Astronomical instrumentation

Abstract:

We have carried out a survey for CO J= 1-0 and J= 2-1 emission in the 260 early-type galaxies of the volume-limited ATLAS3D sample, with the goal of connecting their star formation and assembly histories to their cold gas content. This is the largest volume-limited CO survey of its kind and is the first to include many Virgo cluster members. Sample members are dynamically hot galaxies with a median stellar mass ∼3 × 1010 M⊙; they are selected by their morphology rather than colour, and the bulk of them lie on the red sequence. The overall CO detection rate is 56/259 = 0.22 ± 0.03, with no dependence on the K luminosity and only a modest dependence on the dynamical mass. There are a dozen CO detections among the Virgo cluster members; statistical analysis of their H2 mass distributions and their dynamical status within the cluster shows that the cluster's influence on their molecular masses is subtle at best, even though (unlike spirals) they seem to be virialized within the cluster. We suggest that the cluster members have retained their molecular gas through several Gyr residences in the cluster. There are also a few extremely CO-rich early-type galaxies with H2 masses ≳109 M⊙ and these are in low-density environments. We do find a significant trend between the molecular content and stellar specific angular momentum. The galaxies of low angular momentum also have low CO detection rates, suggesting that their formation processes were more effective at destroying the molecular gas or preventing its re-accretion. We speculate on the implications of these data for the formation of various subclasses of early-type galaxies. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

We demonstrate here using both single-dish and interferometric observations that CO molecules are an excellent kinematic tracer, even in high-mass galaxies, allowing us to investigate for the first time the CO Tully-Fisher relation (CO-TFR) of early-type galaxies. We compare the TFRs produced using both single-dish and interferometric data and various inclination estimation methods, and evaluate the use of the velocity profile shape as a criterion for selecting galaxies in which the molecular gas extends beyond the peak of the rotation curve. We show that the gradient and zero-point of the best-fitting relations are robust, independent of the velocity measure and inclination used, and agree with those of relations derived using stellar kinematics. We also show that the early-type CO-TFR is offset from the CO-TFR of spirals by 0.98 ± 0.22 mag at Ks band, in line with other results. The intrinsic scatter of the relation is found to be ≈0.4 mag, similar to the level found in the spiral galaxy population. Next-generation facilities such as the Large Millimeter Telescope and the Atacama Large Millimeter/Sub-millimeter Array should allow this technique to be used in higher redshift systems, providing a simple new tool to trace the mass-to-light ratio evolution of the most massive galaxies over cosmic time. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

The mass assembly of galaxies leaves imprints in their outskirts, such as shells and tidal tails. The frequency and properties of such fine structures depend on the main acting mechanisms - secular evolution, minor or major mergers - and on the age of the last substantial accretion event. We use this to constrain the mass assembly history of two apparently relaxed nearby early-type galaxies (ETGs) selected from the ATLAS3D sample, NGC 680 and 5557. Our ultra-deep optical images obtained with MegaCam on the Canada-France-Hawaii Telescope reach 29 magarcsec-2 in thegband. They reveal very low surface brightness (LSB) filamentary structures around these ellipticals. Among them, a gigantic 160kpc long, narrow, tail east of NGC 5557 hosts three gas-rich star-forming objects, previously detected in Hi with the Westerbork Synthesis Radio Telescope and in UV withGALEX. NGC 680 exhibits two major diffuse plumes apparently connected to extended Hi tails, as well as a series of arcs and shells. Comparing the outer stellar and gaseous morphology of the two ellipticals with that predicted from models of colliding galaxies, we argue that the LSB features are tidal debris and that each of these two ETGs was assembled during a relatively recent, major wet merger, which most likely occurred after the redshiftz≃ 0.5 epoch. Had these mergers been older, the tidal features should have already fallen back or be destroyed by more recent accretion events. However, the absence of molecular gas and of a prominent young stellar population in the core region of the galaxies indicates that the merger is at least 1-2 Gyr old: the memory of any merger-triggered nuclear starburst has indeed been lost. The star-forming objects found towards the collisional debris of NGC 5557 are then likely tidal dwarf galaxies. Such recycled galaxies here appear to be long-lived and continue to form stars while any star formation activity has stopped in their parent galaxy. The inner kinematics of NGC 680 is typical for fast rotators which make the bulk of nearby ETGs in the ATLAS3D sample. On the other hand, NGC 5557 belongs to the poorly populated class of massive, round, slow rotators that are predicted by semi-analytic models and cosmological simulations to be the end-product of a complex mass accretion history, involving ancient major mergers and more recent minor mergers. Our observations suggest that under specific circumstances a single binary merger may dominate the formation history of such objects and thus that at least some massive ETGs may form at relatively low redshift. Whether the two galaxies studied here are representative of their own sub-class of ETGs is still an open question that will be addressed by an on-going deep optical survey of ATLAS3D galaxies. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

