Susanne Aalto

Black hole accretion and star formation as drivers of gas excitation and chemistry in Markarian 231

We present a full high resolution SPIRE FTS spectrum of the nearby ultraluminous infrared galaxy Mrk 231. In total 25 lines are detected, including CO J = 5-4 through J = 13-12, 7 rotational lines of H2O, 3 of OH+ and one line each of H2O+, CH+, and HF. We find that the excitation of the CO rotational levels up to J = 8 can be accounted for by UV radiation from star formation. However, the approximately flat luminosity distribution of the CO lines over the rotational ladder above J = 8 requires the presence of a separate source of excitation for the highest CO lines. We explore X-ray heating by the accreting supermassive black hole in Mrk 231 as a source of excitation for these lines, and find that it can reproduce the observed luminosities. We also consider a model with dense gas in a strong UV radiation field to produce the highest CO lines, but find that this model strongly overpredicts the hot dust mass in Mrk 231. Our favoured model consists of a star forming disk of radius 560 pc, containing clumps of dense gas exposed to strong UV radiation, dominating the emission of CO lines up to J = 8. X-rays from the accreting supermassive black hole in Mrk 231 dominate the excitation and chemistry of the inner disk out to a radius of 160 pc, consistent with the X-ray power of the AGN in Mrk 231. The extraordinary luminosity of the OH+ and H2O+ lines reveals the signature of X-ray driven excitation and chemistry in this region.

Galaxy interactions - poor starburst triggers III. A study of a complete sample of interacting galaxies

We report on a study of tidally triggered star formation in galaxies based on spectroscopic/photometric observations in the optical/near-IR of a magnitude limited sample of 59 systems of interacting and merging galaxies and a comparison sample of 38 normal isolated galaxies. From a statistical point of view the sample gives us a unique opportunity to trace the effects of tidally induced star formation. In contrast to results from previous investigations, our global UBV colours do not support a significant enhancement of starforming activity in the interacting/merging galaxies. We also show that, contrary to previous claims, there is no significantly increased scatter in the colours of Arp galaxies as compared to normal galaxies. We do find support for moderate (a factor of ∼2-3) increase in star formation in the very centres of the interacting galaxies of our sample, contributing marginally to the total luminosity. The interacting and in particular the merging galaxies are characterized by increased far infrared (hereafter FIR) luminosities and temperatures that weakly correlate with the central activity. The LFIR/LB ratio however, is remarkably similar in the two samples, indicating that true starbursts normally are not hiding in the central regions of the FIR luminous cases. The gas mass-to-luminosity ratio in optical-IR is practically independent of luminosity, lending further support to the paucity of true massive starburst galaxies triggered by interactions/mergers. We estimate the frequency of such cases to be of the order of ∼0.1% of the galaxies in an apparent magnitude limited sample. Our conclusion is that interacting and merging galaxies, from the global star formation aspect, generally do not differ dramatically from scaled up versions of normal, isolated galaxies. No drastic change with redshift is expected. One consequence is that galaxy formation probably continued over a long period of time and did not peak at a specific redshift. The effects of massive starbursts, like blowouts caused by superwinds and cosmic reionization caused by starburst populations would also be less important than what is normally assumed.

CN and HNC line emission in IR luminous galaxies

We have observed HNC 1{0, CN 1{0 & 2{1 line emission in a sample of 13 IR luminous (LIRGs, LIR > 10 11 L) starburst and Seyfert galaxies. HNC 1{0 is detected in 9, CN 1{0 is detected in 10 and CN 2{1 in 7 of the galaxies and all are new detections. We also report the rst detection of HC3N (10{9) emission in Arp 220. The excitation of HNC and CN emission requires densities n> 10 4 cm 3 . We compare their intensities to that of the usual high density tracer HCN. The I(HCN) I(HNC) 1{0 and I(HCN) I(CN) 1{0 line intensity ratios vary signicantly, from 0.5 to >6, among the galaxies. This implies that the actual properties of the dense gas is varying among galaxies who otherwise have similar I(CO) I(HCN) line intensity ratios. We suggest that the HNC emission is not a reliable tracer of cold (10 K) gas at the center of LIR galaxies, as it often is in the disk of the Milky Way. Instead, the HNC abundance may remain substantial, despite high gas temperatures, because the emission is emerging from regions where the HCN and HNC formation and destruction processes are dominated by ion-neutral reactions which are not strongly dependent on kinetic temperature. We nd ve galaxies (Mrk 231, NGC 7469, NGC 7130, IC 694 and NGC 2623) where the I(HCN) I(HNC) intensity ratio is close to unity. Four are classied as active galaxies and one as a starburst. In other active galaxies, however, the I (HCN) I(HNC) is >4. The CN emission is on average a factor of two fainter than the HCN for the luminous IR galaxies, but the variation is large and there seems to be a trend of reduced relative CN luminosity with increasing IR luminosity. This trend is discussed in terms of other PDR tracers such as the (C II) 158 m line emission. One object, NGC 3690, has a CN luminosity twice that of HCN and its ISM is thus strongly aected by UV radiation. We discuss the I(HCN) I(HNC) and I(HCN) I(CN) line ratios as indicators of starburst evolution. However, faint HNC emission is expected both in a shock dominated ISM as well as for a cloud ensemble dominated by dense warm gas in the very early stages of a starburst. Additional information will help resolve the dichotomy.

