We present the HERA CO-Line Extragalactic Survey (HERACLES), an atlas of CO emission from 18 nearby galaxies that are also part of The H I Nearby Galaxy Survey (THINGS) and the Spitzer Infrared Nearby Galaxies Survey (SINGS). We used the HERA multi-pixel receiver on the IRAM 30-m telescope to map the CO J = 2 → 1 line over the full optical disk (defined by the isophotal radius r 25 ) of each target, at 13 ′′ angular resolution and 2.6 km s −1 velocity resolution. Here we describe the observations and reduction of the data and show channel maps, azimuthally averaged profiles, integrated intensity maps, and peak intensity maps. The implied H 2 masses range from 7 × 10 6 to 6 × 10 9 M ⊙ , with four low metallicity dwarf irregular galaxies yielding only upper limits. In the cases where CO is detected, the integrated H 2 -to-H I ratios range from 0.02 -1.13 and H 2 -to-stellar mass ratios from 0.01 to 0.25. Exponential scale lengths of the CO emission for our targets are in the range 0.8 -3.2 kpc, or 0.2 ± 0.05 r 25 . The intensity-weighted mean velocity of CO matches that of H I very well, with a 1σ scatter of only 6 km s −1 . The CO J = 2 → 1/J = 1 → 0 line ratio varies over a range similar to that found in the Milky Way and other nearby galaxies, ∼ 0.6-1.0, with higher values found in the centers of galaxies. The typical line ratio, ∼ 0.8, could be produced by optically thick gas with an excitation temperature of ∼ 10 K.

We use the IRAM HERACLES survey to study CO emission from 33 nearby spiral galaxies down to very low intensities. Using 21 cm line atomic hydrogen (H i) data, mostly from THINGS, we predict the local mean CO velocity based on the mean H i velocity. By re-normalizing the CO velocity axis so that zero corresponds to the local mean H i velocity we are able to stack spectra coherently over large regions. This enables us to measure CO intensities with high significance as low as, an improvement of about one order of magnitude over previous studies. We detect CO out to galactocentric radii r gal ∼ r 25 and find the CO radial profile to follow a remarkably uniform exponential decline with a scale length of ∼0.2 r 25 . Here we focus on stacking as a function of radius, comparing our sensitive CO profiles to matched profiles of H i, Hα, far-UV (FUV), and Infrared (IR) emission at 24 μm and 70 μm. We observe a tight, roughly linear relationship between CO and IR intensity that does not show any notable break between regions that are dominated by molecular gas (Σ H 2 > Σ H i ) and those dominated by atomic gas (Σ H 2 < Σ H i ). We use combinations of FUV + 24 μm and Hα + 24 μm to estimate the recent star formation rate (SFR) surface density, Σ SFR , and find approximately linear relations between Σ SFR and Σ H 2 . We interpret this as evidence of stars forming in molecular gas with little dependence on the local total gas surface density. While galaxies display small internal variations in the SFR-to-H 2 ratio, we do observe systematic galaxy-to-galaxy variations. These galaxy-to-galaxy variations dominate the scatter in relationships between CO and SFR tracers measured at large scales. The variations have the sense that less massive galaxies exhibit larger ratios of SFR-to-CO than massive galaxies. Unlike the SFR-to-CO ratio, the balance between atomic and molecular gas depends strongly on the total gas surface density and galactocentric radius. It must also depend on additional parameters. Our results reinforce and extend to lower surface densities, a picture in which star formation in galaxies can be separated into two processes: the assembly of star-forming molecular clouds and the formation of stars from H 2 . The interplay between these processes yields a total gas-SFR relation with a changing slope, which has previously been observed and identified as a star formation threshold.

