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Register a new userThe Halos and Environments of Nearby Galaxies (HERON) I: Imaging, Sample Characteristics, and Envelope Diameters
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We use a dedicated 0.7-m telescope to image the halos of 119 galaxies in the Local Volume to $mu_r sim 28-30$ mag/arcsec$^2$. The sample is primarily from the 2MASS Large Galaxy Atlas and extended to include nearby dwarf galaxies and more distant giant ellipticals, and spans fully the galaxy colour-magnitude diagram including the blue cloud and red sequence. We present an initial overview, including deep images of our galaxies. Our observations reproduce previously reported low surface brightness structures, including extended plumes in M51, and a newly discovered tidally extended dwarf galaxy in NGC7331. Low surface brightness structures, or envelopes, exceeding 50 kpc in diameter are found mostly in galaxies with $M_V<-20.5$, and classic interaction signatures are infrequent. Defining a halo diameter at the surface brightness 28 mag/arcsec$^2$, we find that halo diameter is correlated with total galaxy luminosity. Extended signatures of interaction are found throughout the galaxy colour-magnitude diagram without preference for the red or blue sequences, or the green valley. Large envelopes may be found throughout the colour-magnitude diagram with some preference for the bright end of the red sequence. Spiral and S0 galaxies have broadly similar sizes, but ellipticals extend to notably greater diameters, reaching 150 kpc. We propose that the extended envelopes of disk galaxies are dominated by an extension of the disk population rather than by a classical population II halo.
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The HERON project is aimed at studying halos and low surface brightness details near galaxies. In this second HERON paper we consider in detail deep imaging (down to surface brightness of ~28 mag/arcsec$^2$ in the r band) for 35 galaxies, viewed edge-on. We confirm a range of low surface brightness features previously described in the literature but also report new ones. We classify the observed outer shapes of the galaxies into three main types (and their prototypes): disc/diamond-like (NGC891), oval (NGC4302), and boxy (NGC3628). We show that the shape of the outer disc in galaxies does not often follow the general 3D model of an exponential disc: 17 galaxies in our sample exhibit oval or even boxy isophotes at the periphery. Also, we show that the less flattened the outer disc is, the more oval or boxy its structure. Many galaxies in our sample have an asymmetric outer structure. We propose that the observed diversity of the galaxy outer shapes is defined by the merger history and its intensity: if no recent multiple minor or single major merging took place, the outer shape is diamond-like or discy. On the contrary, interacting galaxies show oval outer shapes, whereas recent merging appears to transform the outer shape to boxy.
This paper introduces the Multi-wavelength Extreme Starburst Sample (MESS), a new catalog of 138 star-forming galaxies (0.1 < z < 0.3) optically selected from the SDSS using emission line strength diagnostics to have high absolute SFR (minimum 11 solar masses per year, with median SFR approx 61 solar masses per year based on a Kroupa IMF). The MESS was designed to complement samples of nearby star-forming galaxies such as the luminous infrared galaxies (LIRGs), and ultraviolet luminous galaxies (UVLGs). Observations using the multiband imaging photometer (MIPS; 24, 70, and 160{mu}m channels) on the Spitzer Space Telescope indicate the MESS galaxies have IR luminosities similar to those of LIRGs, with an estimated median LTIR ~ 3e11 solar luminosities. The selection criteria for the MESS suggests they may be less obscured than typical far-IR selected galaxies with similar estimated SFRs. 20 out of 70 of the MESS objects detected in the GALEX FUV band also appear to be UV luminous galaxies. We estimate the SFRs based directly on luminosities to determine the agreement for these methods in the MESS. We compare to the emission line strength technique, since effective measurement of dust attenuation plays a central role in these methods. We apply an image stacking technique to the VLA FIRST survey radio data to retrieve 1.4 GHz luminosity information for 3/4 of the sample covered by FIRST including sources too faint, and at too high a redshift, to be detected in FIRST. We also discuss the relationship between the MESS and samples selected through alternative criteria. Morphologies will be the subject of a forthcoming paper.
