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Register a new userLocal Benchmarks for the Evolution of Major-Merger Galaxies -- Spitzer Observations of a K-Band Selected Sample
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We present Spitzer observations for a sample of close major-merger galaxy pairs (KPAIR sample) selected from 2MASS/SDSS-DR3 cross-matches. The goals are to study the star formation activity in these galaxies and to set a local bench mark for the cosmic evolution of close major mergers. The Spitzer KPAIR sample (27 pairs, 54 galaxies) includes all spectroscopically confirmed S+S and S+E pairs in a parent sample that is complete for primaries brighter than K=12.5 mag, projected separations of 5< s < 20 kpc/h, and mass ratios<2.5. The Spitzer data consist of images in 7 bands (3.6, 4.5, 5.8, 8, 24, 70, 160 um). Compared to single spiral galaxies in a control sample, only spiral galaxies in S+S pairs show significantly enhanced specific star formation rate (sSFR=SFR/M), whereas spiral galaxies in S+E pairs do not. Furthermore, the SFR enhancement of spiral galaxies in S+S pairs is highly mass-dependent. Only those with $rm M gsim 10^{10.5} M_sun$ show significant enhancement. Relatively low mass ($rm M sim 10^{10} M_sun$) spirals in S+S pairs have about the same SFR/M compared to their counterparts in the control sample. There is evidence for a correlation between the global star formation activities (but not the nuclear activities) of the component galaxies in massive S+S major-merger pairs (the Holmberg effect). There is no significant difference in the SFR/M between the primaries and the secondaries, nor between spirals of SEP<1 and those of SEP.1. The contribution of KPAIR galaxies to the cosmic SFR density in the local universe is only 1.7%.
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We present long-slit integrated spectroscopy of 238 late-type galaxies belonging to the Herschel Reference Survey, a volume limited sample representative of the nearby universe. This sample has a unique legacy value since ideally defined for any statistical study of the multifrequency properties of galaxies spanning a large range in morphological type and luminosity. The spectroscopic observations cover the spectral range 3600-6900 A at a resolution R ~ 1000 and are thus suitable for separating the underlying absorption from the emission of the Hbeta line as well as the two [NII] lines from the Halpha emission. We measure the fluxes and the equivalent widths of the strongest emission lines ([OII], Hbeta, [OIII], [NII], Halpha, and [SII]). The data are used to study the distribution of the equivalent width of all the emission lines, of the Balmer decrement C(Hbeta) and of the observed underlying Balmer absorption under Hbeta in this sample. Combining these new spectroscopic data with those available at other frequencies, we also study the dependence of C(Hbeta) and E.W.Hbeta_{abs} on morphological type, stellar mass and stellar surface density, star formation rate, birthrate parameter and metallicity in galaxies belonging to different environments (fields vs. Virgo). The distribution of the equivalent width of all the emission lines, of C(Hbeta) and E.W.Hbeta_{abs} are systematically different in cluster and field galaxies. The Balmer decrement increases with stellar mass, stellar surface density, metallicity and star formation rate of the observed galaxies, while it is unexpectedly almost independent from the column density of the atomic and molecular gas. The dependence of C(Hbeta) on stellar mass is steeper than that previously found in other works. The underlying Balmer absorption does not significantly change with any of these physical parameters.
(abridged)The majority of Active Galactic Nuclei (AGN) suffer from significant obscuration by surrounding dust and gas. X-ray surveys in the 2-10 keV band will miss the most heavily-obscured AGN in which the absorbing column density exceeds $sim10^{24}$cm$^{-2}$ (the Compton-thick AGN). It is therefore vital to know the fraction of AGN that are missed in such X-rays surveys and to determine if these AGN represent some distinct population in terms of the fundamental properties of AGN and/or their host galaxies. In this paper we present the analysis of textit{XMM-Newton} X-ray data for a complete sample of 17 low-redshift Type 2 Seyfert galaxies chosen from the Sloan Digital Sky Survey based solely on the high observed flux of the [OIII]$lambda$5007 emission-line. This line is formed in the Narrow Line Region hundreds of parsecs away from the central engine. Thus, unlike the X-ray emission, it is not affected by obscuration due to the torus surrounding the black hole. It therefore provides a useful isotropic indicator of the AGN luminosity. As additional indicators of the intrinsic AGN luminosity, we use the Spitzer Space Telescope to measure the luminosities of the mid-infrared continuum and the [OIV]25.89$mu$m narrow emission-line. We then use the ratio of the 2-10 keV X-ray luminosity to the [OIII], [OIV], and mid-infrared luminosities to assess the amount of X-ray obscuration and to distinguish between Compton-thick and Compton-thin objects. We find that the majority of the sources suffer significant amounts of obscuration: the observed 2-10 keV emission is depressed by more than an order-of-magnitude in 11 of the 17 cases (as expected for Compton-thick sources).
