Intense Star Formation Research Articles

The origin of diverse α-element abundances in galaxy discs

Spectroscopic surveys of the Galaxy reveal that its disc stars exhibit a spread in $\mathrm{[\alpha/Fe]}$ at fixed $\mathrm{[Fe/H]}$, manifest at some locations as a bimodality. The origin of these diverse, and possibly distinct, stellar populations in the Galactic disc is not well understood. We examine the Fe and $\alpha$-element evolution of 133 Milky Way-like galaxies from the EAGLE simulation, to investigate the origin and diversity of their $\mathrm{[\alpha/Fe]}$-$\mathrm{[Fe/H]}$ distributions. We find that bimodal $\mathrm{[\alpha/Fe]}$ distributions arise in galaxies whose gas accretion histories exhibit episodes of significant infall at both early and late times, with the former fostering more intense star formation than the latter. The shorter characteristic consumption timescale of gas accreted in the earlier episode suppresses its enrichment with iron synthesised by Type Ia SNe, resulting in the formation of a high-$\mathrm{[\alpha/Fe]}$ sequence. We find that bimodality in $\mathrm{[\alpha/Fe]}$ similar to that seen in the Galaxy is rare, appearing in approximately 5 percent of galaxies in our sample. We posit that this is a consequence of an early gas accretion episode requiring the mass accretion history of a galaxy's dark matter halo to exhibit a phase of atypically-rapid growth at early epochs. The scarcity of EAGLE galaxies exhibiting distinct sequences in the $\mathrm{[\alpha/Fe]}$-$\mathrm{[Fe/H]}$ plane may therefore indicate that the Milky Way's elemental abundance patterns, and its accretion history, are not representative of the broader population of $\sim L^\star$ disc galaxies.

A Study of Two Dwarf Irregular Galaxies with AsymmetricalStar Formation Distributions

Abstract Two dwarf irregular galaxies, DDO 187 and NGC 3738, exhibit a striking pattern of star formation: intense star formation is taking place in a large region occupying roughly half of the inner part of the optical galaxy. We use data on the H i distribution and kinematics and stellar images and colors to examine the properties of the environment in the high star formation rate (HSF) halves of the galaxies in comparison with the low star formation rate halves. We find that the pressure and gas density are higher on the HSF sides by 30%–70%. In addition we find in both galaxies that the H i velocity fields exhibit significant deviations from ordered rotation and there are large regions of high-velocity dispersion and multiple velocity components in the gas beyond the inner regions of the galaxies. The conditions in the HSF regions are likely the result of large-scale external processes affecting the internal environment of the galaxies and enabling the current star formation there.

ALMA view of a massive spheroid progenitor: a compact rotating core of molecular gas in an AGN host at z = 2.226

We present ALMA observations at 107.291 GHz (band 3) and 214.532 GHz (band 6) of GMASS 0953, a star-forming galaxy at z=2.226 hosting an obscured AGN that has been proposed as a progenitor of compact quiescent galaxies (QG). We measure for the first time the size of the dust and molecular gas emission of GMASS 0953 that we find to be extremely compact ($\sim$1 kpc). This result, coupled with a very high ISM density (n$\sim$10$^{5.5}$ cm$^{-3}$), a low gas mass fraction ($\sim$0.2) and a short gas depletion timescale ($\sim$150 Myr) imply that GMASS 0953 is experiencing an episode of intense star-formation in its central region that will rapidly exhaust its gas reservoirs, likely aided by AGN-induced feedback, confirming its fate as a compact QG. Kinematic analysis of the CO(6-5) line shows evidence of rapidly-rotating gas ($V_{rot}$=320$^{+92}_{-53}$ km s$^{-1}$), as observed also in a handful of similar sources at the same redshift. On-going quenching mechanisms could either destroy the rotation or leave it intact leading the galaxy to evolve into a rotating QG.

