Sandor J. Kruk, Chris J. Lintott, Brooke D. Simmons, Steven P. Bamford, Carolin N. Cardamone, Lucy Fortson, Ross E. Hart, Boris Häußler, Karen L. Masters, Robert C. Nichol, Kevin Schawinski, Rebecca J. Smethurst
Published 2017-04-28, 12 pages, 9 figures, accepted for publication in MNRAS
We use multi-wavelength SDSS images and Galaxy Zoo morphologies to identify asample of $\sim$$270$ late-type galaxies with an off-centre bar. We measureoffsets in the range 0.2-2.5 kpc between the photometric centres of the stellardisc and stellar bar. The measured offsets correlate with global asymmetries ofthe galaxies, with those with largest offsets showing higher lopsidedness.These findings are in good agreement with predictions from simulations ofdwarf-dwarf tidal interactions producing off-centre bars. We find that themajority of galaxies with off-centre bars are of Magellanic type, with a medianmass of $10^{9.6} M_{\odot}$, and 91% of them having $M_{\star}<3\times10^{10}M_{\odot}$, the characteristic mass at which galaxies start having highercentral concentrations attributed to the presence of bulges. We conduct asearch for companions to test the hypothesis of tidal interactions, but findthat a similar fraction of galaxies with offset bars have companions within 100kpc as galaxies with centred bars. Although this may be due to theincompleteness of the SDSS spectroscopic survey at the faint end, alternativescenarios that give rise to offset bars such as interactions with darkcompanions or the effect of lopsided halo potentials should be considered.Future observations are needed to confirm possible low mass companioncandidates and to determine the shape of the dark matter halo, in order to findthe explanation for the off-centre bars in these galaxies.
Qiao Wang, Yougang Wang, Chao Liu, Shude Mao, R. J. Long
Published 2017-05-01, 11 pages, 7 figures and 3 tables, Accepted for publication in MNRAS
With a sample of 48,161 K giant stars selected from the LAMOST DR 2catalogue, we construct torus models in a large volume extending, for the firsttime, from the solar vicinity to a Galactocentric distance of $\sim 20$ kpc,reaching the outskirts of the Galactic disc. We show that the kinematics of theK giant stars match conventional models, e.g. as created by Binney in 2012, inthe Solar vicinity. However such two-disc models fail if they are extended tothe outer regions, even if an additional disc component is utilised. If weloosen constraints in the Sun's vicinity, we find that an effective thick discmodel could explain the anti-centre of the MW. The LAMOST data imply that thesizes of the Galactic discs are much larger, and that the outer disc is muchthicker, than previously thought, or alternatively that the outer structure isnot a conventional disc at all. However, the velocity dispersion $\sigma_{0z}$of the kinematically thick disc in the best-fitting model is about 80 kms$^{-1}$ and has a scale parameter $R_{\sigma}$ for an exponential distributionfunction of $\sim 19$ kpc. Such a height $\sigma_{0z}$ is strongly rejected bycurrent measurements in the solar neighbourhood, and thus a model beyondquasi-thermal, two or three thin or thick discs is required.
Китайская работа в рамках спектрального обзора звезд Галактики на телескопе LAMOST. На основании коллекции из лучевых скоростей 48 тыс. K-гигантов в направлении около антицентра строится модель внешнего диска(-ов) Галактики; применяется метод “торов” Бинни (2012). Получается, что на радиусах Галактики больше 10 кпк звездный диск существенно горячее (и толще), чем в окрестностях Солнца!
