Star Formation in Collision Debris: Insights from the modeling of their Spectral Energy Distribution

During galaxy-galaxy interactions, massive gas clouds can be injected into the intergalactic medium which in turn become gravitationally bound, collapse and form stars, star clusters or even dwarf galaxies. The objects resulting from this process are both "pristine", as they are forming their first generation of stars, and chemically evolved because the metallicity inherited from their parent galaxies is high. Such characteristics make them particularly interesting laboratories to study star formation. After having investigated their star-forming properties, we use photospheric, nebular and dust modeling to analyze here their spectral energy distribution (SED) from the far-ultraviolet to the mid-infrared regime for a sample of 7 star-forming regions. Our analysis confirms that the intergalactic star forming regions in Stephan's Quintet, around Arp 105, and NGC 5291, appear devoid of stellar populations older than 10^9 years. We also find an excess of light in the near-infrared regime (from 2 to 4.5 microns) which cannot be attributed to stellar photospheric or nebular contributions. This excess is correlated with the star formation rate intensity suggesting that it is probably due to emission by very small grains fluctuating in temperature as well as the polycyclic aromatic hydrocarbons (PAH) line at 3.3 micron. Comparing the attenuation via the Balmer decrement to the mid-infrared emission allows us to check the reliability of the attenuation estimate. It suggests the presence of embedded star forming regions in NGC 5291 and NGC 7252. Overall the SED of star-forming regions in collision debris (and Tidal Dwarf Galaxies) resemble more that of dusty star-forming regions in galactic disks than to that of typical star-forming dwarf galaxies.Comment: 22 pages, 24 figures, accepted for publication in A

The Molecular Interstellar Medium of the Local Group Dwarf NGC6822

Do molecular clouds collapse to form stars at the same rate in all environments? In large spiral galaxies, the rate of transformation of H2 into stars (hereafter SFE) varies little. However, the SFE in distant objects (z~1) is much higher than in the large spiral disks that dominate the local universe. Some small local group galaxies share at least some of the characteristics of intermediate-redshift objects, such as size or color. Recent work has suggested that the Star Formation Efficiency (SFE, defined as the SFRate per unit H2) in local Dwarf galaxies may be as high as in the distant objects. A fundamental difficulty in these studies is the independent measure of the H2 mass in metal-deficient environments. At 490 kpc, NGC6822 is an excellent choice for this study; it has been mapped in the CO(2-1) line using the multibeam receiver HERA on the 30 meter IRAM telescope, yielding the largest sample of giant molecular clouds (GMCs) in this galaxy. Despite the much lower metallicity, we find no clear difference in the properties of the GMCs in NGC 6822 and those in the Milky Way except lower CO luminosities for a given mass. Several independent methods indicate that the total H2 mass in NGC 6822 is about 5 x 10^6 Msun in the area we mapped and less than 10^7 Msun in the whole galaxy. This corresponds to a NH2/ICO ~ 4 x 10^{21} cm^-2 /(Kkm/s) over large scales, such as would be observed in distant objects, and half that in individual GMCs. No evidence was found for H2 without CO emission. Our simulations of the radiative transfer in clouds are entirely compatible with these NH2/ICO values. The SFE implied is a factor 5 - 10 higher than what is observed in large local universe spirals.Comment: 16 pages, 13 figures. Accepted for publication in Astronomy and Astrophysic

[CII] emission and star formation in the spiral arms of M31

The CII 158 microns line is the most important coolant of the interstellar medium in galaxies but substantial variations are seen from object to object. The main source of the emission at a galactic scale is still poorly understood. Previous studies of the CII emission in galaxies have a resolution of several kpc or more so the observed emission is an average of different ISM components. The aim of this work is to study, for the first time, the CII emission at the scale of a spiral arm. We want to investigate the origin of this line and its use as a tracer of star formation. We present CII and OI observations of a segment of a spiral arm of M~31 using the Infrared Space Observatory. The CII emission is compared with tracers of neutral gas (CO, HI) and star formation (H\alpha, Spitzer 24 mu.) The similarity of the CII emission with the Ha and 24 mu images is striking when smoothed to the same resolution, whereas the correlation with the neutral gas is much weaker. The CII cooling rate per H atom increases dramatically from ~2.7e-26 ergs/s/atom in the border of the map to ~ 1.4e-25 ergs/s/atom in the regions of star formation. The CII/FIR(42-122) ratio is almost constant at 2%, a factor 3 higher than typically quoted. However, we do not believe that M~31 is unusual. Rather, the whole-galaxy fluxes used for the comparisons include the central regions where the CII/FIR ratio is known to be lower and the resolved observations neither isolate a spiral arm nor include data as far out in the galactic disk as the observations presented here. A fit to published PDR models yields a plausible average solution of G_0~100 and n~3000 for the PDR emission in the regions of star formation in the arm of M31.Comment: 8 pages, 5 figures. To be published by A&A. Low quality figures. High quality version in http://www.obs.u-bordeaux1.fr/Radio/NRodriguez/out/m31.pd

