Il progetto di ricerca GASP ha come scopo principale quello di comprendere come le galassie vicine a noi possano evolvere a seconda dell’ambiente in cui vivono e, in particolare, quali siano i meccanismi fisici che riescono a strappare il gas delle galassie, influenzando la loro forma.
Indice dei contenuti
Dal Gas Strappato alle Galassie Medusa: Come l’Ambiente Modella l’Evoluzione Galattica
di Benedetta Vulcani, Bianca Maria Poggianti, Alessia Moretti, Marco Gullieuszik
Introduzione
Lo studio dell’evoluzione delle galassie è uno dei settori più attivi dell’astrofisica moderna. Studiare l’evoluzione delle galassie è fondamentale per comprendere l’universo e il nostro posto al suo interno. Le galassie sono i mattoni dell’universo; analizzare come si formano, evolvono e interagiscono ci aiuta a svelare i processi che hanno portato alla formazione delle strutture cosmiche su larga scala ovvero dell’Universo stesso. Inoltre, capire l’evoluzione delle galassie può offrire indizi sull’origine e sulla distribuzione della materia oscura, sull’espansione dell’universo e sulle condizioni che hanno permesso la formazione di stelle, pianeti e, in ultima istanza, la vita. Le principali domande che gli astronomi si pongono sulle galassie riguardano la loro formazione, evoluzione e composizione. Ad esempio, ancora non sappiamo quali siano i processi che hanno portato alla nascita delle prime galassie nell’universo primordiale, quali fattori influenzino la loro evoluzione (come ad esempio le interazioni tra galassie o le attività del buco nero supermassiccio centrale), quali siano i meccanismi che regolano la formazione di nuove stelle al loro interno, cosa determini la loro forma e struttura e quale sia il loro destino finale. Queste domande guidano molte delle ricerche attuali in cosmologia e astrofisica, e la loro comprensione può offrire una visione più completa.
Fig. 1 – Osservatorio di Padova
Il progetto di ricerca GASP ha come scopo principale quello di comprendere come le galassie vicine a noi possano evolvere a seconda dell’ambiente in cui vivono e, in particolare, quali siano i meccanismi fisici che riescono a strappare il gas delle galassie, influenzando la loro forma. GASP è l’acronimo di “Gas Stripping Phenomena in Galaxies”, che vuol letteralmente dire “fenomeni fisici che riescono a strappare il gas alle galassie”. Il progetto è guidato dalla dott.ssa Bianca Maria Poggianti, direttrice dell’Osservatorio astronomico di Padova (Fig.1), una delle sedici sedi in Italia dell’Istituto Nazionale di Astrofisica ente di ricerca nazionale dedicato all’astrofisica. Il progetto GASP è stato finanziato dal Consiglio per la ricerca europeo con un ERC Advanced Grant di 2 milioni e mezzo di euro per cinque anni. L’importo è stato sfruttato principalmente per finanziare giovani ricercatrici e ricercatori a collaborare a questo progetto e a disseminare i risultati in conferenze di carattere nazionale e internazionale. Negli ultimi anni, all’Osservatorio di Padova una quindicina di persone tra personale a tempo indeterminato, PostDoc e dottorande/i, ha afferito al gruppo GASP. Al corposo gruppo si sono aggiunti circa venti altri ricercatori di istanza in altri istituti, sia sul suolo italiano che internazionale. La complessità degli studi affrontati infatti ha richiesto la collaborazione di scienziati con esperienze professionali complementari.