J. Neumann, L. Wisotzki, O. S. Choudhury, D. A. Gadotti, C. J. Walcher, J. Bland-Hawthorn, R. García-Benito, R. M. González Delgado, B. Husemann, R. A. Marino, I. Márquez, S. F. Sánchez, B. Ziegler, CALIFA collaboration
Published 2017-05-01, 15 pages, 14 figures, accepted for publication in A&A
Understanding the nature of bulges in disc galaxies can provide importantinsights into the formation and evolution of galaxies. For instance, thepresence of a classical bulge suggests a relatively violent history, incontrast, the presence of simply an inner disc (also referred to as a"pseudobulge") indicates the occurrence of secular evolution processes in themain disc. However, we still lack criteria to effectively categorise bulges,limiting our ability to study their impact on the evolution of the hostgalaxies. Here we present a recipe to separate inner discs from classicalbulges by combining four different parameters from photometric and kinematicanalyses: The bulge S\'ersic index $n_\mathrm{b}$, the concentration index$C_{20,50}$, the Kormendy (1977) relation and the inner slope of the radialvelocity dispersion profile $\nabla\sigma$. With that recipe we provide adetailed bulge classification for a sample of 45 galaxies from theintegral-field spectroscopic survey CALIFA. To aid in categorising bulgeswithin these galaxies, we perform 2D image decomposition to determine bulgeS\'ersic index, bulge-to-total light ratio, surface brightness and effectiveradius of the bulge and use growth curve analysis to derive a new concentrationindex, $C_{20,50}$. We further extract the stellar kinematics from CALIFA datacubes and analyse the radial velocity dispersion profile. The results of thedifferent approaches are in good agreement and allow a safe classification forapproximately $95\%$ of the galaxies. In particular, we show that our new"inner" concentration index performs considerably better than the traditionallyused $C_{50,90}$ when yielding the nature of bulges. We also found that acombined use of this index and the Kormendy (1977) relation gives a very robustindication of the physical nature of the bulge.
Отличный пример, как НЕ надо работать. Уважаемая нами компания обзора CALIFA предлагает новый комплексный(!) подход к классификации балджей галактик, пытаясь многопараметрически(!), сочетая фотометрию и кинематику(!), диагностировать псевдобалджи. В результате, забыв, что закон Серсика можно интегрировать вдоль радиуса, в том числе аналитически для n=1, они накручивают вокруг выбора балджей с n=1 целую свиту критериев, на самом деле однозначно зависимых друг от друга, при этом выбрасывая невзначай единственное, что по Корменди отличает псевдобалджи от классических балджей - пониженную дисперсию скоростей при заданной светимости (отклонение от закона Фабер-Джексона).
Mark B. Peacock, Stephen E. Zepf, Arunav Kundu, Thomas J. Maccarone, Bret D. Lehmer, Claudia Maraston, Anthony H. Gonzalez, Rafael T. Eufrasio, David A. Coulter
Published 2017-05-02, 13 pages, 7 figures, accepted for publication in ApJ
We present constraints on variations in the initial mass function (IMF) ofnine local early-type galaxies based on their low mass X-ray binary (LMXB)populations. Comprised of accreting black holes and neutron stars, these LMXBscan be used to constrain the important high mass end of the IMF. We considerthe LMXB populations beyond the cores of the galaxies ($>0.2R_{e}$; covering$75-90\%$ of their stellar light) and find no evidence for systematicvariations of the IMF with velocity dispersion ($\sigma$). We reject IMFs whichbecome increasingly bottom heavy with $\sigma$, up to steep power-laws(exponent, $\alpha>2.8$) in massive galaxies ($\sigma>300$km/s), forgalactocentric radii $>1/4\ R_{e}$. Previously proposed IMFs that becomeincreasingly bottom heavy with $\sigma$ are consistent with these data if onlythe number of low mass stars $(<0.5M_{\odot}$) varies. We note that our resultsare consistent with some recent work which proposes that extreme IMFs are onlypresent in the central regions of these galaxies. We also consider IMFs thatbecome increasingly top-heavy with $\sigma$, resulting in significantly moreLMXBs. Such a model is consistent with these observations, but additional dataare required to significantly distinguish between this and an invariant IMF.For six of these galaxies, we directly compare with published IMF mismatchparameters from the Atlas3D survey, $\alpha_{dyn}$. We find good agreement withthe LMXB population if galaxies with higher $\alpha_{dyn}$ have more top-heavyIMFs -- although we caution that our sample is quite small. Future LMXBobservations can provide further insights into the origin of $\alpha_{dyn}$variations.