Constraints on UV Absorption in the Intracluster Medium of Abell 1030

We present results from an extensive HST spectroscopic search for UV absorption lines in the spectrum of the quasar B2~1028+313, which is associated with the central dominant galaxy in the cluster Abell~1030 ($z=0.178$). This is one of the brightest known UV continuum sources located in a cluster, and therefore provides an ideal opportunity to obtain stringent constraints on the column densities of any cool absorbing gas that may be associated with the intracluster medium (ICM). Our HST spectra were obtained with the FOS and GHRS, and provide continuous coverage at rest-frame wavelengths from $\sim 975$ to 4060~\AA, thereby allowing the investigation of many different elements and ionization levels. We utilize a new technique that involves simultaneous fitting of large numbers of different transitions for each species, thereby yielding more robust constraints on column densities than can be obtained from a single transition. This method yields upper limits of $\lesssim 10^{11} - 10^{13}$ cm$^{-2}$ on the column densities of a wide range of molecular, atomic and ionized species that may be associated with the ICM. We also discuss a possible \Lya and C IV absorption system associated with the quasar. We discuss the implications of the upper limits on cool intracluster gas in the context of the physical properties of the ICM and its relationship to the quasar.Comment: Astrophysical Journal, in press, 19 pages, includes 5 PostScript figures. Latex format, uses aas2pp4.sty and epsfig.sty file

Particularly Efficient Star Formation in M33

The Star Formation (SF) rate in galaxies is an important parameter at all redshifts and evolutionary stages of galaxies. In order to understand the increased SF rates in intermediate redshift galaxies one possibility is to study star formation in local galaxies with properties frequently found at this earlier epoch like low metallicity and small size. We present sensitive observations of the molecular gas in M 33, a small Local Group spiral at a distance of 840 kpc which shares many of the characteristics of the intermediate redshift galaxies. The observations were carried out in the CO(2--1) line with the HERA heterodyne array on the IRAM 30 m telescope. A 11\arcmin$\times$22\arcmin region in the northern part of M 33 was observed, reaching a detection threshold of a few 10$^{3}$ \msol. The correlation in this field between the CO emission and tracers of SF (8\mum, 24\mum, \Ha, FUV) is excellent and CO is detected very far North, showing that molecular gas forms far out in the disk even in a small spiral with a subsolar metallicity. One major molecular cloud was discovered in an interarm region with no HI peak and little if any signs of SF -- without a complete survey this cloud would never have been found. The radial dependence of the CO emission has a scale length similar to the dust emission, less extended than the \Ha or FUV. If, however, the \ratioo ratio varies inversely with metallicity, then the scale length of the H$_2$ becomes similar to that of the \Ha or FUV. Comparing the SF rate to the H$_2$ mass shows that M 33, like the intermediate redshift galaxies it resembles, has a significantly higher SF efficiency than large local universe spirals.Comment: 16 pages, 15 figure

Non-linear Dependence of L(B) on L(FIR) and M(H2) among Spiral Galaxies and Effects of Tidal Interaction

Through the study of a carefully selected sample of isolated spiral galaxies, we have established that two important global physical quantities for tracing star forming activities, L(FIR) and M(H2), have non-linear dependence on another commonly cited global quantity L(B). Furthermore we show that simple power law relations can effectively describe these non-linear relations for spiral galaxies spanning four orders of magnitude in FIR and M(H2) and nearly three orders of magnitude in L(B). While the existence of non-linear dependence of M(H2) (assuming a constant CO-to-H2 conversion) and L(FIR) on optical luminosity L(B) has been previously noted in the literature, an improper normalization of simple scaling by L(B) has been commonly used in many previous studies to claim enhanced molecular gas content and induced activities among tidally interacting and other types of galaxies. We remove these non-linear effects using the template relations derived from the isolated galaxy sample and conclude that strongly interacting galaxies do not have enhanced molecular gas content, contrary to previous claims. With these non-linear relations among L(B), L(FIR) and M(H2) properly taken into account, we confirm again that the FIR emission and the star formation efficiency L(FIR)/M(H2) are indeed enhanced by tidal interactions. Virgo galaxies show the same level of M(H2) and L(FIR) as isolated galaxies. We do not find any evidence for enhanced star forming activity among barred galaxies.Comment: 19 pages and 5 figures, requires AAS style files, ApJ, accepte

Spitzer Uncovers Active Galactic Nuclei Missed by Optical Surveys in 7 Late-type Galaxies

