clouds. Aims. We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum. Methods. We mapped the emission of gas and dust in M 33 using the far-infrared lines of C II and O I(63 μm) and the total infrared continuum. The line maps were observed with the PACS spectrometer on board the Herschel Space Observatory. These maps have 50 pc resolution and form a ∼370 pc wide stripe along its major axis covering the sites of bright H II regions, but also more quiescent arm and inter-arm regions from the southern arm at 2 kpc galacto-centric distance to the south out to 5.7 kpc distance to the north. Full-galaxy maps of the continuum emission at 24 μm from Spitzer/MIPS, and at 70 μm, 100 μm, and 160 μm from Herschel/PACS were combined to obtain a map of the TIR. Results. TIR and C II intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of G0, obs ∼ 2 to 200 in units of the average Galactic radiation field. The binned C II/TIR ratio drops with rising TIR, with large, but decreasing scatter. The contribution of the cold neutral medium to the C II emission, as estimated from VLA H I data, is on average only 10%. Fits of modified black bodies to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the O I data before comparing it with models of photon dominated regions. Most of the ratios of C II/O I and (C II+O I)/TIR are consistent with two model solutions. The median ratios are consistent with one solution at n ∼ 2 × 102 cm−3, G0 ∼ 60, and a second low-FUV solution at n ∼ 104 cm−3, G0 ∼ 1.5. Conclusions. The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ∼1. A fraction of the gas may, however, be represented by the second solution.
This study is aimed to contribute to a more comprehensive understanding of the molecular hydrogen distribution in the galaxy M33 by introducing novel methods for generating high angular resolution ...($18.2''$, equivalent to $75\ pc $ for a distance of $847\ kpc $) column density maps of molecular hydrogen ($N_ H_2 $). M33 is a local group galaxy that has been observed with Herschel in the far-infrared (FIR) wavelength range from $70$ to $500\ Previous studies have presented total hydrogen column density maps ($N_ H $), using these FIR data (partly combined with mid-IR maps), employing various methods. We first performed a spectral energy distribution (SED) fit to the $160$, $250$, $350,$ and $500\ continuum data obtain $N_ H $, using a technique similar to one previously reported in the literature. We also use a second method which involves translating only the $250\ map into a $N_ H $ map at the same angular resolution of $18.2''$. An $N_ H_2 $ map via each method is then obtained by subtracting the component. Distinguishing our study from previous ones, we adopt a more versatile approach by considering a variable emissivity index, $ and dust absorption coefficient, $ This choice enables us to construct a $ map, thereby enhancing the depth and accuracy of our investigation of the hydrogen column density. We address the inherent biases and challenges within both methods (which give similar results) and compare them with existing maps available in the literature. Moreover, we calculate a map of the carbon monoxide $ CO(1-0) $-to-molecular hydrogen (htwo ) conversion factor ( factor), which shows a strong dispersion around an average value of $1.8 throughout the disk. We obtain column density probability distribution functions ($N$-PDFs) from the $N_ H $, $N_ H_2 $, and $N_ hi $ maps and discuss their shape, consisting of several log-normal and power-law tail components.
ABSTRACT We are conducting COLD GASS, a legacy survey for molecular gas in nearby galaxies. Using the IRAM 30-m telescope, we measure the CO(1-0) line in a sample of 350 nearby (Mpc), massive ...galaxies (log(M*/M) > 10.0). The sample is selected purely according to stellar mass, and therefore provides an unbiased view of molecular gas in these systems. By combining the IRAM data with Sloan Digital Sky Survey (SDSS) photometry and spectroscopy,GALEXimaging and high-quality Arecibo Hi data, we investigate the partition of condensed baryons between stars, atomic gas and molecular gas in 0.1-10L* galaxies. In this paper, we present CO luminosities and molecular hydrogen masses for the first 222 galaxies. The overall CO detection rate is 54 per cent, but our survey also uncovers the existence of sharp thresholds in galaxy structural parameters such as stellar mass surface density and concentration index, below which all galaxies have a measurable cold gas component but above which the detection rate of the CO line drops suddenly. The mean molecular gas fractionof the CO detections is 0.066 ± 0.039, and this fraction does not depend on stellar mass, but is a strong function of (NUV -r) colour. Through stacking, we set a firm upper limit offor red galaxies with NUV -r> 5.0. The average molecular-to-atomic hydrogen ratio in present-day galaxies is 0.3, with significant scatter from one galaxy to the next. The existence of strong detection thresholds in both the Hi and CO lines suggests that 'quenching' processes have occurred in these systems. Intriguingly, atomic gas strongly dominates in the minority of galaxies with significant cold gas that lie above these thresholds. This suggests that some re-accretion of gas may still be possible following the quenching event. PUBLICATION ABSTRACT
We are conducting COLD GASS, a legacy survey for molecular gas in nearby galaxies. Using the IRAM 30-m telescope, we measure the CO(1−0) line in a sample of ∼350 nearby (
Mpc), massive galaxies ...(log(M
*/M⊙) > 10.0). The sample is selected purely according to stellar mass, and therefore provides an unbiased view of molecular gas in these systems. By combining the IRAM data with Sloan Digital Sky Survey (SDSS) photometry and spectroscopy, GALEX imaging and high-quality Arecibo H i data, we investigate the partition of condensed baryons between stars, atomic gas and molecular gas in 0.1-10L* galaxies. In this paper, we present CO luminosities and molecular hydrogen masses for the first 222 galaxies. The overall CO detection rate is 54 per cent, but our survey also uncovers the existence of sharp thresholds in galaxy structural parameters such as stellar mass surface density and concentration index, below which all galaxies have a measurable cold gas component but above which the detection rate of the CO line drops suddenly. The mean molecular gas fraction
of the CO detections is 0.066 ± 0.039, and this fraction does not depend on stellar mass, but is a strong function of (NUV − r) colour. Through stacking, we set a firm upper limit of
for red galaxies with NUV − r > 5.0. The average molecular-to-atomic hydrogen ratio in present-day galaxies is 0.3, with significant scatter from one galaxy to the next. The existence of strong detection thresholds in both the H i and CO lines suggests that 'quenching' processes have occurred in these systems. Intriguingly, atomic gas strongly dominates in the minority of galaxies with significant cold gas that lie above these thresholds. This suggests that some re-accretion of gas may still be possible following the quenching event.
