A 4-Year Longitudinal Neuroimaging Study of Cognitive Control Using Latent Growth Modeling: Developmental Changes and Brain-Behavior Associations

Despite theoretical models suggesting developmental changes in neural substrates of cognitive control in adolescence, empirical research has rarely examined intraindividual changes in cognitive control-related brain activation using multi-wave multivariate longitudinal data. We used longitudinal repeated measures of brain activation and behavioral performance during the multi-source interference task (MSIT) from 167 adolescents (53% male) who were assessed annually over four years from ages 13 to 17 years. We applied latent growth modeling to delineate the pattern of brain activation changes over time and to examine longitudinal associations between brain activation and behavioral performance. We identified brain regions that showed differential change patterns: (1) the fronto-parietal regions that involved bilateral insula, bilateral middle frontal gyrus, left pre-supplementary motor area, left inferior parietal lobule, and right precuneus; and (2) the rostral anterior cingulate cortex (rACC) region. Longitudinal confirmatory factor analyses of the fronto-parietal regions revealed strong measurement invariance across time implying that multivariate functional magnetic resonance imaging data during cognitive control can be measured reliably over time. Latent basis growth models indicated that fronto-parietal activation decreased over time, whereas rACC activation increased over time. In addition, behavioral performance data, age-related improvement was indicated by a decreasing trajectory of intraindividual variability in response time across four years. Testing longitudinal brain-behavior associations using multivariate growth models revealed that better behavioral cognitive control was associated with lower fronto-parietal activation, but the change in behavioral performance was not related to the change in brain activation. The current findings suggest that reduced effects of cognitive interference indicated by fronto-parietal recruitment may be a marker of a maturing brain that underlies better cognitive control performance during adolescence

Mid-Infrared Properties of Luminous Infrared Galaxies II: Probing the Dust and Gas Physics of the GOALS Sample

The Great Observatories All-Sky LIRG Survey (GOALS) is a comprehensive, multiwavelength study of luminous infrared galaxies (LIRGs) in the local universe. Here we present the results of a multi-component, spectral decomposition analysis of the low resolution mid-IR Spitzer IRS spectra from 5-38um of 244 LIRG nuclei. The detailed fits and high quality spectra allow for characterization of the individual PAH features, warm molecular hydrogen emission, and optical depths for silicate dust grains and water ices. We find that starbursting LIRGs, which make up the majority of GOALS, are very consistent in their MIR properties (i.e. tau_9.7um, tau_ice, neon line and PAH feature ratios). However, as their PAH EQW decreases, usually an indicator of an increasingly dominant AGN, LIRGs cover a larger spread in these MIR parameters. The contribution from PAHs to the total L(IR) in LIRGs varies from 2-29% and LIRGs prior to their first encounter show higher L(PAH)/L(IR) ratios on average. We observe a correlation between the strength of the starburst (IR8) and the PAH fraction at 8um but not with the 7.7 to 11.3 PAH ratio, suggesting the fractional PDR emission, and not the overall grain properties, is associated with the rise in IR8 for galaxies off the starburst main sequence. We detect crystalline silicate features in 6% of the sample but only in the most obscured sources (s_9.7um < -1.24). Ice absorption features are observed in 11% (56%) of GOALS LIRGs (ULIRGs). Most GOALS LIRGs have L(H2)/L(PAH) ratios elevated above those observed for normal star-forming galaxies and exhibit a trend for increasing L(H2)/L(PAH) ratio with increasing L(H2). While star formation appears to be the dominant process responsible for exciting the H2 in most of the GOALS galaxies, a subset of LIRGs (10%) show excess H2 emission that is inconsistent with PDR models and may be excited by shocks or AGN-induced outflows.Comment: 21 pages with 20 figures plus 2 table

Spitzer Quasar and ULIRG Evolution Study (QUEST). IV. Comparison of 1-Jy Ultraluminous Infrared Galaxies with Palomar-Green Quasars

We report the results from a comprehensive study of 74 ultraluminous infrared galaxies (ULIRGs) and 34 Palomar-Green (PG) quasars within z ~ 0.3$ observed with the Spitzer Infrared Spectrograph (IRS). The contribution of nuclear activity to the bolometric luminosity in these systems is quantified using six independent methods that span a range in wavelength and give consistent results within ~ +/-10-15% on average. The average derived AGN contribution in ULIRGs is ~35-40%, ranging from ~15-35% among "cool" (f_25/f_60 =< 0.2) optically classified HII-like and LINER ULIRGs to ~50 and ~75% among warm Seyfert 2 and Seyfert 1 ULIRGs, respectively. This number exceeds ~80% in PG QSOs. ULIRGs fall in one of three distinct AGN classes: (1) objects with small extinctions and large PAH equivalent widths are highly starburst-dominated; (2) systems with large extinctions and modest PAH equivalent widths have larger AGN contributions, but still tend to be starburst-dominated; and (3) ULIRGs with both small extinctions and small PAH equivalent widths host AGN that are at least as powerful as the starbursts. The AGN contributions in class 2 ULIRGs are more uncertain than in the other objects, and we cannot formally rule out the possibility that these objects represent a physically distinct type of ULIRGs. A morphological trend is seen along the sequence (1)-(2)-(3), in general agreement with the standard ULIRG - QSO evolution scenario and suggestive of a broad peak in extinction during the intermediate stages of merger evolution. However, the scatter in this sequence, implies that black hole accretion, in addition to depending on the merger phase, also has a strong chaotic/random component, as in local AGN. (abridged)Comment: 61 pages, 39 figures, 16 tables, accepted for publication in ApJS, June 2009 issue. Unabbreviated version can be found at http://www.astro.umd.edu/~veilleux/pubs/quest4.pd

