HI Observations of the starburst galaxy NGC 2146

NGC 2146 is a peculiar spiral galaxy which is currently undergoing a major burst of star formation and is immersed in a extended HI structure that has morphological and kinematical resemblence to a strong tidal interaction. This paper reports aperture synthesis observations carried out in the 21cm line with the Very Large Array (VLA - The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities, Inc. under cooperative agreement with the National Science Foundation.) of two fields positioned to optimally cover the HI streams to the north and south of the galaxy, along with a 300 ft total power spectral mapping program to recover the low surface brightness extended emission. The observations reveal elongated streams of neutral hydrogen towards both the north and the south of the optical galaxy extending out up to 6 Holmberg radii. The streams are not in the principle plane of rotation of the galaxy, but instead are suggestive of a tidal interaction between NGC 2146 and a LSB companion that was destroyed by the encounter and remains undetected at optical wavelengths. Part of the southern stream is turning back to fall into the main galaxy, where it will create a long-lived warp in the HI disk of NGC 2146. Analysis of the trajectory of the outlying gas suggests that the closest encounter took place about 0.8 billion years ago and that infall of debris will continue for a similar time span.Comment: To be published in A&

Shadowing of Ultrahigh Energy Neutrinos

The rise with energy of the neutrino--nucleon cross section implies that at energies above few TeV the Earth is becoming opaque to cosmic neutrinos. The neutrinos interact with the nucleons through the weak charged current, resulting into absorption, and the weak neutral current, which provides a redistribution of the neutrino energy. We Mellin transform the neutrino transport equation and find its exact solution in the moment space. A simple analytical formula is provided, which describes accurately the neutrino spectrum, after the neutrinos have traversed the Earth. The effect of the weak neutral current is most prominent for an initial flat neutrino spectrum and we find that at low energies (around 1 TeV) the neutrino intensity is even enhanced.Comment: gziped, tar file of LaTeX paper plus 2 postscript figures, 13 page

Revealing a Ring-like Cluster Complex in a Tidal Tail of the Starburst Galaxy NGC 2146

We report the discovery of a ring-like cluster complex in the starburst galaxy NGC 2146. The Ruby Ring, so named due to its appearance, shows a clear ring-like distribution of star clusters around a central object. It is located in one of the tidal streams which surround the galaxy. NGC 2146 is part of the Snapshot Hubble U-band Cluster Survey (SHUCS). The WFC3/F336W data has added critical information to the available archival Hubble Space Telescope imaging set of NGC 2146, allowing us to determine ages, masses, and extinctions of the clusters in the Ruby Ring. These properties have then been used to investigate the formation of this extraordinary system. We find evidence of a spatial and temporal correlation between the central cluster and the clusters in the ring. The latter are about 4 Myr younger than the central cluster, which has an age of 7 Myr. This result is supported by the H alpha emission which is strongly coincident with the ring, and weaker at the position of the central cluster. From the derived total H alpha luminosity of the system we constrain the star formation rate density to be quite high, e.g. ~ 0.47 Msun/yr/kpc^2. The Ruby Ring is the product of an intense and localised burst of star formation, similar to the extended cluster complexes observed in M51 and the Antennae, but more impressive because is quite isolated. The central cluster contains only 5 % of the total stellar mass in the clusters that are determined within the complex. The ring-like morphology, the age spread, and the mass ratio support a triggering formation scenario for this complex. We discuss the formation of the Ruby Ring in a "collect & collapse" framework. The predictions made by this model agree quite well with the estimated bubble radius and expansion velocity produced by the feedback from the central cluster, making the Ruby Ring an interesting case of triggered star formation.Comment: 11 pages, 7 figures, 1 table; Accepted for publication in MNRA

Cold Gas Kinematics in an L* Spiral Galaxy at z=0.437: The Nature of Damped Lyman-alpha Absorbers

Westerbork Radio Synthesis Telescope observations of the redshifted 21cm line absorber against the z_em = 0.871 double lobed quasar 3C196 show that the intervening absorber is an L approximately equal L* spiral galaxy (3C196-G1) and that the absorbing layer of cold gas extends to radii of at least 30 h_50^-1 kpc. The new data solve several long standing puzzles about this system by (1) discovering a second 21cm absorption feature, corresponding to absorption against the NE lobe of the background radio source and (2) spatially ``resolving'' the two absorption features to isolate the absorption along the two lines of sight to the opposing radio lobes. These findings resolve the disagreement in redshift between the UV metal and 21cm lines, and as well as demonstrating that the neutral layer does absorb both lobes of the background radio source. Simple kinematic models with an inclined, rotating gas disk match the observed 21cm profile and are also compatible with both the redshift and velocity spread of the absorption measured in UV resonance lines along a third, independent line of sight to the quasar nucleus and with the lack of 21cm absorption in as earlier VLBI experiment that was sensitive to opacity against the hot spot in the northern lobe. The inferred rotation speed and luminosity for the galaxy are compatible with the z approximately 0 Tully-Fisher Relation. This system illustrates well how 21cm absorption against extended background radio sources is a powerful tool in determining the nature of the damped Lyman-alpha class of QSO absorption line system.Comment: 9 pages, accepted at A&

Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe

Observations of the high-redshift Universe with the 21 cm hyperfine line of neutral hydrogen promise to open an entirely new window onto the early phases of cosmic structure formation. Here we review the physics of the 21 cm transition, focusing on processes relevant at high redshifts, and describe the insights to be gained from such observations. These include measuring the matter power spectrum at z~50, observing the formation of the cosmic web and the first luminous sources, and mapping the reionization of the intergalactic medium. The epoch of reionization is of particular interest, because large HII regions will seed substantial fluctuations in the 21 cm background. We also discuss the experimental challenges involved in detecting this signal, with an emphasis on the Galactic and extragalactic foregrounds. These increase rapidly toward low frequencies and are especially severe for the highest redshift applications. Assuming that these difficulties can be overcome, the redshifted 21 cm line will offer unique insight into the high-redshift Universe, complementing other probes but providing the only direct, three-dimensional view of structure formation from z~200 to z~6.Comment: extended review accepted by Physics Reports, 207 pages, 44 figures (some low resolution); version with high resolution figures available at http://pantheon.yale.edu/~srf28/21cm/index.htm; minor changes to match published versio

Cold gas accretion in galaxies

Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: 1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. It may be regarded as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is produced by galactic fountains, it is likely that a part of it is of extragalactic origin. 3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. 4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates needed to sustain the observed star formation (~1 Msol/yr), additional infall of large amounts of gas from the IGM seems to be required.Comment: To appear in Astronomy & Astrophysics Reviews. 34 pages. Full-resolution version available at http://www.astron.nl/~oosterlo/accretionRevie

Study of the use of virtual laboratory environments in teaching physics in compulsory education