Cospatial Star Formation and Supermassive Black Hole Growth in z ∼ 3 Galaxies: Evidence for In Situ Co-evolution

Abstract We present a sub-kiloparsec localization of the sites of supermassive black hole (SMBH) growth in three active galactic nuclei (AGNs) at z ∼ 3 in relation to the regions of intense star formation in their hosts. These AGNs are selected from Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimeter/submillimeter Array (ALMA) observations in the Hubble Ultra-Deep Field and COSMOS, with the centimetric radio emission tracing both star formation and AGN, and the sub/millimeter emission by dust tracing nearly pure star formation. We require radio emission to be more luminous than the level associated with the sub/millimeter star formation to ensure that the radio emission is AGN-dominated, thereby allowing localization of the AGN and star formation independently. In all three galaxies, the AGNs are located within the compact regions of gas-rich, heavily obscured, intense nuclear star formation, with R e = 0.4–1.1 kpc and average star formation rates of ≃100–1200 M ⊙ yr−1. If the current episode of star formation continues at such a rate over the stellar mass doubling time of their hosts, ≃0.2 Gyr, the newly formed stellar mass will be of the order of 1011 M ⊙ within the central kiloparsec region, concurrently and cospatially with significant growth of the SMBH. This is consistent with a picture of in situ galactic bulge and SMBH formation. This work demonstrates the unique complementarity of VLA and ALMA observations to unambiguously pinpoint the locations of AGNs and star formation down to ≃30 mas, corresponding to ≃230 pc at z = 3.

Extragalactic archaeology with the C, N, and O chemical abundances

We predict how the C, N, and O abundances within the interstellar medium of galaxies evolve as functions of the galaxy star formation history (SFH). We adopt a hydrodynamical cosmological simulation, focusing on three star-forming disc galaxies with different SFHs. By assuming failed supernovae, we can predict an increasing trend of the gas-phase N/O–O/H abundance diagram, which was not produced in our previous simulations without failed supernovae. At high redshifts, contrary to the predictions of classical chemical evolution models with instantaneous mixing approximation, we find almost flat trends in the N/O–O/H diagram, which are due to the contribution of intermediate-mass stars together with an inhomogeneous chemical enrichment. Finally, we also predict that the average N/O and C/O steadily increase as functions of time, while the average C/N decreases, due to the mass and metallicity dependence of the yields of asymptotic giant branch stars; such variations are more marked during more intense star formation episodes. Our predictions on the CNO abundance evolution can be used to study the SFH of disc galaxies with the James Webb Space Telescope.

The host of the Type I SLSN 2017egm

Context. Type I superluminous supernova (SLSN) host galaxies are predominantly low-metallicity, highly star-forming (SF) dwarfs. One of the current key questions is whether Type I SLSNe can only occur in such environments and hosts. Aims. Here we present an integral-field study of the massive, high-metallicity spiral NGC 3191, the host of SN 2017egm, the closest Type I SLSN known to date. We use data from PMAS/CAHA and the public MaNGA survey to shed light on the properties of the SLSN site and the origin of star formation in this non-starburst spiral galaxy. Methods. We map the physical properties of different H ii regions throughout the galaxy and characterise their stellar populations using the STARLIGHT fitting code. Kinematical information allows us to study a possible interaction with its neighbouring galaxy as the origin of recent star formation activity which could have caused the SLSN. Results. NGC 3191 shows intense star formation in the western part with three large SF regions of low metallicity. Taking only the properties of emitting gas, the central regions of the host have a higher metallicity, a lower specific star formation rate, and lower ionisation. Modelling the stellar populations gives a different picture: the SLSN region has two dominant stellar populations with different ages, the younger one with an age of 2–10 Myr and lower metallicity, likely the population from which the SN progenitor originated. Emission line kinematics of NGC 3191 show indications of interaction with its neighbour MCG+08-19-017 at ~45 kpc, which might be responsible for the recent starburst. In fact, this galaxy pair has hosted a total of four SNe, 1988B (Type Ia), SN 2003ds (Type Ic in MCG+08-19-017), PTF10bgl (Type II), and 2017egm, underlying the enhanced SF in both galaxies due to interaction. Conclusions. Our study shows that care should be taken when interpreting global host and even gas properties without looking at the stellar population history of the region. The SLSNe seem to be consistent with massive stars (>20 M⊙) requiring low metallicity (<0.6 Z⊙), environments that can also occur in massive late-type galaxies, but not necessarily with starbursts.

