156 research outputs found

    Developing Cryopreservation Methods of Wheat Roots

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    In the midst of record breaking rates of plant species extinction due to climate change and fungal diseases, a universal cryopreservation method would provide a means for preservation of these many different species. The concept of plant root cryopreservation first emerged in the late 1960‚Äôs, and with it came new avenues of preserving tissue for the purposes of agriculture and research. Frozen tissues can be transported and stored more reliably than other more conventional means. When thawed, they have the potential to be cultured and grown. Several different methods of cryopreservation exist. This experiment used the Fast (3¬įC/minute) and Slow freeze (0.3¬įC/minute) method with a controlled freezing unit on a wheat plant species. Cryoprotectant solutions containing 10% DMSO or 10% glycerol were used in conjunction with high (3.11 M) and low (1.5 M) sorbitol concentrations. A no sorbitol group was also tested. After being frozen, samples were transferred to a liquid nitrogen tank for storage, and later thawed. Thawed samples were stained with fluorescent dyes to observe live and dead cells under fluorescent microscopy. Root tips were deemed ‚Äúsurviving‚ÄĚ if multiple live nuclei were present. In the Glycerol group, Slow freezing showed a higher survival rate, while the DMSO group had good survival rates for both freezing rates. The Slow freeze rate showed greater survival rates overall compared to the Fast rate. Future experiments to develop a universal method will include testing on various plant species, such as potato plants, and succulents

    Extragalactic archaeology with the C, N, and O chemical abundances

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    Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO 4 pages, 4 figures, accepted for publication in A&A LettersWe predict how the C, N, and O abundances within the interstellar medium of galaxies evolve as functions of the galaxy star formation history (SFH). We adopt a hydrodynamical cosmological simulation, focusing on three star-forming disc galaxies with different SFHs. By assuming failed supernovae, we can predict an increasing trend of the gas-phase N/O--O/H abundance diagram, which was not produced in our previous simulations without failed supernovae. At high redshifts, contrary to the predictions of classical chemical evolution models with instantaneous mixing approximation, we find almost flat trends in the N/O--O/H diagram, which are due to the contribution of intermediate-mass stars together with an inhomogeneous chemical enrichment. Finally, we also predict that the average N/O and C/O steadily increase as functions of time, while the average C/N decreases, due to the mass and metallicity dependence of the yields of asymptotic giant branch stars; such variations are more marked during more intense star formation episodes. Our predictions on the CNO abundance evolution can be used to study the SFH of disc galaxies with the James Webb Space Telescope.Peer reviewedFinal Accepted Versio

    Nucleosynthesis in ONeMg Novae: Models versus Observations to Constrain the Masses of ONeMg White Dwarfs and Their Envelopes

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    Nucleosynthesis in ONeMg novae has been investigated with the wide ranges of three parameters, i.e., the white dwarf mass, the envelope mass at ignition, and the initial composition. A quasi-analytic one-zone approach is used with an up-to-date nuclear reaction network. The nucleosynthesis results show correlation with the peak temperatures or the cooling timescales during outbursts. Among the combinations of white dwarf and envelope masses which give the same peak temperature, the explosion is more violent for a lower white dwarf mass owing to its smaller gravitational potential. Comparison of the nucleosynthesis results with observations implies that at least two-third of the white dwarf masses for the observed ONeMg novae are ‚ČÉ1.1M‚äô\simeq 1.1 M_\odot, which are significantly lower than estimated by previous hydrodynamic studies but consistent with the observations of V1974 Cyg. Moreover, the envelope masses derived from the comparison are ‚Č≥10‚ąí4M‚äô\gtrsim 10^{-4} M_\odot, which are in good agreement with the ejecta masses estimated from observations but significantly higher than in previous hydrodynamic studies. With such a low mass white dwarf and a high mass envelope, the nova can produce interesting amounts of ő≥\gamma-ray emitters 7^7Be, 22^{22}Na, and 26^{26}Al. We suggest that V1974 Cyg has produced 22^{22}Na as high as the upper limit derived from the COMPTEL survey. In addition, a non-negligible part of the Galactic 26^{26}Al may originate from ONeMg novae, if not the major contributors. Both the future INTEGRAL survey for these ő≥\gamma-ray emitters and abundance estimates derived from ultraviolet, optical, and near infrared spectroscopies will impose a severe constraint on the current nova models.Comment: 21 pages, 23 figures, to appear in the Astrophysical Journal, Vol. 523, No.1, September 20, 1999; preprint with embedded images can be obtained from http://th.nao.ac.jp/~wanajo/journal/onenova.p

