Arcetri Astrophysical Observatory

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Synchrotron emission in molecular cloud cores: the SKA view

Understanding the role of magnetic fields in star-forming regions is of fundamental importance. In the near future, the exceptional sensitivity of the Square Kilometre Array (SKA) will offer a unique opportunity to evaluate the magnetic field strength in molecular clouds and cloud cores through synchrotron emission observations.

During the last decades, many observational techniques have been developed to gather information on the magnetic field strength and geometry in molecular clouds and star-forming regions such as Zeeman splitting of hyperfine molecular transitions, optical and near-infrared polarisation of starlight, polarisation of sub-millimetre thermal dust emission, maser emission polarisation, Goldreich–Kylafis effect, and Faraday rotation. Together, all these techniques contribute to elucidating the still controversial role of magnetic fields in the process of star formation.

An additional method to probe magnetic fields in molecular clouds is via synchrotron radiation produced by relativistic electrons braked by a cloud’s magnetic fields. The intensity of the emission depends only on the electron density per unit energy and the projection of the magnetic field on the plane perpendicular to the line of
sight. However, this technique was constrained by two limitations: the poor knowledge of the interstellar flux of cosmic-ray electrons below about 500 MeV and the limited sensitivity of current radio telescopes.

Today, thanks to the latest data release of the Voyager 1 spacecraft, the flux of cosmic-ray electrons down to about 3 MeV is well known (see Fig. 1). On the instrumental side, with the advent of the Square Kilometre Array (SKA),we are now approaching a new important era for high resolution observations at radio frequencies.

Marco Padovani and Daniele Galli of the INAF-Osservatorio Astrofisico di Arcetri, in a recenlty published paper (Padovani & Galli 2018)  explored the capability of SKA in detecting synchrotron emission in two starless molecular cloud cores in the southern hemisphere, Barnard 68 in Ophiuchus and FeSt 1-457 in the Pipe Nebula, finding that it will be possible to reach signal-to-noise ratios of the order of 2−23 at the lowest frequencies observable by SKA (60−218 MHz) with one hour of integration (see Fig. 2).

They also found a typical spectral index alpha=(pi−1)/2 ~0.6, intermediate between the values 0.2 and 1.1 resulting from the low- and high-energy asymptotic slopes p of the primary cosmic-ray electron flux. Since in starless cores the interstellar cosmic-ray electron flux is not attenuated by losses, the authors expect alpha ~ 0.6 to be a universal quantity, weakly dependent on density and magnetic field strength profiles at SKA1-Low frequencies.

Esyn jespectrum new

Fig.1: Flux of interstellar cosmic-ray electrons (black solid line) as a function of energy. Data: Voyager 1 (solid purple squares) at low energies, AMS-02 (solid cyan circles) at high energies. The red, blue, and green thick lines show the energy range that mostly contributes to synchrotron emission in the frequency range of SKA for the values of the magnetic field strength listed in the legend. The orange dashed line shows the cosmic-ray electron-positron flux obtained from Galactic synchrotron emission.

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Fig.2:  Radial flux density profiles for Barnard 68 (B68) and FeSt 1-457. The observing frequency is shown in black at the top of each column, while numbers in the upper-right corner of each subplot represent the radius-averaged signal-to-noise depending on the assumption on the magnetic field strength profile. The telescope beam is shown in the leftmost column for B68 (short-dashed black line, 330") and FeSt 1-457 (long-dashed black line, 284"). Hatched areas display SKA sensitivities for one hour of integration at different frequencies. The rightmost panel shows the flux density as a function of frequency. Empty (solid) circles refer to a signal-to-noise smaller (larger) than 3, respectively. The spectral index alpha is shown on the right of each curve.

 

Marco Padovani acknowledges funding from the European Unions Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 664931.

An unprecedented view on ionized gas and outflows in the nearby active galaxy NGC 1365

All galaxies in the Universe are thought to host a supermassive black hole (BH) at their center. In some cases the BH can accrete large quantities of surrounding material and reprocess it in the form of energetic electromagnetic radiation, giving rise to a so-called active galactic nucleus (AGN), capable of accelerating fast winds and producing gas outflows. These outflows are believed to have an important role in shaping the formation and evolution of galaxies throughout the history of the Universe. Extended outflows of ionized gas are commonly observed in AGN but the low intrinsic spatial resolution of the observations has generally prevented a detailed characterization of their properties and impact on the host galaxies.

