[en] Combined Spitzer, Chandra, XMM-Newton, and VLA observations of the giant radio galaxy NGC 1316 (Fornax A) show a radio jet and X-ray cavities from active galactic nucleus (AGN) outbursts most likely triggered by a merger with a late-type galaxy at least 0.4 Gyr ago. We detect a weak nucleus with a spectral energy distribution typical of a low-luminosity AGN with a bolometric luminosity of 2.4 x 10⁴² erg s^-1. We examine the Spitzer IRAC and MIPS images of NGC 1316. We find that the dust emission is strongest in regions with little or no radio emission and that the particularly large infrared luminosity relative to the galaxy's K-band luminosity implies an external origin for the dust. The inferred dust mass implies that the merger spiral galaxy had a stellar mass of (1-6) x10¹⁰ M_sun and a gas mass of (2-4) x10⁹ M_sun. X-ray cavities in the Chandra and XMM-Newton images likely result from the expansion of relativistic plasma ejected by the AGN. The soft (0.5-2.0 keV) Chandra images show a small ∼15'' (1.6 kpc) cavity coincident with the radio jet, while the XMM-Newton image shows two large X-ray cavities lying 320'' (34.8 kpc) east and west of the nucleus, each approximately 230'' (25 kpc) in radius. Current radio observations do not show emission within these cavities. The radio lobes lie at radii of 14.'3 (93.3 kpc) and 15.'6 (101 kpc), more distant from the nucleus than the detected X-ray cavities. The relative morphology of the large scale 1.4 GHz and X-ray emission suggests they were products of two distinct outbursts, an earlier one creating the radio lobes and a later one producing the X-ray cavities. Alternatively, if a single outburst created both the X-ray cavities and the radio lobes, this would require that the radio morphology is not fully defined by the 1.4 GHz emission. For the more likely two outbursts scenarios, we use the buoyancy rise times to estimate an age for the more recent outburst that created the X-ray cavities of 0.1 Gyr and the PV work done by the expanding plasma that created the X-ray cavities to estimate the outburst's energy to be 10⁵⁸ erg. The present size and location of the radio lobes imply that the outburst that created them happened ∼0.4 Gyr ago and released ∼5 x 10⁵⁸ erg.

[en] One of the most challenging aspects of studying galaxies in the $z\gtrsim <!--\; ???\; -->7$ universe is the infrequent confirmation of their redshifts through spectroscopy, a phenomenon thought to occur from the increasing opacity of the intergalactic medium to Lyα photons at $z><!--\; >\; -->6.5$. The resulting redshift uncertainties inhibit the efficient search for [C ii] in $z\sim <!--\; ???\; -->7$ galaxies with sub-millimeter instruments such as ALMA, given their limited scan speed for faint lines. One means by which to improve the precision of the inferred redshifts is to exploit the potential impact of strong nebular emission lines on the colors of z ∼ 4 – 8 galaxies as observed by Spitzer/IRAC. At $z\sim <!--\; ???\; -->6.8$, galaxies exhibit IRAC colors as blue as $[3.6]-<!--\; ???\; -->[4.5]\sim <!--\; ???\; -->-<!--\; ???\; -->1$, likely due to the contribution of [O iii]+Hβ to the 3.6 μm flux combined with the absence of line contamination in the 4.5 μm band. In this paper we explore the use of extremely blue $[3.6]-<!--\; ???\; -->[4.5]$ colors to identify galaxies in the narrow redshift window $z\sim <!--\; ???\; -->$ 6.6 – 6.9. When combined with an I-dropout criterion, we demonstrate that we can plausibly select a relatively clean sample of $z\sim <!--\; ???\; -->6.8$ galaxies. Through a systematic application of this selection technique to our catalogs from all five CANDELS fields, we identify 20 probable $z\sim <!--\; ???\; -->$ 6.6 – 6.9 galaxies. We estimate that our criteria select the ∼50% strongest line emitters at $z\sim <!--\; ???\; -->6.8$ and from the IRAC colors we estimate a typical [O iii]$+\mathrm{H}\mathrm{\beta <!--\; ??\; -->}$ rest-frame equivalent width of 1085 Å for this sample. The small redshift uncertainties on our sample make it particularly well suited for follow-up studies with facilities such as ALMA.