Abstract
The C
ii
fine-structure transition at 158
μ
m is frequently the brightest far-infrared line in galaxies. Due to its low ionization potential, C
+
can trace the ionized, atomic, and ...molecular phases of the ISM. We present velocity-resolved C
ii
and N
ii
pointed observations from SOFIA/GREAT on ∼500 pc scales in the nearby galaxies M101 and NGC 6946 and investigate the multiphase origin of C
ii
emission over a range of environments. We show that ionized gas makes a negligible contribution to the C
ii
emission in these positions using N
ii
observations. We spectrally decompose the C
ii
emission into components associated with the molecular and atomic phases using existing CO (2–1) and H
i
data and show that a peak signal-to-noise ratio of 10–15 is necessary for a reliable decomposition. In general, we find that in our pointings ≳50% of the C
ii
emission arises from the atomic phase, with no strong dependence on star formation rate, metallicity, or galactocentric radius. We do find a difference between pointings in these two galaxies, where locations in NGC 6946 tend to have larger fractions of C
ii
emission associated with the molecular phase than in M101. We also find a weak but consistent trend for fainter C
ii
emission to exhibit a larger contribution from the atomic medium. We compute the thermal pressure of the cold neutral medium through the C
ii
cooling function and find
log
(
P
th
/
k
)
=
3.8
–
4.6
K
cm
−
3
, a value slightly higher than similar determinations, likely because our observations are biased toward star-forming regions.
Abstract
We have revisited the chemistry of chlorine-bearing species in the diffuse interstellar medium with new observations of the HCl
+
molecular ion and new astrochemical models. Using the GREAT ...instrument on board SOFIA, we observed the
2
Π
3/2
J
= 5/2 − 3/2 transition of HCl
+
near 1444 GHz toward the bright THz continuum source W49N. We detected absorption by diffuse foreground gas unassociated with the background source, and were able to thereby measure the distribution of HCl
+
along the sight line. We interpreted the observational data using an updated version of an astrochemical model used previously in a theoretical study of Cl-bearing interstellar molecules. The abundance of HCl
+
was found to be almost constant relative to the related H
2
Cl
+
ion, but the observed
n
(H
2
Cl
+
)/
n
(HCl
+
) abundance ratio exceeds the predictions of our astrochemical model by an order of magnitude. This discrepancy suggests that the rate of the primary destruction process for H
2
Cl
+
, dissociative recombination, has been significantly overestimated. For HCl
+
, the model predictions can provide a satisfactory fit to the observed column densities along the W49N sight line while simultaneously accounting for the OH
+
and H
2
O
+
column densities.
We present the first complete, velocity-resolved C ii 158 m image of the M51 grand-design spiral galaxy, observed with the upgraded German Receiver for Astronomy at Terahertz frequencies instrument ...on the Stratospheric Observatory for Infrared Astronomy. C ii is an important tracer of various phases of the interstellar medium (ISM), including ionized gas, neutral atomic, and diffuse molecular regions. We combine the C ii data with H i, CO, 24 m dust continuum, FUV, and NIR K-band observations to study the evolution of the ISM across M51's spiral arms in both position-position and position-velocity space. Our data show strong velocity gradients in H i, 12CO, and C ii at the locations of the stellar arms (traced by K-band data) with a clear offset in position-velocity space between upstream molecular gas (traced by 12CO) and downstream star formation (traced by C ii). We compare the observed position-velocity maps across the spiral arms with synthetic observations from numerical simulations of galaxies with both dynamical and quasi-stationary steady spiral arms that predict both tangential and radial velocities at the location of the spiral arms. We find that our observations, based on the observed velocity gradients and associated offset between CO and C ii, are consistent with the presence of shocks in the spiral arms in the inner parts of M51 and in the arm connecting the companion galaxy, M51b, in the outer parts of M51.
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