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Register a new userA large CO and HCN line survey of Luminous Infrared Galaxies
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Padeli P. Papadopoulos Dr
Publication date
2007
fields
Physics
and research's language is
English
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A large CO, HCN multi-transition survey of 30 Luminous Infrared Galaxies ($rm L_{IR}>10^{11} L_{odot}$) is nearing completion with the James Clerk Maxwell Telescope (JCMT) on Mauna Kea (Hawaii), and the IRAM 30-meter telescope at Pico Veleta (Spain). The CO J=1--0, 2--1, 3--2, 4--3,6--5, $ ^{13}$CO J=2--1, HCN J=1--0, 3--2, 4--3 observations, resulting from $sim 250$ hours of JCMT, $sim 100$ hours of 30-m observing time and data from the literature constitute {it the largest extragalactic molecular line survey to date}, and can be used to address a wide range of issues and eventually allow the construction of reliable Spectral Line Energy Distributions (SLEDs) for the molecular gas in local starbursts. First results suggest that: a) HCN and HCO$^+$ J=1--0 line luminosities can be poor mass estimators of dense molecular gas ($rm ngeq 10^4 cm^{-3}$) unless their excitation is accounted for, b) CO cooling of such gas in ULIRGs may be comparable to that of the CII line at $rm 158 mu m$, and c) low excitation of the {it global} molecular gas reservoir remains possible in such systems. In such cases the expected low CO $rm J+1to J$ line luminosities for $rm J+1geq 4$ can lead to a strong bias against their detection from ULIRGs at high redshifts.
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Preliminary results from a sensitive survey of the CO J=1--0, 2--1, 3--2, 4--3, 6--5, HCN J=1--0, 3--2, 4--3, and CI J=1--0 lines of a sample of 30 Ultraluminous Infrared Galaxies (ULIRGs) are presented. These reveal a tandalizing picture of the physical conditions of the molecular gas in these extraordinary galaxies ($rm L_{FIR}>10^{12} L_{odot}$), with a diffuse phase dominating the low-J CO lines and a much denser and warmer phase dominating the CO 4-3 and 6-5 and all the HCN lines. The CI J=1--0 emission was found to be a robust tracer of their total molecular gas mass under a large range of physical conditions, a potent alternative to the much weaker emission from the $ ^{13}$CO isotopologue, and especially promising as an H$_2$ tracer for similar objects at high redshifts.
We present an analysis of [OI]63, [OIII]88, [NII]122 and [CII]158 far-infrared (FIR) fine-structure line observations obtained with Herschel/PACS, for ~240 local luminous infrared galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey (GOALS). We find pronounced declines -deficits- of line-to-FIR-continuum emission for [NII]122, [OI]63 and [CII]158 as a function of FIR color and infrared luminosity surface density, $Sigma_{rm IR}$. The median electron density of the ionized gas in LIRGs, based on the [NII]122/[NII]205 ratio, is $n_{rm e}$ = 41 cm$^{-3}$. We find that the dispersion in the [CII]158 deficit of LIRGs is attributed to a varying fractional contribution of photo-dissociation-regions (PDRs) to the observed [CII]158 emission, f([CII]PDR) = [CII]PDR/[CII], which increases from ~60% to ~95% in the warmest LIRGs. The [OI]63/[CII]158PDR ratio is tightly correlated with the PDR gas kinetic temperature in sources where [OI]63 is not optically-thick or self-absorbed. For each galaxy, we derive the average PDR hydrogen density, $n_{rm H}$, and intensity of the interstellar radiation field, in units of G$_0$, and find G$_0$/$n_{rm H}$ ratios ~0.1-50 cm$^3$, with ULIRGs populating the upper end of the distribution. There is a relation between G$_0$/$n_{rm H}$ and $Sigma_{rm IR}$, showing a critical break at $Sigma_{rm IR}^{star}$ ~ 5 x 10$^{10}$ Lsun/kpc$^2$. Below $Sigma_{rm IR}^{star}$, G$_0$/$n_{rm H}$ remains constant, ~0.32 cm$^3$, and variations in $Sigma_{rm IR}$ are driven by the number density of star-forming regions within a galaxy, with no change in their PDR properties. Above $Sigma_{rm IR}^{star}$, G$_0$/$n_{rm H}$ increases rapidly with $Sigma_{rm IR}$, signaling a departure from the typical PDR conditions found in normal star-forming galaxies towards more intense/harder radiation fields and compact geometries typical of starbursting sources.
