arXiv subjects

Miguel Pereira-Santaella

Publications and source records attributed to Miguel Pereira-Santaella.

At least 73 records · Page 4Linked to original sources

arXiv2021

SOFIA observations of far-IR fine-structure lines in galaxies to measure metallicity

We present new and archival SOFIA FIFI-LS far-IR spectroscopic observations of 25 local galaxies of either the [OIII]52um and/or the [NIII]57um lines. Including other 31 galaxies from Herschel-PACS, we discuss a local sample of 47 galaxies, including HII region, luminous IR, low-metallicity dwarf and Seyfert galaxies. Analyzing the mid- to far-IR fine-structure lines of this sample, we assess the metallicity and compare with the optical spectroscopy estimates. Using the IR, we find a similar O/H--N/O relation to that known in the optical. As opposite, we find systematically lower N/O IR abundances when compared to the optical determinations, especially at high values of N/O (log(N/O) > -0.8). We explore various hypotheses to account for this difference: (i) difference in ionization structure traced by optical (O+, N+ regions) versus IR lines (O++, N++ regions); (ii) contamination of diffuse ionized gas affecting the optical lines used to compute the N/O abundance; (iii) dust obscuration affecting the optical-based determinations. However, we have not found any correlation of the Delta(N/O)= (N/O)_OPT-(N/O)_IR with either ionization, or electron density, or optical extinction. We speculatively suggest that accretion of metal-poor gas from the circumgalactic medium could provide an explanation for this difference, because the rapid decrease of total abundances during infall is followed by a N/O ratio decrease due to primary production of young - possibly embedded - massive stars, are preferentially traced by the IR diagnostics, while optical diagnostics would better trace the secondary production, when both N/O and O/H abundance ratios will increase.

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arXiv2021

Subarcsecond LOFAR imaging of Arp299 at 150 MHz. Tracing the nuclear and diffuse extended emission of a bright LIRG

We study for the first time the low-frequency ($\sim$150 MHz) radio brightness distribution of Arp~299 at subarcsecond resolution, tracing in both compact and extended emission regions the local spectral energy distribution (SED) in order to characterize the dominant emission and absorption processes. We analysed the spatially resolved emission of Arp 299 revealed by 150 MHz international baseline Low-Frequency Array (LOFAR) and 1.4, 5.0, and 8.4 GHz Very Large Array (VLA) observations. We present the first subarcsecond (0.4"$\sim$100~pc) image of the whole Arp~299 system at 150~MHz. The high surface brightness sensitivity of our LOFAR observations ($\sim$100 $μ$Jy/beam) allowed us to detect all of the nuclear components detected at higher frequencies, as well as the extended steep-spectrum emission surrounding the nuclei. We obtained spatially resolved, two-point spectral index maps for the whole galaxy: the compact nuclei show relatively flat spectra, while the extended, diffuse component shows a steep spectrum. We fitted the radio SED of the nuclear regions using two different models: a continuous free-free medium model and a clumpy model. The continuous model can explain the SED of the nuclei assuming a population of relativistic electrons subjected to synchrotron, bremsstrahlung, and ionization losses. The clumpy model fits assuming relativistic electrons with negligible energy losses, and thermal fractions that are more typical of star-forming galaxies than those required for the continuous model. Our results confirm the usefulness of combining spatially resolved radio imaging at both MHz and GHz frequencies to characterize in detail the radio emission properties of LIRGs from the central 100 pc out to the kiloparsec galaxy-wide scales.