Evidence for CO Shock Excitation in NGC 6240 from Herschel SPIRE Spectroscopy

We present Herschel SPIRE FTS spectroscopy of the nearby luminous infrared galaxy NGC 6240. In total 20 lines are detected, including CO J = 4-3 through J = 13-12, 6 H2O rotational lines, and [C I] and [N II] fine-structure lines. The CO to continuum luminosity ratio is 10 times higher in NGC 6240 than Mrk 231. Although the CO ladders of NGC 6240 and Mrk 231 are very similar, UV and/or X-ray irradiation are unlikely to be responsible for the excitation of the gas in NGC 6240. We applied both C and J shock models to the H-2 v = 1-0 S(1) and v = 2-1 S(1) lines and the CO rotational ladder. The CO ladder is best reproduced by a model with shock velocity v(s) = 10 km s(-1) and a pre-shock density n(H) = 5 x 10(4) cm(-3). We find that the solution best fitting the H-2 lines is degenerate. The shock velocities and number densities range between v(s) = 17-47 km s(-1) and n(H) = 10(7)-5x10(4) cm(-3), respectively. The H-2 lines thus need a much more powerful shock than the CO lines. We deduce that most of the gas is currently moderately stirred up by slow (10 km s(-1)) shocks while only a small fraction (less than or similar to 1%) of the interstellar medium is exposed to the high-velocity shocks. This implies that the gas is rapidly losing its highly turbulent motions. We argue that a high CO line-to-continuum ratio is a key diagnostic for the presence of shocks.

Molecules as tracers of galaxy evolution: an EMIR survey - I. Presentation of the data and first results

Aims. We investigate the molecular gas properties of a sample of 23 galaxies in order to find and test chemical signatures of galaxy evolution and to compare them to IR evolutionary tracers. Methods. Observation at 3 mm wavelengths were obtained with the EMIR broadband receiver, mounted on the IRAM 30 m telescope on Pico Veleta, Spain. We compare the emission of the main molecular species with existing models of chemical evolution by means of line intensity ratios diagrams and principal component analysis. Results. We detect molecular emission in 19 galaxies in two 8 GHz-wide bands centred at 88 and 112 GHz. The main detected molecules are CO, (CO)-C-13, HCN, HNC, HCO+, CN, and C2H. We also detect HC3N J = 10-9 in the galaxies IRAS 17208, IC 860, NGC 4418, NGC 7771, and NGC 1068. The only HC3N detections are in objects with HCO+/HCN 0.8). The brightest HC3N emission is found in IC 860, where we also detect the molecule in its vibrationally excited state. We find low HNC/HCN line ratios (<0.5), that cannot be explained by existing PDR or XDR chemical models. The intensities of HCO+ and HNC appear anti-correlated. No correlation is found between the HNC/HCN line ratio and dust temperature. All HNC-bright objects are either luminous IR galaxies (LIRG) or Seyferts. Galaxies with bright polycyclic aromatic hydrocarbons (PAH) emission show low HNC/HCO+ ratios. The CO/(CO)-C-13 ratio is positively correlated with the dust temperature and is generally higher than in our galaxy. The emission of CN and (CO)-O-18 is correlated. Conclusions. Bright HC3N emission in HCO+-faint objects may imply that these are not dominated by X-ray chemistry. Thus the HCN/HCO+ line ratio is not, by itself, a reliable tracer of XDRs. Bright HC3N and faint HCO+ could be signatures of embedded star-formation, instead of AGN activity. Mechanical heating caused by supernova explosions may be responsible for the low HNC/HCN and high HCO+/HCN ratios in some starbursts. We cannot exclude, however, that the discussed trends are largely caused by optical depth effects or excitation. Chemical models alone cannot explain all properties of the observed molecular emission. Better constraints to the gas spacial distribution and excitation are needed to distinguish abundance and excitation effects.