[en] We present a study of a large, statistically complete sample of star-forming dwarf galaxies using mid-infrared observations from the Spitzer Space Telescope. The relationships between metallicity, star formation rate (SFR), and mid-infrared color in these systems show that the galaxies span a wide range of properties. However, the galaxies do show a deficit of 8.0 μm polycyclic aromatic hydrocarbon emission as is apparent from the median 8.0 μm luminosity which is only 0.004 L*_8.0 while the median B-band luminosity is 0.05 L*_B. Despite many of the galaxies being 8.0 μm deficient, there is about a factor of 4 more extremely red galaxies in the [3.6]-[8.0] color than for a sample of normal galaxies with similar optical colors. We show correlations between the [3.6]-[8.0] color and luminosity, metallicity, and to a lesser extent SFRs that were not evident in the original, smaller sample studied previously. The luminosity-metallicity relation has a flatter slope for dwarf galaxies as has been indicated by previous work. We also show a relationship between the 8.0 μm luminosity and the metallicity of the galaxy which is not expected given the competing effects (stellar mass, stellar population age, and the hardness of the radiation field) that influence the 8.0 μm emission. This larger sample plus a well-defined selection function also allows us to compute the 8.0 μm luminosity function and compare it with the one for the local galaxy population. Our results show that below 10⁹ L_sun, nearly all the 8.0 μm luminosity density of the local universe arises from dwarf galaxies that exhibit strong Hα emission-i.e., 8.0 μm and Hα selection identify similar galaxy populations despite the deficit of 8.0 μm emission observed in these dwarfs.

[en] The recent advent of integral field spectrographs and millimeter interferometers has revealed the internal dynamics of many hundreds of star-forming galaxies. Spatially resolved kinematics have been used to determine the dynamical status of star-forming galaxies with ambiguous morphologies, and constrain the importance of galaxy interactions during the assembly of galaxies. However, measuring the importance of interactions or galaxy merger rates requires knowledge of the systematics in kinematic diagnostics and the visible time with merger indicators. We analyze the dynamics of star-forming gas in a set of binary merger hydrodynamic simulations with stellar mass ratios of 1:1 and 1:4. We find that the evolution of kinematic asymmetries traced by star-forming gas mirrors morphological asymmetries derived from mock optical images, in which both merger indicators show the largest deviation from isolated disks during strong interaction phases. Based on a series of simulations with various initial disk orientations, orbital parameters, gas fractions, and mass ratios, we find that the merger signatures are visible for ∼0.2–0.4 Gyr with kinematic merger indicators but can be approximately twice as long for equal-mass mergers of massive gas-rich disk galaxies designed to be analogs of z ∼ 2–3 submillimeter galaxies. Merger signatures are most apparent after the second passage and before the black holes coalescence, but in some cases they persist up to several hundred Myr after coalescence. About 20%–60% of the simulated galaxies are not identified as mergers during the strong interaction phase, implying that galaxies undergoing violent merging process do not necessarily exhibit highly asymmetric kinematics in their star-forming gas. The lack of identifiable merger signatures in this population can lead to an underestimation of merger abundances in star-forming galaxies, and including them in samples of star-forming disks may bias the measurements of disk properties such as intrinsic velocity dispersion

[en] We present the first systematic comparison of ultraviolet-millimeter spectral energy distributions (SEDs) of observed and simulated interacting galaxies. Our sample is drawn from the Spitzer Interacting Galaxy Survey and probes a range of galaxy interaction parameters. We use 31 galaxies in 14 systems which have been observed with Herschel, Spitzer, GALEX, and 2MASS. We create a suite of GADGET-3 hydrodynamic simulations of isolated and interacting galaxies with stellar masses comparable to those in our sample of interacting galaxies. Photometry for the simulated systems is then calculated with the SUNRISE radiative transfer code for comparison with the observed systems. For most of the observed systems, one or more of the simulated SEDs match reasonably well. The best matches recover the infrared luminosity and the star formation rate of the observed systems, and the more massive systems preferentially match SEDs from simulations of more massive galaxies. The most morphologically distorted systems in our sample are best matched to the simulated SEDs that are close to coalescence, while less evolved systems match well with the SEDs over a wide range of interaction stages, suggesting that an SED alone is insufficient for identifying the interaction stage except during the most active phases in strongly interacting systems. This result is supported by our finding that the SEDs calculated for simulated systems vary little over the interaction sequence.

