Hintze Centre for Astrophysical Surveys

Abstract:

In the companion Paper XV of this series, we derive accurate total mass-to-light ratios (M/L)JAM ≈ (M/L)(r = Re) within a sphere of radius r = Re centred on the galaxy, as well as stellar (M/L)stars (with the dark matter removed) for the volume-limited and nearly massselected (stellarmassM* ≲ 6 × 109M⊙)ATLAS3D sample of 260 early-type galaxies (ETGs, ellipticals Es and lenticulars S0s). Here, we use those parameters to study the two orthogonal projections (MJAM, σe) and (MJAM,R maje ) of the thin Mass Plane (MP) (MJAM, σe,Rmaje ) which describes the distribution of the galaxy population, where MJAM = L × (M/L)JAM ≈ M*. The distribution of galaxy properties on both projections of the MP is characterized by: (i) the same zone of exclusion (ZOE), which can be transformed from one projection to the other using the scalar virial equation. The ZOE is roughly described by two power laws, joined by a break at a characteristic mass MJAM ≈ 3 × 1010M⊙, which corresponds to the minimum Re and maximum stellar density. This results in a break in the meanMJAM-σe relation with trends MJAM α σ2.3e and MJAM α σ4.7e at small and large σe, respectively; (ii) a characteristic mass MJAM ≈ 2 × 1011M⊙ which separates a population dominated by flat fast rotator with discs and spiral galaxies at lower masses, from one dominated by quite round slow rotators at larger masses; (iii) below that mass the distribution of ETGs' properties on the two projections of the MP tends to be constant along lines of roughly constant se, or equivalently along lines with Rmaje α MJAM, respectively (or even better parallel to the ZOE: Rmaje α M0.75JAM); (iv) it forms a continuous and parallel sequence with the distribution of spiral galaxies; (v) at even lower masses, the distribution of fast-rotator ETGs and late spirals naturally extends to that of dwarf ETGs (Sph) and dwarf irregulars (Im), respectively. We use dynamical models to analyse our kinematic maps. We show that σe traces the bulge fraction, which appears to be the main driver for the observed trends in the dynamical (M/L)JAM and in indicators of the (M/L)pop of the stellar population like Hβ and colour, as well as in the molecular gas fraction. A similar variation along contours of σe is also observed for the mass normalization of the stellar initial mass function (IMF), which was recently shown to vary systematically within the ETGs' population. Our preferred relation has the form log10[(M/L)stars/(M/L)Salp] = a + b × log10(σe/130 km s-1) with a=-0.12 ± 0.01 and b = 0.35 ± 0.06. Unless there are major flaws in all stellar population models, this trend implies a transition of the mean IMF from Kroupa to Salpeter in the interval log10(σe/km s-1) ≈ 1.9-2.5 (or σe ≈ 90-290 km s-1), with a smooth variation in between, consistently with what was shown in Cappellari et al. The observed distribution of galaxy properties on the MP provides a clean and novel view for a number of previously reported trends, which constitute special two-dimensional projections of the more general four-dimensional parameters trends on the MP. We interpret it as due to a combination of two main effects: (i) an increase of the bulge fraction, which increases σe, decreases Re, and greatly enhance the likelihood for a galaxy to have its star formation quenched, and (ii) dry merging, increasing galaxy mass and Re by moving galaxies along lines of roughly constant σe (or steeper), while leaving the population nearly unchanged. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We investigate nuclear light profiles in 135 ATLAS3D galaxies for which the Hubble Space Telescope (HST) imaging is available and compare them to the large-scale kinematics obtained with the SAURONintegral-field spectrograph. Specific angular momentum, λR, correlateswith the shape of nuclear light profiles, where, as suggested by previous studies, cores are typically found in slow rotators and core-less galaxies are fast rotators. As also shown before, cores are found only in massive galaxies and only in systems with the stellar mass (measured via dynamical models) M ≳ 8 × 1010 M· Based on our sample, we, however, see no evidence for a bimodal distribution of nuclear slopes. The best predictor for finding a core is based on the stellar velocity dispersion within an effective radius, se, and specific angular momentum, where cores are found for λR ≲ 0.25 and σe ≳ 160 kms-1. We estimate that only about 10 per cent of nearby early-type galaxies contain cores. Furthermore, we show that there is a genuine population of fast rotators with cores. We also show that core fast rotators are morphologically, kinematically and dynamically different from core slow rotators. The cores of fast rotators, however, could harbour black holes of similar masses to those in core slow rotators, but typically more massive than those found in core-less fast rotators. Cores of both fast and slow rotators are made of old stars and found in galaxies typically lacking molecular or atomic gas (with a few exceptions). Core-less galaxies, and especially core-less fast rotators, are underluminous in the diffuse X-ray emission, but the presence of a core does not imply high X-ray luminosities. Additionally, we postulate (as many of these galaxies lack HST imaging) a possible population of core-less galaxies among slow rotators, which cannot be explained as face-on discs, but comprise a genuine sub-population of slow rotators. These galaxies are typically less massive and flatter than core slow rotators, and show evidence for dynamical cold structures and exponential photometric components. Based on our findings, major nondissipative (gas-poor) mergers together with black hole binary evolution may not be the only path for formation of cores in early-type galaxies. We discuss possible processes for formation of cores and their subsequent preservation. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

We present a 30-day monitoring campaign of the optical counterpart of the bright X-ray transient Swift J1745-26, starting only 19min after the discovery of the source.We observe the system peaking at i~17.6 on day six (MJD 561 92) to then decay at a rate of ~0.04 mag d-1. We show that the optical peak occurs at least 3 d later than the hard X-ray (15-50 keV) flux peak. Our measurements result in an outburst amplitude greater than 4.3 mag, which favours an orbital period ≲21 h and a companion star with a spectral type later than ~A0. Spectroscopic observations taken with the Gran Telescopio de Canarias 10.4 m telescope reveal a broad (full width at half-maximum ~1100 km s-1), double-peaked Ha emission line from which we constrain the radial velocity semi-amplitude of the donor to be K2 > 250 km s-1. The breadth of the line and the observed optical and X-ray fluxes suggest that Swift J1745-26 is a new black hole candidate located closer than ~7 kpc. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Abstract:

The black hole candidate, XTE J1752-223, was discovered in 2009 October when it entered an outburst. We obtained radio data from the Australia Telescope Compact Array for the duration of the ~9 month event. The light curves show that the radio emission from the compact jet persisted for the duration of an extended hard state and through the transition to the intermediate state. The flux then rose rapidly by a factor of 10 and the radio source entered a series of at least seven maxima, the first of which was likely to be emission associated with the compact jet. The subsequent six flares were accompanied by variable behaviour in terms of radio spectrum, degree of linear polarization, morphology and associated X-ray behaviour. They were, however, remarkably similar in terms of the estimated minimum power required to launch such an ejection event. We compare the timing of radio peaks with the location of the ejecta, imaged by contemporaneous Very Long Baseline Interferometry experiments. We then discuss the mechanism behind the events, in terms of whether discrete ejections are the most likely description of the behaviour. One ejection, at least, appears to be travelling with apparent superluminal motion. The range of properties, however, suggests that multiple mechanisms may be relevant and that at least some of the emission is coming from shocked interactions amongst the ejecta and between the ejecta and the interstellar medium. We also compare the radio flux density with the X-ray source during the hard state and conclude that XTE J1752-223 is a radio-weak/X-ray-bright outlier on the universal correlation for black hole transient sources. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.