Molecules at z = 0.89 - A 4-mm-rest-frame absorption-line survey toward PKS 1830−211

We present the results of a 7 mm spectral survey of molecular absorption lines originating in the disk of a z = 0.89 spiral galaxy located in front of the quasar PKS 1830-211. Our survey was performed with the Australia Telescope Compact Array and covers the frequency interval 30-50 GHz, corresponding to the rest-frame frequency interval 57-94 GHz. A total of 28 different species, plus 8 isotopic variants, were detected toward the south-west absorption region, located about 2 kpc from the center of the z = 0.89 galaxy, which therefore has the largest number of detected molecular species of any extragalactic object so far. The results of our rotation diagram analysis show that the rotation temperatures are close to the cosmic microwave background temperature of 5.14 K that we expect to measure at z = 0.89, whereas the kinetic temperature is one order of magnitude higher, indicating that the gas is subthermally excited. The molecular fractional abundances are found to be in-between those in typical Galactic diffuse and translucent clouds, and clearly deviate from those observed in the dark cloud TMC 1 or in the Galactic center giant molecular cloud Sgr B2. The isotopic ratios of carbon, nitrogen, oxygen, and silicon deviate significantly from the solar values, which can be linked to the young age of the z = 0.89 galaxy and a release of nucleosynthesis products dominated by massive stars. Toward the north-east absorption region, where the extinction and column density of gas is roughly one order of magnitude lower than toward the SW absorption region, only a handful of molecules are detected. Their relative abundances are comparable to those in Galactic diffuse clouds. We also report the discovery of several new absorption components, with velocities spanning between -300 and +170 km s(-1). Finally, the line centroids of several species (e. g., CH3OH, NH3) are found to be significantly offset from the average velocity. If caused by a variation in the proton-to-electron mass ratio mu with redshift, these offsets yield an upper limit vertical bar Delta mu/mu vertical bar \textless 4x10(-6), which takes into account the kinematical noise produced by the velocity dispersion measured from a large number of molecular species.

Overluminous HNC line emission in Arp 220, NGC 4418 and Mrk 231. Global IR pumping or XDRs?

Context. In recent studies of 3 mm J = 1 - 0 HNC emission from galaxies it is found that the emission is often bright which is unexpected in warm, star forming clouds. We propose that the main cause for the luminous HNC line emission is the extreme radiative and kinematical environment in starburst and active nuclei. Aims. To determine the underlying excitational and chemical causes behind the luminous HNC emission in active galaxies and to establish how HNC emission may serve to identify important properties of the nuclear source. Methods. We present mm and submm JCMT, IRAM 30 m and CSO observations of the J = 3 - 2 line of HNC and its isomer HCN in three luminous galaxies and J = 4 - 3 HNC observations of one galaxy. The observations are discussed in terms of physical conditions and excitation as well as in the context of X-ray influenced chemistry. Results. The ultraluminous mergers Arp 220 and Mrk 231 and the luminous IR galaxy NGC 4418 show the HNC J 3 - 2 emission being brighter than the HCN 3 - 2 emission by factors of 1.5 to 2. We furthermore report the detection of HNC J = 4 - 3 in Mrk 231. Overluminous HNC emission is unexpected in warm molecular gas in ultraluminous galaxies since I( HNC) greater than or similar to I( HCN) is usually taken as a signature of cold (10 - 20 K) dark clouds. Since the molecular gas of the studied galaxies is warm (T(k) greater than or similar to 40 K), we present two alternative explanations to the overluminous HNC: a) HNC excitation is affected by pumping of the rotational levels through the mid-infrared continuum and b) XDRs (X-ray Dominated Regions) influence the abundances of HNC. HNC may become pumped at 21.5 mu m brightness temperatures of T(B) greater than or similar to 50 K, suggesting that HNC-pumping could be common in warm, ultraluminous galaxies with compact IR- nuclei. This means that the HNC emission is no longer dominated by collisions and its luminosity may not be used to deduce information on gas density. On the other hand, all three galaxies are either suspected of having buried AGN - or the presence of AGN is clear (Mrk 231) - indicating that X-rays may affect the ISM chemistry. Conclusions. We conclude that both the pumping and XDR alternatives imply molecular cloud ensembles distinctly different from those of typical starforming regions in the Galaxy, or the ISM of less extreme starburst galaxies. The HNC molecule shows the potential of becoming an additional important tracer of extreme nuclear environments.

