C+/H2 Gas in Star-Forming Clouds and Galaxies

We present analytic theory for the total column density of singly ionized carbon (C+) in the optically thick photon dominated regions (PDRs) of far-UV irradiated (star-forming) molecular clouds. We derive a simple formula for the C+ column as a function of the cloud (hydrogen) density, the far-UV field intensity, and metallicity, encompassing the wide range of galaxy conditions. When assuming the typical relation between UV and density in the cold neutral medium, the C+ column becomes a function of the metallicity alone. We verify our analysis with detailed numerical PDR models. For optically thick gas, most of the C+ column is mixed with hydrogen that is primarily molecular (H2), and this "C+/H2" gas layer accounts for almost all of the `CO-dark' molecular gas in PDRs. The C+/H2 column density is limited by dust shielding and is inversely proportional to the metallicity down to ~0.1 solar. At lower metallicities, H2 line blocking dominates and the C+/H2 column saturates. Applying our theory to CO surveys in low redshift spirals we estimate the fraction of C+/H2 gas out of the total molecular gas to be typically ~0.4. At redshifts 1<z<3 in massive disc galaxies the C+/H2 gas represents a very small fraction of the total molecular gas (<0.16). This small fraction at high redshifts is due to the high gas surface densities when compared to local galaxies.Comment: 15 pages, 14 figures. Accepted to MNRAS. v3 - proofs and minor text modification

Star formation and AGN activity in the most luminous LINERs in the local universe

This work presents the properties of 42 objects in the group of the most luminous, highest star formation rate LINERs at z = 0.04 - 0.11. We obtained long-slit spectroscopy of the nuclear regions for all sources, and FIR data (Herschel and IRAS) for 13 of them. We measured emission line intensities, extinction, stellar populations, stellar masses, ages, AGN luminosities, and star-formation rates. We find considerable differences from other low-redshift LINERs, in terms of extinction, and general similarity to star forming (SF) galaxies. We confirm the existence of such luminous LINERs in the local universe, after being previously detected at z ~ 0.3 by Tommasin et al. (2012). The median stellar mass of these LINERs corresponds to 6 - 7 $\times$ 10$^{10}$M$_{\odot}$ which was found in previous work to correspond to the peak of relative growth rate of stellar populations and therefore for the highest SFRs. Other LINERs although showing similar AGN luminosities have lower SFR. We find that most of these sources have LAGN ~ LSF suggesting co-evolution of black hole and stellar mass. In general among local LINERs being on the main-sequence of SF galaxies is related to their AGN luminosity.Comment: submitted to MNRA

Star formation black hole growth and dusty tori in the most luminous AGNs at z=2-3.5

We report herschel observations of 100 very luminous, optically selected AGNs at z=2-3.5 with log(LUV)(erg/sec)> 46.5, where LUV=L1350A. The distribution in LUV is similar to the general distribution of SDSS AGNs in this redshift and luminosity interval. We measured SF luminosity, LSF, and SFR in 34 detected sources by fitting combined SF and WISE-based torus templates. We also obtained statistically significant stacks for the undetected sources in two luminosity groups. The sample properties are compared with those of very luminous AGNs at z>4.5. The main findings are: 1) The mean and the median SFRs of the detected sources are 1176 and 1010 Msun/yr, respectively. The mean SFR of the undetected sources is 148 Msun/yr. The ratio of SFR to BH accretion rate is approximately 80 for the detected sources and less than 10 for the undetected sources. There is no difference in LAGN and only a very small difference in L(torus) between detected and undetected sources. 2) The redshift distribution of LSF and LAGN for the most luminous, redshift 2-7 AGNs are different. The highest LAGN are found at z=~3. However, LSF of such sources peaks at z=~5. Assuming the objects in our sample are hosted by the most massive galaxies at those redshifts, we find many of them are below the main-sequence of SF galaxies at z=2-3.5. 3) The SEDs of dusty tori at high redshift are similar to those found in low redshift, low luminosity AGNs. Herschel upper limits put strong constraints on the long wavelength SED ruling out several earlier suggested torus templates. 4) We find no evidence for a luminosity dependence of the torus covering factor in sources with log(LAGN)=44-47.5. This conclusion is based on the highly uncertain and non-uniformally treated LAGN in many earlier studies. The median covering factors over this range are 0.68 for isotropic dust emission and 0.4 for anisotropic emission.Comment: 53 pages, 10 diagrams, accepted by Ap

A high-velocity transient outflow in Eta Carinae

We analyze velocity profiles of the X-ray spectral lines emitted by the Eta Carinae stellar binary at four epochs, just before the X-ray minimum (associated with periastron) and more than two years before the minimum (~apastron). The profiles are nicely resolved by the HETGS spectrometer on board Chandra. Far from periastron, we find symmetrical lines that are more or less centered at zero velocity. Closer to periastron, the lines broaden, shift towards the blue, and become visibly asymmetric. While the quiescent X-ray emission and slight (<200 km/sec) centroid shifts can be ascribed to the ordinary continuous binary wind interaction and to the orbital velocity of the secondary star, the observed high-velocity emission up to ~2,000 km/sec and the abrupt flares during which it occurs can not. This leads us to interpret the high-velocity flaring emission as due to a fast collimated outflow of ionized gas.Comment: Accepted by ApJ Letter

Narrow Radiative Recombination Continua: A Signature of Ions Crossing the Contact Discontinuity of Astrophysical Shocks