The incorporation of Information and Communication Technologies (I.C.T.) in Science Education seems to be able to facilitate the shift of new knowledge acquisition from a process of knowledge possession through transmission to a process of knowledge conquest through construction, since I.C.T. are effective tools for involving students in investigative activities scaffolding the construction of new knowledge. In the field of electric circuits in particular, researchers claim that technologically enriched virtual laboratory environments can reduce the students’ cognitive load providing students with suitable support for successfully performing investigative activities and substantially improving their field knowledge. These studies however, focus on the conceptual evolution of students ignoring other aspects of teaching.The present thesis aims at exploring the effects that virtual laboratory environments can have when they are utilized in a teaching-by-inquiry intervention in the field of electric circuits by examining multiple aspects of teaching efficiency when virtual laboratory environments of various kinds are used: real laboratories, 3-D virtual laboratory environments with affordances like the modelspace which bridges virtual circuits with the corresponding circuit schematics, 3-D virtual laboratory environments without the modelspace and virtual laboratories in the form of functional schematic applets. Three teaching sequences are developed and applied to High School student classes in the same way (same instructional method and material) differing only in the electric circuit laboratories used. Teaching effectiveness is assessed by comparing the students’ scores before and after the teaching interventions and through students’ responses in personal interviews. Among classes are compared the students’ conceptual evolution, the students’ ability to construct real circuits, the students’ ability to transform circuits from one representation to another (real, virtual, schematic) and the students’ ability to design and perform experiments with electric circuits.Within the context of a teaching-by-inquiry approach, in most cases all classes achieve similar scores, indicating that all laboratory environments can be effectively utilized in teaching in a similar way. Differences in the students’ scores are recorded in two cases. It seems that teaching with the use of real equipment is advantageous when special handling abilities are required in setting up a real circuit, while teaching with the use of virtual laboratory environments with affordances which support creating connections between the 3-D virtual representation and the schematic representation (modelspace) provides students with an advantage when faced with problems of increased difficulty and transforming an electric circuit from one form to another.Through the comparisons reported, the current study contributes to the assessment of various characteristics of laboratory environments, the improvement of their design and their effective utilization in teaching-by-inquiry activities. The Greek curriculum of physical sciences in compulsory education aims among others in the incorporation of I.C.T. in education and therefore the results of the present study present educational as well as research interest.Η αξιοποίηση των Τεχνολογιών της Πληροφορίας και Επικοινωνίας (Τ.Π.Ε.) στη διδασκαλία των Φυσικών Επιστημών φαίνεται ικανή να βοηθήσει σημαντικά στη μετατόπιση της διαδικασία πρόσκτησης νέων γνώσεων από τη γνώση ως κατοχή μέσω μετάδοσης, στη γνώση ως οικοδόμηση καθώς οι Τ.Π.Ε. αποτελούν αποτελεσματικά εργαλεία για την εμπλοκή των μαθητών σε διδασκαλίες με διερεύνηση και υποβοηθούν τη διαδικασία οικοδόμησης γνώσεων και την εννοιολογική κατανόηση διαφόρων φαινομένων. Ειδικότερα στο χώρο των ηλεκτρικών κυκλωμάτων, ερευνητές υποστηρίζουν ότι τεχνολογικά εμπλουτισμένα εικονικά περιβάλλοντα μπορούν να μειώσουν το γνωστικό φόρτο των μαθητών, προσφέροντας τους την κατάλληλη υποστήριξη για την επιτυχή εκτέλεση διερευνητικών δραστηριοτήτων και την εννοιολογική τους εξέλιξη. Οι μελέτες όμως αυτές επικεντρώνονται στην εννοιολογική εξέλιξη των μαθητών και παραβλέπουν άλλες όψεις της διδασκαλίας.Η παρούσα εργασία στοχεύει να διερευνήσει το ρόλο που μπορούν να διαδραματίσουν τα εικονικά εργαστηριακά περιβάλλοντα όταν ενταχθούν σε διδασκαλία με διερευνητική προσέγγιση στο γνωστικό αντικείμενο των ηλεκτρικών κυκλωμάτων, συγκρίνοντας πολύπλευρα την αποτελεσματικότητα της διδασκαλίας, όταν αυτή χρησιμοποιεί εργαστηριακά περιβάλλοντα διαφόρων χαρακτηριστικών: πραγματικά εργαστήρια, τρισδιάστατα εικονικά εργαστήρια με παροχές όπως ο μοντελοχώρος που συνδέουν τα τρισδιάστατα κυκλώματα με τα αντίστοιχά τους διαγραμματικά, τρισδιάστατα εικονικά περιβάλλοντα χωρίς μοντελοχώρο και εικονικά εργαστήρια με τη μορφή applet. Αναπτύσσονται τρεις διδακτικές σειρές οι οποίες εφαρμόζονται σε μαθητές Γυμνασίου με τον ίδιο τρόπο (ίδια μέθοδος και υλικό διδασκαλίας) σε διαφορετικά τμήματα με χρήση διαφόρων εργαστηριακών περιβαλλόντων ηλεκτρικών κυκλωμάτων. Μέσω συγκρίσεων των επιδόσεων των μαθητών πριν και μετά τις διδασκαλίες αλλά και προσωπικών συνεντεύξεων γίνεται αποτίμηση της εξέλιξης των μαθητών των διαφόρων τμημάτων και εξετάζεται αν οι διδασκαλίες με τα περιβάλλοντα που χρησιμοποιήθηκαν σε αυτές επιφέρουν παρόμοιες βελτιώσεις. Πιο συγκεκριμένα, συγκρίνονται η εννοιολογική εξέλιξη των μαθητών των διαφόρων τμημάτων και οι δεξιότητες κατασκευής πραγματικών ηλεκτρικών κυκλωμάτων, μετασχηματισμού ενός ηλεκτρικού κυκλώματος από μία αναπαράσταση σε άλλη (πραγματική, εικονική, διαγραμματική) και σχεδίασης και εκτέλεσης πειραμάτων με απλά ηλεκτρικά κυκλώματα.Τα αποτελέσματα φανερώνουν ότι στα πλαίσια διδασκαλιών διερευνητικής προσέγγισης στις περισσότερες περιπτώσεις δεν εμφανίζονται διαφοροποιήσεις στις επιδόσεις των διαφορετικών τμημάτων και επομένως τα εργαστήρια μπορούν να χρησιμοποιηθούν αποτελεσματικά στη διδασκαλία με παρόμοιο τρόπο. Διαφορές στις επιδόσεις των μαθητών εμφανίζονται σε δύο περιπτώσεις. Φαίνεται ότι η διδασκαλία με πραγματικά εργαστήρια παρέχει ένα πλεονέκτημα στην κατασκευή πραγματικών κυκλωμάτων όταν απαιτούνται ιδιαίτερες δεξιότητες κατά την κατασκευή, ενώ η διδασκαλία με εικονικά εργαστήρια που συνδυάζουν πολλαπλούς τρόπους αναπαράστασης και συνδέουν την τρισδιάστατη εικονική αναπαράσταση με τη διαγραμματική μέσω κατάλληλων παροχών (μοντελοχώρος) παρέχει ένα πλεονέκτημα στην επίλυση δύσκολων προβλημάτων και το μετασχηματισμό της γνώσης από μία αναπαράσταση σε άλλη.Η παρούσα έρευνα, μέσα από τις συγκρίσεις που παρουσιάζονται, συμβάλλει στην εκτίμηση της χρησιμότητας επί μέρους χαρακτηριστικών των εικονικών περιβαλλόντων, στον ορθότερο σχεδιασμό των εικονικών εργαστηρίων και κατ’ επέκταση στην αποτελεσματικότερη ένταξη και αξιοποίησή τους στην εκπαιδευτική διαδικασία στα πλαίσια διδασκαλιών διερευνητικής προσέγγισης. Το Αναλυτικό Πρόγραμμα Σπουδών για τη διδασκαλία των Φυσικών Επιστημών στην Υποχρεωτική Εκπαίδευση αποσκοπεί μεταξύ άλλων και στην εγκόλπωση των Τ.Π.Ε. στη διδασκαλία και επομένως, τα αποτελέσματα της παρούσας έρευνας εκτός από ερευνητικό, παρουσιάζουν και έντονο εκπαιδευτικό ενδιαφέρον