We report the discovery using Spitzers high resolution spectrograph of 7 Active Galactic Nuclei (AGN) in a sample of 32 late-type galaxies that show no definitive signatures of AGN in their optical spectra. Our observations suggest that the AGN detection rate in late-type galaxies is possibly 4 times larger than what optical spectroscopic observations alone suggest. We demonstrate using photoionization models with an input AGN and an extreme EUV-bright starburst ionizing radiation field that the observed mid-infrared line ratios cannot be replicated unless an AGN contribution, in some cases as little as 10% of the total galaxy luminosity, is included. These models show that when the fraction of the total luminosity due to the AGN is low, optical diagnostics are insensitive to the presence of the AGN. In this regime of parameter space, the mid-infrared diagnostics offer a powerful tool for uncovering AGN missed by optical spectroscopy. The AGN bolometric luminosities in our sample range from ~3 X 10^41 - ~2 X 10^43 ergs s^-1, which, based on the Eddington limit, corresponds to a lower mass limit for the black hole that ranges from ~3 X 10^3Mdot to as high as ~1.5 X 10^5Mdot. These lower mass limits however do not put a strain on the well-known relationship between the black hole mass and the host galaxy's stellar velocity dispersion established in predominantly early-type galaxies. Our findings add to the growing evidence that black holes do form and grow in low-bulge environments and that they are significantly more common than optical studies indicate.Comment: 8 figures, 17 pages, astro-ph\0801.2766 (Abel & Satyapal 2008; ApJ accepted) and this posting designed to form a two-part investigatio

Spitzer Observations of Low Luminosity Isolated and Low Surface Brightness Galaxies

We examine the infrared properties of five low surface brightness galaxies (LSBGs) and compare them with related but higher surface brightness galaxies, using Spitzer Space Telescope images and spectra. All the LSBGs are detected in the 3.6 and 4.5um bands, representing the stellar population. All but one are detected at 5.8 and 8.0um, revealing emission from hot dust and aromatic molecules, though many are faint or point-like at these wavelengths. Detections of LSBGs at the far-infrared wavelengths, 24, 70, and 160um, are varied in morphology and brightness, with only two detections at 160um, resulting in highly varied spectral energy distributions. Consistent with previous expectations for these galaxies, we find that detectable dust components exist for only some LSBGs, with the strength of dust emission dependent on the existence of bright star forming regions. However, the far-infrared emission may be relatively weak compared with normal star-forming galaxies.Comment: 20 pages, 8 figures, accepted to Ap

A Search for Molecular Gas in the Nucleus of M87 and Implications for the Fueling of Supermassive Black Holes

Supermassive black holes in giant elliptical galaxies are remarkably faint given their expected accretion rates. This motivates models of radiatively inefficient accretion, due to either ion-electron thermal decoupling, generation of outflows that inhibit accretion, or settling of gas to a gravitationally unstable disk that forms stars in preference to feeding the black hole. The latter model predicts the presence of cold molecular gas in a thin disk around the black hole. Here we report Submillimeter Array observations of the nucleus of the giant elliptical galaxy M87 that probe 230 GHz continuum and CO(J=2--1) line emission. Continuum emission is detected from the nucleus and several knots in the jet, including one that has been undergoing flaring behavior. We estimate a conservative upper limit on the mass of molecular gas within ~100pc and +-400km/s line of sight velocity of the central black hole of ~8x10^6Msun, which includes an allowance for possible systematic errors associated with subtraction of the continuum. Ignoring such errors, we have a 3 sigma sensitivity to about 3x10^6Msun. In fact, the continuum-subtracted spectrum shows weak emission features extending up to 4 sigma above the RMS dispersion of the line-free channels. These may be artifacts of the continuum subtraction process. Alternatively, if they are interpreted as CO emission, then the implied molecular gas mass is ~5x10^6Msun spread out over a velocity range of 700km/s. These constraints on molecular gas mass are close to the predictions of the model of self-gravitating, star-forming accretion disks fed by Bondi accretion (Tan & Blackman 2005).Comment: 10 pages, accepted to ApJ Main Journa

Efficiency of the dynamical mechanism

The most extreme starbursts occur in galaxy mergers, and it is now acknowledged that dynamical triggering has a primary importance in star formation. This triggering is due partly to the enhanced velocity dispersion provided by gravitational instabilities, such as density waves and bars, but mainly to the radial gas flows they drive, allowing large amounts of gas to condense towards nuclear regions in a small time scale. Numerical simulations with several gas phases, taking into account the feedback to regulate star formation, have explored the various processes, using recipes like the Schmidt law, moderated by the gas instability criterion. May be the most fundamental parameter in starbursts is the availability of gas: this sheds light on the amount of external gas accretion in galaxy evolution. The detailed mechanisms governing gas infall in the inner parts of galaxy disks are discussed.Comment: 6 pages, 3 figures, to be published in "Starbursts - From 30 Doradus to Lyman break galaxies", ed. R. de Grijs and R. Gonzalez-Delgad