J. P. Nogueira-Cavalcante, T. S. Gonçalves, K. Menéndez-Delmestre, K. Sheth
Published 2017-09-20, 14 pages, 9 figures, 1 table. Accepted for publication in MNRAS (14 Sep. 2017)
We calculate the star formation quenching timescales in green valley galaxiesat intermediate redshifts ($z\sim0.5-1$) using stacked zCOSMOS spectra ofdifferent galaxy morphological types: spheroidal, disk-like, irregular andmerger, dividing disk-like galaxies further into unbarred, weakly-barred andstrongly-barred, assuming a simple exponentially-decaying star formationhistory model and based on the H$_{\delta}$ absorption feature and the $4000$\AA ~break. We find that different morphological types present different starformation quenching timescales, reinforcing the idea that the galaxy morphologyis strongly correlated with the physical processes responsible for quenchingstar formation. Our quantification of the star formation quenching timescaleindicates that disks have typical timescales $60\%$ to 5 times longer than thatof galaxies presenting spheroidal, irregular or merger morphologies. Barredgalaxies in particular present the slowest transition timescales through thegreen valley. This suggests that although secular evolution may ultimately leadto gas exhaustion in the host galaxy via bar-induced gas inflows that triggerstar formation activity, secular agents are not major contributors in the rapidquenching of galaxies at these redshifts. Galaxy interaction, associated withthe elliptical, irregular and merger morphologies contribute, to a moresignificant degree, to the fast transition through the green valley at theseredshifts. In the light of previous works suggesting that both secular andmerger processes are responsible for the star formation quenching at lowredshifts, our results provide an explanation to the recent findings that starformation quenching happened at a faster pace at $z\sim0.8$.
Published 2017-09-20, 11 pages, 12 figures, accepted to MNRAS
New and more reliable distances and proper motions of a large number of starsin the Tycho-Gaia Astrometric Solution (TGAS) catalogue allow to calculate thelocal matter density distribution more precisely than earlier. We devised amethod to calculate the stationary gravitational potential distributionperpendicular to the Galactic plane by comparing the vertical probabilitydensity distribution of a sample of observed stars with the theoreticalprobability density distribution computed from their vertical coordinates andvelocities. We applied the model to idealised test stars and to the realobservational samples. Tests with two mock datasets proved that the method isviable and provides reasonable results. Applying the method to TGAS data wederived that the total matter density in the Solar neighbourhood is $0.09\pm0.02 \text{M}_\odot\text{pc}^{-3}$ being consistent with the results fromliterature. The matter surface density within $|z|\le 0.75 \text{kpc}$ is$42\pm 4 \text{M}_\odot\text{pc}^{-2}$. This is slightly less than the resultsderived by other authors but within errors is consistent with previousestimates. Our results show no firm evidence for significant amount of darkmatter in the Solar neighbourhood. However, we caution that our calculations at$|z| \leq 0.75$ kpc rely on an extrapolation from the velocity distributionfunction calculated at $|z| \leq 25$ pc. This extrapolation can be verysensitive to our assumption that the stellar motions are perfectly decoupled inR and z, and to our assumption of equilibrium. Indeed, we find that $\rho (z)$within $|z|\le 0.75$ kpc is asymmetric with respect to the Galactic plane atdistances $|z| = 0.1-0.4$ kpc indicating that the density distribution may beinfluenced by density perturbations.
C. P. Haines, A. Finoguenov, G. P. Smith, A. Babul, E. Egami, P. Mazzotta, N. Okabe, M. J. Pereira, M. Bianconi, S. L. McGee, F. Ziparo, L. E. Campusano, C. Loyola
Published 2017-09-14, 20 pages, 17 figures. Submitted to MNRAS. Comments welcome
Galaxy clusters are expected to form hierarchically in a LCDM universe,growing primarily through mergers with lower mass clusters and the continualaccretion of group-mass halos. Galaxy clusters assemble late, doubling theirmasses since z~0.5, and so the outer regions of clusters should be replete withinfalling group-mass systems. We present an XMM-Newton survey to search forX-ray groups in the infall regions of 23 massive galaxy clusters at z~0.2,identifying 39 X-ray groups that have been spectroscopically confirmed to lieat the cluster redshift. These groups have mass estimates in the range2x10^13-7x10^14Msun, and group-to-cluster mass ratios as low as 0.02. Thecomoving number density of X-ray groups in the infall regions is ~25x higherthan that seen for isolated X-ray groups from the XXL survey. The average massper cluster contained within these X-ray groups is 2.2x10^14Msun, or 19% of themass within the primary cluster itself. We estimate that ~10^15Msun clustersincrease their masses by 16% between z=0.223 and the present day due to theaccretion of groups with M200>10^13.2Msun. This represents about half of theexpected mass growth rate of clusters at these late epochs. The other half islikely to come from smooth accretion of matter not bound in halos. The massfunction of the infalling X-ray groups appears significantly top-heavy withrespect to that of field X-ray systems, consistent with expectations fromnumerical simulations, and the basic consequences of collapsed massive darkmatter halos being biased tracers of the underlying large-scale densitydistribution.