P CYGNI PROFILES OF MOLECULAR LINES TOWARD ARP 220 NUCLEI

We report ~100 pc (0farcs3) resolution observations of (sub)millimeter HCO+ and CO lines in the ultraluminous infrared galaxy Arp 220. The lines peak at two merger nuclei, with HCO+ being more spatially concentrated than CO. Asymmetric line profiles with blueshifted absorption and redshifted emission are discovered in HCO+(3-2) and (4-3) toward the two nuclei and in CO(3-2) toward one nucleus. We suggest that these P Cygni profiles are due to ~100 km s–1 outward motion of molecular gas from the nuclei. This gas is most likely outflowing from the inner regions of the two nuclear disks rotating around individual nuclei, clearing the shroud around the luminosity sources there.

PACS and SPIRE photometer maps of M33: First results of the HERschel M33 Extended Survey (HERM33ES)

Context. Within the framework of the HERM33ES key program, we are studying the star forming interstellar medium in the nearby, metal-poor spiral galaxy M33, exploiting the high resolution and sensitivity of Herschel. Aims. We use PACS and SPIRE maps at 100, 160, 250, 350, and 500 mu m wavelength, to study the variation of the spectral energy distributions (SEDs) with galacto-centric distance. Methods. Detailed SED modeling is performed using azimuthally averaged fluxes in elliptical rings of 2 kpc width, out to 8 kpc galacto-centric distance. Simple isothermal and two-component grey body models, with fixed dust emissivity index, are fitted to the SEDs between 24 mu m and 500 mu m using also MIPS/Spitzer data, to derive first estimates of the dust physical conditions. Results. The far-infrared and submillimeter maps reveal the branched, knotted spiral structure of M33. An underlying diffuse disk is seen in all SPIRE maps (250-500 mu m). Two component fits to the SEDs agree better than isothermal models with the observed, total and radially averaged flux densities. The two component model, with beta fixed at 1.5, best fits the global and the radial SEDs. The cold dust component clearly dominates; the relative mass of the warm component is less than 0.3% for all the fits. The temperature of the warm component is not well constrained and is found to be about 60 K +/- 10 K. The temperature of the cold component drops significantly from similar to 24 K in the inner 2 kpc radius to 13 K beyond 6 kpc radial distance, for the best fitting model. The gas-to-dust ratio for beta = 1.5, averaged over the galaxy, is higher than the solar value by a factor of 1.5 and is roughly in agreement with the subsolar metallicity of M33.

The Herschel Comprehensive (U)LIRG Emission Survey (HERCULES): CO Ladders, Fine Structure Lines, and Neutral Gas Cooling

(Ultra) luminous infrared galaxies ((U)LIRGs) are objects characterized by their extreme infrared (8-1000 mu m) luminosities (L-LIRG > 10(11) L-circle dot and L-ULIRG > 10(12) L-circle dot). The Herschel Comprehensive ULIRG Emission Survey (PI: van derWerf) presents a representative flux-limited sample of 29 (U)LIRGs that spans the full luminosity range of these objects (10(11)L(circle dot) <= L-IR <= 10(13)L(circle dot)). With the Herschel Space Observatory, we observe [CII] 157 mu m, [O I] 63 mu m, and [O I] 145 mu m line emission with Photodetector Array Camera and Spectrometer, CO J = 4-3 through J = 13-12, [C I] 370 mu m, and [C I] 609 mu m with SPIRE, and low-J CO transitions with ground-based telescopes. The CO ladders of the sample are separated into three classes based on their excitation level. In 13 of the galaxies, the [O I] 63 mu m emission line is self absorbed. Comparing the CO excitation to the InfraRed Astronomical Satellite 60/100 mu m ratio and to far infrared luminosity, we find that the CO excitation is more correlated to the far infrared colors. We present cooling budgets for the galaxies and find fine-structure line flux deficits in the [C II], [Si II], [O I], and [C I] lines in the objects with the highest far IR fluxes, but do not observe this for CO 4 <= J(upp) <= 13. In order to study the heating of the molecular gas, we present a combination of three diagnostic quantities to help determine the dominant heating source. Using the CO excitation, the CO J = 1-0 linewidth, and the active galactic nucleus (AGN) contribution, we conclude that galaxies with large CO linewidths always have high-excitation CO ladders, and often low AGN contributions, suggesting that mechanical heating is important.