Arcetri Astrophysical Observatory

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Dynamical decomposition of galaxies reveals the unquiet life of disks

The motions of stars in galaxies are the result of their formation processes, of the internal dynamical evolution of galaxies, and of the episodes of accretion and merger that galaxies may experience along their life. Hence analyzing how stars at different locations and with different ages or chemical compositions orbit within galaxies provide invaluable information to understand how galaxies formed and evolve. This task, however, is extremely challenging and requires advanced instrumentation and observational techniques to map the kinematics and the properties of stars, and utterly complex analysis methods to model the physical state and evolution of the systems.

A group of scientists, led by Ling Zhu of the Max Planck Institute for Astronomy in Heidelberg and including Stefano Zibetti from the INAF-Arcetri Astrophysical Observatory, has made a big step in this direction with the results presented in a paper published on January 1st, 2018 in Nature Astronomy, "The stellar orbit distribution in present-day galaxies inferred from the CALIFA survey" (also available here). They have analyzed the statistical distribution of the orbits of stars for the first time in a representative sample of present-day galaxies. The team has exploited the wealth of exquisite Integral-Field Spectroscopic data (i.e. spatially resolved spectroscopy) collected by the CALIFA (Calar Alto Legacy Integral Field Area) survey to obtain detailed kinematic maps of 300 galaxies. The observations were modeled using the Schwarzschild orbit-superposition method to decompose the stellar population of each galaxy into three different families of orbits: "cold" orbits, representing the ordered circular motions of stars in galactic disks; "hot" orbits, characteristic of chaotic motions occurring in spheroidal components, such as the bulges and the haloes; "warm" orbits, with a significant rotational component, yet with non-negligible random motions, typical of thick disks and "pseudo-bulges" (see Figure 1).

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Figure 1: Models of stellar orbits (left) are matched to the observed images and maps of stellar velocity and velocity dispersion measured from the CALIFA spectra (right; example for the galaxy NGC001 in the sample). Credit: Instituto de Astrofisica de Andalucia (IAA-CSIC). Adapted from figure 1 of Zhu et al, 2018.

The statistical distribution of stars in these three orbital families, as a function of galaxy properties, is a key benchmark and will be of fundamental importance to constrain galaxy evolution models and simulations of the Universe (see Figure 2). Interestingly, this study finds that no galaxy is fully dominated by ordered circular motions. Even those galaxies that appear structurally dominated by a thin disk, actually have a majority of stars in warm orbits and a fraction on hot ones. This supports the idea that, although stars are born mainly in thin purely rotating disks, the so-called dynamical "secular evolution" driven by instabilities (spiral arms, bars) and minor interactions with external galaxies plays a major role in reshaping the orbital distribution of stars and in "warming up" the galactic disks.

Zhu Fig3Figure 2: Average fractional contribution of each of the three orbital components, plus counter-rotating (CR) orbits, as a function of the galaxy stellar mass. Source: figure 3 of Zhu et al, 2018.

For furher info, see "Quando le stelle perdono la bussola" on Media INAF.


Edited by S. Zibetti and A. Gallazzi

 

Phosphorus-bearing molecules in the Galactic Center

Phosphorus (P) is one of the essential elements for life due to its central role in bio-chemical processes. Recent searches have shown that P-bearing molecules (in particular PN and PO) are present in star-forming regions, although their formation routes remain poorly understood. A group of researchers of the Arcetri Observatory, led by Victor Rivilla and including Francesco Fontani and Maria Teresa Beltran, has reported observations of PN and PO towards seven molecular clouds located in the Galactic Center, which are characterizedby different types of chemistry. PN is detected in five out of seven sources, whose chemistry is thought to be shock-dominated. The two sources with PN non-detectionscorrespond to clouds exposed to  intense UV/X-rays/cosmic-ray radiation. PO is detected only towards the cloud G+0.693-0.03, with a PO/PN abundance ratio of  1.5. They conclude that P-bearing molecules likely form in shocked gas as a result of dustgrain sputtering, while are  destroyed by intense UV/X-ray/cosmic ray radiation.