Galaxy dynamo in inhomogeneous interstellar medium

In some galaxies there are magnetic fields of several microgauss. Their evolution is connected with dynamo mechanism which is based on joint action of alpha-effect and differential rotation. The equations of the dynamo theory usually include some averaged kinematic characteristics of the interstellar turbulence. This approach is quite suitable for galaxies with "calm" interstellar medium. As for the galaxies with intensive star formation or supernovae explosions it will not give proper results. For this case the HII regions are quite small, exist for small-time and their localization can be assumed random. So it is useful to take the dynamo equations with random coefficients, which take one of two values (the first one is connected with HI regions, and the second one - with HII). We have studied the magnetic field evolution in the stochastic dynamo model for some typical cases. From the mathematical point of view, the results show some special effects. Firstly, the magnetic field evolution demonstrates the intermittency effect: higher statistical moments of the field grow faster than the lower ones. Moreover, the magnetic field in this model can have large fluctuations, so we have also described the correlation function of the field.

High Dense Gas Fraction in Intensely Star-forming Dusty Galaxies

Abstract We present ALMA and VLA observations of the dense molecular gas tracers HCN, HCO+, and HNC in two lensed, high-redshift starbursts selected from the Herschel-ATLAS survey: H-ATLAS J090740.0−004200 (SDP.9, ) and H-ATLAS J091043.1−000321 (SDP.11, ). In SDP.9 we have detected all transitions and also HCN(1–0) and HCO+(1–0). In SDP.11 we have detected HCN(3–2) and HCO+(3–2). The amplification factors for both galaxies have been determined from subarcsecond-resolution CO and dust emission observations carried out with NOEMA and the SMA. The HNC(1–0)/HCN(1–0) line ratio in SDP.9 suggests the presence of photon-dominated regions, as happens in most local (U)LIRGs. The CO, HCN, and HCO+ spectral line energy distribution (SLEDs) of SDP.9 are compatible to those found for many local, IR-bright galaxies, indicating that the molecular gas in local and high-redshift dusty starbursts can have similar excitation conditions. We obtain that the correlation between total IR ( ) and dense line ( ) luminosity in SDP.9 and SDP.11 and local star-forming galaxies can be represented by a single relation. We argue that the scatter of the correlation, together with the lack of sensitive dense molecular gas tracer observations for a homogeneous sample of high-redshift galaxies, prevents us from distinguishing differential trends with redshift. Our results suggest that the intense star formation found in some high-redshift, dusty, luminous starbursts is associated with more massive dense molecular gas reservoirs and higher dense molecular gas fractions.

ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud

Abstract We present high-resolution (subparsec) observations of a giant molecular cloud in the nearest star-forming galaxy, the Large Magellanic Cloud. ALMA Band 6 observations trace the bulk of the molecular gas in 12CO(2–1) and the high column density regions in 13CO(2–1). Our target is a quiescent cloud (PGCC G282.98−32.40, which we refer to as the “Planck cold cloud” or PCC) in the southern outskirts of the galaxy where star formation activity is very low and largely confined to one location. We decompose the cloud into structures using a dendrogram and apply an identical analysis to matched-resolution cubes of the 30 Doradus molecular cloud (located near intense star formation) for comparison. Structures in the PCC exhibit roughly 10 times lower surface density and five times lower velocity dispersion than comparably sized structures in 30 Dor, underscoring the non-universality of molecular cloud properties. In both clouds, structures with relatively higher surface density lie closer to simple virial equilibrium, whereas lower surface-density structures tend to exhibit supervirial line widths. In the PCC, relatively high line widths are found in the vicinity of an infrared source whose properties are consistent with a luminous young stellar object. More generally, we find that the smallest resolved structures (“leaves”) of the dendrogram span close to the full range of line widths observed across all scales. As a result, while the bulk of the kinetic energy is found on the largest scales, the small-scale energetics tend to be dominated by only a few structures, leading to substantial scatter in observed size–line-width relationships.