Mid-Infrared Spectroscopic Properties of Ultra-Luminous Infrared Quasars

We analyse mid-infrared (MIR) spectroscopic properties for 19 ultra-luminous infrared quasars (IR QSOs) in the local universe based on the spectra from the Infrared Spectrograph on board the Spitzer Space Telescope. The MIR properties of IR QSOs are compared with those of optically-selected Palomar-Green QSOs (PG QSOs) and ultra-luminous infrared galaxies (ULIRGs). The average MIR spectral features from ~ 5 to 30um, including the spectral slopes, 6.2um PAH emission strengths and [NeII] 12.81um luminosities of IR QSOs, differ from those of PG QSOs. In contrast, IR QSOs and ULIRGs have comparable PAH and [NeII] luminosities. These results are consistent with IR QSOs being at a transitional stage from ULIRGs to classical QSOs. We also find that the colour index alpha(30, 15) is a good indicator of the relative contribution of starbursts to AGNs for all QSOs. Correlations between the [NeII] 12.81um and PAH 6.2um luminosities and those between the [NeII], PAH with 60um luminosities for ULIRGs and IR QSOs indicate that both [NeII] and PAH luminosities are approximate star formation rate indicators for IR QSOs and starburst-dominated galaxies; the scatters are, however, quite large (~ 0.7 to 0.8 dex). Finally the correlation between the EW(PAH 6.2um) and outflow velocities suggests that star formation activities are suppressed by feedback from AGNs and/or supernovae.Comment: 19 pages, 8 figures, 6 tables, accepted for publication in MNRA

Spitzer and ISO Galaxy Counts in the Mid-Infrared

Galaxy source counts that simultaneously fit the deep mid-infrared surveys at 24 microns and 15 microns made by the Spitzer Space Telescope and the Infrared Space Observatory (ISO) respectively are presented for two phenomenological models. The models are based on starburst and luminous infrared galaxy dominated populations. Both models produce excellent fits to the counts in both wavebands and provide an explanation for the high redshift population seen in the longer Spitzer 24 micron band supporting the hypothesis that they are luminous-ultraluminous infrared galaxies at z=2-3, being the mid-infrared counterparts to the sub-mm galaxy population. The source counts are characterized by strong evolution to redshift unity, followed by less drastic evolution to higher redshift. The number-redshift distributions in both wavebands are well explained by the effect of the many mid-infrared features passing through the observation windows. The sharp upturn at around a milliJansky in the 15 micron counts in particular depends critically on the distribution of mid-infrared features around 12 microns, in the assumed spectral energy distribution.Comment: 6 pages, 2 figures, accepted for publication MNRA

Mid-Infrared Spectral Diagnostics of Luminous Infrared Galaxies

We present a statistical analysis of the mid-infrared (MIR) spectra of 248 luminous infrared (IR) galaxies (LIRGs) which comprise the Great Observatories All-sky LIRG Survey (GOALS) observed with the Infrared Spectrograph (IRS) on-board the Spitzer Space Telescope. The GOALS sample enables a direct measurement of the relative contributions of star-formation and active galactic nuclei (AGN) to the total IR emission from a large sample of local LIRGs. The AGN contribution to the MIR emission (f-AGN) is estimated by employing several diagnostics based on the properties of the [NeV], [OIV] and [NeII] fine structure gas emission lines, the 6.2 microns PAH and the shape of the MIR continuum. We find that 18% of all LIRGs contain an AGN and that in 10% of all sources the AGN contributes more than 50% of the total IR luminosity. Summing up the total IR luminosity contributed by AGN in all our sources suggests that AGN supply ~12% of the total energy emitted by LIRGs. The average spectrum of sources with an AGN looks similar to the average spectrum of sources without an AGN, but it has lower PAH emission and a flatter MIR continuum. AGN dominated LIRGs have higher IR luminosities, warmer MIR colors and are found in interacting systems more often than pure starbursts LIRGs. However we find no linear correlations between these properties and f-AGN. We used the IRAC colors of LIRGs to confirm that finding AGN on the basis of their MIR colors may miss ~40% of AGN dominated (U)LIRGsComment: accepted for publication in ApJ, 34 pages, 12 figure

