In Sung Jang, Roelof S. de Jong, I. Minchev, Eric F. Bell, Antonela Monachesi, Benne W. Holwerda, Jeremy Bailin, Adam Smercina, Richard D'Souza
Published 2020-07-27, 6 pages, 5 figures, 1 appendix, accepted for publication in A&A Letters
NGC 300 is a low-mass disk galaxy in the Sculptor group. In the literature,it has been identified as a pure exponential disk galaxy, as its luminosityprofile could be well fitted with a single exponential law over many disk scalelengths (Type I). We investigate the stellar luminosity distribution of NGC 300using $Hubble$ $Space$ $Telescope$ (HST) archive data, reaching farther anddeeper than any other previous studies. Color magnitude diagrams show asignificant population of old red giant branch (RGB) stars in all fields out to$R\sim19$ kpc ($32'$), as well as younger populations in the inner regions. Weconstruct the density profiles of the young, intermediate-aged, and old stellarpopulations. We find two clear breaks in the density profiles of the old RGBand intermediate-aged stars: one down-bending (Type II) at $R\sim5.9$ kpc, andanother up-bending (Type III) at $R\sim8.3$ kpc. Moreover, the old RGB starsexhibit a negative radial color gradient with an up-bending at $R\sim8$~kpc,beyond which the stellar populations are uniformly old ($>$7~Gyr) andmetal-poor ($\rm[Fe/H] = -1.6^{+0.2}_{-0.4}$ dex). The outer stellar componentat $R\gtrapprox8$ kpc is, therefore, well separated from the inner disk interms of the stellar density and stellar populations. While our results castdoubt on the currently established wisdom that NGC\,300 is a pure exponentialdisk galaxy, a more detailed survey should be carried out to identify theoutskirts as either a disk or a stellar halo.
Published 2020-07-17, 17 pages, 15 figures; submitted to MNRAS
Optical diagnostic diagrams are powerful tools to separate different ionizingsources in galaxies. However, the model-constraining power of the mostwidely-used diagrams is very limited and challenging to visualize. In addition,there have always been classification inconsistencies between diagrams based ondifferent line ratios, and ambiguities between regions purely ionized by activegalactic nuclei (AGNs) and composite regions. We present a simple reprojectionof the 3D line ratio space composed of [N II]$\lambda 6583$/H$\alpha$, [SII]$\lambda \lambda$6716, 6731/H$\alpha$, and [O III]$\lambda$ 5007/H$\beta$,which reveals its model-constraining power and removes the ambiguity for thetrue composite objects. It highlights the discrepancy between many theoreticalmodels and the data loci. With this reprojection, we can put strong constraintson the photoionization models and the secondary nitrogen abundanceprescription. We find that a single nitrogen prescription cannot fit both thestar-forming locus and AGN locus simultaneously, with the latter requiringhigher N/O ratios. The true composite regions stand separately from bothmodels. We can compute the fractional AGN contributions for the compositeregions, and define demarcations with specific upper limits on contaminationfrom AGN or star formation. When the discrepancy about nitrogen prescriptionsgets resolved in the future, it would also be possible to make robustmetallicity measurements for composite regions and AGNs.
Для визуализации диагностических отношений линий вместо классических BPT диаграмм авторы предлагают использовать альтернативную проекцию в 3-мерном пространстве параметров [OIII]/Hb=R3, [NII]/Ha=N2 и [SII]/Ha=S2. Они показывают, что в плоскости P2 vs P1, где P1 = 0.63 N2 + 0.51 S2 + 0.59 R3; P2 = −0.63 N2 + 0.78 S2, модели для чистого звездообразования и AGN прекрасно разделяются, и хорошо выделяются области со смешанным механизмом ионизации. В этом пространстве возможно оценить металличность для таких композитных областей. Авторы отмечают, что для согласования наблюдений и моделей нужно предположить разный вклад вторичного азота в отношение N/O для областей звездообразования и AGN.
J. M. Pérez-Martínez, B. Ziegler, H. Dannerbauer, A. Böhm, M. Verdugo, A. I. Díaz, C. Hoyos
Published 2020-07-26, 26 pages, 13 figures. Accepted for publication in A&A
We present new gas kinematic observations with the OSIRIS instrument at theGTC for galaxies in the Cl1604 cluster system at z=0.9. These observationstogether with a collection of other cluster samples at different epochsanalyzed by our group are used to study the evolution of the Tully-Fisher,velocity-size and stellar mass-angular momentum relations in dense environmentsover cosmic time. We use 2D and 3D spectroscopy to analyze the kinematics ofour cluster galaxies and extract their maximum rotation velocities (Vmax). Ourmethods are consistently applied to all our cluster samples which make themideal for an evolutionary comparison. Up to redshift one, our cluster samplesshow evolutionary trends compatible with previous observational results in thefield and in accordance with semianalytical models and hydrodynamicalsimulations concerning the Tully-Fisher and velocity-size relations. However,we find a factor 3 drop in disk sizes and an average B-band luminosityenhancement of 2 mag by z=1.5. We discuss the role that differentcluster-specific interactions may play in producing this observational result.In addition, we find that our intermediate-to-high redshift cluster galaxiesfollow parallel sequences with respect to the local specific angularmomentum-stellar mass relation, although displaying lower angular momentumvalues in comparison with field samples at similar redshifts. This can beunderstood by the stronger interacting nature of dense environments withrespect to the field.
Zhen Wan, Geraint F. Lewis, Ting S. Li, Jeffrey D. Simpson, Sarah L. Martell, Daniel B. Zucker, Jeremy R. Mould, Denis Erkal, Andrew B. Pace, Dougal Mackey, Alexander P. Ji, Sergey E. Koposov, Kyler Kuehn, Nora Shipp, Eduardo Balbinot, Joss Bland-Hawthorn, Andrew R. Casey, Gary S. Da Costa, Prajwal Kafle, Sanjib Sharma, Gayandhi M. De Silva
Published 2020-07-29, Authors' version of an Article published in Nature on July 29th, 2020
Globular clusters are some of the oldest bound stellar structures observed inthe Universe. They are ubiquitous in large galaxies and are believed to traceintense star formation events and the hierarchical build-up of structure.Observations of globular clusters in the Milky Way, and a wide variety of othergalaxies, have found evidence for a `metallicity floor', whereby no globularclusters are found with chemical (`metal') abundances below approximately 0.3to 0.4 per cent of that of the Sun. The existence of this metallicity floor mayreflect a minimum mass and a maximum redshift for surviving globular clustersto form, both critical components for understanding the build-up of mass in theuniverse. Here we report measurements from the Southern Stellar StreamsSpectroscopic Survey of the spatially thin, dynamically cold Phoenix stellarstream in the halo of the Milky Way. The properties of the Phoenix stream areconsistent with it being the tidally disrupted remains of a globular cluster.However, its metal abundance ([Fe/H] = -2.7) is substantially below that of theempirical metallicity floor. The Phoenix stream thus represents the debris ofthe most metal-poor globular cluster discovered so far, and its progenitor isdistinct from the present-day globular cluster population in the localUniverse. Its existence implies that globular clusters below the metallicityfloor have probably existed, but were destroyed during Galactic evolution.