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arXiv2021

The properties of Polycyclic Aromatic Hydrocarbons in galaxies: constraints on PAH sizes, charge and radiation fields

Based on theoretical spectra computed using Density Functional Theory we study the properties of Polycyclic Aromatic Hydrocarbons (PAH). In particular using bin-average spectra of PAH molecules with varying number of carbons we investigate how the intensity of the mid-infrared emission bands, 3.3, 6.2, 7.7 and 11.3 microns, respond to changes in the number of carbons, charge of the molecule, and the hardness of the radiation field that impinges the molecule. We confirm that the 6.2/7.7 band ratio is a good predictor for the size of the PAH molecule (based on the number of carbons present). We also investigate the efficacy of the 11.3/3.3 ratio to trace the size of PAH molecules and note the dependence of this ratio on the hardness of the radiation field. While the ratio can potentially also be used to trace PAH molecular size, a better understanding of the impact of the underlying radiation field on the 3.3 microns feature and the effect of the extinction on the ratio should be evaluated. The newly developed diagnostics are compared to band ratios measured in a variety of galaxies observed with the Infrared Spectrograph on board the Spitzer Space Telescope. We demonstrate that the band ratios can be used to probe the conditions of the interstellar medium in galaxies and differentiate between environments encountered in normal star forming galaxies and Active Galactic Nuclei. Our work highlights the immense potential that PAH observations with the James Webb Space Telescope will have on our understanding of the PAH emission itself and of the physical conditions in galaxies near and far.

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arXiv2021

Stellar kinematics in the nuclear regions of nearby LIRGs with VLT-SINFONI. Comparison with gas phases and implications for dynamical mass estimations

In this work we use seeing-limited SINFONI H- and K-band spectroscopy to analyse the spatially resolved kinematics of the stellar component in the inner r<1-2 kpc of ten nearby (mean z=0.014) LIRGs. We compare the stellar kinematics with those for different gas phases, and analyse the relative effects of using different tracers when estimating dynamical masses. The stellar velocity and velocity dispersion maps are extracted in both near-IR bands by fitting the continuum emission using pPXF while we use the gas kinematics from previous works. We find that the different gas phases have similar kinematics, whereas the stellar component is rotating with slightly lower velocities (V$_*$~0.8V$_g$) but in significantly warmer orbits ($σ_*$~2$σ_g$) than the gas phases. These values indicate that stars are rotating in thick discs while the gas phases are confined in dynamically cooler rotating discs. However, these differences do not lead to significant discrepancies between the dynamical mass estimations based on the stellar and gas kinematics. This result suggests that the gas kinematics can be used to estimate M$_{dyn}$ also in z~2 SFGs, a galaxy population that shares many structural and kinematic properties with local LIRGs.

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arXiv2021

Mid-IR cosmological spectrophotometric surveys from space: Measuring AGN and star formation at the Cosmic Noon with a SPICA-like mission

We use the SPace Infrared telescope for Cosmology and Astrophysics (SPICA) project as a template to demonstrate how deep spectrophotometric surveys covering large cosmological volumes over extended fields (1-15 square degrees) with a mid-IR imaging spectrometer (17-36 micron) in conjunction with deep 70 micron photometry with a far-IR camera, at wavelengths which are not affected by dust extinction can answer the most crucial questions in current galaxy evolution studies. A SPICA-like mission will be able for the first time to provide an unobscured three dimensional (3-D, i.e. x, y and redshift z) view of galaxy evolution back to an age of the Universe of less than ~2 Gyrs, in the mid-IR rest-frame. This survey strategy will produce a full census of the Star formation Rate (SFR) in the Universe, using Polycyclic Aromatic Hydrocarbons (PAH) bands and fine-structure ionic lines, reaching the characteristic knee of the galaxy luminosity function, where the bulk of the population is distributed, at any redshift up to z ~3.5. Deep follow-up pointed spectroscopic observations with grating spectrometers { onboard the satellite}, across the full IR spectral range (17-210 micron), would simultaneously measure Black Hole Accretion Rate (BHAR), from high-ionization fine-structure lines, and SFR, from PAH and low- to mid-ionization lines in thousands of galaxies from solar to low metallicities, down to the knee of their luminosity functions. The analysis of the resulting atlas of IR spectra will reveal the physical processes at play in evolving galaxies across cosmic time, especially its heavily dust-embedded phase during the activity peak at the cosmic noon (z ~1-3), through IR emission lines and features that are insensitive to the dust obscuration.

