A prediction on the age of thick discs as a function of the stellar mass of the host galaxy

One of the suggested thick disc formation mechanisms is that they were born quickly and in situ from a turbulent clumpy disc. Subsequently, thin discs formed slowly within them from leftovers of the turbulent phase and from material accreted through cold flows and minor mergers. In this letter, I propose an observational test to verify this hypothesis. By combining thick disc and total stellar masses of edge-on galaxies with galaxy stellar mass functions calculated in the redshift range of $z\leq3.0$, I derived a positive correlation between the age of the youngest stars in thick discs and the stellar mass of the host galaxy; galaxies with a present-day stellar mass of $\mathcal{M}_\star(z=0)<10^{10}\,\mathcal{M}_\odot$ have thick disc stars as young as $4-6\,{\rm Gyr}$, whereas the youngest stars in the thick discs of Milky-Way-like galaxies are $\sim10\,{\rm Gyr}$ old. I tested this prediction against the scarcely available thick disc age estimates, all of them are from galaxies with $\mathcal{M}_\star(z=0)\gtrsim10^{10}\,\mathcal{M}_\odot$, and I find that field spiral galaxies seem to follow the expectation. On the other hand, my derivation predicts ages that are too low for the thick discs in lenticular galaxies, indicating a fast early evolution for S0 galaxies. I propose the idea of conclusively testing whether thick discs formed quickly and in situ by obtaining the ages of thick discs in field galaxies with masses of $\mathcal{M}_\star(z=0)\sim10^{9.5}\,\mathcal{M}_\odot$ and by checking whether they contain $\sim5\,{\rm Gyr}$-old stars.Comment: Letter accepted for publication in A&

A monolithic collapse origin for the thin/thick disc structure of ESO 243-49

ESO 243-49 is a high-mass (circular velocity $v_{\rm c}\approx200\,{\rm km\,s^{-1}}$) edge-on S0 galaxy in the Abell 2877 cluster at a distance of $\sim95\,{\rm Mpc}$. To elucidate the origin of its thick disc, we use MUSE science verification data to study its kinematics and stellar populations. The thick disc emits $\sim80\%$ of the light at heights in excess of $3.5^{\prime\prime}$ ($1.6\,{\rm kpc}$). The rotation velocities of its stars lag by $30-40\,{\rm km\,s^{-1}}$ compared to those in the thin disc, which is compatible with the asymmetric drift. The thick disc is found to be more metal-poor than the thin disc, but both discs have old ages. We suggest an internal origin for the thick disc stars in high-mass galaxies. We propose that the thick disc formed either ${\rm a)}$ first in a turbulent phase with a high star formation rate and that a thin disc formed shortly afterwards, or ${\rm b)}$ because of the dynamical heating of a thin pre-existing component. Either way, the star formation in ESO 243-49 was quenched just a few Gyrs after the galaxy was born and the formation of a thin and a thick disc must have occurred before the galaxy stopped forming stars. The formation of the discs was so fast that it could be described as a monolithic collapse where several generations of stars formed in a rapid succession.Comment: Accepted for publication in A&A. The reduced data-cube as well as the data necessary to build the kinematic and stellar population maps are available at https://etsin.avointiede.fi/dataset/urn-nbn-fi-csc-kata2016092414291163237

Oxygen and silicon abundances in Cygnus OB2: Chemical homogeneity in a sample of OB slow rotators

Cygnus OB2 is a rich OB association in the Galaxy which has experienced intense star formation in the last 20-25 Myr. Its stellar population shows a correlation between age and Galactic longitude. Exploring the chemical composition of its stellar content we will be able to check the degree of homogeneity of the natal molecular cloud and possible effects of self-enrichment processes. Our aim is to determine silicon and oxygen abundances for a sample of eight early-type slow rotators in Cygnus OB2 in order to check possible inhomogeneities across the whole association and whether there exists a correlation of chemical composition with Galactic longitude. We have performed a spectroscopic analysis of a sample of late O and early B stars with low rotational velocity, which have been chosen so as to cover the whole association area. We have carried out an analysis based on equivalent widths of metal lines, the wings of the H Balmer lines and FASTWIND stellar atmosphere models to determine their stellar fundamental parameters as well as the silicon and oxygen surface abundances. We derive a rather homogeneous distribution of silicon and oxygen abundances across the region, with average values of 12+log(Si/H)=7.53$\pm$0.08 dex and 12+log(O/H)=8.65$\pm$0.12 dex. We find a homogeneous chemical composition in Cygnus OB2 with no clear evidence for significant chemical self-enrichment, despite indications of strong stellar winds and possible supernovae during the history of the region. Comparison with different scenarios of chemical enrichment by stellar winds and supernovae point to star forming efficiencies not significantly above 10%. The degree of homogeneity that we find is consistent with the observed Milky Way oxygen gradient based on HII regions. We also find that the oxygen scatter within Cygnus OB2 is at least of the same order than among HII regions at similar Galactocentric distance.Comment: 15 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

