ALBERT

Description: Context. Recent surveys of the Galactic plane in the dust continuum and CO emission lines reveal that large (≳50 pc) and massive (≳105 M⊙) filaments, know as giant molecular filaments (GMFs), may be linked to Galactic dynamics and trace the mid-plane of the gravitational potential in the Milky Way. Yet our physical understanding of GMFs is still poor. Aims. We investigate the dense gas properties of one GMF, with the ultimate goal of connecting these dense gas tracers with star formation processes in the GMF. Methods. We imaged one entire GMF located at l ~ 52–54° longitude, GMF54 (~68 pc long), in the empirical dense gas tracers using the HCN(1–0), HNC(1–0), and HCO+(1–0) lines, and their 13C isotopologue transitions, as well as the N2H+(1–0) line. We studied the dense gas distribution, the column density probability density functions (N-PDFs), and the line ratios within the GMF. Results. The dense gas molecular transitions follow the extended structure of the filament with area filling factors between 0.06 and 0.28 with respect to 13CO(1–0). We constructed the N-PDFs of H2 for each of the dense gas tracers based on their column densities and assumed uniform abundance. The N-PDFs of the dense gas tracers appear curved in log–log representation, and the HCO+ N-PDF has the flattest power-law slope index. Studying the N-PDFs for sub-regions of GMF54, we found an evolutionary trend in the N-PDFs that high-mass star-forming and photon-dominated regions have flatter power-law indices. The integrated intensity ratios of the molecular lines in GMF54 are comparable to those in nearby galaxies. In particular, the N2H+/13CO ratio, which traces the dense gas fraction, has similar values in GMF54 and all nearby galaxies except Ultraluminous Infrared Galaxies. Conclusions. As the largest coherent cold gaseous structure in our Milky Way, GMFs, are outstanding candidates for connecting studies of star formation on Galactic and extragalactic scales. By analyzing a complete map of the dense gas in a GMF we have found that: (1) the dense gas N-PDFs appear flatter in more evolved regions and steeper in younger regions, and (2) its integrated dense gas intensity ratios are similar to those of nearby galaxies.

Description: The detection of intermediate-mass black holes in the centres of globular clusters is highly controversial, as complementary observational methods often deliver significantly different results. In order to understand these discrepancies, we develop a procedure to simulate integral field unit (IFU) observations of globular clusters: Simulating IFU Star Cluster Observations ( sisco ). The inputs of our software are realistic dynamical models of globular clusters that are then converted in a spectral data cube. We apply sisco to Monte Carlo cluster simulations with a realistic number of stars and concentrations. Using independent realizations of a given simulation we are able to quantify the stochasticity intrinsic to the problem of observing a partially resolved stellar population with integrated-light spectroscopy. We show that the luminosity-weighted IFU observations can be strongly biased by the presence of a few bright stars that introduce a scatter in the velocity dispersion measurements up to ~=40 per cent around the expected value, preventing any sound assessment of the central kinematic and a sensible interpretation of the presence/absence of an intermediate-mass black hole. Moreover, we illustrate that, in our mock IFU observations, the average kinematic tracer has a mass of ~=0.75 M , only slightly lower than the mass of the typical stars examined in studies of resolved line-of-sight velocities of giant stars. Finally, in order to recover unbiased kinematic measurements we test different masking techniques that allow us to remove the spaxels dominated by bright stars, bringing the scatter down to a level of only a few per cent. The application of sisco will allow us to investigate state-of-the-art simulations as realistic observations.

Description: Studying star formation beyond the optical radius of galaxies allows us to test empirical relations in extreme conditions with low average gas density and low molecular fraction. Previous studies discovered galaxies with extended ultraviolet (XUV) discs, which often contain star-forming regions with lower Hα-to-far-UV (FUV) flux ratios compared to inner disc star-forming regions. However, most previous studies lack measurements of molecular gas, which is presumably the component of the interstellar medium out of which stars form. We analysed published CO measurements and upper limits for 15 star-forming regions in the XUV or outer disc of three nearby spiral galaxies and a new CO upper limit from the IRAM (Institut de Radioastronomie Millimétrique) 30 m telescope in one star-forming region at r = 3.4 r 25 in the XUV disc of NGC 4625. We found that the star-forming regions are in general consistent with the same molecular-hydrogen Kennicutt–Schmidt law that applies within the optical radius, independent of whether we used Hα or FUV as the star formation rate (SFR) tracer. However, a number of the CO detections are significantly offset towards higher SFR surface density for their molecular-hydrogen surface density. Deeper CO data may enable us to use the presence or absence of molecular gas as an evolutionary probe to break the degeneracy between age and stochastic sampling of the initial mass function as the explanation for the low Hα-to-FUV flux ratios in XUV discs.