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arXiv2021

Predicting the Observability of Population III Stars with ELT-HARMONI via the Helium $1640{\rmÅ}$ emission line

Population III (Pop. III) stars, as of yet, have not been detected, however as we move into the era of extremely large telescopes this is likely to change. One likely tracer for Pop. III stars is the HeII$\lambda1640$ emission line, which will be detectable by the HARMONI spectrograph on the European Extremely Large Telescope (ELT) over a broad range of redshifts ($2\leq z\leq14$). By post-processing galaxies from the cosmological, AMR-hydrodynamical simulation NewHorizon with theoretical spectral energy distributions (SED) for Pop. III stars and radiative transfer (i.e. the Yggdrasil Models and CLOUDY look-up tables respectively) we are able to compute the flux of HeII$\lambda1640$ for individual galaxies. From mock 10 hour observations of these galaxies we show that HARMONI will be able to detect Pop. III stars in galaxies up to $z\sim10$ provided Pop. III stars have a top heavy Initial Mass Function (IMF). Furthermore, we find that should Pop. III stars instead have an IMF similar to those of the Pop. I stars, the HeII$\lambda1640$ line would only be observable for galaxies with Pop. III stellar masses in excess of $10^{7}{\,M}_\odot\,{\rm yr}^{-1}$, average stellar age $<1{\rm Myr}$ at $z=4$. Finally, we are able to determine the minimal intrinsic flux required for HARMONI to detect Pop. III stars in a galaxy up to $z=10$.

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arXiv2020

A proto-pseudobulge in ESO 320-G030 fed by a massive molecular inflow driven by a nuclear bar

Galaxies with nuclear bars are believed to efficiently drive gas inward, generating a nuclear starburst and possibly an active galactic nucleus (AGN). We confirm this scenario for the isolated, double-barred, luminous infrared galaxy ESO 320-G030 based on an analysis of Herschel and ALMA spectroscopic observations. Herschel/PACS and SPIRE observations of ESO 320-G030 show absorption/emission in 18 lines of H2O, which we combine with the ALMA H2O 423-330 448 GHz line (Eupper~400 K) and continuum images to study the nuclear region. Radiative transfer models indicate that 3 nuclear components are required to account for the H2O and continuum data. An envelope, with R~130-150 pc, T_dust~50 K, and N_H2~2x10^{23} cm^{-2}, surrounds a nuclear disk with R~40 pc and tau_100um~1.5-3 (N_H2~2x10^{24} cm^{-2}) and an extremely compact (R~12 pc), warm (~100 K), and buried (tau_100um>5, N_H2>~5x10^{24} cm^{-2}) core component. The three nuclear components account for 70% of the galaxy L_IR (SFR~16-18 Msun yr^{-1}). The nucleus is fed by a molecular inflow observed in CO 2-1 with ALMA, which is associated with the nuclear bar. With decreasing radius (r=450-225 pc), the mass inflow rate increases up to ~20 Msun yr^{-1}, which is similar to the nuclear SFR, indicating that the starburst is sustained by the inflow. At lower r, the inflow is best probed by the far-infrared OH ground-state doublets, with an estimated inflow rate of ~30 Msun yr^{-1}. The short timescale of ~20 Myr for nuclear gas replenishment indicates quick secular evolution, and indicates that we are witnessing an intermediate stage (<100 Myr) proto-pseudobulge fed by a massive inflow that is driven by a strong nuclear bar. We also apply the H2O model to the Herschel far-infrared spectroscopic observations of H2^{18}O, OH, $^{18}OH, OH+, H2O^+, H3O^+, NH, NH2, NH3, CH, CH^+, ^{13}CH^+, HF, SH, and C3, and estimate their abundances.