[en] We measured and modeled spectral energy distributions (SEDs) in 28 bands from the ultraviolet to the far-infrared (FIR) for 31 interacting galaxies in 14 systems. The sample is drawn from the Spitzer Interacting Galaxy Survey, which probes a range of galaxy interaction parameters at multiple wavelengths with an emphasis on the infrared bands. The subset presented in this paper consists of all galaxies for which FIR Herschel SPIRE observations are publicly available. Our SEDs combine the Herschel photometry with multi-wavelength data from Spitzer, GALEX, Swift UVOT, and 2MASS. While the shapes of the SEDs are broadly similar across our sample, strongly interacting galaxies typically have more mid-infrared emission relative to their near-infrared and FIR emission than weakly or moderately interacting galaxies. We modeled the full SEDs to derive host galaxy star formation rates (SFRs), specific star formation rates (sSFRs), stellar masses, dust temperatures, dust luminosities, and dust masses. We find increases in the dust luminosity and mass, SFR, and cold (15-25 K) dust temperature as the interaction progresses from moderately to strongly interacting and between non-interacting and strongly interacting galaxies. We also find increases in the SFR between weakly and strongly interacting galaxies. In contrast, the sSFR remains unchanged across all the interaction stages. The ultraviolet photometry is crucial for constraining the age of the stellar population and the SFR, while dust mass is primarily determined by SPIRE photometry. The SFR derived from the SED modeling agrees well with rates estimated by proportionality relations that depend on infrared emission.

[en] We have observed warm molecular hydrogen in two nearby edge-on disk galaxies, NGC 4565 and NGC 5907, using the Spitzer high-resolution infrared spectrograph. The 0-0 S(0) 28.2 μm and 0-0 S(1) 17.0 μm pure rotational lines were detected out to 10 kpc from the center of each galaxy on both sides of the major axis, and in NGC 4565 the S(0) line was detected at r = 15 kpc on one side. This location is beyond the transition zone where diffuse neutral atomic hydrogen starts to dominate over cold molecular gas and marks a transition from a disk dominated by high surface-brightness far-infrared (far-IR) emission to that of a more quiescent disk. It also lies beyond a steep drop in the radio continuum emission from cosmic rays (CRs) in the disk. Despite indications that star formation activity decreases with radius, the H₂ excitation temperature and the ratio of the H₂ line and the far-IR luminosity surface densities, Σ(L_H2)/Σ(L_TIR), change very little as a function of radius, even into the diffuse outer region of the disk of NGC 4565. This suggests that the source of excitation of the H₂ operates over a large range of radii and is broadly independent of the strength and relative location of UV emission from young stars. Although excitation in photodissociation regions is the most common explanation for the widespread H₂ emission, CR heating or shocks cannot be ruled out. At r = 15 kpc in NGC 4565, outside the main UV- and radio-continuum-dominated disk, we derived a higher than normal H₂ to 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission ratio, but this is likely due to a transition from mainly ionized PAH molecules in the inner disk to mainly neutral PAH molecules in the outer disk. The inferred mass surface densities of warm molecular hydrogen in both edge-on galaxies differ substantially, being 4(-60) M_sun pc^-2 and 3(-50) M_sun pc^-2 at r = 10 kpc for NGC 4565 and NGC 5907, respectively. The higher values represent very unlikely point-source upper limits. The point-source case is not supported by the observed emission distribution in the spectral slits. These mass surface densities cannot support the observed rotation velocities in excess of 200 km s^-1. Therefore, warm molecular hydrogen cannot account for dark matter in these disk galaxies, contrary to what was implied by a previous Infrared Space Observatory study of the nearby edge-on galaxy NGC 891.