Triggering active galactic nuclei in galaxy clusters

We model the triggering of Active Galactic Nuclei (AGN) in galaxy clusters using the semi- analytic galaxy formation model SAGE (?). We prescribe triggering methods based on the ram pressure galaxies experience as they move throughout the intracluster medium, which is hypothesized to trigger star formation and AGN activity. The clustercentric radius and velocity distribution of the simulated active galaxies produced by these models are compared with that of AGN and galaxies with intense star formation from a sample of low-redshift, relaxed clusters from the Sloan Digital Sky Survey. The ram pressure triggering model that best explains the clustercentric radius and velocity distribution of these observed galaxies has AGN and star formation triggered if $2.5\times10^{-14} < P_{ram} < 2.5\times10^{-13}$ Pa and $P_{ram} > 2P_{internal}$; this is consistent with expectations from hydrodynamical simulations of ram-pressure induced star formation. Our results show that ram pressure is likely to be an important mechanism for triggering star formation and AGN activity in clusters.

Positron annihilation in the nuclear outflows of the Milky Way

Abstract Observations of soft gamma rays emanating from the Milky Way from SPI/INTEGRAL reveal the annihilation of ∼2 × 1043 positrons every second in the Galactic bulge. The origin of these positrons, which annihilate to produce a prominent emission line centred at 511 keV, has remained mysterious since their discovery almost 50 yr ago. A plausible origin for the positrons is in association with the intense star formation ongoing in the Galactic centre. Moreover, there is strong evidence for a nuclear outflow in the Milky Way. We find that advective transport and subsequent annihilation of positrons in such an outflow cannot simultaneously replicate the observed morphology of positron annihilation in the Galactic bulge and satisfy the requirement that 90 per cent of positrons annihilate once the outflow has cooled to 104 K.

Atomic hydrogen bridge fueling NGC 4418 with gas from VV 655

The galaxy NGC 4418 harbours a compact ($<20$ pc) core with a very high bolometric luminosity ($\sim10^{11}$L$_\odot$). As most of the galaxy's energy output comes from this small region, it is of interest to determine what fuels this intense activity. An interaction with VV 655 has been proposed, where gas aquired by NGC 4418 could trigger intense star formation and/or black hole accretion in the centre. We aim to constrain the interaction hypothesis by studying neutral hydrogen structures around the two galaxies. We present observations at 1.4 GHz with the Very Large Array of radio continuum as well as emission and absorption from atomic hydrogen. Gaussian distributions are fitted to observed HI emission and absorption spectra. An atomic HI bridge is seen in emission, connecting NGC 4418 to VV 655. While NGC 4418 is bright in continuum emission and seen in HI absorption, VV 655 is barely detected in the continuum but show bright HI emission (M$_\mathrm{HI}\sim10^9$ M$_\odot$). We estimate SFRs from 1.4 GHz of 3.2 M$_\odot$ yr$^{-1}$ and 0.13 M$_\odot$ yr$^{-1}$ for NGC 4418 and VV 655 respectively. Systemic HI velocities of 2202$\pm$20 km s$^{-1}$ (emission) and 2105.4$\pm$10 km s$^{-1}$ (absorption) are measured for VV 655 and NGC 4418 respectively. Redshifted HI absorption is seen towards NGC 4418, suggesting gas infall. Blueshifted HI-emission is seen north-west of NGC 4418, which we interpret as a continuation of the outflow previously discussed by Sakamoto et al. (2013). The morphology and velocity structure seen in HI is consistent with an interaction scenario, where gas was transferred from VV 655 to NGC 4418, and may fuel the activity in the centre.