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arXiv2020

$Herschel$/PACS OH Spectroscopy of Seyfert, LINER, and Starburst Galaxies

We investigated the 65$μ$m, 71$μ$m, 79$μ$m, 84$μ$m, 119$μ$m, and 163$μ$m OH doublets of 178 local (0 < $z$ < 0.35) galaxies. They were observed using the $Herschel$/PACS spectrometer, including Seyfert galaxies, LINERs, and star-forming galaxies. We observe these doublets exclusively in absorption (OH71), primarily in absorption (OH65, OH84), mostly in emission (OH79), only in emission (OH163) and an approximately even mix of the both (OH119). In 19 galaxies we find P-Cygni or reverse P-Cygni line profiles in the OH doublets. We use several galaxy observables to probe spectral classification, brightness of a central AGN/starburst component, and radiation field strength. We find that OH79, OH119, and OH163 are more likely to display strong emission for bright, unobscured AGN. For less luminous, obscured AGN and non-active galaxies, we find populations of strong absorption (OH119), weaker emission (OH163), and a mix of weak emission and weak absorption (OH79). For OH65, OH71 and OH84, we do not find significant correlations with the observables listed above. We do find relationships between OH79 and OH119 with both the 9.7$μ$m silicate feature and Balmer decrement dust extinction tracers in which more dust leads to weaker emission / stronger absorption. The origin of emission for the observed OH doublets, whether from collisional excitation, or from radiative pumping by infrared photons, is discussed.

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arXiv2020

Simulating gas kinematic studies of high-redshift galaxies with the HARMONI Integral Field Spectrograph

We present simulated observations of gas kinematics in galaxies formed in 10 pc resolution cosmological simulations with the hydrodynamical + N-body code RAMSES, using the new RAMSES2HSIM pipeline with the simulated observing pipeline (HSIM) for the ELT HARMONI IFU spectrograph. We post-process the galaxy's gas kinematics and Hα line emission for each simulation cell, and integrate the emission to produce an extinction-corrected input cube. We then simulate observations of the input cube with HARMONI, for a range of exposure times, spatial sampling, and spectral resolution. We analyze the mock observations to recover galaxy properties such as its kinematics and compare with the known simulation values. We investigate the cause of biases between the 'real' and 'observed' kinematic values, demonstrating the sensitivity of the inferred rotation curve to knowledge of the instrument's point spread function.

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arXiv2020

Extinction in the 11.2 micron PAH band and the low L_11.2/L_IR in ULIRGs

We present a method for recovering the intrinsic (extinction-corrected) luminosity of the 11.2 micron PAH band in galaxy spectra. Using 105 high S/N Spitzer/IRS spectra of star-forming galaxies, we show that the equivalent width ratio of the 12.7 and 11.2 micron PAH bands is independent on the optical depth, with small dispersion of ~5% indicative of a nearly constant intrinsic flux ratio R_int = (f_12.7/f_11.2)_int = 0.377 +/- 0.020. Conversely, the observed flux ratio, R_obs = (f_12.7/f_11.2)_obs strongly correlates with the silicate strength (S_sil) confirming that differences in R_obs reflect variation in the optical depth. The relation between R_obs and S_sil reproduces predictions for the Galactic Centre extinction law but disagrees with other laws. We calibrate the total extinction affecting the 11.2 micron PAH from R_obs, which we apply to another sample of 215 galaxies with accurate measurements of the total infrared luminosity (L_IR) to investigate the impact of extinction on L_11.2/L_IR. Correlation between L_11.2/L_IR and R_obs independently on L_IR suggests that increased extinction explains the well known decrease in the average L_11.2/L_IR at high L_IR. The extinction-corrected L_11.2 is proportional to L_IR in the range L_IR/L_sun = 10^9--10^13. These results consolidate L_11.2 as a robust tracer of star formation in galaxies.

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arXiv2017

No AGN evidence in NGC1614 from deep radio VLBI observations

We present deep dual-band 5.0 and 8.4GHz European VLBI Network (EVN) observations of NGC1614, a local luminous infrared galaxy with a powerful circumnuclear starburst ring, and whose nuclear engine origin is still controversial. We aim at detecting and characterizing compact radio structures both in the nuclear region and in the circumnuclear ring. We do not find any compact source in the central 200pc region, setting a very tight 5 sigma upper limit of $3.7\times10^{36}$erg s$^{-1}$ and $5.8\times10^{36}$erg s$^{-1}$, at 5.0 and 8.4GHz, respectively. However, we report a clear detection at both frequencies of a compact structure in the circumnuclear ring, 190pc to the north of the nucleus, whose luminosity and spectral index are compatible with a core-collapse supernova, giving support to the high star formation rate in the ring. Our result favors the pure starburst scenario, even for the nucleus of NGC1614, and shows the importance of radio VLBI observations when dealing with the obscured environments of dusty galaxies.