Description: Two-body interactions play a major role in shaping the structural and dynamical properties of globular clusters (GCs) over their long-term evolution. In particular, GCs evolve towards a state of partial energy equipartition that induces a mass dependence in their kinematics. By using a set of Monte Carlo cluster simulations evolved in quasi-isolation, we show that the stellar mass dependence of the velocity dispersion ( m ) can be described by an exponential function 2 exp (– m / m eq ), with the parameter m eq quantifying the degree of partial energy equipartition of the systems. This simple parametrization successfully captures the behaviour of the velocity dispersion at lower as well as higher stellar masses, that is, the regime where the system is expected to approach full equipartition. We find a tight correlation between the degree of equipartition reached by a GC and its dynamical state, indicating that clusters that are more than about 20 core relaxation times old, have reached a maximum degree of equipartition. This equipartition–dynamical state relation can be used as a tool to characterize the relaxation condition of a cluster with a kinematic measure of the m eq parameter. Vice versa, the mass dependence of the kinematics can be predicted knowing the relaxation time solely on the basis of photometric measurements. Moreover, any deviations from this tight relation could be used as a probe of a peculiar dynamical history of a cluster. Finally, our novel approach is important for the interpretation of state-of-the-art Hubble Space Telescope proper motion data, for which the mass dependence of kinematics can now be measured, and for the application of modelling techniques which take into consideration multimass components and mass segregation.

Description: We present near-infrared ( H - and K -band) integral-field observations of the inner ~700 pc of the active spiral galaxy NGC 613, obtained with SINFONI on the Very Large Telescope. We use emission-line ratios to determine the dominant excitation mechanisms in different regions within our field of view, in particular, the active nucleus and the star-forming circumnuclear ring. Diagnostic diagrams involving [Fe ii ] and H 2 fluxes indicate that the gas is not only photoionized by the active galactic nucleus (AGN) in the nucleus of NGC 613, but also shock heated. On the other hand, the emission-line ratios measured in the ‘hotspots’ along the ring are fully consistent with them being young star-forming regions. We find no sign of radial gas transport from the ring into the core region dominated by the AGN. The ring morphology appears disturbed by a radial outflow of material from the AGN, which is confirmed by the existence of a weak jet in archival radio maps. However, this jet does not seem to have any significant effect on the morphology of the large (~8 10 7 M ) reservoir of molecular gas that has accumulated inside the central ~100 pc. Such a concentration of molecular gas around an AGN is unusual, and supports a scenario in which star formation is recurrent and episodic in spiral galaxies. In this context, NGC 613 appears to be in final stages of the gas accumulation phase and is likely to undergo a nuclear starburst in the near future.

Description: We present 90 cm Very Large Array imaging of the COSMOS field, comprising a circular area of 3.14 square degrees at 8.0arcsec x 6.0arcsec angular resolution with an average rms of 0.5 mJy beam –1 . The extracted catalogue contains 182 sources (down to 5.5), 30 of which are multicomponent sources. Using Monte Carlo artificial source simulations, we derive the completeness of the catalogue, and we show that our 90 cm source counts agree very well with those from previous studies. Using X-ray, NUV–NIR and radio COSMOS data to investigate the population mix of our 90 cm radio sample, we find that our sample is dominated by active galactic nuclei. The average 90–20 cm spectral index ( S α , where S is the flux density at frequency and α the spectral index) of our 90 cm selected sources is –0.70, with an interquartile range from –0.90 to –0.53. Only a few ultra-steep-spectrum sources are present in our sample, consistent with results in the literature for similar fields. Our data do not show clear steepening of the spectral index with redshift. Nevertheless, our sample suggests that sources with spectral indices steeper than –1 all lie at z 1, in agreement with the idea that ultra-steep-spectrum radio sources may trace intermediate-redshift galaxies ( z 1).