[en] We have carried out a search for galaxies at z ∼ 7-10 in ∼14.4 arcmin² of new NICMOS parallel imaging taken in the Great Observatories Origins Deep Survey (5.9 arcmin²), the Cosmic Origins Survey (7.2 arcmin²), and SSA22 (1.3 arcmin²). These images reach 5σ sensitivities of J ₁₁₀ = 26.0-27.5 (AB), and combined they increase the amount of deep near-infrared data by more than 60% in fields where the investment in deep optical data has already been made. We find no z > 7 candidates in our survey area, consistent with the Bouwens et al. measurements at z ∼ 7 and 9 (over 23 arcmin²), which predict 0.7 galaxies at z ∼ 7 and <0.03 galaxies at z ∼ 9. We estimate that 10%-20% of z > 7 galaxies are missed by this survey, due to incompleteness from foreground contamination by faint sources. For the case of luminosity evolution, assuming a Schecter parameterization with a typical φ* = 10^-3 Mpc^-3, we find M*> - 20.0 for z ∼ 7 and M*> - 20.7 for z ∼ 9 (68% confidence). This suggests that the downward luminosity evolution of Lyman break galaxies continues to z ∼ 7, although our result is marginally consistent with the z ∼ 6 luminosity function of Bouwens et al. In addition, we present newly acquired deep MMT/Megacam imaging of the z ∼ 9 candidate JD2325+1433, first presented in Henry et al. The resulting weak but significant detection at i' indicates that this galaxy is most likely an interloper at z ∼ 2.7.

[en] Low-frequency radio selection finds radio-bright galaxies regardless of the amount of obscuration by gas and dust. We report Chandra observations of a complete 178 MHz–selected, and so orientation-unbiased, sample of 44 0.5 < z < 1 3CRR sources. The sample is comprised of quasars and narrow-line radio galaxies (NLRGs) with similar radio luminosities, and the radio structure serves as both an age and an orientation indicator. Consistent with unification, intrinsic obscuration (measured by N _H, X-ray hardness ratio, and X-ray luminosity) generally increases with inclination. However, the sample includes a population not seen in high-z 3CRR sources: NLRGs viewed at intermediate inclination angles with N _H < 10²² cm⁻². Multiwavelength analysis suggests that these objects have lower L/L _Edd than typical NLRGs at similar orientation. Thus, both orientation and L/L _Edd are important, and a “radiation-regulated unification” provides a better explanation of the sample’s observed properties. In comparison with the 3CRR sample at 1 < z < 2, our lower-redshift sample shows a higher fraction of Compton-thin NLRGs (45% versus 29%) but a similar Compton-thick fraction (20%), implying a larger covering factor of Compton-thin material at intermediate viewing angles and thus a more “puffed-up” torus atmosphere. We posit that this is due to a range of L/L _Edd extending to lower values in this sample. In contrast, at high redshifts, the narrower range and high L/L _Edd values allowed orientation (and so simple unification) to dominate the sample’s observed properties.

[en] We measure the faint-end slope of the galaxy luminosity function (LF) for cluster galaxies at 1 < z < 1.5 using Spitzer IRAC data. We investigate whether this slope, α, differs from that of the field LF at these redshifts, and with the cluster LF at low redshifts. The latter is of particular interest as low-luminosity galaxies are expected to undergo significant evolution. We use seven high-redshift spectroscopically confirmed galaxy clusters drawn from the IRAC Shallow Cluster Survey to measure the cluster-galaxy LF down to depths of M* + 3 (3.6 μm) and M* + 2.5 (4.5 μm). The summed LF at our median cluster redshift (z = 1.35) is well fit by a Schechter distribution with α_3.6μm = –0.97 ± 0.14 and α_4.5μm = –0.91 ± 0.28, consistent with a flat faint-end slope and is in agreement with measurements of the field LF in similar bands at these redshifts. A comparison to α in low-redshift clusters finds no statistically significant evidence of evolution. Combined with past studies which show that M* is passively evolving out to z ∼ 1.3, this means that the shape of the cluster LF is largely in place by z ∼ 1.3. This suggests that the processes that govern the buildup of the mass of low-mass cluster galaxies have no net effect on the faint-end slope of the cluster LF at z ∼< 1.3.