Arcetri Astrophysical Observatory

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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).

gw

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

The globular cluster anti-correlation as seen by the Gaia-ESO Survey

A group of researchers led by Elena Pancinopart of theGaia-ESO Survey consortium, have recently published on A&A a work (Pancino et al. 2017) based on the fourth internal release (iDR4)  data on globular clusters (GCs).  Their work was a science demonstration case of the quality of the derived atmospheric parameters and abundance ratios for relatively metal-poor stars. They also present for the first time data for NGC 5927, which is one of the most metal-rich globular clusters studied in the literature so far, with [Fe/H]=−0.39±0.04 dex; this cluster was included to connect with the open cluster regime in the Gaia-ESO Survey internal calibration.

Elena Pancino and her collaborators have studied the shape of the Mg-Al anti-correlation, which is extremely important for understanding the still unexplained multiple population phenomenon in globular clusters (see Figure 1).

The anti-correlation is thought to be produced by the hotter reactions in the CNO hydrogen burning cycle, that require at least 80 million degrees to operate and therefore cannot be produced within the low-mass stars we observe today.

They thus studied the dependency of the Mg-Al anti-correlation extension with metallicity, present-day mass, and age of the clusters, using Gaia-ESO data in combination with a large set of homogenized literature measurements. They found a dependency with both metallicity and mass, which is evident when fitting for the two parameters simultaneously (see Figure 2), but we did not find significant dependency with age. They confirm that the Mg-Al anti-correlation is not seen in all clusters, but disappears for the less massive or most metal-rich clusters.

fig MgAl

Figure 1: The Mg-Al anticorrelation in Galactic globular clusters observed by the Gaia-ESO survey. Each panel represents one of the GCs in iDR4 dataset, with large cyan symbols representing UVES high-resolution measurements, and large magenta ones representing GIRAFFE medium resolution ones. The smaller symbols in the background represent previous literature measurements. The GCs are sorted by metallicity, with NGC 4833 at [Fe/H]=-1.92~dex and NGC 5927 at [Fe/H]=-0.39 dex: it appears clearly that higher metallic.

fig ext

Figure 2: Behaviour of the extent of the Mg-Al anticorrelation in GCs as a function of mass and metallicity. The extent is represented by two indicators: the standard deviation  σ[Al/Ma] and the maximum variation Δ[Fe/H]. Points are coloured with mass in the left panels (yellow is less massive and orange more massive) and with metallicity in the right panels (blue is less metal-rich and red is more metal-rich). As can be seen, the Mg-Al anticorrelation extension changes with the mass and the metallicity of GCs, to the point of disappearing for the less massive and/or more metal-rich GCs.

Thanks to the wide variety of checks and applications done by the group of Elena Pancino on globular clusters, we can firmly conclude that Gaia-ESO iDR4 data already meet the requirements set by the main survey goals and that they can be also used to the metal poor stellar populations, including a detailed study of globular clusters. 

References:

The Gaia-ESO Survey. Mg-Al anti-correlation in iDR4 globular clusters, Pancino, E.; Romano, D.; Tang, B.; Tautvaišienė, G.; Casey, A. R.; Gruyters, P.; Geisler, D.; San Roman, I.; Randich, S.; Alfaro, E. J, et al., 2017, A&A, 601, 112

The uniformity of dust properties at high galactic latitude in the HeViCS field

  

Within the Herschel Virgo Cluster Survey (HeViCS) project, 84 square degrees of the sky on the Virgo cluster of galaxies were observed with the Herschel Space Telescope in 5 different wavelengths from 70 to 500um.

After several works devoted to study the dust properties of the galaxy population of the cluster, the 20th paper of the collaboration has switched its attention to the foreground emission from the Milky Way. The work is led by Simone Bianchi and with the participation of Carlo Giovanardi, Viviana Casasola, Sperello di Serego Alighieri, Leslie K. Hunt, Laura Magrini, Stefano Zibetti of the Arcetri Observatory, and other colleagues from the HeViCS and DustPedia  consortia. It combines SPIRE images from HeVICS with data from the ALFALFA HI survey and the Planck and IRAS satellites.

The correlation between observations of dust emission in the FIR/submm and the atomic gas column densityhas allowed to measure the dust emissivity (surface brightness per column density of hydrogen) at the unprecedented resolution of 5 arcminutes, thanks to the relatively high resolution of the ALFALFA data from the Arecibo radiotelescope. The work provides the first determination of the dust emissivity at 250 micron, which is found to be consistent with the measures from the Planck consortium at longer submm wavelengths. This is remarkable, since the HeViCS fields cover just 0.2% of the sky, while the Planck determinations are based on 50% of the sky. Thus, the dust emissivity - despite field-to-field variations, appears to be remarkably similar over the high latitude sky.

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Figure 1: dust emission at 250 micron (left), dust emission associated with HI (center) and the residuals.

The measurements presented in the work, together with those from Planck, will force the revision of the current dust model, which was constructed to fit previous, higher, estimates of the dust emissivity from observations of the COBE satellite in the early nineties. After subtracting the dust emission associated with atomic gas, considerable residuals at scales smaller than 20 arcminutes are found (see Figure 1). Most of these residuals are likely due to dust associated to molecular gas, as confirmed by a recent observing run at the 30m IRAM telescope. An analysis of the properties and on the nature of these small, dusty, high-latitude molecular clouds is ongoing.