Description: We report the first counts of faint submillimetre galaxies (SMGs) in the 870-μm band derived from arcsecond-resolution observations with the Atacama Large Millimeter Array (ALMA). We have used ALMA to map a sample of 122 870-μm-selected submillimetre sources drawn from the 0 ${^{\circ}_{.}}$ 5 x 0 ${^{\circ}_{.}}$ 5 the Large Apex BOlometer CAmera (LABOCA) Extended Chandra Deep Field South submillimetre survey (LESS). These ALMA maps have an average depth of 870 μm ~ 0.4 mJy, some approximately three times deeper than the original LABOCA survey and critically the angular resolution is more than an order of magnitude higher, FWHM of ~1.5 arcsec compared to ~19 arcsec for the LABOCA discovery map. This combination of sensitivity and resolution allows us to precisely pinpoint the SMGs contributing to the submillimetre sources from the LABOCA map, free from the effects of confusion. We show that our ALMA-derived SMG counts broadly agree with the submillimetre source counts from previous, lower resolution single-dish surveys, demonstrating that the bulk of the submillimetre sources are not caused by blending of unresolved SMGs. The difficulty which well-constrained theoretical models have in reproducing the high surface densities of SMGs, thus remains. However, our observations do show that all of the very brightest sources in the LESS sample, S 870 μm 12 mJy, comprise emission from multiple, fainter SMGs, each with 870-μm fluxes of 9 mJy. This implies a natural limit to the star formation rate in SMGs of 10 3  M  yr –1 , which in turn suggests that the space densities of z  〉 1 galaxies with gas masses in excess of ~5  x 10 10  M is 〈10 –5  Mpc –3 . We also discuss the influence of this blending on the identification and characterization of the SMG counterparts to these bright submillimetre sources and suggest that it may be responsible for previous claims that they lie at higher redshifts than fainter SMGs.

Description: We present a study of the radio properties of 870 μm-selected submillimetre galaxies (SMGs), observed at high resolution with Atacama Large Millimeter Array (ALMA) in the Extended Chandra Deep Field South. From our initial sample of 76 ALMA SMGs, we detect 52 SMGs at 〉3 significance in Karl G. Jansky Very Large Array 1400 MHz imaging, of which 35 are also detected at 〉3 in new 610 MHz Giant Metre-Wave Radio Telescope imaging. Within this sample of radio-detected SMGs, we measure a median radio spectral index $\alpha _{610}^{1400} = -0.79 \pm 0.06$ , (with inter-quartile range α = [–1.16, –0.56]) and investigate the far-infrared/radio correlation via the parameter q IR , the logarithmic ratio of the rest-frame 8–1000 μm flux and monochromatic radio flux. Our median q IR  = 2.56 ± 0.05 (inter-quartile range q IR  = [2.42, 2.78]) is higher than that typically seen in single-dish 870 μm-selected sources ( q IR  ~ 2.4), which may reflect the fact that our ALMA-based study is not biased to radio-bright counterparts, as previous samples were. Finally, we search for evidence that q IR and α evolve with age in a codependent manner, as predicted by starburst models: the data populate the predicted region of parameter space, with the stellar mass tending to increase along tracks of q IR versus α in the direction expected, providing the first observational evidence in support of these models.

Description: We exploit Atacama Large Millimeter Array (ALMA) 870 μm observations of sub-millimetre sources in the Extended Chandra Deep Field South to investigate the far-infrared properties of high-redshift sub-millimetre galaxies (SMGs). Using the precisely located 870 μm ALMA positions of 99 SMGs, together with 24μm and radio imaging, we deblend the Herschel /SPIRE imaging to extract their far-infrared fluxes and colours. The median redshifts for ALMA LESS (ALESS) SMGs which are detected in at least two SPIRE bands increases with wavelength of the peak in their spectral energy distributions (SEDs), with z  = 2.3 ± 0.2, 2.5 ± 0.3 and 3.5 ± 0.5 for the 250, 350 and 500 μm peakers, respectively. 34 ALESS SMGs do not have a 〉3 counterpart at 250, 350 or 500 μm. These galaxies have a median photometric redshift derived from the rest-frame UV–mid-infrared SEDs of z  = 3.3 ± 0.5, which is higher than the full ALESS SMG sample; z  = 2.5 ± 0.2. We estimate the far-infrared luminosities and characteristic dust temperature of each SMG, deriving L IR  = (3.0 ± 0.3) 10 12 L (SFR = 300 ± 30 M yr –1 ) and T d  = 32 ± 1 K. The characteristic dust temperature of these high-redshift SMGs is T d  = 3–5 K lower than comparably luminous galaxies at z  = 0, reflecting the more extended star formation in these systems. We show that the contribution of S 870 μm ≥ 1 mJy SMGs to the cosmic star formation budget is 20 per cent of the total over the redshift range z ~ 1–4. Adopting an appropriate gas-to-dust ratio, we estimate a typical molecular mass of the ALESS SMGs of $M_{\rm H_2}$  = (4.2 ± 0.4) 10 10 M . Finally, we show that SMGs with S 870 μm 〉 1 mJy ( L IR  10 12 L ) contain ~ 10 per cent of the z ~ 2 volume-averaged H 2 mass density.