We review the physical properties of nearby, relatively luminous galaxies, using results from newly available massive data sets together with more detailed observations. First, we present the global distribution of properties, including the optical and ultraviolet luminosity, stellar mass, and atomic gas mass functions. Second, we describe the shift of the galaxy population from late galaxy types in underdense regions to early galaxy types in overdense regions. We emphasize that the scaling relations followed by each galaxy type change very little with environment, with the exception of some minor but detectable effects. The shift in the population is apparent even at the densities of small groups and therefore cannot be exclusively due to physical processes operating in rich clusters. Third, we divide galaxies into four crude types -- spiral, lenticular, elliptical, and merging systems -- and describe some of their more detailed properties. We attempt to put these detailed properties into the global context provided by large surveys.
The optical time-domain astronomy has grown rapidly in the past decade but the dynamic infrared sky is rarely explored. Aiming to construct a sample of mid-infrared outburst in nearby galaxies (MIRONG), we have conducted a systematical search of low-redshift ($z<0.35$) SDSS spectroscopic galaxies that have experienced recent MIR flares using their Wide-field Infrared Survey Explorer (WISE) light curves. A total of 137 galaxies have been selected by requiring a brightening amplitude of 0.5 magnitude in at least one WISE band with respect to their quiescent phases. Only a small faction (10.9%) has corresponding optical flares. Except for the four supernova (SNe) in our sample, the MIR luminosity of remaining sources ($L_{rm 4.6mu m}>10^{42}~rm erg~s^{-1}$) are markedly brighter than known SNe and their physical locations are very close to the galactic center (median <0.1). Only four galaxies are radio-loud indicating that synchrotron radiation from relativistic jets could contribute MIR variability. We propose that these MIR outburst are dominated by the dust echoes of transient accretion onto supermassive black holes, such as tidal disruption events (TDEs) and turn-on (changing-look) AGNs. Moreover, the inferred peak MIR luminosity function is generally consistent with the X-ray and optical TDEs at high end albeit with large uncertainties. Our results suggest that a large population of transients have been overlooked by optical surveys, probably due to dust obscuration or intrinsically optical weakness. Thus, a search in the infrared band is crucial for us to obtain a panoramic picture of nuclear outburst. The multiwavength follow-up observations of the MIRONG sample are in progress and will be presented in a series of subsequent papers.
Spectroscopy of background QSO sightlines passing close to foreground galaxies is a potent technique for studying the circumgalactic medium (CGM). QSOs are effectively point sources, however, limiting their potential to constrain the size of circumgalactic gaseous structures. Here we present the first large Keck/LRIS and VLT/FORS2 spectroscopic survey of bright (B_AB < 22.3) background galaxies whose lines of sight probe MgII 2796, 2803 absorption from the CGM around close projected foreground galaxies at transverse distances 10 kpc < R_perp < 150 kpc. Our sample of 72 projected pairs, drawn from the PRIsm MUlti-object Survey (PRIMUS), includes 48 background galaxies which do not host bright AGN, and both star-forming and quiescent foreground galaxies with stellar masses 9.0 < log M_*/M_sun < 11.2 at redshifts 0.35 < z_f/g < 0.8. We detect MgII absorption associated with these foreground galaxies with equivalent widths 0.25 Ang < W_2796 < 2.6 Ang at >2sigma significance in 20 individual background sightlines passing within R_perp < 50 kpc, and place 2sigma upper limits on W_2796 of <0.5 Ang in an additional 11 close sightlines. Within R_perp < 50 kpc, W_2796 is anticorrelated with R_perp, consistent with analyses of MgII absorption detected along background QSO sightlines. Subsamples of these foreground hosts divided at log M_*/M_sun = 9.9 exhibit statistically inconsistent W_2796 distributions at 30 kpc < R_perp < 50 kpc, with the higher-M_* galaxies yielding a larger median W_2796 by 0.9 Ang. Finally, we demonstrate that foreground galaxies with similar stellar masses exhibit the same median W_2796 at a given R_perp to within <0.2 Ang toward both background galaxies and toward QSO sightlines drawn from the literature. Analysis of these datasets constraining the spatial coherence scale of circumgalactic MgII absorption is presented in a companion paper.
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