Fuel Star Formation Research Articles

Molecular Gas in Infrared-Excess, Optically Selected and the Quasars Connection with Infrared-Luminous Galaxies

The initial results of a millimeter (CO) survey of infrared-excess, optically selected quasars from the Palomar-Green (PG) Bright Quasar Survey with redshifts in the range 0.04 0.36, in which the contribution to the bolometric luminosity of infrared thermal dust emission for all PG QSOs is typically 20%–40%. Six out of ten of the QSOs observed are detected in the CO (1 → 0) emission line; two detections confirm previous, less sensitive detections of CO (1 → 0) in PG 1613+658 and 0838+770, and four additional QSOs are detected for the first time (PG 1119+120, 1351+640, 1415+451, and 1440+356). These six detections, plus two previous detections of CO in I Zw1 and Mrk 1014, bring the total number of 0.04 < z < 0.17 infrared-excess PG QSOs detected in CO to date to eight and provide possible evidence that, in addition to fueling star formation, molecular gas may also serve as a primary source of fuel for QSO activity. Both the eight QSOs detected in CO and the four QSOs with nondetections have high infrared-to-CO luminosity ratios, LIR/L, relative to most infrared luminous galaxies of the same LIR. The placement of these QSOs on the LIR/L–LIR plane may be due to significant contributions from dust heated by the QSO in their host galaxies, due to dust heated by massive stars formed with high efficiency (i.e., per unit molecular gas mass) relative to most infrared luminous galaxies, or a combination of both. If the observed high values of LIR/L are primarily due to dust heating by QSOs, a significant fraction of ULIGs with similar values of LIR/L may also contain buried active galactic nuclei. Alternatively, if high LIR/L is due primarily to star formation, then an enhanced star formation rate may be intimately connected to the QSO phenomenon. A comparison of the infrared and CO luminosities of the eight detected and four undetected QSOs with the optical morphologies of their host galaxies shows that the three QSOs with LIR and L similar to ULIGs appear to reside in morphologically disturbed galaxies (i.e., ongoing major mergers involving two or more gas-rich disk galaxies), whereas the host galaxies of the remaining eight QSOs with lower LIR and L appear to be a mixture of barred spiral host galaxies, elliptical galaxies, galaxies with an indeterminate classification, and at least one ongoing major merger.

A massive reservoir of low-excitation molecular gas at high redshift.

Molecular hydrogen (H2) is an important component of galaxies because it fuels star formation and the accretion of gas onto active galactic nuclei (AGN), the two processes that can generate the large infrared luminosities of gas-rich galaxies. Observations of spectral-line emission from the tracer molecule carbon monoxide (CO) are used to probe the properties of this gas. But the lines that have been studied in the local Universe-mostly the lower rotational transitions of J = 1 --> 0 and J = 2 --> 1-have hitherto been unobservable in high-redshift galaxies. Instead, higher transitions have been used, although the densities and temperatures required to excite these higher transitions may not be reached by much of the gas. As a result, past observations may have underestimated the total amount of molecular gas by a substantial amount. Here we report the discovery of large amounts of low-excitation molecular gas around the infrared-luminous quasar APM08279+5255 at redshift z = 3.91, using the two lowest excitation lines of 12 CO (J = 1 --> 0 and J = 2 --> 1). The maps confirm the presence of hot and dense gas near the nucleus, and reveal an extended reservoir of molecular gas with low excitation that is 10 to 100 times more massive than the gas traced by the higher-excitation observations. This raises the possibility that significant amounts of low-excitation molecular gas may exist in the environments of high-redshift (z > 3) galaxies.

The Origin of Star Formation Gradients in Rich Galaxy Clusters

We examine the origin of clustercentric gradients in the star formation rates and colors of rich cluster galaxies within the context of a simple model where clusters are built through the ongoing accretion of field galaxies. The model assumes that after galaxies enter the cluster their star formation rates decline on a timescale of a few gigayears, the typical gas consumption timescale of disk galaxies in the field. Such behavior might be expected if tides and ram pressure strip off the gaseous envelopes that normally fuel star formation in spirals over a Hubble time. Combining these timescales with mass accretion histories derived from N-body simulations of cluster formation in a ΛCDM universe, we reproduce the systematic differences observed in the color distribution of cluster and field galaxies, as well as the strong suppression of star formation in cluster galaxies and its dependence on clustercentric radius. The simulations also indicate that a significant fraction of galaxies beyond the virial radius of the cluster may have been within the main body of the cluster in the past, a result that explains naturally why star formation in the outskirts of clusters (and as far out as 2 virial radii) is systematically suppressed relative to the field. The agreement with the data beyond the cluster virial radius is also improved if we assume that stripping happens within lower mass systems, before the galaxy is accreted into the main body of the cluster. We conclude that the star formation rates of cluster galaxies depend primarily on the time elapsed since their accretion onto massive virialized systems and that the cessation of star formation may have taken place gradually over a few gigayears.

A molecular gas bar fuelling starburst activity in galaxy IC342

STARBURST galaxies are those that are presently forming stars at a higher rate than that averaged over the galactic lifetime. Starburst galaxies usually exhibit strong CO line emission, indicating that they contain large amounts of molecular gas which fuels star formation. We have mapped CO(J = 1 → 0) line emission from the central region of a late-type spiral galaxy IC342 using the Nobeyama Millimetre Array at a resolution of 2.4". We find a pair of narrow ridges of molecular gas, offset and forming a ring-like structure in the galactic nucleus; the motion of the gas in these bars is highly non-circular. This structure is very similar to that of the gas in the elliptical gravitational field of barred spiral galaxies. Strong radio continuum emission in the nuclear region suggests that it may be a site of starburst activity; if so, it is possible that the activity is being fuelled by the molecular gas bars.