ABSTRACT As part of the Megamaser Cosmology Project, we present VLBI maps of nuclear water masers toward five galaxies. The masers originate in sub-parsec circumnuclear disks. For three of the ...galaxies, we fit Keplerian rotation curves to estimate their supermassive black hole (SMBH) masses, and determine (2.9 0.3) × 106 M for J0437+2456, (1.7 0.1) × 107 M for ESO 558-G009, and (1.1 0.2) × 107 M for NGC 5495. In the other two galaxies, Mrk 1029 and NGC 1320, the geometry and dynamics are more complicated and preclude robust black hole mass estimates. Including our new results, we compiled a list of 15 VLBI-confirmed disk maser galaxies with robust SMBH mass measurements. With this sample, we confirm the empirical relation of Rout ∝ 0.3MSMBH reported in Wardle & Yusef-Zadeh. We also find a tentative correlation between maser disk outer radii and Wide-Field Infrared Survey Explorer luminosity. We find no correlations of maser disk size with X-ray 2-10 keV luminosity or O iii luminosity.
We used the Atacama Pathfinder Experiment (APEX) 12 m telescope to observe the JKAKc = 303 → 202, 322 → 221, and 321 → 220 transitions of para-H2CO at 218GHz simultaneously to determine kinetic ...temperatures of the dense gas in the central molecular zone (CMZ) of our Galaxy. The map extends over approximately 40′ × 8′ (~100 × 20pc2) along the Galactic plane with a linear resolution of 1.2pc. The strongest of the three lines, the H2CO (303 → 202) transition, is found to be widespread, and its emission shows a spatial distribution similar to ammonia. The relative abundance of para-H2CO is 0.5−1.2 × 10-9, which is consistent with results from lower frequency H2CO absorption lines. Derived gas kinetic temperatures for individual molecular clouds range from 50K to values in excess of 100K. While a systematic trend toward (decreasing) kinetic temperature versus (increasing) angular distance from the Galactic center (GC) is not found, the clouds with highest temperature (Tkin> 100K) are all located near the nucleus. For the molecular gas outside the dense clouds, the average kinetic temperature is 65 ± 10K. The high temperatures of molecular clouds on large scales in the GC region may be driven by turbulent energy dissipation and/or cosmic-rays instead of photons. Such a non-photon-driven thermal state of the molecular gas provides an excellent template for the more distant vigorous starbursts found in ultraluminous infrared galaxies (ULIRGs).
We compare the relations among various integrated characteristics of ∼25 000 low-redshift (
z
≲ 1.0) compact star-forming galaxies (CSFGs) from Data Release 16 (DR16) of the Sloan Digital Sky Survey ...and of high-redshift (
z
≳ 1.5) star-forming galaxies (SFGs) with respect to oxygen abundances, stellar masses
M
⋆
, far-UV absolute magnitudes
M
FUV
, star-formation rates SFR and specific star-formation rates sSFR, Lyman-continuum photon production efficiencies (
ξ
ion
), UV continuum slopes
β
, O
III
λ
5007/O
II
λ
3727 and Ne
III
λ
3868/O
II
λ
3727 ratios, and emission-line equivalent widths EW(O
II
λ
3727), EW(O
III
λ
5007), and EW(H
α
). We find that the relations for low-
z
CSFGs with high equivalent widths of the H
β
emission line, EW(H
β
) ≥ 100 Å, and high-
z
SFGs are very similar, implying close physical properties in these two categories of galaxies. Thus, CSFGs are likely excellent proxies for the SFGs in the high-
z
Universe. They also extend to galaxies with lower stellar masses, down to ∼10
6
M
⊙
, and to absolute FUV magnitudes as faint as −14 mag. Thanks to their proximity, CSFGs can be studied in much greater detail than distant SFGs. Therefore, the relations between the integrated characteristics of the large sample of CSFGs studied here can prove very useful for our understanding of high-
z
dwarf galaxies in future observations with large ground-based and space telescopes.
We present the direct measurement of the Hubble constant, yielding the direct measurement of the angular-diameter distance to NGC 6264 using the H sub(2)O megamaser technique. Our measurement is ...based on sensitive observations of the circumnuclear megamaser disk from four observations with the Very Long Baseline Array, the Green Bank Telescope (GBT), and the Effelsberg telescope. We also monitored the maser spectral profile for 2.3 years using the GBT to measure accelerations of maser lines by tracking their line-of-sight velocities as they change with time. The measured accelerations suggest that the systemic maser spots have a significantly wider radial distribution than in the archetypal megamaser in NGC 4258. We model the maser emission as arising from a circumnuclear disk with orbits dominated by the central black hole. The best fit of the data gives a Hubble constant of H sub(0) = 68 + or - 9 km s super(-1) Mpc super(-1), which corresponds to an angular-diameter distance of 144 + or - 19 Mpc. In addition, the fit also gives a mass of the central black hole of (3.09 + or - 0.42) x 10 super(7) M sub(middot in circle). The result demonstrates the feasibility of measuring distances to galaxies located well into the Hubble flow by using circumnuclear megamaser disks.
