The Diverse Infrared Properties of a Complete Sample of Star-Forming Dwarf Galaxies

We present mid-infrared Spitzer Space Telescope observations of a complete sample of star-forming dwarf galaxies selected from the KPNO International Spectroscopic Survey. The galaxies span a wide range in mid-infrared properties. Contrary to expectations, some of the galaxies emit strongly at 8 micron indicating the presence of hot dust and/or PAHs. The ratio of this mid-infrared dust emission to the stellar emission is compared with the galaxies' luminosity, star-formation rate, metallicity, and optical reddening. We find that the strength of the 8.0 micron dust emission to the stellar emission ratio is more strongly correlated with the star-formation rate than it is with the metallicity or the optical reddening in these systems. Nonetheless, there is a correlation between the 8.0 micron luminosity and metallicity. The slope of this luminosity-metallicity correlation is shallower than corresponding ones in the B-band and 3.6 micron. The precise nature of the 8.0 micron emission seen in these galaxies (i.e., PAH versus hot dust or some combination of the two) will require future study, including deep mid-IR spectroscopy.Comment: 14 pages, accepted Ap

The Spitzer South Pole Telescope Deep Field Survey: Linking galaxies and halos at z=1.5

We present an analysis of the clustering of high-redshift galaxies in the recently completed 94 deg$^2$ Spitzer-SPT Deep Field survey. Applying flux and color cuts to the mid-infrared photometry efficiently selects galaxies at $z\sim1.5$ in the stellar mass range $10^{10}-10^{11}M_\odot$, making this sample the largest used so far to study such a distant population. We measure the angular correlation function in different flux-limited samples at scales $>6^{\prime \prime}$ (corresponding to physical distances $>0.05$ Mpc) and thereby map the one- and two-halo contributions to the clustering. We fit halo occupation distributions and determine how the central galaxy's stellar mass and satellite occupation depend on the halo mass. We measure a prominent peak in the stellar-to-halo mass ratio at a halo mass of $\log(M_{\rm halo} / M_\odot) = 12.44\pm0.08$, 4.5 times higher than the $z=0$ value. This supports the idea of an evolving mass threshold above which star formation is quenched. We estimate the large-scale bias in the range $b_g=2-4$ and the satellite fraction to be $f_\mathrm{sat}\sim0.2$, showing a clear evolution compared to $z=0$. We also find that, above a given stellar mass limit, the fraction of galaxies that are in similar mass pairs is higher at $z=1.5$ than at $z=0$. In addition, we measure that this fraction mildly increases with the stellar mass limit at $z=1.5$, which is the opposite of the behavior seen at low-redshift.Comment: 32 pages, 22 figures. Published in MNRA

Simulated Galaxy Interactions as Probes of Merger Spectral Energy Distributions

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 simulated SEDs close to coalescence, while less evolved systems match well with SEDs over a wide range of interaction stages, suggesting that an SED alone is insufficient to identify 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.Comment: 24 pages, 16 figures, 2 tables, accepted for publication in ApJ. Animations of the evolution of the simulated SEDs can be found at http://www.cfa.harvard.edu/~llanz/sigs_sim.htm

Probabilistic Classification of Infrared-selected targets for SPHEREx mission: In search of YSOs