X-rays from planetary nebulae (PNs) are believed to originate from a shock driven into the fast stellar wind (v ~ 1000 km/s) as it collides with an earlier circumstellar slow wind (v ~ 10 km/s). In theory, the shocked fast wind (hot bubble) and the ambient cold nebula can remain separated by magnetic fields along a surface referred to as the contact discontinuity (CD) that inhibits diffusion and heat conduction. The CD region is extremely difficult to probe directly owing to its small size and faint emission. This has largely left the study of CDs, stellar-shocks, and the associated micro-physics in the realm of theory. This paper presents spectroscopic evidence for ions from the hot bubble (kT ~ 100 eV) crossing the CD and penetrating the cold nebular gas (kT ~ 1 eV). Specifically, a narrow radiative recombination continuum (RRC) emission feature is identified in the high resolution X-ray spectrum of the PN BD+30 3639 indicating bare C VII ions are recombining with cool electrons at kT_e=1.7+-1.3 eV. An upper limit to the flux of the narrow RRC of H-like C VI is obtained as well. The RRCs are interpreted as due to C ions from the hot bubble of BD+30 3639 crossing the CD into the cold nebula, where they ultimately recombine with its cool electrons. The RRC flux ratio of C VII to C VI constrains the temperature jump across the CD to Delta kT > 80 eV, providing for the first time direct evidence for the stark temperature disparity between the two sides of an astrophysical CD, and constraining the role of magnetic fields and heat conduction accordingly. Two colliding-wind binaries are noted to have similar RRCs suggesting a temperature jump and CD crossing by ions may be a common feature of stellar wind shocks.Comment: 14 pages, 5 figures, accepted to ApJ. Corrected typos, minor modifications to eq. 5 and corresponding tex

The X-Ray Spectrum of a Planetary Nebula at High Resolution: Chandra Gratings Spectroscopy of BD+30 3639

We present the results of the first X-ray gratings spectroscopy observations of a planetary nebula (PN), the X-ray-bright, young BD+30 3639. We observed BD+30 3639 for a total of 300 ks with the Chandra X-ray Observatory's Low Energy Transmission Gratings in combination with its Advanced CCD Imaging Spectrometer(LETG/ACIS-S). The LETG/ACIS-S spectrum of BD+30 3639 is dominated by H-like resonance lines of O viii and C sc vi and the He-like triplet line complexes of Ne ix and O vii. Other H-like resonance lines, such as N vii, as well as lines of highly ionized Fe, are weak or absent. Continuum emission is evident over the range 6-18 A. Spectral modeling indicates the presence of a range of plasma temperatures from T~1.7x10^6 K to 2.9x10^6 K and an intervening absorbing column N_H~2.4x10^21 cm-2. The same modeling conclusively demonstrates that C and Ne are highly enhanced, with abundance ratios of C/O~15-45 and Ne/O~3.3-5.0 (90% confidence ranges, relative to the solar ratios), while N and Fe are depleted, N/O~0.0-1.0 and Fe/O~0.1-0.4. The intrinsic luminosity of the X-ray source determined from the modeling and the measured flux (F_X = 4.1x10^-13 ergs cm-2 s-1) is L_X~8.6x10^32 erg s-1(assuming D = 1.2kpc). These gratings spectroscopy results are generally consistent with earlier results obtained from X-ray CCD imaging spectroscopy of BD+30 3639, but are far more precise. The tight constraints placed on the (nonsolar) abundances directly implicate the present-day central star -- hence, ultimately, the intershell region of the progenitor asymptotic giant branch star -- as the origin of the shocked plasma now emitting in X-rays.Comment: Accepted for publication in ApJ (29 pages, 8 figures, 4 tables, Abstract abridged

On Star Formation Rates and Star Formation Histories of Galaxies out to z ~ 3

We compare multi-wavelength SFR indicators out to z~3 in GOODS-South. Our analysis uniquely combines U-to-8um photometry from FIREWORKS, MIPS 24um and PACS 70, 100, and 160um photometry from the PEP survey, and Ha spectroscopy from the SINS survey. We describe a set of conversions that lead to a continuity across SFR indicators. A luminosity-independent conversion from 24um to total infrared luminosity yields estimates of LIR that are in the median consistent with the LIR derived from PACS photometry, albeit with significant scatter. Dust correction methods perform well at low to intermediate levels of star formation. They fail to recover the total amount of star formation in systems with large SFR_IR/SFR_UV ratios, typically occuring at the highest SFRs (SFR_UV+IR \gtrsim 100 Msun/yr) and redshifts (z \gtrsim 2.5) probed. Finally, we confirm that Ha-based SFRs at 1.5<z<2.6 are consistent with SFR_SED and SFR_UV+IR provided extra attenuation towards HII regions is taken into account (Av,neb = Av,continuum / 0.44). With the cross-calibrated SFR indicators in hand, we perform a consistency check on the star formation histories inferred from SED modeling. We compare the observed SFR-M relations and mass functions at a range of redshifts to equivalents that are computed by evolving lower redshift galaxies backwards in time. We find evidence for underestimated stellar ages when no stringent constraints on formation epoch are applied. We demonstrate how resolved SED modeling, or alternatively deep UV data, may help to overcome this bias. The age bias is most severe for galaxies with young stellar populations, and reduces towards older systems. Finally, our analysis suggests that SFHs typically vary on timescales that are long (at least several 100 Myr) compared to the galaxies' dynamical time.Comment: Accepted for publication in The Astrophysical Journal, 19 pages, 15 figure