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Register a new userDust properties in the cold and hot gas phases of the ATLAS3D early-type galaxies as revealed by AKARI
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The properties of the dust in the cold and hot gas phases of early-type galaxies (ETGs) are key to understand ETG evolution. We thus conducted a systematic study of the dust in a large sample of local ETGs, focusing on relations between the dust and the molecular, atomic, and X-ray gas of the galaxies, as well as their environment. We estimated the dust temperatures and masses of the 260 ETGs from the ATLAS3D survey, using fits to their spectral energy distributions primarily constructed from AKARI measurements. We also used literature measurements of the cold (CO and HI) and X-ray gas phases. Our ETGs show no correlation between their dust and stellar masses, suggesting inefficient dust production by stars and/or dust destruction in X-ray gas. The global dust-to-gas mass ratios of ETGs are generally lower than those of late-type galaxies, likely due to dust-poor HI envelopes in ETGs. They are also higher in Virgo Cluster ETGs than in group and field ETGs, but the same ratios measured in the central parts of the galaxies only are independent of galaxy environment. Slow-rotating ETGs have systematically lower dust masses than fast-rotating ETGs. The dust masses and X-ray luminosities are correlated in fast-rotating ETGs, whose star formation rates are also correlated with the X-ray luminosities. The correlation between dust and X-rays in fast-rotating ETGs appears to be caused by residual star formation, while slow-rotating ETGs are likely well evolved, and thus exhausting their dust. These results appear consistent with the postulated evolution of ETGs, whereby fast-rotating ETGs form by mergers of late-type galaxies and associated bulge growth, while slow-rotating ETGs form by (dry) mergers of fast-rotating ETGs. Central cold dense gas appears to be resilient against ram pressure stripping, suggesting that Virgo Cluster ETGs may not suffer strong related star formation suppression.
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We present a study of the cold gas contents of the Atlas3D early-type galaxies, in the context of their optical colours, near-UV colours, and Hbeta absorption line strengths. Early-type (elliptical and lenticular) galaxies are not as gas-poor as previously thought, and at least 40% of local early-type galaxies are now known to contain molecular and/or atomic gas. This cold gas offers the opportunity to study recent galaxy evolution through the processes of cold gas acquisition, consumption (star formation), and removal. Molecular and atomic gas detection rates range from 10% to 34% in red sequence early-type galaxies, depending on how the red sequence is defined, and from 50% to 70% in blue early-type galaxies. Notably, massive red sequence early-type galaxies (stellar masses > 5e10 Msun, derived from dynamical models) are found to have HI masses up to M(HI)/Mstar ~ 0.06 and H_2 masses up to M(H$_2$)/Mstar ~ 0.01. Some 20% of all massive early-type galaxies may have retained atomic and/or molecular gas through their transition to the red sequence. However, kinematic and metallicity signatures of external gas accretion (either from satellite galaxies or the intergalactic medium) are also common, particularly at stellar masses <= 5e10 Msun, where such signatures are found in ~ 50% of H$_2$-rich early-type galaxies. Our data are thus consistent with a scenario in which fast rotator early-type galaxies are quenched former spiral galaxies which have undergone some bulge growth processes, and in addition, some of them also experience cold gas accretion which can initiate a period of modest star formation activity. We discuss implications for the interpretation of colour-magnitude diagrams.