Phosphorus (P) is essential for life because it plays a centralrole in the formation ofmacromolecules such as phospholipids (the structural components of cellular membranes)and the deoxyribonucleic acid (DNA, Macia et al. 1997). For decades PN  remained as the only P-bearingspecies observed in these regions (Turner & Bally 1987; Ziurys1987; Yamaguchi et al. 2011Fontani et al. 2016), whilePO has been discovered just recently in the surroundingsof both high- and low-mass protostars  (with PO/PN abundanceratios of 1-3; Rivilla et al. 2016; Lefloch et al. 2016).

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  Figure 1: Sample of Galactic Center clouds we have observed, overplotted on an Spitzer-IRAC 4 image.

The formation of P-bearing molecules is still poorly understood. Three routes have been proposed: (i) shock-induced desorptionof P-bearing species (e.g. PH3) from dust grainsand subsequent gasphase formation (Aota & Aikawa 2012;Lefloch  et al. 2016); (ii) high-temperature gas-phase chemistryafter the thermal desorption of PH3 from ices (Charnley& Millar  1994); and (iii) gas-phase formation of PN andPO during the cold collapse phase andsubsequent thermaldesorption (at  temperatures 35 K) by protostellar heating (Rivilla et al. 2016). Due to the limited number of observationsavailable, and the  limited range of physical conditionsof the observed regions with detected P-bearing molecules,the formation routes for PN  and PO are strongly debated.

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 Figure 2:PN (2-1) and 29SiO (2-1) lines measured towards the Galactic Center clouds. The Local Thermodynamic Equilibrium best fits are shown with red lines. The PN molecule is only detected towards the sources dominated by shocks.

Victor Rivilla and collaborators have presented new observations of PN and POtowards seven regions spread across the  Central MolecularZone (CMZ) in the Galactic Center (GC) (see Figure 1). These sourcesare excellent laboratories to test the  chemistry of P-bearingmolecules since they show different physical properties (highkinetic temperatures, low dust  temperatures and moderatedensities) and chemistries dominated by either UV photons,cosmic-rays (CR), X-rays or shock  waves. The selected sample includes two different types of sources:(i) Shock-dominated regions; and (ii) Radiation dominated regions.

They have carried out observations at 3mm and 2mm using the radiotelescope IRAM 30m located at Pico Veleta (Granada,  Spain). PN is detected towards five of the seven sources (see Figure 2). PO is detected only towardsone of the sources,  G+0.693-0.03 (see Figure 3), which is thought to be therichest source of O-bearing molecules in the Galactic Center.The  derived PO/PN abundance ratio is 1.5, similar to valuespreviously found in star-forming regions.

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 Figure 3: PO detection towards G+0.693-0.03 (lower panel) compared with the detection towards the hot molecular core W51  e1/e2from Rivilla et al. (2016) (upper panel). The PO quadruplet is shown with vertical blue lines. Other molecular  species are labeled in theupper panel. TheLocal Thermodynamic Equilibrium synthetic spectrum of PO in both sources is  shown with red lines.

The regions whereP-bearing species have been detected are clouds thought tobe affected by shock waves, and rich in the  well-knownshock tracer 29SiO (see Figure 4). The two sources where no P-bearingmolecules were detected are regions  exposed to intense radiation,and exhibit lower abundances of 29SiO. Wethus conclude that P-bearing species are formed in  thegas phase after the shock-induced sputtering of the grainmantles,and that they are efficiently destroyed by thehigh  cosmic-rays/X-rays/UV-photon radiation expected inthe Galactic Center.

Arcetri-highlight-fig4.jpg

Figure 4:Column density ratios of PN and 29SiO with respectto C34S. The different type of sources are Shock-dominated GCclouds (red dots) and Radiation-dominated regions (greenstars). The L1157-B1 shock(magenta open star) and the L1544 pre-stellar core (opendiamond) havealso been added. Arrows indicate 3 upper limits.