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The jellyfish galaxy JW39 hangs serenely in this image from the NASA/ESA Hubble Space Telescope. This galaxy lies over 900 million light-years away in the constellation Coma Berenices, and is one of several jellyfish galaxies that Hubble has been studying over the past two years. Despite this jellyfish galaxy’s serene appearance, it is adrift in a ferociously hostile environment; a galaxy cluster. Compared to their more isolated counterparts, the galaxies in galaxy clusters are often distorted by the gravitational pull of larger neighbours, which can twist galaxies into a variety of weird and wonderful shapes. If that was not enough, the space between galaxies in a cluster is also pervaded with a searingly hot plasma known as the intracluster medium. While this plasma is extremely tenuous, galaxies moving through it experience it almost like swimmers fighting against a current, and this interaction can strip galaxies of their star-forming gas. This interaction between the intracluster medium and the galaxies is called ram-pressure stripping, and is the process responsible for the trailing tendrils of this jellyfish galaxy. As JW39 has moved through the cluster the pressure of the intracluster medium has stripped away gas and dust into long trailing ribbons of star formation that now stretch away from the disc of the galaxy. Astronomers using Hubble’s Wide Field Camera 3 studied these trailing tendrils in detail, as they are a particularly extreme environment for star formation. Surprisingly, they found that star formation in the ‘tentacles’ of jellyfish galaxies was not noticeably different from star formation in the galaxy disc. [Image Description: A spiral galaxy. It is large in the centre with a lot of detail visible. The core glows brightly and is surrounded by concentric rings of dark and light dust. The spiral arms are thick and puffy with grey dust and glowing blue areas of star formation. They wrap around the galaxy to form a ring. Part of the arm isThe galaxy JW100 features prominently in this image from the NASA/ESA Hubble Space Telescope, with streams of star-forming gas dripping from the disc of the galaxy like streaks of fresh paint. These tendrils of bright gas are formed by a process called ram pressure stripping, and their resemblance to dangling tentacles has led astronomers to refer to JW100 as a ‘jellyfish’ galaxy. It is located in the constellation Pegasus, over 800 million light-years away. Ram pressure stripping occurs when galaxies encounter the diffuse gas that pervades galaxy clusters. As galaxies plough through this tenuous gas it acts like a headwind, stripping gas and dust from the galaxy and creating the trailing streamers that prominently adorn JW100. The bright elliptical patches in the image are other galaxies in the cluster that hosts JW100. As well as JW100’s bright tendrils, this image also contains a remarkably bright area of diffuse light towards the top of this image which contains two bright blotches at its core. This is the core of IC 5338, the brightest galaxy in the galaxy cluster, known as a cD galaxy. It’s not unusual for cD galaxies to exhibit multiple nuclei, as they are thought to grow by consuming smaller galaxies, the nuclei of which can take a long time to be absorbed. The bright points of light studding its outer fringes are a rich population of globular clusters. This observation took advantage of the capabilities of Hubble’s Wide Field Camera 3, and is part of a sequence of observations designed to explore star formation in the tendrils of jellyfish galaxies. These tendrils represent star formation under extreme conditions, and could help astronomers understand the process of star formation elsewhere in the universe. [Image Description: A thin spiral galaxy is seen edge-on in the lower right. Its bulge and arms are very bright, mixing reddish and bluish light. Patchy blue trails extend below it, resembling tentacles, made from star-forming regions. Six smA jellyfish galaxy with trailing tentacles of stars hangs in inky blackness in this image from the NASA/ESA Hubble Space Telescope. As Jellyfish galaxies move through intergalactic space they are slowly stripped of gas, which trails behind the galaxy in tendrils illuminated by clumps of star formation. These blue tendrils are visible drifting below the core of this galaxy, and give it its jellyfish-like appearance. This particular jellyfish galaxy — known as JO201 — lies in the constellation Cetus, which is named after a sea monster from ancient Greek mythology. This sea-monster-themed constellation adds to the nautical theme of this image. The tendrils of jellyfish galaxies extend beyond the bright disc of the galaxy core. This particular observation comes from an investigation into the sizes, masses and ages of the clumps of star formation in the tendrils of jellyfish galaxies. Astronomers hope that this will provide a breakthrough in understanding the connection between ram-pressure stripping — the process that creates the tendrils of jellyfish galaxies — and star formation. This galactic seascape was captured by Wide Field Camera 3 (WFC3), a versatile instrument that captures images at ultraviolet and visible wavelengths. WFC3 is the source of some of Hubble’s most spectacular images, from a view of Jupiter and Europa to a revisit to the Pillars of Creation. [Image description: A spiral galaxy lies just off-centre. It has large, faint, reddish spiral arms and a bright, reddish core. These lie over two brighter blue spiral arms. These are patchy, with blotches of star formation. Long trails of these bright blotches trail down from the lower spiral arm, resembling tendrils. The background is black, lightly scattered with small galaxies and stars, and a larger elliptical galaxy in one corner.] Links First science paper in the Astrophysical Journal Second science paper in the Astrophysical Journal Zoom: Galactic SeascapeThe jellyfish galaxy JO206 trails across this image from the NASA/ESA Hubble Space Telescope, showcasing a colourful