For early-type galaxies, the ability to sustain a corona of hot, X-ray emitting gas could have played a key role in quenching their star-formation history. Yet, it is still unclear what drives the precise amount of hot gas around these galaxies. By combining photometric and spectroscopic measurements for the early-type galaxies observed during the Atlas3D integral-field survey with measurements of their X-ray luminosity based on X-ray data of both low and high spatial resolution we conclude that the hot-gas content of early-type galaxies can depend on their dynamical structure. Specifically, whereas slow rotators generally have X-ray halos with luminosity L_X,gas and temperature T values that are in line with what is expected if the hot-gas emission is sustained by the thermalisaton of the kinetic energy carried by the stellar-mass loss material, fast rotators tend to display L_X,gas values that fall consistently below the prediction of this model, with similar T values that do not scale with the stellar kinetic energy as observed in the case of slow rotators. Considering that fast rotators are likely to be intrinsically flatter than slow rotators, and that the few L_X,gas-deficient slow rotators also happen to be relatively flat, the observed L_X,gas deficiency in these objects would support the hypothesis whereby flatter galaxies have a harder time in retaining their hot gas. We discuss the implications that a different hot-gas content could have on the fate of both acquired and internally-produced gaseous material, considering in particular how the L_X,gas deficiency of fast rotators would make them more capable to recycle the stellar-mass loss material into new stars than slow rotators. This is consistent with the finding that molecular gas and young stars are detected only in fast rotators in the Atlas3D sample, and that fast rotators tend to dustier than slow rotators. [Abridged]
Observations of neutral hydrogen (HI) and molecular gas show that 50% of all nearby early-type galaxies (ETGs) contain some cold gas. Molecular gas is always found in small gas discs in the central region of the galaxy, while neutral hydrogen is often distributed in a low-column density disc or ring typically extending well beyond the stellar body. Dust is frequently found in ETGs as well. The goal of our study is to understand the link between dust and cold gas in nearby ETGs as a function of HI content. We analyse deep optical $g-r$ images obtained with the MegaCam camera at the Canada-France-Hawaii Telescope for a sample of 21 HI-rich and 41 HI-poor ETGs. We find that all HI-rich galaxies contain dust seen as absorption. Moreover, in 57 percent of these HI-rich galaxies, the dust is distributed in a large-scale spiral pattern. Although the dust detection rate is relatively high in the HI-poor galaxies ($sim$59 percent), most of these systems exhibit simpler dust morphologies without any evidence of spiral structures. We find that the HI-rich galaxies possess more complex dust morphology extending to almost two times larger radii than HI-poor objects. We measured the dust content of the galaxies from the optical colour excess and find that HI-rich galaxies contain six times more dust (in mass) than HI-poor ones. In order to maintain the dust structures in the galaxies, continuous gas accretion is needed, and the substantial HI gas reservoirs in the outer regions of ETGs can satisfy this need for a long time. We find that there is a good correspondence between the observed masses of the gas and dust, and it is also clear that dust is present in regions further than 3~Reff. Our findings indicate an essential relation between the presence of cold gas and dust in ETGs and offer a way to study the interstellar medium in more detail than what is possible with HI observations.
I present an overview of new observations of atomic and molecular gas in early-type galaxies, focusing on the Atlas3D project. Our data on stellar kinematics, age and metallicity, and ionized gas kinematics allow us to place the cold gas into the broader context of early-type galaxy assembly and star formation history. The cold gas data also provide valuable constraints for numerical simulations of early-type galaxies.
The star formation properties of early-type galaxies (ETGs) are currently the subject of considerable interest, particularly whether they differ from those of gas-rich spirals. We perform a systematic study of star formation in a large sample of local ETGs using polycyclic aromatic hydrocarbon (PAH) and dust emission, focusing on the galaxies star formation rates (SFRs) and star formation efficiencies (SFEs). Our sample is composed of the 260 ETGs from the ATLAS3D survey, from which we use the cold gas measurements (HI and CO). The SFRs are estimated from stellar, PAH and dust fits to spectral energy distributions created from new AKARI measurements and literature data from WISE and 2MASS. The mid-infrared luminosities of non-CO-detected galaxies are well correlated with their stellar luminosities, showing that they trace (circum)stellar dust emission. CO-detected galaxies show an excess above these correlations, uncorrelated with their stellar luminosities, indicating that they likely contain PAHs and dust of interstellar origin. PAH and dust luminosities of CO-detected galaxies show tight correlations with their molecular gas masses, and the derived current SFRs are typically 0.01-1 Msun/yr. These SFRs systematically decrease with stellar age at fixed stellar mass, while they correlate nearly linearly with stellar mass at fixed age. The majority of local ETGs follow the same star-formation law as local star-forming galaxies, and their current SFEs do not depend on either stellar mass or age. Our results clearly indicate that molecular gas is fueling current star formation in local ETGs, that appear to acquire this gas via mechanisms regulated primarily by stellar mass. The current SFEs of local ETGs are similar to those of local star-forming galaxies, indicating that their low SFRs are likely due to smaller cold gas fractions rather than a suppression of star formation.
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