More info:

Contact: Víctor M. Rivilla, https://www.arcetri.astro.it/~rivilla/

Paper:Phosphorous-bearing molecules in the Galactic Center”, accepted for publication in Monthly Notices of the Royal Astronomical Society Letters; https://arxiv.org/abs/1712.07006

Open clusters with Gaia and the Gaia-ESO Survey: a way forward to stellar age calibration

A group of researchers at Arcetri, led by Sofia Randich, the Co-PI of the Gaia-ESO Spectroscopic Survey, have carried out the first study making use of the data products of the Survey together with the dataset from the first release (DR1) of the Gaia mission. The Gaia-ESO project is a large public spectroscopic survey, to which 340 nights on the ESO VLT have been allocated. It is the largest stellar survey performed on an 8-m class telescope. The Survey, which is close to completion, is observing more than 100,000 stars, systematically covering all the major components of the Milky Way, from halo to star-forming regions, providing the first homogeneous overview of the distributions of kinematics and elemental abundances. This alone is revolutionising knowledge of Galactic and stellar evolution: when combined with Gaia astrometry, the Survey is going to quantify the formation history and evolution of young, mature and ancient Galactic populations.

One of the top level goals of the Gaia-ESO Survey is the use of star clusters to calibrate stellar ages, which in turn is vital in addressing many of the most important topics in modern astrophysics. Star clusters indeed represent key age calibrators for stars in all evolutionary phases. Members of a given cluster cover different masses and evolutionary stages, at the cluster age and metallicity. Each cluster thus represents a snapshot of stellar evolution; linking together observations of many clusters at different ages and chemical compositions empirically reveals the story of stellar evolution, to be compared with the predictions of theoretical models.

Randich and collaborators exploited the Gaia-DR1 TGAS catalogue and the Gaia-ESO Survey data of targets in the fields of eight open star clusters, to infer the cluster parallaxes and distances to the Sun, and to derive clean sequences of cluster members. These were compared with different sets of stellar evolutionary models using a statistical approach (see Figure 1) and allowing the determination of the ages of the clusters. The systematic parallax errors inherent in the Gaia DR1 data still limited the precision of the results; nevertheless, the eight clusters were put onto the same age scale for the first time. Also, the approach used in this pilot study appeared extremely promising, demonstrating the potential of combining Gaia and ground-based spectroscopic datasets.

IC2391 JKs H siBin ov0.150 finali fitJHKsIC2602 VKs V siBin ov0.150 finali fitJHKsIC2602 HR siBin ov0.150 finali fitJHKs

Figure 1: Comparison between isochrones and observational data in the (J-Ks,H), (V-Ks,V), and (log Teff, Ks) diagrams for the cluster IC 2602 (30 Myr). Members from the Gaia-ESO Survey are indicated as blue full circles and red squares, while Gaia-TGAS members are plotted as magenta open triangles. Photometric data are taken from the 2MASS catalogue, AAVSO Photometric All Sky Survey (APASS) DR9, and ASCC-2.5, 3rd version compilation. An unresolved binary sequence is shown as a dashed line. The Figure shows a good, global agreement between theory - isochrones with the most probable ages and reddening values - and observations. Noticeably, this is true for the different selected colour-magnitude diagrams (for the entire sequences) and even in the Ks versus Teff diagrams. Source: Randich et al (2017).

The study has involved many researchers from the Arcetri Astrophysical Observatory (Randich, Pancino, Sacco, Magrini, Franciosini, Morbidelli, Roccatagliata, Bravi) and has been carried out in collaboration with people at the University of Pisa and with researchers in different INAF institutes. The results of this study are published in the article "The Gaia-ESO Suvery: open clusters in Gaia-DR1 - a way forward to stellar age calibration" led by Sofia Randich.


Edited by Sofia Randich and Anna Gallazzi

 

CO excitation in the Seyfert galaxy NGC 34: stars, shock or AGN driven?

In the last decades, intensive observational and theoretical investigations have demonstrated how the Star Formation (SF) of galaxies and the Active Galactic Nuclei (AGN) phenomena are deeply connected. There are many observational pieces of evidence that support this connection, such as the tight relations between the super massive black hole (BH) mass and the host galaxy properties or the similar shape of the SF and the BH accretion density as a function of the cosmic.