We present the results of a study on the properties and evolution of massive (M_* > 10^11 M_0) galaxies at z~0.4 - 2 utilising Keck spectroscopy, near-Infrared Palomar imaging, and Hubble, Chandra, and Spitzer data covering fields targeted by the DEEP2 galaxy spectroscopic survey. Our sample is K band selected based on wide-area NIR imaging from the Palomar Observatory Wide-Field Infrared Survey, which covers 1.53 deg^2 to K_s,vega~20.5. Our major findings include: (i) statistically the mass and number densities of M_* > 10^11 M_0 galaxies show little evolution between z = 0 - 1, and from z ~ 0 - 2 for M_* > 10^11.5 M_0 galaxies. (ii) Using Hubble ACS imaging, we find that M_* > 10^11 selected galaxies show a nearly constant elliptical fraction of ~70-90% at all redshifts. The remaining objects are peculiars possibly undergoing mergers at z > 0.8, while spirals dominate the remainder at lower redshifts. (iii) We find that only a fraction (~60%) of massive galaxies with M_* > 10^11 M_0 are on the red-sequence at z~1.4, while nearly 100% evolve onto it by z~0.4. (iv) By utilising Spitzer MIPS imaging and [OII] line fluxes we argue that M_* > 10^11.5 galaxies have a steeply declining star formation rate density ~(1+z)^6. By examining the contribution of star formation to the evolution of the mass function, as well as the merger history through the CAS parameters, we determine that M_* >10^11 M_0 galaxies undergo on average 0.9^+0.7_-0.5 major mergers at 0.4 < z < 1.4. (v) A high (5%) fraction of all M_* > 10^11 M_0 galaxies are X-ray emitters. Roughly half of these are morphologically distorted ellipticals or peculiars. We compare our results with the Millennium simulation, finding that the number and mass densities of M_* > 10^11.5 M_0 galaxies are under predicted by a factor of > 100.
We discuss infrared Spitzer observations of early type galaxies in the SAURON sample at 24, 60 and 170 microns. When compared with 2MASS Ks band luminosities, lenticular (S0) galaxies exhibit a much wider range of mid to far-infrared luminosities then elliptical (E) galaxies. Mid and far-infrared emission from E galaxies is a combination of circumstellar or interstellar emission from local mass-losing red giant stars, dust buoyantly transported from the galactic cores into distant hot interstellar gas and dust accreted from the environment. The source of mid and far-IR emission in S0 galaxies is quite different and is consistent with low levels of star formation, 0.02 - 0.2 Msol/yr, in cold, dusty gaseous disks. The infrared 24micron-70micron color is systematically lower for (mostly S0) galaxies with known molecular disks. Our observations support the conjecture that cold dusty gas in some S0 galaxies is created by stellar mass loss at approximately the same rate that it is consumed by star formation, so the mass depletion of these disks by star formation will be slow. Unlike E galaxies, the infrared luminosities of S0 galaxies correlate with both the mass of molecular gas and the stellar Hbeta spectral index, and all are related to the recent star formation rate. However, star formation rates estimated from the Hbeta emission line luminosities L_{Hbeta} in SAURON S0 galaxies are generally much smaller. Since L_{Hbeta} does not correlate with 24 microns emission from dust heated by young stars, optical emission lines appear to be a poor indicator of star formation rates in SAURON S0 galaxies. The absence of Hbeta emission may be due to a relative absence of OB stars in the initial mass function or to dust absorption of Hbeta emission lines.
Direct submm imaging has recently revealed the 850-micron background to be mostly composed of a population of distant ultraluminous infrared galaxies, but identifying the optical/NIR counterparts to these sources has proved difficult due to the poor submm spatial resolution. However, the proportionality of both cm and submm data to the star formation rate suggests that high resolution radio continuum maps with subarcsecond positional accuracy can be exploited to locate submm sources. In this paper we present results from a targeted SCUBA survey of micro-Jansky radio sources in the flanking fields of the Hubble Deep Field. Even with relatively shallow 850-micron SCUBA observations (>6 mJy at 3-sigma), we were successful at making submm detections of optical/NIR-faint (I>24 and K~21-22) radio sources, and our counts closely match the bright counts from submm surveys. Redshift estimates can be made from the ratio of the submm flux to the radio flux across the 100 GHz break in the spectral energy distribution. This millimetric redshift estimation places the bright submm population at z=1-3 where it forms the high redshift tail of the faint radio population. The star formation rate density (SFRD) due to ultraluminous infrared galaxies increases by more than two orders of magnitude from z~0 to z~1-3. The SFRD at high redshift inferred from our >6 mJy submm observations is comparable to that observed in the UV/optical. (Abridged)
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