We study the formation of early-type galaxies (ETGs) through mergers with a sample of 70 high-resolution (softening length <60pc and 12 × 106 particles) numerical simulations of binary mergers of disc galaxies (with 10 per cent of gas) and 16 simulations of ETG remergers. These simulations, designed to accompany observations and models conducted within the ATLAS3D project, encompass various mass ratios (from 1:1 to 6:1), initial conditions and orbital parameters. The progenitor disc galaxies are spiral-like with bulge-to-disc ratios typical of Sb and Sc galaxies and high central baryonic angular momentum. We find that binary mergers of disc galaxies with mass ratios of 3:1 and 6:1 are nearly always classified as fast rotators according to the ATLAS3D criterion (based on the λR parameter - ATLAS3D Paper III): they preserve the structure of the input fast rotating spiral progenitors. They have intrinsic ellipticities larger than 0.5, cover intrinsic λR values between 0.2 and 0.6, within the range of observed fast rotators. The distribution of the observed fastest rotators does in fact coincide with the distribution of our disc progenitors. Major disc mergers (mass ratios of 2:1 and 1:1) lead to both fast and slow rotators. Most of the fast rotators produced in major mergers have intermediate flattening, with ellipticities ε between 0.4 and 0.6. Most slow rotators formed in these binary disc mergers hold a stellar kinematically distinct core (KDC) in their ~1-3 central kiloparsec: these KDCs are built from the stellar components of the progenitors. However, these remnants are still very flat with ε often larger than 0.45 and sometimes as high as 0.65. Besides a handful of specific observed systems - the counter-rotating discs (2σ galaxies, ATLAS3D Paper II) - these therefore cannot reproduce the observed population of slow rotators in the nearby Universe. This sample of simulations supports the notion of slow and fast rotators: these two families of ETGs present distinct characteristics in term of their angular momentum content (at all radii) and intrinsic properties - the slow rotators are not simply velocity-scaled down versions of fast rotators. The mass ratio of the progenitors is a fundamental parameter for the formation of slow rotators in these binary mergers, but it also requires a retrograde spin for the earlier-type (Sb) progenitor galaxy with respect to the orbital angular momentum. We also study remergers of these merger remnants: these produce relatively round fast rotators or systems near the threshold for slow rotators. In such cases, the orbital angular momentum dominates the central region, and these systems no longer exhibit a KDC, as KDCs are destroyed during the remergers and do not re-form in these relatively dry events. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

In Paper I of this series we introduced a volume-limited parent sample of 871 galaxies from which we extracted the ATLAS3D sample of 260 early-type galaxies (ETGs). In Papers II and III we classified the ETGs using their stellar kinematics, in a way that is nearly insensitive to the projection effects, and we separated them into fast and slow rotators. Here we look at galaxy morphology and note that the edge-on fast rotators generally are lenticular galaxies. They appear like spiral galaxies with the gas and dust removed, and in some cases are flat ellipticals (E5 or flatter) with discy isophotes. Fast rotators are often barred and span the same full range of bulge fractions as spiral galaxies. The slow rotators are rounder (E4 or rounder, except for counter-rotating discs) and are generally consistent with being genuine, namely spheroidal-like, elliptical galaxies. We propose a revision to the tuning-fork diagram by Hubble as it gives a misleading description of ETGs by ignoring the large variation in the bulge sizes of fast rotators. Motivated by the fact that only one third (34 per cent) of the ellipticals in our sample are slow rotators, we study for the first time the kinematic morphology-density T-Σ relation using fast and slow rotators to replace lenticulars and ellipticals. We find that our relation is cleaner than using classic morphology. Slow rotators are nearly absent at the lowest density environments [per cent] and generally constitute a small fraction [f(SR) ≈ 4 per cent] of the total galaxy population in the relatively low-density environments explored by our survey, with the exception of the densest core of the Virgo cluster [f(SR) ≈ 20 per cent]. This contrasts with the classic studies that invariably find significant fractions of (misclassified) ellipticals down to the lowest environmental densities. We find a clean log-linear relation between the fraction f(Sp) of spiral galaxies and the local galaxy surface density Σ3, within a cylinder enclosing the three nearest galaxies. This holds for nearly four orders of magnitude in the surface density down to Σ3≈ 0.01Mpc-2, with f(Sp) decreasing by 10 per cent per dex in Σ3, while f(FR) correspondingly increases. The existence of a smooth kinematic T-Σ relation in the field excludes processes related to the cluster environment, like e.g. ram-pressure stripping, as main contributors to the apparent conversion of spirals into fast rotators in low-density environments. It shows that the segregation is driven by local effects at the small-group scale. This is supported by the relation becoming shallower when using a surface density estimator Σ10 with a cluster scale. Only at the largest densities in the Virgo core does the f(Sp) relation break down and steepen sharply, while the fraction of slow rotators starts to significantly increase. This suggests that a different mechanism is at work there, possibly related to the stripping of the gas from spirals by the hot intergalactic medium in the cluster core and the corresponding lack of cold accretion. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.