We apply machine learning algorithms to classify Infrared (IR)-selected targets for NASA's upcoming SPHEREx mission. In particular, we are interested in classifying Young Stellar Objects (YSOs), which are essential for understanding the star formation process. Our approach differs from previous work, which has relied heavily on broadband color criteria to classify IR-bright objects, and are typically implemented in color-color and color-magnitude diagrams. However, these methods do not state the confidence associated with the classification and the results from these methods are quite ambiguous due to the overlap of different source types in these diagrams. Here, we utilize photometric colors and magnitudes from seven near and mid-infrared bands simultaneously and employ machine and deep learning algorithms to carry out probabilistic classification of YSOs, Asymptotic Giant Branch (AGB) stars, Active Galactic Nuclei (AGN) and main-sequence (MS) stars. Our approach also sub-classifies YSOs into Class I, II, III and flat spectrum YSOs, and AGB stars into carbon-rich and oxygen-rich AGB stars. We apply our methods to infrared-selected targets compiled in preparation for SPHEREx which are likely to include YSOs and other classes of objects. Our classification indicates that out of $8,308,384$ sources, $1,966,340$ have class prediction with probability exceeding $90\%$, amongst which $\sim 1.7\%$ are YSOs, $\sim 58.2\%$ are AGB stars, $\sim 40\%$ are (reddened) MS stars, and $\sim 0.1\%$ are AGN whose red broadband colors mimic YSOs. We validate our classification using the spatial distributions of predicted YSOs towards the Cygnus-X star-forming complex, as well as AGB stars across the Galactic plane.Comment: 17 pages, 12 figures, Accepted for publication in MNRA

Variations of the ISM Compactness Across the Main Sequence of Star-Forming Galaxies: Observations and Simulations

(abridged) The majority of star-forming galaxies follow a simple empirical correlation in the star formation rate (SFR) versus stellar mass ($M_*$) plane, usually referred to as the star formation Main Sequence (MS). Here we combine a set of hydro-dynamical simulations of interacting galactic disks with state-of-the-art radiative transfer codes to analyze how the evolution of mergers is reflected upon the properties of the MS. We present \textsc{Chiburst}, a Markov Chain Monte Carlo (MCMC) Spectral Energy Distribution (SED) code that fits the multi-wavelength, broad-band photometry of galaxies and derives stellar masses, star formation rates, and geometrical properties of the dust distribution. We apply this tool to the SEDs of simulated mergers and compare the derived results with the reference output from the simulations. Our results indicate that changes in the SEDs of mergers as they approach coalescence and depart from the MS are related to an evolution of dust geometry in scales larger than a few hundred parsecs. This is reflected in a correlation between the specific star formation rate (sSFR), and the compactness parameter $\mathcal{C}$, that parametrizes this geometry and hence the evolution of dust temperature ($T_{\rm{dust}}$) with time. As mergers approach coalescence, they depart from the MS and increase their compactness, which implies that moderate outliers of the MS are consistent with late-type mergers. By further applying our method to real observations of Luminous Infrared Galaxies (LIRGs), we show that the merger scenario is unable to explain these extreme outliers of the MS. Only by significantly increasing the gas fraction in the simulations are we able to reproduce the SEDs of LIRGs.Comment: 18 pages, 10 figures, accepted in Ap

High-precision Photometric Redshifts from Spitzer/IRAC: Extreme [3.6]-[4.5] Colors Identify Galaxies in the Redshift Range z~6.6-6.9

One of the most challenging aspects of studying galaxies in the z>~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 Lya photons at z>6.5. The resulting redshift uncertainties inhibit the efficient search for [C II] in z~7 galaxies with sub-mm 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~6.8, galaxies exhibit IRAC colors as blue as [3.6]-[4.5] ~-1, likely due to the contribution of [O III]+Hb to the 3.6 mum flux combined with the absence of line contamination in the 4.5 mum 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~6.6-6.9. When combined with an I-dropout criterion, we demonstrate that we can plausibly select a relatively clean sample of z~6.8 galaxies. Through a systematic application of this selection technique to our catalogs from all five CANDELS fields, we identify 20 probable z~6.6-6.9 galaxies. We estimate that our criteria select the ~50% strongest line emitters at z~6.8 and from the IRAC colors we estimate a typical [O III]+Hb rest-frame equivalent width of 1085A for this sample. The small redshift uncertainties on our sample make it particularly well suited for follow-up studies with facilities such as ALMA.Comment: In submission to the Astrophysical Journal, updated in response to the referee report, 13 pages, 11 figures, 1 tabl