An ALMA survey of submillimetre galaxies in the COSMOS field: Physical properties derived from energy balance spectral energy distribution modelling

We determine the physical properties of a sample of SMGs in the COSMOS field that were pre-selected at the observed wavelength of $\lambda_{\rm obs}=1.1$ mm, and followed up at $\lambda_{\rm obs}=1.3$ mm with ALMA. We used MAGPHYS to fit the panchromatic (ultraviolet to radio) SEDs of 124 of the target SMGs, 19.4% of which are spectroscopically confirmed. The SED analysis was complemented by estimating the gas masses of the SMGs by using the $\lambda_{\rm obs}=1.3$ mm emission as a tracer of the molecular gas. The sample median and 16th-84th percentile ranges of the stellar masses, SFRs, dust temperatures, and dust and gas masses were derived to be $\log(M_{\star}/{\rm M}_{\odot})=11.09^{+0.41}_{-0.53}$, ${\rm SFR}=402^{+661}_{-233}$ ${\rm M}_{\odot}~{\rm yr}^{-1}$, $T_{\rm dust}=39.7^{+9.7}_{-7.4}$ K, $\log(M_{\rm dust}/{\rm M}_{\odot})=9.01^{+0.20}_{-0.31}$, and $\log(M_{\rm gas}/{\rm M}_{\odot})=11.34^{+0.20}_{-0.23}$, respectively. The median gas-to-dust ratio and gas fraction were found to be $120^{+73}_{-30}$ and $0.62^{+0.27}_{-0.23}$, respectively. We found that 57.3% of our SMGs populate the main sequence (MS) of star-forming galaxies, while 41.9% of the sources lie above the MS by a factor of >3 (one source lies below the MS). The largest 3 GHz radio sizes are found among the MS sources. Those SMGs that appear irregular in the rest-frame UV are predominantly starbursts, while the MS SMGs are mostly disk-like. The larger radio-emitting sizes of the MS SMGs compared to starbursts is a likely indication of their more widespread, less intense star formation. The irregular UV morphologies of the starburst SMGs are likely to echo their merger nature. Our results suggest that the transition from high-$z$ SMGs to local ellipticals via compact, quiescent galaxies (cQGs) at $z \sim 2$ might not be universal, and the latter population might also descend from the so-called blue nuggets.

Planck’s dusty GEMS

We present an analysis of high-resolution ALMA interferometry of CO(4–3) line emission and dust continuum in the “Ruby” (PLCK_G244.8+54.9), a bright, gravitationally lensed galaxy at z = 3.0 discovered with the Planck all-sky survey. The Ruby is the brightest of Planck’s dusty GEMS, a sample of 11 of the brightest gravitationally lensed high-redshift galaxies on the extragalactic sub-mm sky. We resolve the high-surface-brightness continuum and CO line emission of the Ruby in several extended clumps along a partial, nearly circular Einstein ring with 1.4′′ diameter around a massive galaxy at z = 1.5. Local star-formation intensities are up to 2000 M⊙ yr-1 kpc-2, amongst the highest observed at high redshift, and clearly in the range of maximal starbursts. Gas-mass surface densities are a few × 104M⊙ pc-2. The Ruby lies at, and in part even above, the starburst sequence in the Schmidt-Kennicutt diagram, and at the limit expected for star formation that is self-regulated through the kinetic energy injection from radiation pressure, stellar winds, and supernovae. We show that these processes can also inject sufficient kinetic energy and momentum into the gas to explain the turbulent line widths, which are consistent with marginally gravitationally bound molecular clouds embedded in a critically Toomre-stable disk. The star-formation efficiency is in the range 1–10% per free-fall time, consistent with the notion that the pressure balance that sets the local star-formation law in the Milky Way may well be universal out to the highest star-formation intensities. AGN feedback is not necessary to regulate the star formation in the Ruby, in agreement with the absence of a bright AGN component in the infrared and radio regimes.