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arXiv2016

VLT-SINFONI sub-kpc study of the star formation in local LIRGs and ULIRGs: Analysis of the global $Σ_{\rm SFR}$ structure and characterisation of individual star-forming clumps

We present a 2D study of star formation at kpc and sub-kpc scales of a sample of local (z<0.1) U/LIRGs, based on near-IR VLT-SINFONI observations. We obtained integrated measurements of the star formation rate (SFR) and star formation rate surface density, together with their 2D distributions, based on Br_gamma and Pa_alpha emission. We observe a tight linear correlation between the SFR derived from our extinction-corrected measurements and that derived from 24 micron data, and a reasonable agreement with SFR derived from total IR luminosity. Our near-IR measurements are on average a factor 3 larger than optical SFR, even when extinction corrections are applied. We found that LIRGs have a median-observed star formation rate surface density of 1.72 Msun/yr/kpc^2 for the extinction-corrected distribution, whilst ULIRGs have 0.23 Msun/yr/kpc^2, respectively. These median values for ULIRGs increase up to 2.90 Msun/yr/kpc^2, when only their inner regions, covering the same size as the average FoV of LIRGs, are considered. We identified a total of 95 individual SF clumps in our sample, with sizes within 60-1500pc, and extinction-corrected Pa_alpha luminosities of 10^5-10^8 Lsun. Star-forming clumps in LIRGs are about ten times larger and thousands of times more luminous than typical clumps in spiral galaxies. Clumps in ULIRGs have sizes similar (x0.5-1) to those of high-z clumps, having Pa_alpha luminosities similar to some high-z clumps, and about 10 times less luminous than the most luminous high-z clumps identified so far. We also observed a change in the slope of the L-r relation. A likely explanation is that most luminous galaxies are interacting and merging, and therefore their size represents a combination of the distribution of the star-forming clumps within each galaxy in the system plus the effect of the projected distance.

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arXiv2015

Star-formation histories of local luminous infrared galaxies

We present the analysis of the integrated spectral energy distribution (SED) from the ultraviolet (UV) to the far-infrared and H$α$ of a sample of 29 local systems and individual galaxies with infrared (IR) luminosities between 10^11 Lsun and 10^11.8 Lsun. We have combined new narrow-band H$α$+[NII] and broad-band g, r optical imaging taken with the Nordic Optical Telescope (NOT), with archival GALEX, 2MASS, Spitzer, and Herschel data. The SEDs (photometry and integrated H$α$ flux) have been fitted with a modified version of the MAGPHYS code using stellar population synthesis models for the UV-near-IR range and thermal emission models for the IR emission taking into account the energy balance between the absorbed and re-emitted radiation. From the SED fits we derive the star-formation histories (SFH) of these galaxies. For nearly half of them the star-formation rate appears to be approximately constant during the last few Gyrs. In the other half, the current star-formation rate seems to be enhanced by a factor of 3-20 with respect to that occured ~1 Gyr ago. Objects with constant SFH tend to be more massive than starbursts and they are compatible with the expected properties of a main-sequence (M-S) galaxy. Likewise, the derived SFHs show that all our objects were M-S galaxies ~1 Gyr ago with stellar masses between 10^10.1 and 10^11.5 Msun. We also derived from our fits the average extinction (A_v=0.6-3 mag) and the polycyclic aromatic hydrocarbons (PAH) luminosity to L(IR) ratio (0.03-0.16). We combined the A_v with the total IR and H$α$ luminosities into a diagram which can be used to identify objects with rapidly changing (increasing or decreasing) SFR during the last 100 Myr.