Обзор в рентгене 23 площадок, содержащих массивные скопления галактик на красном смещении
0.2. Выделяются окрестные, тоже рентгеновские, группы, которые, судя по кинематике, падают
на скопления. По этой статистике оценивается темп роста массы скоплений галактик в последние
2 млрд лет - есть расхождение с LCDM предсказаниями.
Gregory Rudnick, Jacqueline Hodge, Fabian Walter, Ivelina Momcheva, Kim-Vy Tran, Casey Papovich, Elisabete da Cunha, Roberto Decarli, Amelie Saintonge, Christopher Willmer, Jennifer Lotz, Lindley Lentati
Published 2017-09-20, 19 pages, 11 figures, 3 tables. Accepted for publication in the Astrophysical Journal
We present an extremely deep CO(1-0) observation of a confirmed $z=1.62$galaxy cluster. We detect two spectroscopically confirmed cluster members inCO(1-0) with $S/N>5$. Both galaxies have log(${\cal M_{\star}}$/\msol)$>11$ andare gas rich, with ${\cal M}_{\rm mol}$/(${\cal M_{\star}}+{\cal M}_{\rmmol}$)$\sim 0.17-0.45$. One of these galaxies lies on the star formation rate(SFR)-${\cal M_{\star}}$ sequence while the other lies an order of magnitudebelow. We compare the cluster galaxies to other SFR-selected galaxies with COmeasurements and find that they have CO luminosities consistent withexpectations given their infrared luminosities. We also find that they havecomparable gas fractions and star formation efficiencies (SFE) to what isexpected from published field galaxy scaling relations. The galaxies arecompact in their stellar light distribution, at the extreme end for all highredshift star-forming galaxies. However, their SFE is consistent with otherfield galaxies at comparable compactness. This is similar to two other sourcesselected in a blind CO survey of the HDF-N. Despite living in a highly quenchedproto-cluster core, the molecular gas properties of these two galaxies, one ofwhich may be in the processes of quenching, appear entirely consistent withfield scaling relations between the molecular gas content, stellar mass, starformation rate, and redshift. We speculate that these cluster galaxies cannothave any further substantive gas accretion if they are to become members of thedominant passive population in $z<1$ clusters.
В массивном скоплении галактик на красном смещении 1.62 найдены две массивные галактики с
большим (до 45%) содержанием молекулярного газа. В них идет совершенно нормальное звездообразование.
Авторы ломают голову, как бы его прекратить в течение 2 млрд лет, чтобы вписаться в любимую
парадигму queinching'a.
Meng Gu, Charlie Conroy, David Law, Pieter van Dokkum, Renbin Yan, David Wake, Kevin Bundy, Allison Merritt, Roberto Abraham, Jielai Zhang, Matthew Bershady, Dmitry Bizyaev, Jonathan Brinkmann, Niv Drory, Kathleen Grabowski, Karen Masters, Kaike Pan, John Parejko, Anne-Marie Weijmans, Kai Zhang
Published 2017-09-20,
A large population of ultra-diffuse galaxies (UDGs) was recently discoveredin the Coma cluster. Here we present optical spectra of three such UDGs, DF7,DF44 and DF17, which have central surface brightnesses of $\mu_g \approx24.4-25.1$ mag arcsec$^{-2}$. The spectra were acquired as part of an ancillaryprogram within the SDSS-IV MaNGA Survey. We stacked 19 fibers in the centralregions from larger integral field units (IFUs) per source. With over 13.5hours of on-source integration we achieved a mean signal-to-noise ratio (S/N)in the optical of $9.5$\AA$^{-1}$, $7.9$\AA$^{-1}$ and $5.0$\AA$^{-1}$,respectively, for DF7, DF44 and DF17. Stellar population models applied tothese spectra enable measurements of recession velocities, ages andmetallicities. The recession velocities of DF7, DF44 and DF17 are$6599^{+40}_{-25}$km/s, $6402^{+41}_{-39}$km/s and $8315^{+43}_{-43}$km/s,spectroscopically confirming that all of them reside in the Coma cluster. Thestellar populations of these three galaxies are old and metal-poor, with agesof $7.9^{+3.6}_{-2.5}$Gyr, $8.9^{+4.3}_{-3.3}$Gyr and $9.1^{+3.9}_{-5.5}$Gyr,and iron abundances of $\mathrm{[Fe/H]}$ $-1.0^{+0.3}_{-0.4}$,$-1.3^{+0.4}_{-0.4}$ and $-0.8^{+0.5}_{-0.5}$, respectively. Their stellarmasses are $3$-$6\times10^8 M_\odot$. The UDGs in our sample are as old orolder than galaxies at similar stellar mass or velocity dispersion (only DF44has an independently measured dispersion). They all follow the well-establishedstellar mass$-$stellar metallicity relation, while DF44 lies below the velocitydispersion-metallicity relation. These results, combined with the fact thatUDGs are unusually large for their stellar mass, suggest that stellar massplays a more important role in setting stellar population properties for thesegalaxies than either size or surface brightness.
Для трех ультрадиффузных галактик в скоплении Coma получены интегральные спектры с относительно
приличным S/N и оценены возраст и металличность звездного населения. Металличность - на порядок
ниже солнечной, возраст - 8-10 млрд лет. Мы не привыкли к таким большим средним возрастам звезд
у таких маломассивных галактик.