    Scaling relations of metallicity, stellar mass, and star formation rate in metal-poor starbursts: II. Theoretical models

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    Scaling relations of metallicity (O/H), star formation rate (SFR), and stellar mass give important insight on galaxy evolution. They are obeyed by most galaxies in the Local Universe and also at high redshift. In a companion paper, we compiled a sample of ~1100 galaxies from redshift 0 to ~3, spanning almost two orders of magnitude in metal abundance, a factor of ‚ąľ106\sim10^6 in SFR, and of ~10^5 in stellar mass. We have characterized empirically the star-formation "main sequence" (SFMS) and the mass-metallicity relation (MZR) for this sample, and also identified a class of low-metallicity starbursts, rare locally but more common in the distant universe. These galaxies deviate significantly from the main scaling relations, with high SFR and low metal content for a given M*. In this paper, we model the scaling relations and explain these deviations from them with a set of multi-phase chemical evolution models based on the idea that, independently of redshift, initial physical conditions in a galaxy's evolutionary history can dictate its location in the scaling relations. Our models are able to successfully reproduce the O/H, M*, and SFR scaling relations up to z~3, and also successfully predict the molecular cloud fraction as a function of stellar mass. These results suggest that the scaling relations are defined by different modes of star formation: an "active" starburst mode, more common at high redshift, and a quiescent "passive" mode that is predominant locally and governs the main trends.Comment: 17 pages, 7 figures, accepted for publication by MNRA

    HI study of extremely metal-deficient dwarf galaxies. I. The Nancay Radio Telescope observations of twenty-two objects

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    The goal of this study is to measure parameters of the integrated HI emission for twenty-two dwarf galaxies with oxygen abundance 12+log(O/H) in the range of 7.42 to 7.65, which are representatives of the eXtremely Metal-Deficient (XMD) galaxy group. Some of them are expected to be similar to the well-known candidates for local young galaxies, IZw18 and SBS 0335-052 that have most of their baryon mass in the form of neutral gas. Therefore, the HI 21-cm line observations are crucial to understanding their group and individual properties. The Nancay Radio Telescope (NRT) with the upgraded focal receiver was used for observations of the 21-cm HI-line. This permitted the detection of the faintest sources with rms of ~1 mJy per 10.5 km/s resolution element. For eighteen detected galaxies we present the parameters of their integrated HI line emission and describe the data on individual objects in more detail. For four undetected XMD galaxies, we give upper limits on their M$(HI). For 70% of the twenty studied non low surface brightness XMD galaxies, we find evidence (both from HI and optical data) for their interaction with neighboring objects. In the brief discussion of the group HI properties of the observed subsample (the total O/H range is of 0.23 dex, or a factor of 1.7), we underline the broad distributions of the HI mass (range is of 2 orders of magnitude), of the ratio M(HI)/L_B (of 1 order of magnitude), and of the blue luminosity (range is of 2 orders of magnitude). We also obtained HI parameters of six galaxies that do not belong to the XMD sample. These data increase the number of XMD galaxies with known integrated HI parameters (or upper limits) by a factor of two. This allows us to address statistical properties of this group, which will be presented in a forthcoming paper. (Abridged).Comment: 11 pages, including 2 tables and 2 postscript figures. Accepted to Astronomy and Astrophysic