AKARI Near-Infrared Spectroscopy of Luminous Infrared Galaxies

We present the AKARI near-infrared (NIR; 2.5-5 micron) spectroscopic study of 36 (ultra)luminous infrared galaxies [(U)LIRGs] at z=0.01-0.4. We measure the NIR spectral features including the strengths of 3.3 micron polycyclic aromatic hydrocarbon (PAH) emission and hydrogen recombination lines (Br\alpha. and Br\beta), optical depths at 3.1 and 3.4 micron, and NIR continuum slope. These spectral features are used to identify optically elusive, buried AGN. We find that half of the (U)LIRGs optically classified as non-Seyferts show AGN signatures in their NIR spectra. Using a combined sample of (U)LIRGs with NIR spectra in the literature, we measure the contribution of buried AGN to the infrared luminosity from the SED-fitting to the IRAS photometry. The contribution of these buried AGN to the infrared luminosity is 5-10%, smaller than the typical AGN contribution of (U)LIRGs including Seyfert galaxies (10-40%). We show that NIR continuum slopes correlate well with WISE [3.4]-[4.6] colors, which would be useful for identifying a large number of buried AGN using the WISE data.Comment: ApJ, accepted. 37 pages, 11 figure

H_2 emission arises outside photodissociation regions in ultra-luminous infrared galaxies

Ultra-luminous infrared galaxies are among the most luminous objects in the local universe and are thought to be powered by intense star formation. It has been shown that in these objects the rotational spectral lines of molecular hydrogen observed at mid-infrared wavelengths are not affected by dust obscuration, leaving unresolved the source of excitation of this emission. Here I report an analysis of archival Spitzer Space Telescope data on ultra-luminous infrared galaxies and demonstrate that star formation regions are buried inside optically thick clouds of gas and dust, so that dust obscuration affects star-formation indicators but not molecular hydrogen. I thereby establish that the emission of H_2 is not co-spatial with the buried starburst activity and originates outside the obscured regions. This is rather surprising in light of the standard view that H_2 emission is directly associated with star-formation activity. Instead, I propose that H_2 emission in these objects traces shocks in the surrounding material, which are in turn excited by interactions with nearby galaxies, and that powerful large-scale shocks cooling by means of H_2 emission may be much more common than previously thought. In the early universe, a boost in H_2 emission by this process may speed up the cooling of matter as it collapsed to form the first stars and galaxies and would make these first structures more readily observable.Comment: Main text and supplemental information, 21 pages including 6 figures, 2 table

HERUS: the far-IR/submm spectral energy distributions of local ULIRGs and photometric atlas

We present the Herschel-SPIRE photometric atlas for a complete flux limited sample of 43 local ultraluminous infrared galaxies (ULIRGs), selected at 60 μm by IRAS, as part of the HERschel ULIRG Survey (HERUS). Photometry observations were obtained using the SPIRE instrument at 250, 350, and 500 μm. We describe these observations, present the results, and combine the new observations with data from IRAS to examine the far-infrared spectral energy distributions (SEDs) of these sources. We fit the observed SEDs of HERUS objects with a simple parametrized modified blackbody model, where temperature and emissivity β are free parameters. We compare the fitted values to those of non-ULIRG local galaxies, and find, in agreement with earlier results, that HERUS ULIRGs have warmer dust (median temperature T = 37.9 ± 4.7 K compared to 21.3 ± 3.4 K) but a similar β distribution (median β = 1.7 compared to 1.8) to the Herschel reference sample (HRS, Cortese et al. 2014) galaxies. Dust masses are found to be in the range of 107.5–109 M⊙, significantly higher than that of HRS sources. We compare our results for local ULIRGs with higher redshift samples selected at 250 and 850 μm. These latter sources generally have cooler dust and/or redder 100-to-250  μm colours than our 60 μm-selected ULIRGs. We show that this difference may in part be the result of the sources being selected at different wavelengths rather than being a simple indication of rapid evolution in the properties of the population

Exploring the active galactic nucleus and starburst content of local ultraluminous infrared galaxies through 5-8 micron spectroscopy

We present a 5-8 micron analysis of the Spitzer-IRS spectra of 71 ultraluminous infrared galaxies (ULIRGs) with redshift z < 0.15, devoted to the study of the role of active galactic nuclei (AGN) and starbursts (SB) as the power source of the extreme infrared emission. Around 5 micron an AGN is much brighter (by a factor 30) than a starburst of equal bolometric luminosity. This allows us to detect the presence of even faint accretion-driven cores inside ULIRGs: signatures of AGN activity are found in 70 per cent of our sample (50/71 sources). Through a simple analytical model we are also able to obtain a quantitative estimate of the AGN/SB contribution to the overall energy output of each source. Although the main fraction of ULIRG luminosity is confirmed to arise from star formation events, the AGN contribution is non-negligible (23 per cent) and is shown to increase with luminosity. The existence of a rather heterogeneous pattern in the composition and geometrical structure of the dust among ULIRGs is newly supported by the comparison between individual absorption features and continuum extinction.Comment: 56 pages, 13 figures, 4 tables. Accepted for publication in MNRA