High resolution (a few arcseconds) observations of CO(1-0) and HCN(1-0) emission from nearby Seyfert galaxies have been conducted with the Nobeyama Millimeter Array. Based on the observed CO distributions and kinematics,we suggest that a small scale (a few 100 pc - a few kpc) distortion of the underlying potential seems to be necessary for Seyfert activity, although it is not a sufficient condition. We also find that the Toomres Q values in the centers of Seyfert galaxies tend to be larger than unity, indicating the circumnuclear molecular gas disks around Seyfert nuclei would be gravitationally stable. The HCN/CO integrated intensity ratios (R_HCN/CO) range over an order of magnitude, from 0.086 to 0.6. The Seyfert galaxies with high R_HCN/CO may have an extended (r ~ 100 pc scale) envelope of obscuring material. The presence of kpc scale jet/ outflow might be also related to the extremely high R_HCN/CO.
We present the first results of a high-resolution Karl G. Jansky Very Large Array (VLA) imaging survey of luminous and ultra-luminous infrared galaxies (U/LIRGs) in the Great Observatories All-Sky LIRG Survey (GOALS). From the full sample of 68 galaxies, we have selected 25 LIRGs that show resolved extended emission at sufficient sensitivity to image individual regions of star-formation activity beyond the nucleus.~With wideband radio continuum observations, which sample the frequency range from $3-33$ GHz, we have made extinction-free measurements of the luminosities and spectral indicies for a total of 48 individual star-forming regions identified as having de-projected galactocentric radii ($r_{G}$) that lie outside the 13.2$mu$m core of the galaxy.~The median $3-33$ GHz spectral index and 33 GHz thermal fraction measured for these extranuclear regions is $-0.51 pm 0.13$ and $65 pm 11%$ respectively.~These values are consistent with measurements made on matched spatial scales in normal star-forming galaxies, and suggests that these regions are more heavily-dominated by thermal free-free emission relative to the centers of local ULIRGs.~Further, we find that the median star-formation rate derived for these regions is $sim 1 M_{odot}$ yr$^{-1}$, and when we place them on the sub-galactic star-forming main sequence of galaxies (SFMS), we find they are offset from their host galaxies globally-averaged specific star-formation rates (sSFRs).~We conclude that while nuclear starburst activity drives LIRGs above the SFMS, extranuclear star-formation still proceeds in a more extreme fashion relative to what is seen in local spiral galaxies.
In this paper we present results from an IRAM Plateau de Bure millimetre-wave Interferometer (PdBI) survey for CO emission towards radio-detected submillimetre galaxies (SMGs) with known optical and near-infrared spectroscopic redshifts. Five sources in the redshift range z~1-3.5 were detected, nearly doubling the number of SMGs detected in CO. We summarise the properties of all 12 CO-detected SMGs, as well as 6 sources not detected in CO by our survey, and use this sample to explore the bulk physical properties of the SMG population as a whole. The median CO line luminosity of the SMGs is <L_CO> = (3.8 +- 2.0) x 10^10 K km/s pc^2. Using a CO-to-H_2 conversion factor appropriate for starburst galaxies, this corresponds to a molecular gas mass <M(H_2)> = (3.0 +- 1.6) x 10^10 Msun within a ~2kpc radius, about four times greater than the most luminous local ultraluminous infrared galaxies (ULIRGs) but comparable to that of the most extreme high-redshift radio galaxies and QSOs. The median CO fwhm linewidth is broad, <fwhm> = 780 +- 320 km/s, and the SMGs often have double peaked line profiles, indicative of either a merger or a disk. From their median gas reservoirs (~3 x 10^10 Msun) and star-formation rates (>700 Msun/yr) we estimate a lower limit on the typical gas-depletion time scale of >40Myr in SMGs. This is marginally below the typical age expected for the starbursts in SMGs, and suggests that negative feedback processes may play an important role in prolonging the gas consumption time scale. We find a statistically-significant correlation between the far-infrared and CO luminosities of the SMGs which extends the observed correlation for local ULIRGs to higher luminosities and higher redshifts. [ABRIDGED]
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