    Chemical Abundances of Seven Irregular and Three Tidal Dwarf Galaxies in the M81 Group

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    We have derived nebular abundances for 10 dwarf galaxies belonging to the M81 Group, including several galaxies which do not have abundances previously reported in the literature. For each galaxy, multiple H \ii regions were observed with GMOS-N at the Gemini Observatory in order to determine abundances of several elements (oxygen, nitrogen, sulfur, neon, and argon). For seven galaxies, at least one H \ii region had a detection of the temperature sensitive [OIII] őĽ\lambda4363 line, allowing a "direct" determination of the oxygen abundance. No abundance gradients were detected in the targeted galaxies and the observed oxygen abundances are typically in agreement with the well known metallicity-luminosity relation. However, three candidate "tidal dwarf" galaxies lie well off this relation, UGC 5336, Garland, and KDG 61. The nature of these systems suggests that UGC 5336 and Garland are indeed recently formed systems, whereas KDG 61 is most likely a dwarf spheroidal galaxy which lies along the same line of sight as the M81 tidal debris field. We propose that these H \ii regions formed from previously enriched gas which was stripped from nearby massive galaxies (e.g., NGC 3077 and M81) during a recent tidal interaction.Comment: 37 pages, 10 figures, accepted for publication in ApJ. Slit positions in Table 2 have been update

    The role of mergers in driving morphological transformation over cosmic time

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    Accepted for publication in MNRASUnderstanding the processes that trigger morphological transformation is central to understanding how and why the Universe transitions from being disc-dominated at early epochs to having the morphological mix that is observed today. We use Horizon-AGN, a cosmological hydrodynamical simulation, to perform a comprehensive study of the processes that drive morphological change in massive (M*/M ‚äô > 10 10) galaxies over cosmic time. We show that (1) essentially all the morphological evolution in galaxies that are spheroids at z = 0 is driven by mergers with mass ratios greater than 1: 10; (2) major mergers alone cannot produce today's spheroid population - minor mergers are responsible for a third of all morphological transformation over cosmic time and are its dominant driver after z ~ 1; (3) prograde mergers trigger milder morphological transformation than retrograde mergers - while both types of event produce similar morphological changes at z > 2, the average change due to retrograde mergers is around twice that due to their prograde counterparts at z ~ 0; (4) remnant morphology depends strongly on the gas fraction of a merger, with gas-rich mergers routinely re-growing discs; and (5) at a given stellar mass, discs do not exhibit drastically different merger histories from spheroids - disc survival in mergers is driven by acquisition of cold gas (via cosmological accretion and gas-rich interactions) and a preponderance of prograde mergers in their merger histories.Peer reviewedFinal Accepted Versio

    A general-purpose mechanism of visual feature association in visual word identification and beyond

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    As writing systems are a relatively novel invention (slightly over 5 kya),1 they could not have influenced the evolution of our species. Instead, reading might recycle evolutionary older mechanisms that originally supported other tasks2,3 and preceded the emergence of written language. Accordingly, it has been shown that baboons and pigeons can be trained to distinguish words from nonwords based on orthographic regularities in letter co-occurrence.4,5 This suggests that part of what is usually considered reading-specific processing could be performed by domain-general visual mechanisms. Here, we tested this hypothesis in humans: if the reading system relies on domain-general visual mechanisms, some of the effects that are often found with orthographic material should also be observable with non-orthographic visual stimuli. We performed three experiments using the same exact design but with visual stimuli that progressively departed from orthographic material. Subjects were passively familiarized with a set of composite visual items and tested in an oddball paradigm for their ability to detect novel stimuli. Participants showed robust sensitivity to the co-occurrence of features (\u201cbigram\u201d coding) with strings of letter-like symbols but also with made-up 3D objects and sinusoidal gratings. This suggests that the processing mechanisms involved in the visual recognition of novel words also support the recognition of other novel visual objects. These mechanisms would allow the visual system to capture statistical regularities in the visual environment.6\u20139 We hope that this work will inspire models of reading that, although addressing its unique aspects, place it within the broader context of vision. Vidal et al. show that an effect usually studied in the context of reading\u2014sensitivity to bigram frequencies\u2014is also found when participants are presented with images of objects and circular sinusoidal gratings. This suggests that some mechanisms implied in the processing of novel words are in fact of general purpose

    How do minor mergers promote inside-out growth of ellipticals, transforming the size, density profile and dark matter fraction?

