ALBERT

Description: We present a detailed study of the physical properties of the molecular gas in a sample of 18 molecular gas-rich early-type galaxies (ETGs) from the ATLAS 3D sample. Our goal is to better understand the star formation processes occurring in those galaxies, starting here with the dense star-forming gas. We use existing integrated 12 CO (1–0, 2–1), 13 CO (1–0, 2–1), HCN (1–0) and HCO + (1–0) observations and new 12 CO (3–2) single-dish data. From these, we derive for the first time the average kinetic temperature, H 2 volume density and column density of the emitting gas in a significant sample of ETGs, using a non-local thermodynamical equilibrium theoretical model. Since the CO lines trace different physical conditions than of those the HCN and HCO + lines, the two sets of lines are treated separately. For most of the molecular gas-rich ETGs studied here, the CO transitions can be reproduced with kinetic temperatures of 10–20 K, H 2 volume densities of 10 3–4 cm –3 and CO column densities of $10^{18\text{--}20}$ cm –2 . The physical conditions corresponding to the HCN and HCO + gas component have large uncertainties and must be considered as indicative only. We also compare for the first time the predicted CO spectral line energy distributions and gas properties of our molecular gas-rich ETGs with those of a sample of nearby well-studied disc galaxies. The gas excitation conditions in 13 of our 18 ETGs appear analogous to those in the centre of the Milky Way, hence the star formation activity driving these conditions is likely of a similar strength and nature. Such results have never been obtained before for ETGs and open a new window to explore further star-formation processes in the Universe. The conclusions drawn should nevertheless be considered carefully, as they are based on a limited number of observations and on a simple model. In the near future, with higher CO transition observations, it should be possible to better identify the various gas components present in ETGs, as well as more precisely determine their associated physical conditions. To achieve these goals, we show here from our theoretical study, that mid- J CO lines [such as the 12 CO (6–5) line] are particularly useful.

Description: We present the Combined Array for Research in Millimeter Astronomy (CARMA) ATLAS 3D molecular gas imaging survey, a systematic study of the distribution and kinematics of molecular gas in CO-rich early-type galaxies. Our full sample of 40 galaxies (30 newly mapped and 10 taken from the literature) is complete to a 12 CO(1–0) integrated flux of 18.5 Jy km s –1 , 1 and it represents the largest, best studied sample of its type to date. A comparison of the CO distribution of each galaxy to the g  – r colour image (representing dust) shows that the molecular gas and dust distributions are in good agreement and trace the same underlying interstellar medium. The galaxies exhibit a variety of CO morphologies, including discs (50 per cent), rings (15 per cent), bars+rings (10 per cent), spiral arms (5 per cent) and mildly (12.5 per cent) and strongly (7.5 per cent) disrupted morphologies. There appear to be weak trends between galaxy mass and CO morphology, whereby the most massive galaxies in the sample tend to have molecular gas in a disc morphology. We derive a lower limit to the total accreted molecular gas mass across the sample of 2.48  x 10 10 M , or approximately 8.3  x 10 8 M per minor merger within the sample, consistent with minor merger stellar mass ratios.

Description: We present a detailed two-dimensional stellar dynamical analysis of a sample of 44 cosmological hydrodynamical simulations of individual central galaxies with stellar masses of 2 10 10 M    M *   6 10 11 M . Kinematic maps of the stellar line-of-sight velocity, velocity dispersion and higher order Gauss–Hermite moments h 3 and h 4 are constructed for each central galaxy and for the most massive satellites. The amount of rotation is quantified using the R -parameter. The velocity, velocity dispersion, h 3 and h 4 fields of the simulated galaxies show a diversity similar to observed kinematic maps of early-type galaxies in the ATLAS 3D survey. This includes fast (regular), slow and misaligned rotation, hot spheroids with embedded cold disc components as well as galaxies with counter-rotating cores or central depressions in the velocity dispersion. We link the present-day kinematic properties to the individual cosmological formation histories of the galaxies. In general, major galaxy mergers have a significant influence on the rotation properties resulting in both a spin-down as well as a spin-up of the merger remnant. Lower mass galaxies with significant (18 per cent) in situ formation of stars since z 2, or with additional gas-rich major mergers – resulting in a spin-up – in their formation history, form elongated ( ~ 0.45) fast rotators ( R  ~ 0.46) with a clear anticorrelation of h 3 and v /. An additional formation path for fast rotators includes gas-poor major mergers leading to a spin-up of the remnants ( R  ~ 0.43). This formation path does not result in anticorrelated h 3 and v /. The formation histories of slow rotators can include late major mergers. If the merger is gas rich, the remnant typically is a less flattened slow rotator with a central dip in the velocity dispersion. If the merger is gas poor, the remnant is very elongated ( ~ 0.43) and slowly rotating ( R  ~ 0.11). The galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. In general, more massive galaxies have less in situ star formation since z  ~ 2, rotate slower and have older stellar populations. We discuss general implications for the formation of fast and slowly rotating galaxies as well as the weaknesses and strengths of the underlying models.