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arXiv2014

Spatially resolved physical conditions of molecular gas and potential star formation tracers in M83, revealed by the Herschel SPIRE FTS

Since the launch of the Herschel Space Observatory, our understanding about the photo-dissociation regions (PDR) has taken a step forward. In the bandwidth of the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) on board Herschel, ten CO rotational transitions, including J=4-3 to J=13-12, and three fine structure lines, including [CI] 609, [CI] 370, and [NII] 250 micron, are covered. In this paper, we present our findings from the FTS observations at the nuclear region of M83, based on the spatially resolved physical parameters derived from the CO spectral line energy distribution (SLED) map and the comparisons with the dust properties and star-formation tracers. We discuss (1) the potential of using [NII] 250 and [CI] 370 micron as star-formation tracers; (2) the reliability of tracing molecular gas with CO; (3) the excitation mechanisms of warm CO; (4) the possibility of studying stellar feedback by tracing the thermal pressure of molecular gas in the nuclear region of M83.

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arXiv2014

Far-infrared line spectra of Seyfert galaxies from the Herschel-PACS Spectrometer

We present spectroscopic observations of FIR fine-structure lines of 26 Seyfert galaxies obtained with the Herschel-PACS spectrometer. These observations are complemented by spectroscopy with Spitzer-IRS and Herschel-SPIRE. The ratios of the OIII, NII, SIII and NeV lines have been used to determine electron densities in the ionised gas regions. The CI lines, observed with SPIRE, have been used to measure the densities in the neutral gas, while the OI lines provide a measure of the gas temperature, at densities below 10000 cm-3. Using the OI145/63um and SIII33/18um line ratios we find an anti-correlation of the temperature with the gas density. Using various fine-structure line ratios, we find that density stratification is common in these active galaxies. On average, the electron densities increase with the ionisation potential of the ions producing the NII, SIII and NeV emission. The infrared emission lines arise partly in the Narrow Line Region (NLR) photoionised by the AGN central engine, partly in HII regions photo ionised by hot stars and partly in neutral gas in photo-dissociated regions (PDRs). We attempt to separate the contributions to the line emission produced in these different regions by comparing our emission line ratios to empirical and theoretical values. In particular, we tried to separate the contribution of AGN and star formation by using a combination of Spitzer and Herschel lines, and we found that, besides the well known mid-IR line ratios, the mixed mid-IR/far-IR line ratio of OIII88um/OIV26um can reliably discriminate the two emission regimes, while the far-IR line ratio of CII157um/OI63um is only able to mildly separate the two regimes. By comparing the observed CII157um/NII205um ratio with photoionisation models, we also found that most of the CII emission in the galaxies we examined is due to PDRs.

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arXiv2014

Outflow of hot and cold molecular gas from the obscured secondary nucleus of NGC3256: closing in on feedback physics

The nuclei of merging galaxies are often deeply buried in dense layers of gas and dust. In these regions, gas outflows driven by starburst and AGN activity are believed to play a crucial role in the evolution of these galaxies. However, to fully understand this process it is essential to resolve the morphology and kinematics of such outflows. Using near-IR integral-field spectroscopy obtained with VLT/SINFONI, we detect a kpc-scale structure of high-velocity molecular hydrogen (H2) gas associated with the deeply buried secondary nucleus of the IR-luminous merger NGC3256. We show that this structure is likely the hot component of a molecular outflow, which is detected also in the cold molecular gas by Sakamoto et al. This outflow, with a molecular gas mass of M(H2)~2x10^7 Msun, is among the first to be spatially resolved in both the hot H2 gas with VLT/SINFONI and the cold CO-emitting gas with ALMA. The hot and cold components share a similar morphology and kinematics, with a hot-to-cold molecular gas mass ratio of ~6x10^-5. The high (~100 pc) resolution at which we map the geometry and velocity structure of the hot outflow reveals a biconical morphology with opening angle ~40 deg and gas spread across a FWZI~1200 km/s. Because this collimated outflow is oriented close to the plane of the sky, the molecular gas may reach maximum intrinsic outflow velocities of ~1800 km/s, with an average mass outflow rate of at least ~20 Msun/yr. By modeling the line-ratios of various near-IR H2 transitions, we show that the H2 gas in the outflow is heated through shocks or X-rays to a temperature of ~1900K. The energy needed to drive the outflow is likely provided by a hidden Compton-thick AGN or by the nuclear starburst. We show that the global kinematics of the molecular outflow in NGC3256 mimic those of CO-outflows that have been observed at low spatial resolution in starburst- and active galaxies.