A team of astronomers from the Arcetri Astrophysical Observatory has started an observational campaign, called MAGNUM (Measuring Active Galactic Nuclei Under MUSE Microscope), to overcome these limitations by targeting the nearest galaxies hosting an AGN with the new-generation Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT) in Chile. MUSE provides a total of about 90000 spectra in the optical and near-infrared band on a wide area in the sky (1′⨉1′). This allowed the team to inspect theionized gas in the central kiloparsecs of nearby active galaxies in unprecedented detail, resolving its structure and kinematics down to the scale of single gas clouds (about 10 parsec). The first results of the MAGNUM survey are published in a work led by G. Venturi in collaboration with E. Nardini as well as A. Marconi, M. Mingozzi, G. Cresci, F. Mannucci, G. Risaliti, A. Gallazzi, M. Perna, P. Tozzi and S. Zibetti from the Arcetri Astrophysical Observatory. The paper (Venturi et al. 2018) focuses on the barred spiral galaxy NGC 1365 hosting an AGN.

Among the various ionized gas emission lines observable within MUSE wavelength range, Hα allows to trace star formation in the galaxy, while [OIII] emission is sensitive to the ionization from AGN photons. Moreover, from the Doppler shift of the emission lines it is possible to infer the kinematics of the ionized gas. Fig. 1 (left panel) displays the distribution of the ionized gas emission in the central 5 kpc of NGC 1365 covered by MUSE. Star formation predominantly occurs in an S-shaped region along the bar and in a circumnuclear ring (red), while the gas ionized by the energetic radiation from the spatially-unresolved AGN located in the nucleus (at the position of the cross) is distributed in a perpendicular double cone (green). The kinematics of the ionized gas reveals a bipolar outflow in correspondence of the ionization cones, with velocities up to about 200 km/s (Fig. 1 right panel). The biconical outflow approaches the observer to the bottom-left of the nucleus (negative velocities), while it recedes to the top-right (positive velocities). The former resides in fact above the disk of the galaxy, while the latter behind it, partially obscured by dust in the disk, as shown by their different brightness in the left panel.

NGC1365 maps
Figure 1: Ionized gas distribution and kinematics in the central 5 kpc of NGC 1365 obtained with MUSE at VLT. Left panel: Hα emission (red), tracing star formation in the galaxy, and [OIII] emission (green), due to the energetic ionizing radiation from the active galactic nucleus. Right panel: [OIII] velocity, revealing a biconical ionized outflow, approaching to the bottom-left (negative velocities), receding to the top-right (positive velocities).

Comparison with data from the Chandra X-ray space telescope, tracing the hot diffuse gas emitting in X-rays (with a temperature of about 106-107 K), shows that the diffuse X-ray emission predominantly arises from star-forming regions. However, the authors were able to isolate the hot gas component ionized by the AGN and show that this nicely matches the optical counterpart from MUSE emitting in [OIII], given by warm ionized gas (with a temperature of about 104 K). Moreover, the presence of a nuclear spatially-unresolved wind of hot, highly-ionized gas is traced by absorption lines in the X-rays, indicating a velocity of about 3000 km/s for this nuclear wind.

The spatially-resolved information provided by MUSE allowed the team to investigate the properties of the extended ionized outflow as a function of distance from the AGN. In particular, the radial profiles of the mass outflow rate and of the kinetic energy rate, that is, the rate at which the mass of ionized gas participating in the outflow and its associated kinetic energy flow away from the nucleus, are displayed in Fig. 2. After an initial peak at a radius of about 1 kpc, the mass outflow rate and the kinetic energy rate drop at larger distances, implying either that the outflow slows down with radius or that the AGN activity has intensified recently. Finally, the mass outflow rate of the nuclear X-ray wind is consistent with that of the extended ionized outflow within 1 kpc, suggesting that the nuclear and galactic-scale outflows are only two different phases of the same wind accelerated by the AGN.

NGC1365 profiles
Figure 2: Radial profiles of the mass outflow rate (left panel) and kinetic energy rate (right panel) of the galactic ionized outflow as a function of distance from the active galactic nucleus. Blue and red points indicate the approaching and the receding cone, respectively (see right panel of Fig. 1). The green dashed line marks the mass outflow rate of the fast nuclear wind measured from X-rays.

