L. Bizzocchi

The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and Goals

We present the Spitzer Extragalactic Representative Volume Survey (SERVS), an 18 square degrees medium-deep survey at 3.6 and 4.5 microns with the post-cryogenic Spitzer Space Telescope to ~2 microJy (AB=23.1) depth of five highly observed astronomical fields (ELAIS-N1, ELAIS-S1, Lockman Hole, Chandra Deep Field South and XMM-LSS). SERVS is designed to enable the study of galaxy evolution as a function of environment from z~5 to the present day, and is the first extragalactic survey both large enough and deep enough to put rare objects such as luminous quasars and galaxy clusters at z>1 into their cosmological context. SERVS is designed to overlap with several key surveys at optical, near- through far-infrared, submillimeter and radio wavelengths to provide an unprecedented view of the formation and evolution of massive galaxies. In this paper, we discuss the SERVS survey design, the data processing flow from image reduction and mosaicing to catalogs, as well as coverage of ancillary data from other surveys in the SERVS fields. We also highlight a variety of early science results from the survey.

Deuterated methanol in the pre-stellar core L1544

Context. High methanol (CH3OH) deuteration has been revealed in Class 0 protostars with the detection of singly, doubly, and even triply D-substituted forms. Methanol is believed to form during the pre-collapse phase via gas-grain chemistry and then eventually injected into the gas when the heating produced by the newly formed protostar sublimates the grain mantles. The molecular deuterium fraction of the warm gas is thus a relic of the cold pre-stellar era and provides hints of the past history of the protostars. Aims. Pre-stellar cores represent the preceding stages in the process of star formation. We aim at measuring methanol deuteration in L1544, a prototypical dense and cold core on the verge of gravitational collapse. The aim is to probe the deuterium fractionation process while the “frozen” molecular reservoir is accumulated onto dust grains. Methods. Using the IRAM 30 m telescope, we mapped the methanol emission in the pre-stellar core L1544 and observed singly deuterated methanol (CH2DOH and CH3OD) towards the dust peak of L1544. Non-LTE radiative transfer modelling was performed on three CH3OH emissions lines at 96.7 GHz, using a Bonnor–Ebert sphere as a model for the source. We have also assumed a centrally decreasing abundance profile to take the molecule freeze-out in the inner core into account. The column density of CH2DOH was derived assuming LTE excitation and optically thin emission. Results. The CH3OH emission has a highly asymmetric morphology, resembling a non-uniform ring surrounding the dust peak, where CO is mainly frozen onto dust grains. The observations provide an accurate measure of methanol deuteration in the cold pre-stellar gas. The derived abundance ratio is [CH2DOH]/[CH3OH] = 0.10 ± 0.03, which is significantly smaller than the ones found in lowmass Class 0 protostars and smaller than the deuterium fraction measured in other molecules towards L1544. The singly-deuterated form CH3OD was not detected at 3σ sensitivity of 7 mK km s −1 , yielding a lower limit of [CH2DOH]/[CH3OD] ≥ 10, consistent with previous measurements towards Class 0 protostars. Conclusions. The low deuterium fractionation observed in L1544 and the morphology of the CH3OH emission suggest that we are mainly tracing the outer parts of the core, where CO just started to freeze-out onto dust grains.

ULTRA STEEP SPECTRUM RADIO SOURCES IN THE LOCKMAN HOLE: SERVS IDENTIFICATIONS AND REDSHIFT DISTRIBUTION AT THE FAINTEST RADIO FLUXES

Ultra Steep Spectrum (USS) radio sources have been successfully used to select powerful radio sources at high redshifts (z > 2). Typically restricted to large-sky surveys and relatively bright radio flux densities, it has gradually become possible to extend the USS search to sub-mJy levels, thanks to the recent appearance of sensitive low-frequency radio facilities. Here we present a first detailed analysis of the nature of the faintest USS sources. By using GMRT and VLA radio observations of the Lockman Hole (LH) at 610 MHz and 1.4 GHz, a sample of 58 micro-Jansky USS sources is assembled. Deep infrared data at 3.6 and 4.5 μm from the Spitzer Extragalactic Representative Volume Survey (SERVS) is used to reliably identify counterparts for 48 (83%) of these sources, showing an average magnitude of [3. 6] = 19. 7 mag(AB). Spectroscopic redshifts for 14 USS sources, together with photometric redshift estimates, improved by the use of the deep SERVS data, for a further 19 objects, show redshifts ranging from z = 0. 1 to z = 2. 8, peaking at z ∼ 0. 6 and tailing off at high redshifts.