We combine new sensitive, wide-field CO data from the HERACLES survey with ultraviolet and infrared data from GALEX and Spitzer to compare the surface densities of H 2 , Σ H2 , and the recent star formation rate, Σ SFR , over many thousands of positions in 30 nearby disk galaxies. We more than quadruple the size of the galaxy sample compared to previous work and include targets with a wide range of galaxy properties. Even though the disk galaxies in this study span a wide range of properties, we find a strong, and approximately linear correlation between Σ SFR and Σ H2 at our common resolution of 1 kpc. This implies a roughly constant median H 2 consumption time, τ H2 Dep = Σ H2 /Σ SFR , of ∼ 2.35 Gyr (including heavy elements) across our sample. At 1 kpc resolution, there is only a weak correlation between Σ H2 and τ H2 Dep over the range Σ H2 ≈ 5-100 M ⊙ pc −2 , which is probed by our data. We compile a broad set of literature measurements that have been obtained using a variety of star formation tracers, sampling schemes and physical scales and show that overall, these data yield almost exactly the same results, although with more scatter. We interpret these results as strong, albeit indirect evidence that star formation proceeds in a uniform way in giant molecular clouds in the disks of spiral galaxies.

We present maps of 12 CO J = 2 − 1 emission covering the entire star-forming disks of 16 nearby dwarf galaxies observed by the IRAM HERACLES survey. The data have 13 ′′ angular resolution, ∼ 250 pc at our average distance of D = 4 Mpc, and sample the galaxies by 10 − 1000 resolution elements. We apply stacking techniques to perform the first sensitive search for CO emission in dwarf galaxies outside the Local Group ranging from individual lines-of-sight, stacking over IR-bright regions of embedded star formation, and stacking over the entire galaxy. We detect 5 galaxies in CO with total CO luminosities of L CO 2−1 = 3 − 28 × 10 6 K km s −1 pc 2 . The other 11 galaxies remain undetected in CO even in the stacked images and have L CO 2−1 0.4 − 8 × 10 6 K km s −1 pc 2 . We combine our sample of dwarf galaxies with a large sample of spiral galaxies from the literature to study scaling relations of L CO with M B and metallicity. We find that dwarf galaxies with metallicities of Z ≈ 1/2 − 1/10 Z ⊙ have L CO of 2 − 4 orders of magnitude smaller than massive spiral galaxies and that their L CO per unit L B is 1 − 2 orders of magnitude smaller. A comparison with tracers of star formation (FUV and 24µm) shows that L CO per unit SFR is 1 − 2 orders of magnitude smaller in dwarf galaxies. One possible interpretation is that dwarf galaxies form stars much more efficiently, we argue that the low L CO /SFR ratio is due to the fact that the CO-to-H 2 conversion factor, α CO , changes significantly in low metallicity environments. Assuming that a constant H 2 depletion time of τ dep = 1.8 Gyr holds in dwarf galaxies (as found for a large sample of nearby spirals) implies α CO values for dwarf galaxies with Z ≈ 1/2 − 1/10 Z ⊙ that are more than one order of magnitude higher than those found in solar metallicity spiral galaxies. Such a significant increase of α CO at low metallicity is consistent with previous studies, in particular those of Local Group dwarf galaxies which model dust emission to constrain H 2 masses. Even though it is difficult to parameterize the dependence of α CO on metallicity given the currently available data the results suggest that CO is increasingly difficult to detect at lower metallicities. This has direct consequences for the detectability of star-forming galaxies at high redshift which presumably have on average sub-solar metallicity.

We analyze the behavior of the parsec-scale jet of the quasar 3C 454.3 during pronounced flaring activity in [2005][2006][2007][2008]. Three major disturbances propagated down the jet along different trajectories with Lorentz factors Γ >10. The disturbances show a . High-amplitude optical events in the R-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long-lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 Autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and γ-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the mm-wave core lies at the end of the jet's acceleration and collimation zone. We infer that the X-ray emission is produced via inverse Compton scattering by relativistic electrons of photons both from within the jet (synchrotron self-Compton) and external to the jet (external Compton, or EC); which one dominates depends on the physical parameters of the jet. A broken power-law model of the γ-ray spectrum reflects a steepening of the synchrotron emission spectrum from near-IR to soft UV wavelengths. We propose that the γ-ray emission is dominated by the EC mechanism, with the sheath of the jet supplying seed photons for γ-ray events that occur near the mm-wave core.