The results of this work are published in:

Venturi G. et al., ‘MAGNUM survey: A MUSE-Chandra resolved view on ionized outflows and photoionization in the Seyfert galaxy NGC1365’, 2018, A&A, 619, 74  (also available here)

Further MAGNUM publications on the topic:

Mingozzi M. et al., ‘The MAGNUM survey: different gas properties in the outflowing and disk components in nearby active galaxies with MUSE’, A&A, accepted (available here)

Venturi G. et al., 'Ionized gas outflows from the MAGNUM survey: NGC 1365 and NGC 4945’, 2017, FrASS, 4, 46 (also available here)


Edited by G. Venturi and A. Gallazzi

Dust, so little, so powerful!

Sub-micron solid particles, collectively known as dust, constitutes only a minor fraction of the mass of a galaxy, yet they have profound effects on its appearance and spectrum. Dust grains absorb starlight in the ultraviolet/optical and reemit this energy at longer wavelengths, mainly in the far-infrared and submm. On average, 20% of the total energy output of a galaxy is due to dust emission, with values running from a few percent for ellipticals up to 90% for bright spiral galaxies.

These are the results of a recent work led by Simone Bianchi and with the participation of Viviana Casasola, of the Arcetri Observatory. They measured the fraction of energy absorbed/emitted by dust for 814 galaxies of different morphological type. The objects are part of DustPedia, a sample including almost all the galaxies of the Local Universe observed in the far-infrared and submm by the Herschel Space Observatory.

Fig1.png

Fig.1. A fit to the SED of one DustPedia galaxy. The blue curve is the CIGALE fit, used as a reference in the work. Other method have been used to confirm the results (red and purple lines).

Thanks to Herschel and ancillary data, Dustpedia galaxies have an extensive coverage of the Spectral Energy Distribution (SED), with an average of 21 photometric datapoints per object. Each SED (Fig. 1) was fitted with modelling tools to derive fabs as the ratio of the luminosity due to dust emission at wavelength larger than 4 micron, and the total luminosity from the UV to the submm.

 

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Fig. 2.The fraction of absorbed energy (fabs) as a function of the total, bolometric, luminosity.

The large number of objects in DustPedia has allowed to detect unexpected trends: the fraction of absorbed energy has been found to correlate broadly with the total luminosity of the galaxy, at least for galaxies of later types (spirals and irregulars), with a structure dominated by a disk, rich in gas and with high specific star-formation rates (Fig. 2).  Instead, no correlation was found for Elliptical galaxies, while lenticular and earlier type spirals in part follow the correlation, in part do not. While it is expected that more luminous, evolved, spirals have a larger dust mass, it is not clear why that dust should be more effective in absorbing starlight, unless an evolution in the relative stellar and dust geometry is also at play. Various scenarios are currently being explored using samples from cosmological simulations coupled to the radiative transfer model SKIRT.

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Fig.3:The fraction of absorbed energy (fabs) as a function of the gas fraction.

Indeed, evolution is suggested by analyzing fabs as a function of the gas fraction, fgas, the ratio between the mass of atomic gas and that of the baryons (gas and stars, Fig. 3). As more and more gas is converted into stars (a decreasing fgas), the fraction of absorbed energy increases. An abrupt drop in fabs is seen for the evolved lenticulars and ellipticals. If galaxies with fgas < 0.1 are supposed to have had, at an earlier time, a fabs along the trend for fgas>0.1, the luminosity-weighted average of the fraction of absorbed energy would rise to 45%. This value is close to what has been estimated from measurements of the extragalactic background light, showing that almost half of the starlight produced during the evolution of the Universe has been reprocessed by dust.

References:

[1] Bianchi et al., "Fraction of bolometric luminosity absorbed by dust in DustPedia galaxies", A&A, in press

[2] DustPedia   is   a   collaborative   focused   research   project supported  by  the  European  Union  under  the  Seventh  Framework  Programme  (2007-  2013)  call  (proposal  no.  606824,  P.I.  J.  I.  Davies, http://www.dustpedia.com). The photometric data used in this work is publicly available at http://dustpedia.astro.noa.gr . Soon, ancillary data on gas and metallicity, as well as the result of global and resolved SED modelling will become available.