ISM Properties of a Massive Dusty Star-forming Galaxy Discovered at z ∼ 7

Abstract We report the discovery and constrain the physical conditions of the interstellar medium of the highest-redshift millimeter-selected dusty star-forming galaxy to date, SPT-S J031132−5823.4 (hereafter SPT0311−58), at . SPT0311−58 was discovered via its 1.4 mm thermal dust continuum emission in the South Pole Telescope (SPT)-SZ survey. The spectroscopic redshift was determined through an Atacama Large Millimeter/submillimeter Array 3 mm frequency scan that detected CO(6–5), CO(7–6), and (2–1), and subsequently was confirmed by detections of CO(3–2) with the Australia Telescope Compact Array and with APEX. We constrain the properties of the ISM in SPT0311−58 with a radiative transfer analysis of the dust continuum photometry and the CO and line emission. This allows us to determine the gas content without ad hoc assumptions about gas mass scaling factors. SPT0311−58 is extremely massive, with an intrinsic gas mass of . Its large mass and intense star formation is very rare for a source well into the epoch of reionization.

Gas kinematics in powerful radio galaxies at z ~ 2: Energy supply from star formation, AGN, and radio jets

We compare the kinetic energy and momentum injection rates from intense star formation, bolometric AGN radiation, and radio jets with the kinetic energy and momentum observed in the warm ionized gas in 24 powerful radio galaxies at z ~ 2. These galaxies are among our best candidates for being massive galaxies near the end of their active formation period, when intense star formation, quasar activity, and powerful radio jets all co-exist. All galaxies have VLT/SINFONI imaging spectroscopy of the rest-frame optical line emission, showing extended emission-line regions with large velocity offsets (up to 1500 km s-1) and line widths (typically 800–1000 km s-1) consistent with very turbulent, often outflowing gas. As part of the HeRGÉ sample, they also have FIR estimates of the star formation and quasar activity obtained with Herschel/PACS and SPIRE, which enables us to measure the relative energy and momentum release from each of the three main sources of feedback in massive, star-forming AGN host galaxies during their most rapid formation phase. We find that star formation falls short by factors 10−1000 of providing the energy and momentum necessary to power the observed gas kinematics. The obscured quasars in the nuclei of these galaxies provide enough energy and momentum in about half of the sample, however, only if both are transferred to the gas relatively efficiently. We compare with theoretical and observational constraints on the efficiency of the energy and momentum transfer from jet and AGN radiation, which favors the radio jets as main drivers of the gas kinematics.

Uniform Contribution of Supernova Explosions to the Chemical Enrichment of Abell 3112 out to R200

Abstract The spatial distribution of the metals residing in the intra-cluster medium (ICM) of galaxy clusters records all the information on a cluster’s nucleosynthesis and chemical enrichment history. We present measurements from a total of 1.2 Ms Suzaku XIS and 72 ks Chandra observations of the cool-core galaxy cluster Abell 3112 out to its virial radius (∼1470 kpc). We find that the ratio of the observed supernova type Ia explosions to the total supernova explosions has a uniform distribution at a level of 12%–16% out to the cluster’s virial radius. The observed fraction of type Ia supernova explosions is in agreement with the corresponding fraction found in our Galaxy and the chemical enrichment of our Galaxy. The non-varying supernova enrichment suggests that the ICM in cluster outskirts was enriched by metals at an early stage before the cluster itself was formed during a period of intense star formation activity. Additionally, we find that the 2D delayed detonation model CDDT produce significantly worse fits to the X-ray spectra compared to simple 1D W7 models. This is due to the relative overestimate of Si, and the underestimate of Mg in these models with respect to the measured abundances.