First measurements of 15N Fractionation in N2H+ toward high-mass star-forming cores

We report on the first measurements of the isotopic ratio 14N/15N in N2H+ toward a statistically significant sample of high-mass star forming cores. The sources belong to the three main evolutionary categories of the high-mass star formation process: high-mass starless cores, high-mass protostellar objects, and ultracompact HII regions. Simultaneous measurements of 14N/15N in CN have been made. The 14N/15N ratios derived from N2H+ show a large spread (from ~180 up to ~1300), while those derived from CN are in between the value measured in the terrestrial atmosphere (~270) and that of the proto-Solar nebula (~440) for the large majority of the sources within the errors. However, this different spread might be due to the fact that the sources detected in the N2H+ isotopologues are more than those detected in the CN ones. The 14N/15N ratio does not change significantly with the source evolutionary stage, which indicates that time seems to be irrelevant for the fractionation of nitrogen. We also find a possible anticorrelation between the 14N/15N (as derived from N2H+) and the H/D isotopic ratios. This suggests that 15N enrichment could not be linked to the parameters that cause D enrichment, in agreement with the prediction by recent chemical models. These models, however, are not able to reproduce the observed large spread in 14N/15N, pointing out that some important routes of nitrogen fractionation could be still missing in the models.

Accurate rotational rest-frequencies of CH2NH at submillimetre wavelengths

Context. Methanimine (CH2NH) has been detected in different astronomical sources, both galactic (as in several “hot cores”, the circumstellar enevolope IRC+10216, and the L183 pre-stellar core) and extragalactic, and is considered a pre-biotic interstellar molecule. Its ground-state rotational spectrum has been studied in the laboratory up to 172 GHz, well below the spectral ranges covered by Herschel/HIFI and the ALMA bands 9 and 10. Aims. In this laboratory study, we extend into the submillimetre-wave region the detection of the rotational spectrum of CH2NH in its vibrational ground state. Methods. The investigation was carried out using a source-modulation microwave spectrometer equipped with a cell coupled to a pyrolysis apparatus working at 1150 ◦ C. The spectrum was recorded in the frequency range 329–629 GHz, with the detection of 58 transitions. Results. The newly measured transition frequencies, along with those available from previous microwave studies, allow us to determine fairly accurate rotational constants of CH2NH and the complete sets of quartic and sextic centrifugal distortion constants, in addition to two octic constants. Several transitions have an hyperfine structure due to the 14 N nucleus, which was accounted for in the analysis. Conclusions. The determined spectroscopic constants make it possible to build a list of very accurate rest-frequencies for astrophysical purposes in the THz region with 1σ uncertainties lower than 0.01 km s −1 in radial equivalent velocity.

INTERSTELLAR DETECTION OF c-C3D2 *

We report the first interstellar detection of c-C3D2. The doubly deuterated cyclopropenylidene, a carbene, has been detected toward the starless cores TMC- 1C and L1544 using the IRAM 30m telescope. The J(Ka,Kc) = 3(0,3)-2(1,2), 3(1,3)-2(0,2), and 2(2,1)-1(1,0) transitions of this species have been observed at 3 mm in both sources. The expected 1:2 intensity ratio has been found in the 3(0,3)-2(1,2) and 3(1,3)-2(0,2) lines, belonging to the para and ortho species respectively. We also observed lines of the main species, c-C3H2, the singly deuterated c-C3HD, and the species with one 13C off of the principal axis of the molecule, c-H13CC2H. The lines of c-C3D2 have been observed with high signal to noise ratio, better than 7.5 sigma in TMC-1C and 9 sigma in L1544. The abundance of doubly deuterated cyclopropenylidene with respect to the normal species is found to be (0.4 - 0.8)% in TMC-1C and (1.2 - 2.1)% in L1544. The deuteration of this small hydrocarbon ring is analysed with a comprehensive gas-grain model, the first including doubly deuterated species. The observed abundances of c-C3D2 can be explained solely by gas-phase processes, supporting the idea that c-C3H2 is a good indicator of gas-phase deuteration.

Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

We present

Chemical differentiation in a prestellar core traces non-uniform illumination

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The Spitzer Extragalactic Representative Volume Survey (SERVS): Survey Definition and Goals (PASP, 124, 714, [2012])

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The Herschel Virgo Cluster Survey XVII. SPIRE point-source catalogs and number counts

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