[3] A version of CIGALE, including the DustPedia reference grain model THEMIS  is available here 

Complex starts with simple:

First ALMA maps of HCO, an important precursor of complex organic molecules, towards IRAS 16293-2422

An international group of scientists from the Arcetri Astrophysical Observatory (INAF-OAA, Florence, Italy), the Ural Federal University (Russia), the Max-Planck Institute for Extraterrestrial Physics (MPE, Garching, Germany) and the University College of London (UK) has obtained for the first time maps of the emission of the formyl radical, HCO, towardsa Solar-type protostellar binary. This simple molecule is the basic precursor of more complex organic molecules related to the origin of Life.

The formation of complex organic molecules (COMs) − carbon-based compounds with more than 5 atoms - is being intensively debated in astrochemistry. COMs play a central role in prebiotic chemistry and are thought to be directly linked to the origin of life. Numerous efforts have been done in the last years to understand how COMs are formed in the interstellar medium (ISM), by combining observations, chemical models and laboratory experiments. However, despite all efforts, our understanding about the synthesis of COMs in the ISM is still very limited. Two  paradigms have been proposed: i) gas-phase chemistry triggered by the evaporation (thermal or non-thermal) of interstellar ices; and ii) hydrogenation and/or radical-radical reactions on dust grain surfaces.

A key step to understand how complex molecules are built up in the ISM is to study their molecular precursors, namely, the basic pieces that lead to their formation. Many chemical models and laboratory experiments have suggested that the simple formyl radical, HCO, is the precursor of COMs like e.g. methanol, the sugar-like molecule glycolaldehyde, the sugar-alcohol ethylene glycol, or formamide. However, despite the importance of HCO to build-up chemical complexity, little is known so far about its formation.

To better understand the role of HCO in the formation of COMs, an international group of astronomers have studied this molecule towards the Solar-type protostellar binary IRAS 16293−2422 using the Atacama Large Millimeter/submillimeter Array (ALMA). The work is led by the Marie Skłodowska-Curie fellow Víctor M. Rivilla, and includes also the participation of Maite Beltrán, Francesco Fontani and Riccardo Cesaroni(INAF-Arcetri Astrophysical observatory). The high angular resolution (∼1′′, ∼150 au) maps reveal compact HCO emission arising from the two protostars (see Figure 1). The line profiles also show redshifted absorption produced by foreground material of a circumbinary envelope that is infalling towards the protostars. Additionally, IRAM 30m single-dish data revealed a more extended HCO component arising from the common circumbinary envelope.

   fig rivilla1

 Fig. 1: ALMA maps of the HCO emission and absorption towards the two members, A and B, of theSolar-type protostellar binary IRAS 16293−2422.

The comparison between the observed molecular abundances and the chemical model  (see Figure 2) suggests that whereas the extended HCO from the envelope can be formed via gas-phase reactions during the cold collapse of the natal core, the HCO in the hot corinos surrounding the protostars is predominantly formed by the hydrogenation of CO on the surface of dust grains and subsequent thermal desorption during the protostellar phase (see Figure 3).The derived abundance of HCO in the dust grains is high enough to produce efficiently more complex species such as formaldehyde (H2CO), methanol (CH3OH), and glycolaldehyde (CH2OHCHO) by surface chemistry

fig rivilla2
Fig. 2: Chemical routes discussed in the article and implemented in the chemical model to study the formation and HCO (in white) and glycoladehyde, CH2OHCHO (in green). The solid and dashed arrows indicate gas-phase and grain surface reactions, respectively.

fig rivilla3

Fig. 3:  Results of the different chemical models (I, II and III) developed to investigate the formation of HCO during the warm-up phase of a protostar. The evolution of the molecular abundance of HCO is represented as a function of temperature. The area within the dotted curves delimit the uncertainty of the chemical model. Model I (blue) includes only gas-phase reactions; model II (red) includes gas-phase reactions and the surface hydrogenation of CO; and model III (green) includes gas-phase reactions and the surface reaction H2CO + OH HCO + H2CO. The horizontal light gray band indicates the range of HCO abundances found in the ALMA observations.