The Serpens filament, as one of the nearest infrared dark clouds, is regarded as a pristine filament at a very early evolutionary stage of star formation. In order to study its molecular content and ...dynamical state, we mapped this filament in seven species: C18O, HCO+, HNC, HCN, N2H+, CS, and CH3OH. Among them, HCO+, HNC, HCN, and CS show self-absorption, while C18O is most sensitive to the filamentary structure. A kinematic analysis demonstrates that this filament forms a velocity-coherent (trans)sonic structure, a large part of which is one of the most quiescent regions in the Serpens cloud. Widespread C18O depletion is found throughout the Serpens filament. Based on the Herschel dust-derived H2 column density map, the line mass of the filament is 36–41 M⊙ pc−1, and its full width at half maximum is 0.17 ± 0.01 pc, while its length is ≈1.6 pc. The inner radial column density profile of this filament can be well fitted with a Plummer profile with an exponent of 2.2 ± 0.1, a scale radius of 0.018 ± 0.003 pc, and a central density of (4.0 ± 0.8) × 104 cm−3. The Serpens filament appears to be slightly supercritical. The widespread blue-skewed HNC and CS line profiles and HCN hyperfine line anomalies across this filament indicate radial infall in parts of the Serpens filament. C18O velocity gradients also indicate accretion flows along the filament. The velocity and density structures suggest that such accretion flows are likely due to a longitudinal collapse parallel to the filament’s long axis. Both the radial infall rate (~72 M⊙ Myr−1, inferred from HNC and CS blue-skewed profiles) and the longitudinal accretion rate (~10 M⊙ Myr−1, inferred from C18O velocity gradients) along the Serpens filament are lower than all previously reported values in other filaments. This indicates that the Serpens filament lies at an early evolutionary stage when collapse has just begun, or that thermal and nonthermal support are effective in providing support against gravity.
Context. Observations of chemical species can provide insights into the physical conditions of the emitting gas however it is important to understand how their abundances and excitation vary within ...different heating environments. C2H is a molecule typically found in PDR regions of our own Galaxy but there is evidence to suggest it also traces other regions undergoing energetic processing in extragalactic environments. Aims. As part of the ALCHEMI ALMA large program, we map the emission of C2H in the central molecular zone of the nearby starburst galaxy NGC 253 at 1.6″ (28 pc) resolution and characterize it to understand its chemical origins. Methods. We used spectral modeling of the N = 1−0 through N = 4−3 rotational transitions of C2H to derive the C2H column densities towards the dense clouds in NGC 253. We then use chemical modeling, including photodissociation region (PDR), dense cloud, and shock models to investigate the chemical processes and physical conditions that are producing the molecular emission. Results. We find high C2H column densities of ∼1015 cm−2 detected towards the dense regions of NGC 253. We further find that these column densities cannot be reproduced if it is assumed that the emission arises from the PDR regions at the edge of the clouds. Instead, we find that the C2H abundance remains high even in the high visual extinction interior of these clouds and that this is most likely caused by a high cosmic-ray ionization rate.
We present a survey of atomic carbon (C I) emission in high-redshift (z > 2) submillimeter galaxies and quasar host galaxies. Sensitive observations of the C I (3 P 1 -> 3 P 0) and C I (3 P 2 -> 3 P ...1) lines have been obtained at the IRAM Plateau de Bure interferometer and the IRAM 30 m telescope. A total of 16 C I lines have been targeted in 10 sources, leading to a total of 10 detected lines--this doubles the number of C I observations at high redshift to date. We include previously published C I observations (an additional five detected sources) in our analysis. Our main finding is that the C I properties of the high-redshift galaxies studied here do not differ significantly from what is found in low-redshift systems, including the Milky Way. The C I (3 P 2 -> 3 P 1)/C I (3 P 1 -> 3 P 0) and the C I (3 P 1 -> 3 P 0)/12CO(3-2) line luminosity (L') ratios change little in our sample, with respective ratios of 0.55 ? 0.15 and 0.32 ? 0.13. The C I lines are not an important contributor to cooling of the molecular gas (average L C I /L FIR ~ (7.7 ? 4.6) X 10--6). We derive a mean carbon excitation temperature of 29.1 ? 6.3 K, broadly consistent with dust temperatures derived for high-redshift star-forming systems, but lower than gas temperatures typically derived for starbursts in the local universe. The carbon abundance of XC I/XH2 ~ (8.4?3.5)X10--5 is of the same order as found in the Milky Way and nearby galaxies. This implies that the high-z galaxies studied here are significantly enriched in carbon on galactic scales, even though the look-back times are considerable (the average redshift of the sample sources corresponds to an age of the universe of ~2 Gyr).