In this context, Matilde Mingozzi – Ph.D. student at the University of Bologna and at the Arcetri Astrophysical Observatory – and her collaborators (including Giovanni Cresci from the Arcetri Astrophysical Observatory) have studied the local galaxy NGC 34, where the two phenomena co-exist. The main aim of their work is assessing whether and to what extent the molecular gas emission, traced mainly by carbon monoxide (CO), is influenced by the radiation produced by the accretion onto the BH. In fact, molecular gas is a key component of the interstellar medium (ISM), as it is the fuel of SF and possibly of AGN accretion. In particular, the so-called CO Spectral Line Energy Distribution (CO SLED) – i.e. the CO line luminosity as a function of the CO upper rotational level – is a fundamental tool to constrain the physical properties of the molecular gas, such as density, temperature and the main source that causes the emission.

In their work, they analyse the X-ray and CO emission, using mainly archival data from XMM-Newton, NuSTAR, ALMA, and Herschel. On the one hand, the X-ray data allow to properly include the effect of AGN radiation in the modelling of the CO SLED, on the other hand, the CO emission, traced by ALMA in the central region of NGC 34 at high spatial resolution, is crucial to spatially constrain the region where the contribution of the AGN actually dominates. In order to fit the shape of the CO SLED, they make use of grids of models of Photo-Dissociation Regions (PDRs) and X-ray Dominated Regions (XDRs) – i.e. regions whose physics and chemistry are dominated by far ultraviolet radiation due to young stars and X-ray radiation coming from the AGN, respectively. In addition, they take into account also a grid of shock models, since shocks, originated from the supersonic injection of mass into the ISM, can also compress and heat the gas.

Matilde picture1
Figure 1: Fiducial model (PDR + XDR) over-plotted on the observed data. The light-blue dashed line and the red dotted line represent the low-density PDR and the high-density XDR, respectively. The black solid line indicates the sum of the two components and the shaded areas indicate the ±1σ uncertainty range on the normalization of each component. Taken from Mingozzi et al (2017).

In a recent paper, “CO excitation in the Seyfert galaxy NGC 34: stars, shock or AGN driven?” led by M. Minghozzi and accepted in MNRAS, the authors conclude that the AGN contribution is significant in heating the molecular gas in NGC 34 (Fig. 1 shows the best-fit of the CO SLED, composed by PDR and XDR components). Their results shed light on the great potential of combining self-consistent multi-band and multi-resolution data in order to assess the importance of AGN and SF activity, and mechanical heating produced by shocks for the physics of molecular gas.


Edited by M. Mingozzi and A. Gallazzi

The merger of two neutron stars opens the era of multimessenger astronomy

 

On August 17 at 12:41:04 UT the two LIGO and the Virgo gravitational wave interferometers detected the merging of two neutron stars, which occurred in the lenticular galaxy NGC 4993 at a distance of 40 Mpc (see Fig. 1). Two seconds later the Fermi satellite saw a short gamma-ray burst.

Figure 1: Artist view of a neutron star merger (Credit: NASA/Swift/Dana Berry).

This simultaneity and the coincidence of the position of the signals indicated that this was a single event; within minutes astronomers had turned their telescopes to the source, to capture for the first time the electromagnetic counterpart of an object detected in gravitational waves. Ultimately more than 70 observing facilities both ground- and space-based were used to study the electromagnetic counterpart, and verify the existence of a "kilonova" that had never before been clearly detected. INAF astronomers led many of the followup studies (see MediaINAF), and four scientists from the Arcetri Astrophysical Observatory were also involved. 

In particular Leslie Hunt participated in the study of optical and near-infraredspectra obtained with Xshooter at the VLT (Pian et al. 2017, see the Figure 2), Viviana Casasola collaborates with the team which observed the source in the radio with SRT (Aresu et al. 2017), Sperello di Serego Alighieri contributed to the optical polarimetry of the source obtained with FORS2 at the VLT (Covino et al. 2017), and Marco Padovani, who took part to the follow-up with gamma rays (HESS collaboration, 2017).

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Figure 2: Optical-IR spectra of the neutron stars merger obtained on consecutive days, as indicated on the right (Credit: Pian et al. 2017).