Description: One quarter of all nearby early-type galaxies (ETGs) outside Virgo host a disc/ring of H i with size from a few to tens of kpc and mass up to ~10 9 M . Here we investigate whether this H i is related to the presence of a stellar disc within the host making use of the classification of ETGs in fast and slow rotators (FR/SR). We find a large diversity of H i masses and morphologies within both families. Surprisingly, SRs are detected as often, host as much H i and have a similar rate of H i discs/rings as FRs. Accretion of H i is therefore not always linked to the growth of an inner stellar disc. The weak relation between H i and stellar disc is confirmed by their frequent kinematical misalignment in FRs, including cases of polar and counterrotating gas. In SRs the H i is usually polar. This complex picture highlights a diversity of ETG formation histories which may be lost in the relative simplicity of their inner structure and emerges when studying their outer regions. We find that CDM hydrodynamical simulations have difficulties reproducing the H i properties of ETGs. The gas discs formed in simulations are either too massive or too small depending on the star formation feedback implementation. Kinematical misalignments match the observations only qualitatively. The main point of conflict is that nearly all simulated FRs and a large fraction of all simulated SRs host corotating H i . This establishes the H i properties of ETGs as a novel challenge to simulations.

Description: We present measurements of the star formation rate (SFR) in the early-type galaxies (ETGs) of the ATLAS 3D sample, based on Wide-field Infrared Survey Explorer ( WISE ) 22 μm and Galaxy Evolution Explorer far-ultraviolet emission. We combine these with gas masses estimated from 12 CO and H i data in order to investigate the star formation efficiency (SFE) in a larger sample of ETGs than previously available. We first recalibrate (based on WISE data) the relation between old stellar populations (traced at K s band) and 22 μm luminosity, allowing us to remove the contribution of 22 μm emission from circumstellar dust. We then go on to investigate the position of ETGs on the Kennicutt–Schmidt (KS) relation. Molecular gas-rich ETGs have comparable star formation surface densities to normal spiral galaxy centres, but they lie systematically offset from the KS relation, having lower SFEs by a factor of 2.5 (in agreement with other authors). This effect is driven by galaxies where a substantial fraction of the molecular material is in the rising part of the rotation curve, and shear is high. We show here for the first time that although the number of stars formed per unit gas mass per unit time is lower in ETGs, it seems that the amount of stars formed per free-fall time is approximately constant. The scatter around this dynamical relation still correlates with galaxy properties such as the shape of the potential in the inner regions. This leads us to suggest that dynamical properties (such as shear or the global stability of the gas) may be important second parameters that regulate star formation and cause much of the scatter around star formation relations.

Description: We present a study of the cold gas contents of the Atlas 3D early-type galaxies, in the context of their optical colours, near-ultraviolet colours and Hβ absorption line strengths. Early-type (elliptical and lenticular) galaxies are not as gas poor as previously thought, and at least 40 per cent 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 per cent in red sequence early-type galaxies, depending on how the red sequence is defined, and from 50 to 70 per cent in blue early-type galaxies. Notably, massive red sequence early-type galaxies (stellar masses 〉5 10 10 M , derived from dynamical models) are found to have H i masses up to M (H i )/ M * ~ 0.06 and H 2 masses up to M (H 2 )/ M * ~ 0.01. Some 20 per cent 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 ≤5 10 10 M , where such signatures are found in ~50 per cent 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.

Description: The standard interstellar ratio of deuterium to hydrogen (D/H) atoms is ~1.5 10 –5 . However, the deuterium fractionation is in fact found to be enhanced, to different degrees, in cold, dark cores, hot cores around massive star-forming regions, lukewarm cores, and warm cores (hereafter hot corinos) around low-mass star-forming regions. In this paper, we investigate the overall differences in the deuterium chemistry between hot cores and hot corinos. We have modelled the chemistry of dense gas around low-mass and massive star-forming regions using a gas-grain chemical model. We investigate the influence of varying the core density, the depletion efficiency of gaseous species on to dust grains, the collapse mode and the final mass of the protostar on the chemical evolution of star-forming regions. We find that the deuterium chemistry is, in general, most sensitive to variations of the depletion efficiency on to grain surfaces, in agreement with observations. In addition, the results showed that the chemistry is more sensitive to changes in the final density of the collapsing core in hot cores than in hot corinos. Finally, we find that ratios of deuterated sulphur bearing species in dense gas around hot cores and corinos may be good evolutionary indicators in a similar way as their non-deuterated counterparts.