Aims. We study the polarization of the SiO maser emission in a representative sample of evolved stars in order to derive an estimate of the strength of the magnetic field, and thus determine the influence of this magnetic field on evolved stars. Methods. We made simultaneous spectroscopic measurements of the 4 Stokes parameters, from which we derived the circular and linear polarization levels. The observations were made with the IF polarimeter installed at the IRAM 30 m telescope. Results. A discussion of the existing SiO maser models is developed in the light of our observations. Under the Zeeman splitting hypothesis, we derive an estimate of the strength of the magnetic field. The averaged magnetic field varies between 0 and 20 Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughout the circumstellar envelope. As a consequence, the magnetic field may play the role of a shaping, or perhaps collimating, agent of the circumstellar envelopes in evolved objects.

We report the detection of variable emission from Sgr A* in almost all wavelength bands (i.e. centimeter, millimeter, submillimeter, near-IR and X-rays) during a multi-wavelength observing campaign. Three new moderate flares are detected simultaneously in both near-IR and X-ray bands. The ratio of X-ray to near-IR flux in the flares is consistent with inverse Compton scattering of near-IR photons by submillimeter emitting relativistic particles which follow scaling relations obtained from size measurements of Sgr A*. We also find that the flare statistics in near-IR wavelengths is consistent with the probability of flare emission being inversely proportional to the flux. At millimeter wavelengths, the presence of flare emission at 43 GHz (7mm) using VLBA with milli-arcsecond spatial resolution indicates the first direct evidence that hourly time scale flares are localized within the inner 30×70 Schwarzschild radii of Sgr A*. We also show several cross correlation plots between near-IR, millimeter and submillimeter light curves that collectively demonstrate the presence of time delays between the peaks of emission up to three hours. The evidence for time delays at millimeter and submillimeter wavelengths are consistent with the source of emission being optically thick initially followed by a transition to an optically thin regime. In particular, there is an intriguing correlation between the optically thin near-IR and X-ray flare and optically thick radio flare at 43 GHz that occurred on 2007 April 4. This would be the first evidence of a radio flare emission at 43 GHz delayed with respect to the near-IR and X-ray flare emission. The time delay measurements support the expansion of hot self-absorbed synchrotron plasma blob and weaken the hot spot model of flare emission. In addition, a simultaneous fit to 43 and 84 GHz light curves, using an adiabatic expansion model of hot plasma, appears to support a power law rather than a relativistic Maxwellian distribution of particles.

During the dawn of chemistry 1,2 when the temperature of the young Universe had fallen below ~4000 K, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With its higher ionization potentials, He ++ (54.5 eV) and He + (24.6 eV) combined first with free electrons to form the first neutral atom, prior to the recombination of hydrogen (13.6 eV). At that time, in this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH + , by radiative association with protons (He + H + → HeH + + hν). As recombination progressed, the destruction of HeH + (HeH + + H → He + H 2 + ) created a first path to the formation of molecular hydrogen, marking the beginning of the Molecular Age. Despite its unquestioned importance for the evolution of the early Universe, the HeH + molecule has so far escaped unequivocal detection in interstellar space. In the laboratory the ion was discovered as long ago as 1925 3 , but only in the late seventies was the possibility that HeH + might exist in local astrophysical plasmas discussed 4,5,6,7 . In particular, the conditions in planetary nebulae were shown to be suitable for the production of potentially detectable HeH + column densities: the hard radiation field from the central hot white dwarf creates overlapping Strömgren spheres, where HeH + is predicted to form, primarily by radiative association of He + and H. With the GREAT spectrometer 8.9 on board SOFIA 10 the HeH + rotational ground-state transition at λ149.1 µm is now accessible. We report here its detection towards the planetary nebula NGC7027. The mere fact of its proven existence in nearby interstellar space constrains our understanding of the chemical networks controlling the formation of this very special molecular ion.To be published in Nature 568, pages 357-359 (2019) | KOSMA/Universität zu Köln, in cooperation with the DLR Institut für Optische Sensorsysteme.