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Register a new userDust in a merging galaxy sequence: the SCUBA view
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We investigate the cold and warm dust properties during galaxy interactions using a merging galaxy sample ordered into a chronological sequence from pre- to post-mergers. Our sample comprises a total of 29 merging systems selected to have far-infrared and sub-millimeter observations. We use the 100-to-850 micron flux density ratio, f100/f850, as a proxy to the mass fraction of the warm and the cold dust in these systems. We find evidence for an increase in f100/f850 along the merging sequence from early to advanced mergers and interpret this trend as an increase of the warm relative to the cold dust mass. We argue that the two key parameters affecting the f100/f850 flux ratio is the star-formation rate and the dust content of individual systems relative to the stars. Using a sophisticated model for the absorption and re-emission of the stellar UV radiation by dust we show that these parameters can indeed explain both the increase and the observed scatter in the f100/f850 along the merging galaxy sequence.
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We investigate the cold and warm dust properties during galaxy interactions using a merging galaxy sample ordered into a chronological sequence from pre- to post-mergers. Our sample comprises a total of 29 merging systems selected to have far-infrared and sub-millimeter observations. The sub-millimeter data are mainly culled from the literature while for 5 galaxies (NGC 3597, NGC 3690, NGC 6090, NGC 6670 and NGC 7252) the sub-millimeter observations are presented here for the first time. We use the 100-to-850 micron flux density ratio, f_{100}/f_{850}, as a proxy to the mass fraction of the warm and the cold dust in these systems. We find evidence for an increase in f_{100}/f_{850} along the merging sequence from early to advanced mergers and interpret this trend as an increase of the warm relative to the cold dust mass. We argue that the two key parameters affecting the f_{100}/f_{850} flux ratio is the star-formation rate and the dust content of individual systems relative to the stars. Using a sophisticated model for the absorption and re-emission of the stellar UV radiation by dust we show that these parameters can indeed explain both the increase and the observed scatter in the f_{100}/f_{850} along the merging galaxy sequence. We also discuss our results under the hypothesis that elliptical galaxies are formed via disc galaxy mergers.
We present new results from the SCUBA Local Universe Galaxy Survey (SLUGS), the first large systematic submillimetre survey of the local Universe. Since our initial survey of a sample of 104 IRAS-selected galaxies we have now completed a survey of a sample of 81 optically-selected galaxies, observed with the SCUBA camera on the James Clerk Maxwell Telescope. Since SCUBA is sensitive to the 90% of dust too cold to radiate significantly in the IRAS bands our new sample represents the first unbiased survey of dust in galaxies along the whole length of the Hubble sequence. We find little change in the properties of dust in galaxies along the Hubble sequence and detected 6 out of 11 elliptical galaxies. As in our earlier work on IRAS galaxies we find that the IRAS and submm fluxes are well-fitted by a two-component dust model with dust emissivity index beta=2. The major difference from our earlier work is that we find the ratio of the mass of cold dust to the mass of warm dust is much higher for our optically-selected galaxies and can reach values of ~1000. Comparison of the results for the IRAS- and optically-selected samples shows that there is a population of galaxies containing a large proportion of cold dust that is unrepresented in the IRAS sample. We derive local submm luminosity and dust mass functions, both directly from our optically-selected SLUGS sample, and by extrapolation from the IRAS PSCz survey using the method of Serjeant & Harrison, and find excellent agreement between the two. We find them to be well-fitted by Schechter functions except at the highest luminosities. We find that as a consequence of the omission of cold galaxies from the IRAS sample the luminosity function presented in our earlier work is too low by a factor of 2.
We present the results of N-body simulations of dissipationless galaxy merging in Modified Newtonian Dynamics (MOND). For comparison, we also studied Newtonian merging between galaxies embedded in dark matter halos, with internal dynamics equivalent to the MOND systems. We found that the merging timescales are significantly longer in MOND than in Newtonian gravity with dark matter, suggesting that observational evidence of rapid merging could be difficult to explain in MOND. However, when two galaxies eventually merge, the MOND merging end-product is hardly distinguishable from the final stellar distribution of an equivalent Newtonian merger with dark matter.
Galaxies in dense environments, such as groups and clusters, experience various processes by which galaxies gain and lose gas. Using data from the SDSS-IV MaNGA survey, we previously reported the discovery of a giant (6 -- 8 kpc in diameter) H$alpha$ blob, Totoro, about 8 kpc away from a pair of galaxies (Satsuki and Mei) residing in a galaxy group which is experiencing a group-group merger. Here, we combine interferometric $^{12}$CO(1--0) molecular gas data, new wide-field H$alpha$, $u$-band data, and published X-ray data to determine the origin of the blob. Several scenarios are discussed to account for its multi-wavelength properties, including (1) H$alpha$ gas being stripped from galaxy Satsuki by ram-pressure; (2) a separated low-surface-brightness galaxy; (3) gas being ejected or ionized by an active galactic nucleus (AGN); and (4) a cooling intra-group medium (IGM). Scenarios (1) and (2) are less favored by the present data. Scenario (3) is also less likely as there is no evidence for an active ongoing AGN in the host galaxy. We find that the CO (cold) and H$alpha$ (warm) gas coexist with X-ray (hot) structures; moreover, the derived cooling time is within the regime where molecular and H$alpha$ gas are expected. The coexistence of gas with different temperatures also agrees with that of cooling gas in other systems. Our multi-wavelength results strongly suggest that the CO and H$alpha$ gas are the product of cooling from the IGM at its current location, i.e., cooling has occurred, and may be ongoing, well outside the host-galaxy core.
A common assumption is that galaxies fall in two distinct regions on a plot of specific star-formation rate (SSFR) versus galaxy stellar mass: a star-forming Galaxy Main Sequence (GMS) and a separate region of `passive or `red and dead galaxies. Starting from a volume-limited sample of nearby galaxies designed to contain most of the stellar mass in this volume, and thus being a fair representation of the Universe at the end of 12 billion years of galaxy evolution, we investigate the distribution of galaxies in this diagram today. We show that galaxies follow a strongly curved extended GMS with a steep negative slope at high galaxy stellar masses. There is a gradual change in the morphologies of the galaxies along this distribution, but there is no clear break between early-type and late-type galaxies. Examining the other evidence that there are two distinct populations, we argue that the `red sequence is the result of the colours of galaxies changing very little below a critical value of the SSFR, rather than implying a distinct population of galaxies, and that Herschel observations, which show at least half of early-type galaxies contain a cool interstellar medium, also imply continuity between early-type and late-type galaxies. This picture of a unitary population of galaxies requires more gradual evolutionary processes than the rapid quenching processes needed to to explain two distinct populations. We challenge theorists to reproduce the properties of this `Galaxy End Sequence.
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