References:

HeViCS XX: Dust and gas in the foreground Galactic cirrus, S. Bianchi et al., 2017, A&A, 597, A130

HeViCS I: Luminosity functions, Davies et al., 2010, A&A 518, L48

DustPedia releases its rich database

The European  DustPedia project is capitalising on the legacy of the Herschel Space Observatory, using cutting-edge modelling techniques to study dust in the 875 DustPedia galaxies - representing the vast majority of extended galaxies within 3000 km/s that were observed by Herschel. This work requires a database of multiwavelength imagery and photometry that greatly exceeds the scope (in terms of wavelength coverage and number of galaxies) of any previous local-Universe survey.

The DustPedia team, including Simone Bianchi (co-PI of DustPedia) and Viviana Casasola of the Arcetri Observatory, constructed a database containing dedicated Herschel reductions, along with standardised archival observations from GALEX, SDSS, DSS, 2MASS, WISE, Spitzer, and Planck. The Arcetri DustPedia node was busy in the quality check of final data.

Using these data, the DustPedia team performed consistent aperture-matched photometry, which was combined with external supplementary photometry from IRAS and Planck. The DustPedia paper Clark et al. (2017), led by Christopher Clark of the Cardiff University, presents the multiwavelength imagery and photometry across 42 UV-microwave bands for the 875 DustPedia galaxies. Figure 1 shows the DustPedia spectral coverage.

Transmission_Curves.png

Figure 1: Illustration of the spectral coverage provided by the DustPedia database, showing filter response functions of all bands for which DustPedia presents data. The data provides complete sampling of over five orders of magnitude in wavelength. Response functions of the bands for which DustPedia presents both imagery and aperture-matched photometry are traced with solid lines. Bands for which DustPedia presents supplementary external photometry are traced with dashed lines. Bands for which DustPedia presents imagery only are traced with dotted lines.

The aperture-matched photometry, combined with the external supplementary photometry, represents a total of 21,857 photometric measurements. A typical DustPedia galaxy has multiwavelength photometry spanning 25 bands. An example of the final master aperture for a given source, applied to all bands, is shown in Figure 2 for the galaxy NGC 4559.

 NGC4559_Thumbnail_Grid.jpg

Figure 2: Photometry thumbnail image grid for the galaxy NGC 4559. DustPedia gives an image like this for every source. Solid lines show master aperture, whilst dashed lines demark background annulus.

The science within DustPedia is fast 'exploding' (e.g., Davies et al. 2017, Casasola et al. 2017, Clark et al. 2017), and 8 other works in advanced stage of preparation, on low metallicity galaxies, elliptical galaxies, morphological classification, extended dust in edge-on systems, radiative transfer modelling, scaling laws, global dust absorption, and environmental effects.

The rich DustPedia database of imagery and photometry is being made available to the community, and the DustPedia team encourages its use.

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References:

DustPedia: Multiwavelength Photometry and Imagery of 875 Nearby Galaxies in 42 Ultraviolet--Microwave Bands, Clark et al. (2017), arXiv170805335C, A&A in press

Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies, Casasola et al. (2017),  A&A 605, A18

DustPedia: A Definitive Study of Cosmic Dust in the Local Universe, Davies et al. (2017), PASP, 129, 044102

                     

DustPedia measures the scale-lengths in the Local Universe

The  paper Casasola et al. (2017), recently accepted by A&A,  is the first scientific DustPedia  publication, realized by exploiting the multi-wavelength (from UV to sub-mm wavelengths) DustPedia database. It is led by  Viviana Casasola and involves Simone Bianchi (co-PI of DustPedia), Laura Magrini, and the whole European DustPedia team.

This work presents the spatial variations of dust, stars, gas, and star-formation rate (SFR) of a sample of 18 nearby, spiral, face-on galaxies extracted from the DustPedia sample, by analyzing their surface brightness and mass surface density profiles. It provides the first 'direct' confirmation of the large gradients in the dust disks, which were 'indirectly' found by fitting the extinction lanes in optical images of edge-on galaxies.

Although each galaxy has its own peculiar behaviour, we identified some common trends of the exponential scale-lengths vs. wavelength, highlighted in Figure 1.

 all sl w02 stdevmean Umin3

Figure 1: Mean scale-length normalized with respect to r25 of all sample galaxies as a function of wavelength. The scale-lengths of mass (of dust, gas, and stars) and SFR surface density profiles and the scale-length of radiation field (U) profile, normalized with respect to r25, are plotted as with horizontal lines because they are not associated with a wavelength. They and the corresponding error bars are drawn at λ = 2000 micron.

The main result is that, on average, the dust mass surface density scale-length is about 1.8 times the stellar one, and close to that in the UV. This result is somewhat surprising and unexpected.

A mechanism that could explain our findings is a change in the typical lifetimes of grains against destruction by shocks, with a longer lifetime at larger radii.Because of the similarity between UV and dust scale-lengths, it is tempting to associate this mechanism to the inside-out galaxy formation scenario that is used to explain, together with dust extinction, the larger scale-lengths of radiation from the younger stellar component.

The radiative transfer analysis of galaxies we are planning to do within DustPedia consortium will shed a light on the true contribution of dust extinction to the shaping of the UV and optical appearance of spiral galaxies, and provide a detailed description of the disk surface brightness across the spectral energy distribution of dust emission.

References:

Casasola, V., Cassarà, L. P., Bianchi, S., Verstocken, S., Xilouris, E., Magrini, L., et al. 2017, “Radial distribution of dust, stars, gas, and star-formation rate in DustPedia face-on galaxies”, Accepted for publication in Astronomy & Astrophysics.

Davies, J. I., Baes, M., Bianchi, S., et al. 2017, PASP, 129, 044102,"DustPedia: A Definitive Study of Cosmic Dust in the Local Universe"