Accretion in Brown Dwarfs: an Infrared View

This paper presents a study of the accretion properties of 19 very low mass objects (Mstar .01-0.1 Msun) in the regions Chamaeleon I and rho-Oph For 8 objects we obtained high resolution Halpha profiles and determined mass accretion rate Macc and accretion luminosity Lacc. Pabeta is detected in emission in 7 of the 10 rho-Oph objects, but only in one in Cha I. Using objects for which we have both a determination of Lacc from Halpha and a Pabeta detection, we show that the correlation between the Pabeta luminosity and luminosity Lacc, found by Muzerolle et al. (1998) for T Tauri stars in Taurus, extends to objects with mass approx 0.03 Msun; L(Pab) can be used to measure Lacc also in the substellar regime. The results were less conclusive for Brgamma, which was detected only in 2 objects, neither of which had an Halpha estimate of Macc. Using the relation between L(Pab) and Lacc we determined the accretion rate for all the objects in our sample (including those with no Halpha spectrum), more than doubling the number of substellar objects with known Macc. When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lower Macc for lower Mstar, although with a large spread. Some of the spread is probably due to an age effect; our very young objects in rho-Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Halpha measured at 10% peak intensity is not only a qualitative indicator of accretion, but can be used to obtain a quantitative estimate of Macc over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss.Comment: 11 pages, 8 figures, A&A in pres

MUSE-AO view of the starburst-AGN connection: NGC 7130

We present the discovery of a small kinematically decoupled core of 0.2$^{\prime\prime}$ (60 pc) in radius as well as an outflow jet in the archetypical AGN-starburst "composite" galaxy NGC 7130 from integral field data obtained with the adaptive optics-assisted MUSE-NFM instrument on the VLT. Correcting the already good natural seeing at the time of our science verification observations with the four-laser GALACSI AO system, we reach an unprecedented spatial resolution at optical wavelengths of around 0.15$^{\prime\prime}$. We confirm the existence of star-forming knots arranged in a ring of 0.58$^{\prime\prime}$ (185 pc) in radius around the nucleus, previously observed from UV and optical Hubble Space Telescope and CO(6-5) ALMA imaging. We determine the position of the nucleus as the location of a peak in gas velocity dispersion. A plume of material extends towards the NE from the nucleus until at least the edge of our field of view at 2$^{\prime\prime}$ (640 pc) radius which we interpret as an outflow jet originating in the AGN. The plume is not visible morphologically, but is clearly characterised in our data by emission-line ratios characteristic of AGN emission, enhanced gas velocity dispersion, and distinct non-circular gas velocities. Its orientation is roughly perpendicular to the line of nodes of the rotating host galaxy disc. A circumnuclear area of positive and negative velocities of 0.2$^{\prime\prime}$ in radius indicates a tiny inner disc, which can only be seen after combining the integral field spectroscopic capabilities of MUSE with adaptive optics.Comment: Accepted for publication in A&A letter

Binary energy source of the HH 250 outflow and its circumstellar environment

Herbig-Haro flows are signposts of recent major accretion and outflow episodes. We aim to determine the nature and properties of the little-known outflow source HH 250-IRS, which is embedded in the Aquila clouds. We have obtained adaptive optics-assisted L-band images with the NACO instrument on the Very Large Telescope (VLT), together with N- and Q-band imaging with VISIR also on the VLT. Using the SINFONI instrument on the VLT we carried out H- and K-band integral field spectroscopy of HH 250-IRS, complemented with spectra obtained with the SpeX instrument at the InfraRed Telescope Facility (IRTF) in the JHKL bands. Finally, the SubMillimeter Array (SMA) interferometer was used to study the circumstellar environment of HH 250-IRS at 225 and 351 GHz with CO (2-1) and CO (3-2) maps and 0.9 mm and 1.3 mm continuum images. The HH 250-IRS source is resolved into a binary with 0''53 separation, corresponding to 120 AU at the adopted distance of 225 pc. The individual components show heavily veiled spectra with weak CO absorption indicative of late-type stars. Both are Class I sources, but their spectral energy distributions between 1.5 $\mu$m and 19 $\mu$m differ markedly and suggest the existence of a large cavity around one of the components. The millimeter interferometric observations indicate that the gas mainly traces a circumbinary envelope or disk, while the dust emission is dominated by one of the circumstellar envelopes. HH 250-IRS is a new addition to the handful of multiple systems where the individual stellar components, the circumstellar disks and a circumbinary disk can be studied in detail, and a rare case among those systems in which a Herbig-Haro flow is present.Comment: Accepted for publication by Astronomy and Astrophysic

Thick discs in galaxies were most likely not accreted

The origin of thick discs in galaxies remains shrouded in mystery. A variety of formation scenarios has been proposed. Here we aim to test one such scenario where the thick disc stars are proposed to be accreted from satellite galaxies. In this scenario, in at least some galaxies a fraction of thick disc stars would rotate in a retrograde way, which would cause a large thick disc velocity lag. Here, we compare the rotation curves of the thin and the thick discs of eight edge-on galaxies observed with MUSE at the VLT. We find that the velocity lags of the thick discs are compatible with those expected from asymmetric drift. If we consider the galaxies with thick disc rotation curves in the literature, only one in about fifteen shows clear signs of an accreted thick disc. Based on simulations in the literature we estimate that if thick discs were accreted, at least one in six would show clear signs of retrograde material. Thus, there is a growing tension between the observations and the hypothesis that thick discs are made of accreted stars