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arXiv2014

Warm molecular gas temperature distribution in six local infrared bright Seyfert galaxies

We simultaneously analyze the spectral line energy distributions (SLEDs) of CO and H2 of six local luminous infrared (IR) Seyfert galaxies. For the CO SLEDs, we used new Herschel/SPIRE FTS data (from J=4-3 to J=13-12) and ground-based observations for the lower-J CO transitions. The H2 SLEDs were constructed using archival mid-IR Spitzer/IRS and near-IR VLT/SINFONI data for the rotational and ro-vibrational H2 transitions, respectively. In total, the SLEDs contain 26 transitions with upper level energies between 5 and 15000 K. A single, constant density, model (n$_{H_2}$ ~ 10$^{4.5-6}$ cm$^{-3}$) with a broken power-law temperature distribution reproduces well both the CO and H2 SLEDs. The power-law indices are $β_1$ ~ 1-3 for warm molecular gas (20 K < T < 100 K) and $β_2$ ~ 4-5 for hot molecular gas (T > 100 K). We show that the steeper temperature distribution (higher $β$) for hot molecular gas can be explained by shocks and photodissociation region (PDR) models, however, the exact $β$ values are not reproduced by PDR or shock models alone and a combination of both is needed. We find that the three major mergers among our targets have shallower temperature distributions for warm molecular gas than the other three spiral galaxies. This can be explained by a higher relative contribution of shock excitation, with respect to PDR excitation, for the warm molecular gas in these mergers. For only one of the mergers, IRASF 05189-2524, the shallower H2 temperature distribution differs from that of the spiral galaxies. The presence of a bright active galactic nucleus in this source might explain the warmer molecular gas observed.

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arXiv2013

Herschel SPIRE-FTS Observations of Excited CO and [CI] in the Antennae (NGC 4038/39): Warm and Cold Molecular Gas

We present Herschel SPIRE-FTS observations of the Antennae (NGC 4038/39), a well studied, nearby ($22$ Mpc) ongoing merger between two gas rich spiral galaxies. We detect 5 CO transitions ($J=4-3$ to $J=8-7$), both [CI] transitions and the [NII]$205μm$ transition across the entire system, which we supplement with ground based observations of the CO $J=1-0$, $J=2-1$ and $J=3-2$ transitions, and Herschel PACS observations of [CII] and [OI]$63μm$. Using the CO and [CI] transitions, we perform both a LTE analysis of [CI], and a non-LTE radiative transfer analysis of CO and [CI] using the radiative transfer code RADEX along with a Bayesian likelihood analysis. We find that there are two components to the molecular gas: a cold ($T_{kin}\sim 10-30$ K) and a warm ($T_{kin} \gtrsim 100$ K) component. By comparing the warm gas mass to previously observed values, we determine a CO abundance in the warm gas of $x_{CO} \sim 5\times 10^{-5}$. If the CO abundance is the same in the warm and cold gas phases, this abundance corresponds to a CO $J=1-0$ luminosity-to-mass conversion factor of $α_{CO} \sim 7 \ M_{\odot}{pc^{-2} \ (K \ km \ s^{-1})^{-1}}$ in the cold component, similar to the value for normal spiral galaxies. We estimate the cooling from H$_2$, [CII], CO and [OI]$63μm$ to be $\sim 0.01 L_{\odot}/M_{\odot}$. We compare PDR models to the ratio of the flux of various CO transitions, along with the ratio of the CO flux to the far-infrared flux in NGC 4038, NGC 4039 and the overlap region. We find that the densities recovered from our non-LTE analysis are consistent with a background far-ultraviolet field of strength $G_0\sim 1000$. Finally, we find that a combination of turbulent heating, due to the ongoing merger, and supernova and stellar winds are sufficient to heat the molecular gas.

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