Publication:First ALMA maps of HCO, an important precursor of complex organic molecules, towards IRAS 16293-2422V. M. Rivilla, M. T. Beltrán, A. Vasyunin, P. Caselli, S. Viti, F. Fontani, R. Cesaroni, article accepted in Monthly Notices of the Royal Astronomical Society, in press, arXiv:1811.01650, https://arxiv.org/abs/1811.01650

Contact:  

Víctor M. Rivilla Rodríguez

Marie Sklodowska-Curie ASTROFIT2 Fellow

Osservatorio Astrofisico di Arcetri, Florence, Italy

Website:http://www.arcetri.astro.it/~rivilla/

e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Acknowledgments: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 664931

Cosmic-ray ionisation in circumstellar discs

Galactic cosmic rays are a ubiquitous source of ionisation of the interstellar gas, competing with UV and X-ray photons as well as natural radioactivity in determining the fractional abundance of electrons, ions, and charged dust grains in molecular clouds and circumstellar discs.

The ionisation fraction is a fundamental quantity for the dynamics of the interstellar gas, in particular during the earliest stages of star formation, from the collapse of a molecular cloud core to the formation of an accretion disc. Before the formation of a protostar, cosmic-ray ionisation regulates the degree of coupling between gas and magnetic field in the densest parts of a cloud core, setting the timescale of magnetic field diffusion, see e.g. [1], and controlling the amount of magnetic braking of collapsing rotating envelopes [2,3].

Previous studies on cosmic-ray propagation [4,5,6,7,8] neglected the contribution of electron-positron pairs generated by photon decay and that of relativistic protons and electrons. A group of researchers - led by Marco Padovani and including Daniele Galli of the Osservatorio Astrofisico di Arcetri - showed that, while this approximation is appropriate for diffuse and dense molecular clouds, it becomes invalid at the high values of column densities characteristic of circumstellar discs, where cosmic-ray ionisation is dominated by both relativistic and secondary particles (see Fig. 1).

 fig1

Fig.1: Ionisation diagram, explaining the effect of secondary particles that are generated (directly or indirectly) by CR protons and electrons through ionisation, pion decay, bremsstrahlung (BS), and pair production (pair). The secondary particles include electrons, positrons and electrons, and photons, all contributing to the respective ionisation routes.

Padovani et al. (2018)  modelled the various components of Galactic cosmic rays versus the column density of the gas, focusing on their propagation at high densities, above a few g cm-2, relevant for circumstellar discs.
They found that the cosmic-ray ionisation rate is determined by cosmic-ray protons and their secondary electrons below 130 g cm−2 and by electron-positron pairs created by photon decay above 600 g~cm−2. They showed that the cosmic-ray ionisation rate in high-density environments, such as the inner parts of collapsing molecular clouds or the mid-plane of circumstellar discs, is higher than previously assumed [9]. It does not decline exponentially with increasing column density, but follows a more complex behaviour because of the interplay of the different processes governing the generation and propagation of secondary particles (see Fig. 2).

fig2

Fig.2: Ionisation rate per H2 molecule due to primary and secondary cosmic-ray species plotted vs. the surface density  (bottom scale) and the column density (top scale). The black line shows the total ionisation rate. Partial contributions include ionisation due to primary CR protons and electrons (blue and red lines, respectively), ionisation due to secondary electrons created by primary cosmic rays (orange line), and ionisation due to electrons and positrons created by charged pion decay and pair production (green line). The blue dashed line shows the proton contribution calculated with the continuous slowing down approximation approach. The horizontal dashed line at 1.4 10−22 s−1 indicates the total ionisation rate set by long-lived radioactive nuclei (LLR). For comparison, the total ionisation rate per H2 molecule ([9]; grey dashed line) is shown.

The main result of this paper is the characterisation of the CR ionisation rate at high column densities that turns out to be useful for numerical simulations and chemical models in order to interpret observations of circumstellar discs.

[1] Pinto et al. (2008)
[2] Galli et al. (2006) 
[3] Li et al. (2016) 
[4] Padovani et al. (2009) 
[5] Padovani & Galli (2011)
[6] Padovani & Galli (2013) 
[7] Padovani et al. (2013) 
[8] Padovani et al. (2014) 
[9] Umebayashi & Nakano (1981)