We present the distribution and kinematics of the molecular gas in the circumnuclear disk (CND; 400 pc × 200 pc) of Centaurus A with resolutions of ∼5 pc (0 3) and shed light onto the mechanism ...feeding the active galactic nucleus (AGN) using CO(3-2), HCO+(4-3), HCN(4-3), and CO(6-5) observations obtained with ALMA. Multiple filaments or streamers of tens to a hundred parsec scale exist within the CND, which form a ring-like structure with an unprojected diameter of 9″ × 6″ (162 pc × 108 pc) and a position angle P.A. 155°. Inside the nuclear ring, there are two leading and straight filamentary structures with lengths of about 30-60 pc at P.A. 120° on opposite sides of the AGN, with a rotational symmetry of 180° and steeper position-velocity diagrams, which are interpreted as nuclear shocks due to non-circular motions. Along the filaments, and unlike other nearby AGNs, several dense molecular clumps present low HCN/HCO+(4-3) ratios ( 0.5). The filaments abruptly end in the probed transitions at r 20 pc from the AGN, but previous near-IR H2(J = 1-0)S(1) maps show that they continue in an even warmer gas phase (T ∼ 1000 K), winding up in the form of nuclear spirals, and forming an inner ring structure with another set of symmetric filaments along the N-S direction and within r 10 pc. The molecular gas is governed primarily by non-circular motions, being the successive shock fronts at different scales where loss of angular momentum occurs, a mechanism that may feed efficiently powerful radio galaxies down to parsec scales.
We introduce the Dwarf Galaxy Survey with Amateur Telescopes (DGSAT) project and report the discovery of eleven low surface brightness (LSB) galaxies in the fields of the nearby galaxies NGC 2683, ...NGC 3628, NGC 4594 (M104), NGC 4631, NGC 5457 (M101), and NGC 7814. The DGSAT project aims to use the potential of small-sized telescopes to probe LSB features around large galaxies and to increase the sample size of the dwarf satellite galaxies in the Local Volume. Using long exposure images, fields of the target spiral galaxies are explored for extended LSB objects. After identifying dwarf galaxy candidates, their observed properties are extracted by fitting models to their light profiles. We find three, one, three, one, one, and two new LSB galaxies in the fields of NGC 2683, 3628, 4594, 4631, 5457, and 7814, respectively. To determine whether these LSB galaxies are indeed satellites of the above mentioned massive galaxies, their distances need to be determined via further observations.
To investigate the relative amount of ejecta from high-mass versus intermediate-mass stars and to trace the chemical evolution of the Galaxy, we have performed a systematic study of Galactic ...interstellar 18O/17O ratios toward a sample of 421 molecular clouds with IRAM 30 m and the 10 m Submillimeter Telescope, covering a galactocentric distance range of ∼1–22 kpc. The results presented in this paper are based on the J = 2–1 transition and encompass 364 sources showing both C18O and C17O detections. The previously suggested 18O/17O gradient is confirmed. For the 41 sources detected with both facilities, good agreement is obtained. A correlation of the 18O/17O ratios with heliocentric distance is not found, indicating that beam dilution and linear beam sizes are not relevant. For the subsample of IRAM 30 m high-mass star-forming regions with accurate parallax distances, an unweighted fit gives 18O/17O = (0.12 ± 0.02)RGC + (2.38 ± 0.13) with a correlation coefficient of R = 0.67. While the slope is consistent with our J = 1–0 measurement, the ratios are systematically lower. This should be caused by larger optical depths of C18O 2–1 lines with respect to the corresponding 1–0 transitions, which is supported by RADEX calculations and the fact that C18O/C17O is positively correlated with 13CO/C18O. When we consider that optical depth effects with C18O J = 2–1 typically reach an optical depth of ∼0.5, the corrected 18O/17O ratios from the J = 1–0 and J = 2–1 lines are consistent. A good numerical fit to the data is provided by the MWG-12 model, which includes both rotating stars and novae.
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