star-forming disc surrounded by a pale, luminous cloud of dust. A handful of bright stars with criss-cross diffraction spikes stand out against an inky black backdrop at the bottom of the image. JO206 lies over 700 million light-years from Earth in the constellation Aquarius, and this image of the galaxy is the sixth and final instalment in a series of observations of jellyfish galaxies. Some of Hubble’s other observations of these peculiar galaxies — which range from grandiose to ghostly — are available here. Jellyfish galaxies are so-called because of their resemblance to their aquatic namesakes. In this image, the disc of JO206 is trailed by long tendrils of bright star formation that stretch towards the bottom right of this image, just as jellyfish trail tentacles behind them. The tendrils of jellyfish galaxies are formed by the interaction between galaxies and the intra-cluster medium, a tenuous superheated plasma that pervades galaxy clusters. As galaxies move through galaxy clusters they ram into the intracluster medium, which strips gas from the galaxies and draws it into the long tendrils of star formation. The tentacles of jellyfish galaxies give astronomers a unique opportunity to study star formation under extreme conditions, far from the influence of the main disc of the galaxy. Surprisingly, Hubble revealed that there are no striking differences between star formation in the discs of jellyfish galaxies and star formation in their tentacles, which suggests the environment of newly-formed stars has only a minor influence on their formation. [Image Description: A spiral galaxy that is tilted partially toward us. Its inner disc is bright and colourful, with bluish and reddish spots of star formation throughout the arms. An outer disc of pale, dim dust surrounds it. It has many arms, which are being pulled away from the disc, down and tThe jellyfish galaxy JO175 appears to hang suspended in this image from the NASA/ESA Hubble Space Telescope. This galaxy lies over 650 million light-years from Earth in the appropriately-named constellation Telescopium, and was captured in crystal-clear detail by Hubble’s Wide Field Camera 3. A handful of more distant galaxies are lurking throughout the scene, and a bright four-pointed star lies to the lower right side. Jellyfish galaxies get their unusual name from the tendrils of star-forming gas and dust that trail behind them, just like the tentacles of a jellyfish. These bright tendrils contain clumps of star formation and give jellyfish galaxies a particularly striking appearance. Unlike their ocean-dwelling namesakes, jellyfish galaxies make their homes in galaxy clusters, and the pressure of the tenuous superheated plasma that permeates these galaxy clusters is what draws out the jellyfish galaxies’ distinctive tendrils. Hubble recently completed a deep dive into jellyfish clusters, specifically the star-forming clumps of gas and dust that stud their tendrils. By studying the origins and fate of the stars in these clumps, astronomers hoped to better understand the processes underpinning star formation elsewhere in the Universe. Interestingly, their research suggests that star formation in the discs of galaxies is similar to star formation in the extreme conditions found in the tendrils of jellyfish galaxies. [Image Description: A spiral galaxy. Its spiral arms are studded with many pink spots, especially around the top of the galaxy. One arm is sticking out below the galaxy. From it and around the bottom of the galaxy, faint gas streams away, while little gas is visible above the galaxy. The galaxy is quite small in the centre of a dark background, where a few smaller galaxies of various shapes and sizes hang.] Links Pan: Ghostly galactic jellyfishHere we see JO204, a ‘jellyfish galaxy’ so named for the bright tendrils of gas that appear in this image to be drifting lazily below JO204’s bright central bulk. The galaxy lies almost 600 million light-years away in the constellation Sextans. This image was captured by the NASA/ESA Hubble Space Telescope, and it is the third of a series of Pictures of the Week featuring jellyfish galaxies. This series of images is possible thanks to a survey in which observations were made of six of these fascinating galaxies, including JO204. This survey was performed with the intention of better understanding star formation under extreme conditions. Given the dreamy appearance of this image, it would be understandable to wonder why jellyfish galaxies should be such a crucible for star formation. The answer is that — as is often the case with astronomy — first appearances can be deceiving. Whilst the delicate ribbons of gas beneath JO204 may look like floating jellyfish tentacles, they are in fact the outcome of an intense astronomical process known as ram pressure stripping. Ram pressure is a particular type of pressure exerted on a body when it moves relative to a fluid. An intuitive example is the sensation of pressure you experience when you are standing in an intense gust of wind — the wind is a moving fluid, and your body feels pressure from it. An extension of this analogy is that your body will remain whole and coherent, but the more loosely bound things — like your hair and your clothes — will flap in the wind. The same is true for jellyfish galaxies. They experience ram pressure because of their movement against the intergalactic medium that fills the spaces between galaxies in a galaxy cluster. The galaxies experience intense pressure from that movement, and as a result their more loosely bound gas is stripped away. This gas is mostly the colder and denser gas in the galaxy — gas which, when stirred and compressed by the ram pressure, collapses and
Fig. 9. Esempi di galassie che risentono della cosiddetta ram pressure stripping. Scie di materiale perso dalla galassia nel suo moto attraverso l’ammasso sono evidenti. GASP ha scoperto come in queste code si possano formare nuove stelle. (Credit: ESA/Hubble & NASA, M. Gullieuszik and the GASP team).
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