Description: In this paper we use observations of molecular tracers in metal-rich and α-enhanced galaxies to study the effect of abundance changes on molecular chemistry. We selected a sample of metal-rich spiral and star-bursting objects from the literature, and present here new data for a sample of early-type galaxies (ETGs) previously studied by Crocker et al. We conducted the first survey of carbon monosulphide (CS) and methanol emission in ETGs, detecting seven objects in at least one CS transition, and methanol emission in five ETGs. We find that ETGs whose gas is dominated by ionization from star formation have enhanced CS emission, compared to their hydrogen cyanide (HCN) emission, supporting the hypothesis that CS is a better tracer of dense star-forming gas than HCN. We suggest that the methanol emission in these sources is driven by dust mantle destruction due to ionization from high-mass star formation in dense molecular clouds, but cannot rule out a component due to shocks dominating in some sources. We construct rotation diagrams for each early-type source where at least two transitions of a given species were detected. The rotational temperatures we derive for linear molecules vary between 3 and 9 K, with the majority of sources having rotational temperatures around 5 K. Despite the large uncertainty inherent in this method, the derived source-averaged CS and methanol column densities are similar to those found by other authors for normal spiral and starburst galaxies. This may suggest dense clouds are little affected by the differences between early- and late-type galaxies. Finally, we used the total column density ratios for both our ETG and literature galaxy sample to show for the first time that some molecular tracers do seem to show systematic variations that appear to correlate with metallicity, and that these variations roughly match those predicted by chemical models. Using this fact, the chemical models of Bayet et al. and assumptions about the optical depth we are able to roughly predict the metallicity of our spiral and ETG sample, with a scatter of 0.3 dex. We provide the community with linear approximations to the relationship between the HCN and CS column density ratio and metallicity. Further study will clearly be required to determine if this, or any, molecular tracer can be used to robustly determine gas-phase metallically, but that a relationship exists at all suggests that in the future it may be possible to calibrate a metallicity indicator for the molecular interstellar medium.

Description: We analyse the spectral line energy distributions of 13 CO and C 18 O for the J  = 1-〉0 up to J  = 7-〉6 transitions in the gravitationally lensed ultraluminous infrared galaxy SMM J2135–0102 at z  = 2.3. This is the first detection of 13 CO and C 18 O in a high-redshift star-forming galaxy. These data comprise observations of six transitions taken with Plateau de Bure Interferometer and we combine these with ~33 GHz Jansky Very Large Array data and our previous spatially resolved 12 CO and continuum emission information to better constrain the properties of the interstellar medium (ISM) within this system. We study both the velocity-integrated and kinematically decomposed properties of the galaxy and coupled with a large velocity gradient (LVG) model we find that the star-forming regions in the system vary in their cold gas properties, in particular in their chemical abundance ratios. We find strong C 18 O emission both in the velocity-integrated emission and in the two kinematic components at the periphery of the system, where the C 18 O line flux is equivalent to or higher than the 13 CO. We derive an average velocity-integrated flux ratio of 13 CO/C 18 O ~ 1 which suggests an abundance ratio of [ 13 CO]/[C 18 O] which is at least seven times lower than that in the Milky Way. This is suggestive of enhanced C 18 O abundance, perhaps indicating star formation preferentially biased to high-mass stars. We estimate the relative contribution to the ISM heating from cosmic rays and UV of (30–3300) 10 –25 erg s –1 and 45 10 –25 erg s –1 per H 2 molecule respectively and find them to be comparable to the total cooling rate of (0.8–20) 10 –25 erg s –1 from the CO. However, our LVG models indicate high (〉100 K) temperatures and densities (〉10 3 ) cm –3 in the ISM which may suggest that cosmic rays play a more important role than UV heating in this system. If cosmic rays dominate the heating of the ISM, the increased temperature in the star-forming regions may favour the formation of massive stars and so explain the enhanced C 18 O abundance. This is a potentially important result for a system which may evolve into a local elliptical galaxy.

Description: Combining observations of multiple CO lines with radiative transfer modelling is a very powerful tool to investigate the physical properties of the molecular gas in galaxies. Using new observations and literature data, we provide the most complete CO ladders ever generated for eight star-forming regions in the spiral arms and inter-arms of the spiral galaxy NGC 6946, with observations of the CO(1–0), CO(2–1), CO(3–2), CO(4–3), CO(6–5), 13 CO(1–0) and 13 CO(2–1) transitions. For each region, we use the large velocity gradient assumption to derive beam-averaged molecular gas physical properties, namely the gas kinetic temperature ( T K ), H 2 number volume density ( n (H 2 )) and CO number column density ( N (CO)). Two complementary approaches are used to compare the observations with the model predictions: 2 minimization and likelihood. The physical conditions derived vary greatly from one region to the next: T K  = 10–250 K, n (H 2 ) = 10 2.3 –10 7.0  cm –3 and N (CO) = 10 15.0 –10 19.3  cm –2 . The spectral line energy distribution (SLED) in some of these extranuclear regions indicate a star formation activity that is more intense than that at the centre of our own Milky Way. The molecular gas in regions with a large SLED turnover transition ( J max  〉 4) is hot but tenuous with a high CO column density, while that in regions with a low SLED turnover transition ( J max  ≤ 4) is cold but dense with a low CO column density. We finally discuss and find some correlations between the physical properties of the molecular gas in each region and the presence of young stellar population indicators (supernova remnants, H ii regions, H i holes, etc.).