Zooniverse

Abstract:

We explore the properties of dust and associated molecular gas in 352 nearby (0.01 < z < 0.07) early-type galaxies (ETGs) with prominent dust lanes, drawn from the Sloan Digital Sky Survey (SDSS). Two-thirds of these 'dusty ETGs' (D-ETGs) are morphologically disturbed, which suggests a merger origin, making these galaxies ideal test beds for studying the merger process at low redshift. The D-ETGs preferentially reside in lower density environments, compared to a control sample drawn from the general ETG population. Around 80per cent of D-ETGs inhabit the field (compared to 60per cent of the control ETGs) and less than 2per cent inhabit clusters (compared to 10per cent of the control ETGs). Compared to their control-sample counterparts, D-ETGs exhibit bluer ultraviolet-optical colours (indicating enhanced levels of star formation) and an active galactic nucleus fraction that is more than an order of magnitude greater (indicating a strikingly higher incidence of nuclear activity). The mass of clumpy dust residing in large-scale dust features is estimated, using the SDSS r-band images, to be in the range 104.5-106.5M⊙. A comparison to the total (clumpy + diffuse) dust masses - calculated using the far-infrared fluxes of 15per cent of the D-ETGs that are detected by the Infrared Astronomical Satellite (IRAS) - indicates that only 20per cent of the dust is typically contained in these large-scale dust features. The dust masses are several times larger than the maximum value expected from stellar mass loss, ruling out an internal origin. The dust content shows no correlation with the blue luminosity, indicating that it is not related to a galactic scale cooling flow. Furthermore, no correlation is found with the age of the recent starburst, suggesting that the dust is accreted directly in the merger rather than being produced in situ by the triggered star formation. Using molecular gas-to-dust ratios of ETGs in the literature, we estimate that the median current molecular gas fraction in the IRAS-detected ETGs is ∼1.3per cent. Adopting reasonable values for gas depletion time-scales and starburst ages, the median initial gas fraction in these D-ETGs is ∼4per cent. Recent work has suggested that the merger © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Abstract:

We present the second of two papers concerning the origin and evolution of local early-type galaxies exhibiting dust features. We use optical and radio data to examine the nature of active galactic nucleus (AGN) activity in these objects, and compare these with a carefully constructed control sample. We find that dust lane early-type galaxies are much more likely to host emission-line AGN than the control sample galaxies. Moreover, there is a strong correlation between radio and emission-line AGN activity in dust lane early types, but not the control sample. Dust lane early-type galaxies show the same distribution of AGN properties in rich and poor environments, suggesting a similar triggering mechanism. By contrast, this is not the case for early types with no dust features. These findings strongly suggest that dust lane early-type galaxies are starburst systems formed in gas-rich mergers. Further evidence in support of this scenario is provided by enhanced star formation and black hole accretion rates in these objects. Dust lane early types therefore represent an evolutionary stage between starbursting and quiescent galaxies. In these objects, the AGN has already been triggered but has not as yet completely destroyed the gas reservoir required for star formation. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Abstract:

We present an analysis of the environmental dependence of bars and bulges in disc galaxies, using a volume-limited catalogue of 15810 galaxies at z < 0.06 from the Sloan Digital Sky Survey with visual morphologies from the Galaxy Zoo 2 project. We find that the likelihood of having a bar, or bulge, in disc galaxies increases when the galaxies have redder (optical) colours and larger stellar masses, and observe a transition in the bar and bulge likelihoods at M*= 2 × 1010M⊙, such that massive disc galaxies are more likely to host bars and bulges. In addition, while some barred and most bulge-dominated galaxies are on the 'red sequence' of the colour-magnitude diagram, we see a wider variety of colours for galaxies that host bars. We use galaxy clustering methods to demonstrate statistically significant environmental correlations of barred, and bulge-dominated, galaxies, from projected separations of 150kpch-1 to 3Mpch-1. These environmental correlations appear to be independent of each other: i.e. bulge-dominated disc galaxies exhibit a significant bar-environment correlation, and barred disc galaxies show a bulge-environment correlation. As a result of sparse sampling tests - our sample is nearly 20 times larger than those used previously - we argue that previous studies that did not detect a bar-environment correlation were likely inhibited by small number statistics. We demonstrate that approximately half of the bar-environment correlation can be explained by the fact that more massive dark matter haloes host redder disc galaxies, which are then more likely to have bars; this fraction is estimated to be 50 ± 10per cent from a mock catalogue analysis and 60 ± 5per cent from the data. Likewise, we show that the environmental dependence of stellar mass can only explain a smaller fraction (25 ± 10per cent) of the bar-environment correlation. Therefore, a significant fraction of our observed environmental dependence of barred galaxies is not due to colour or stellar mass dependences, and hence must be due to another galaxy property, such as gas content, or to environmental influences. Finally, by analysing the projected clustering of barred and unbarred disc galaxies with halo occupation models, we argue that barred galaxies are in slightly higher mass haloes than unbarred ones, and some of them (approximately 25per cent) are satellite galaxies in groups. We discuss the implications of our results on the effects of minor mergers and interactions on bar formation in disc galaxies. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Abstract:

We present a statistical observational study of the tidal dwarf (TD) population in the nearby Universe by exploiting a large, homogeneous catalogue of galaxy mergers compiled from the Sloan Digital Sky Survey. 95percent of TD-producing mergers involve two spiral progenitors (typically both in the blue cloud), while most remaining systems have at least one spiral progenitor. The fraction of TD-producing mergers where both parents are early-type galaxies is less than 2percent, suggesting that TDs are unlikely to form in such mergers. The bulk of TD-producing mergers inhabit a field environment and have mass ratios greater than ~1:7 (the median value is 1:2.5). TDs forming at the tidal-tail tips are ~4 times more massive than those forming at the base of the tails. TD stellar masses are less than 10percent of the stellar masses of their parents (the median is 0.6percent) and lie within 15 optical half-light radii of their parent galaxies. The TD population is typically bluer than the parents, with a median offset of ~0.3mag in the (g-r) colour and the TD colours are not affected by the presence of active galactic nuclei (AGN) activity in their parents. An analysis of their star formation histories indicates that TDs contain both newly formed stars (with a median age of ~30Myr) and old stars drawn from the parent discs, each component probably contributing roughly equally to the stellar mass of the object. Thus TDs are not formed purely through gas condensation in tidal tails but host a significant component of old stars from the parent discs. Finally, an analysis of the TD contribution to the observed dwarf-to-massive galaxy ratio in the local Universe indicates that ~6percent of dwarfs in nearby clusters may have a tidal origin, if TD production rates in nearby mergers are representative of those in the high-redshift Universe. Even if TD production rates at high redshift were several factors higher, it seems unlikely that the entire dwarf galaxy population today is a result of merger activity over the lifetime of the Universe. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.