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    There is observational evidence for inside-out growth of elliptical galaxies since z‚Č≥2‚ąí3z \gtrsim 2-3, which is not driven by in-situ star formation. Many systems at high redshift have small sizes ‚ąľ1kpc\sim 1kpc and surface brightness profiles with low Sersic indices n. The most likely descendants have, on average, grown by a factor of two in mass and a factor of four in size, indicating r‚ąĚMőĪr \propto M^{\alpha} with őĪ‚Č≥2\alpha \gtrsim 2. They also have surface brightness profiles with n‚Č≥5n \gtrsim 5. This evolution can be qualitatively explained on the basis of two assumptions: compact ellipticals predominantly grow by collisionless minor or intermediate 'dry' mergers, and they are embedded in massive dark matter halos. We draw these conclusions from idealized collisionless mergers spheroidal galaxies - with and without dark matter - with mass ratios of 1:1, 1:5, and 1:10. The sizes evolve as r‚ąĚMőĪr \propto M^{\alpha} with őĪ<2\alpha < 2 for mass-ratios of 1:1. For minor mergers of galaxies embedded in dark matter halos, the sizes grow significantly faster and the profile shapes change more rapidly. Mergers with moderate mass-ratios of 1:5 give őĪ‚ąľ2.3\alpha \sim 2.3 and a final Sersic index of n=9.5n = 9.5 after doubling the stellar mass. This is accompanied by a significant increase of the dark matter fraction within the stellar half-mass radius, driven by the strong size increase probing larger, dark matter dominated regions. Only a few intermediate mass-ratio mergers of galaxies embedded in massive dark matter halos can result in the observed concurrent inside-out growth and the rapid evolution in profile shapes. Apart from negative stellar metallicity gradients such a 'minor' merger scenario also predicts significantly lower dark matter fractions for z‚ąľ2z \sim 2 compact quiescent galaxies and their rare present day analogues (abbreviated).Comment: accepted for publicatio

    Color and stellar population gradients in galaxies. Correlation with mass

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    We analyze the color gradients (CGs) of ~50000 nearby SDSS galaxies. From synthetic spectral models based on a simplified star formation recipe, we derive the mean spectral properties, and explain the observed radial trends of the color as gradients of the stellar population age and metallicity (Z). The most massive ETGs (M_* > 10^{11} Msun) have shallow CGs in correspondence of shallow (negative) Z gradients. In the stellar mass range 10^(10.3-10.5) < M_* < 10^(11) Msun, the Z gradients reach their minimum of ~ -0.5 dex^{-1}. At M_* ~ 10^{10.3-10.5} Msun, color and Z gradient slopes suddenly change. They turn out to anti-correlate with the mass, becoming highly positive at the very low masses. We have also found that age gradients anti-correlate with Z gradients, as predicted by hierarchical cosmological simulations for ETGs. On the other side, LTGs have gradients which systematically decrease with mass (and are always more negative than in ETGs), consistently with the expectation from gas infall and SN feedback scenarios. Z is found to be the main driver of the trend of color gradients, especially for LTGs, but age gradients are not negligible and seem to play a significant role too. We have been able to highlight that older galaxies have systematically shallower age and Z gradients than younger ones. Our results for high-mass galaxies are in perfect agreement with predictions based on the merging scenario, while the evolution of LTGs and younger and less massive ETGs seems to be mainly driven by infall and SN feedback. (Abridged)Comment: 20 pages, 16 figures, accepted for publication on MNRAS. This version includes revisions after the referee's report
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