Constraining the Stellar Populations and Star Formation Histories of Blue Compact Dwarf Galaxies with SED Fits

Abstract We discuss and test possible evolutionary connections between blue compact dwarf galaxies (BCDs) and other types of dwarf galaxies. BCDs provide ideal laboratories to study intense star formation episodes in low-mass dwarf galaxies, and have sometimes been considered a short-lived evolutionary stage between types of dwarf galaxies. To test these connections, we consider a sample of BCDs as well as a comparison sample of nearby galaxies from the Local Volume Legacy (LVL) survey for context. We fit the multi-wavelength spectral energy distributions (SED, far-ultra-violet to far-infrared) of each galaxy with a grid of theoretical models to determine their stellar masses and star formation properties. We compare our results for BCDs with the LVL galaxies to put BCDs in the context of normal galaxy evolution. The SED fits demonstrate that the star formation events currently underway in BCDs are at the extreme of the continuum of normal dwarf galaxies, both in terms of the relative mass involved and in the relative increase over previous star formation rates. Today’s BCDs are distinctive objects in a state of extreme star formation that is rapidly transforming them. This study also suggests ways to identify former BCDs whose star formation episodes have since faded.

Interactions of the Galactic bar and spiral arm in NGC 3627

Aims: To gain insight into the expected gas dynamics at the interface of the Galactic bar and spiral arms in our own Milky Way galaxy, we examine as an extragalactic counterpart the evidence for multiple distinct velocity components in the cold, dense molecular gas populating a comparable region at the end of the bar in the nearby galaxy NGC3627. Methods: We assemble a high resolution view of molecular gas kinematics traced by CO(2-1) emission and extract line-of-sight velocity profiles from regions of high and low gas velocity dispersion. Results: The high velocity dispersions arise with often double-peaked or multiple line-profiles. We compare the centroids of the different velocity components to expectations based on orbital dynamics in the presence of bar and spiral potential perturbations. A model of the region as the interface of two gas-populated orbits families supporting the bar and the independently rotating spiral arms provides an overall good match to the data. An extent of the bar to the corotation radius of the galaxy is favored. Conclusions: Using NGC3627 as an extragalactic example, we expect situations like this to favor strong star formation events such as observed in our own Milky Way since gas can pile up at the crossings between the orbit families. The relative motions of the material following these orbits is likely even more important for the build up of high density in the region. The surface densities in NGC3627 are also so high that shear at the bar end is unlikely to significantly weaken the star formation activity. We speculate that scenarios in which the bar and spiral rotate at two different pattern speeds may be the most favorable for intense star formation at such interfaces.

ALMA observations of the dense and shocked gas in the nuclear region of NGC 4038 (Antennae galaxies)

Abstract We present 1″(&amp;lt;100 pc) resolution maps of millimeter emission from five molecules—CN, HCN, HCO+, CH3OH, and HNCO—obtained towards NGC 4038, which is the northern galaxy of the mid-stage merger, Antennae galaxies, with the Atacama Large Millimeter/submillimeter Array. Three molecules (CN, CH3OH, and HNCO) were detected for the first time in the nuclear region of NGC 4038. High-resolution mapping reveals a systematic difference in distributions of different molecular species and continuum emission. Active star-forming regions identified by the 3 mm and 850 μm continuum emission are offset from the gas-rich region associated with the HCN (1–0) and CO (3–2) peaks. The CN (1–0)/HCN (1–0) line ratios are enhanced (CN/HCN ≃ 0.8–1.2) in the star-forming regions, suggesting that the regions are photon dominated. The large molecular gas mass (108 M⊙) within a 0${^{\prime\prime}_{.}}$6 (∼60 pc) radius of the CO (3–2) peak and a high dense gas fraction (&amp;gt;20%) suggested by the HCN (1–0)/CO (3–2) line ratio may signify a future burst of intense star formation there. The shocked gas traced in the CH3OH and HNCO emission indicates sub-kpc-scale molecular shocks. We suggest that the molecular shocks may be driven by collisions between inflowing gas and the central massive molecular complex.