Дальнейшее “закрытие” зависимости наклона начальной функции масс звезд от их дисперсии скоростей, через статистику двойных маломассивных рентгеновских звездных систем в 9 галактиках ранних типов. Продвижение вперед в данной работе основано на увеличении “базы” - интервала дисперсии скоростей звезд в исследуемой выборке галактик, а также на понижении предела по потоку для выборки рентгеновских источников Chandra. В результате в каждой индивидуальной галактике найдено от нескольких штук до десятков штук рентгеновских источников, даны индивидуальные оценки доли массивных звезд в общем количестве звезд, и по-прежнему не найдено никакой зависимости этой самой доли от дисперсии скоростей звезд в галактике.
A. Adamo, J. E. Ryon, M. Messa, H. Kim, K. Grasha, D. O. Cook, D. Calzetti, J. C. Lee, B. C. Whitmore, B. G. Elmegreen, L. Ubeda, L. J. Smith, S. N. Bright, A. Runnholm, J. E. Andrews, M. Fumagalli, D. A. Gouliermis, L. Kahre, P. Nair, D. Thilker, R. Walterbos, A. Wofford, A. Aloisi, G. Ashworth, T. M. Brown, R. Chandar, C. Christian, M. Cignoni, G. C. Clayton, D. A. Dale, S. E. de Mink, C. Dobbs, D. M. Elmegreen, A. S. Evans, J. S. Gallagher III, E. K. Grebel, A. Herrero, D. A. Hunter, K. E. Johnson, R. C. Kennicutt, M. R. Krumholz, D. Lennon, K. Levay, C. Martin, A. Nota, G. Ostlin, A. Pellerin, J. Prieto, M. W. Regan, E. Sabbi, E. Sacchi, D. Schaerer, D. Schiminovich F. Shabani, M. Tosi, S. D. Van Dyk, E. Zackrisson
Published 2017-05-03, accepted for publication in ApJ; 27 pages; complete stellar cluster catalogues will be released in June 2017 at https://legus.stsci.edu
We report the large effort which is producing comprehensive high-level youngstar cluster (YSC) catalogues for a significant fraction of galaxies observedwith the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. Wepresent the methodology developed to extract cluster positions, verify theirgenuine nature, produce multiband photometry (from NUV to NIR), and derivetheir physical properties via spectral energy distribution fitting analyses. Weuse the nearby spiral galaxy NGC628 as a test case for demonstrating the impactthat LEGUS will have on our understanding of the formation and evolution ofYSCs and compact stellar associations within their host galaxy. Our analysis ofthe cluster luminosity function from the UV to the NIR finds a steepening atthe bright end and at all wavelengths suggesting a dearth of luminous clusters.The cluster mass function of NGC628 is consistent with a power-law distributionof slopes $\sim -2$ and a truncation of a few times $10^5$ M$_\odot$. Aftertheir formation YSCs and compact associations follow different evolutionarypaths. YSCs survive for a longer timeframe, confirming their being potentiallybound systems. Associations disappear on time scales comparable tohierarchically organized star-forming regions, suggesting that they areexpanding systems. We find mass-independent cluster disruption in the innerregion of NGC628, while in the outer part of the galaxy there is little or nodisruption. We observe faster disruption rates for low mass ($\leq$ $10^4$M$_\odot$) clusters suggesting that a mass-dependent component is necessary tofully describe the YSC disruption process in NGC628.
Первые результаты - и отличная реклама! - обзора LEGUS - поиска молодых звездных скоплений в близких галактиках с пятицветной фотометрией Хаббловского космического телескопа, включая ультрафиолет. Представлены результаты для NGC 628 - около тысячи скоплений, в интервале масс от 5000 до 2 млн солнечных масс и в интервале возрастов от 2 до 200 млн лет. Обнаружено разное поведение (разная статистика) для массивных скоплений и менее массивных ОВ-ассоциаций.