Probing the Active Massive Black Hole Candidate in the Center of NGC 404 with VLBI

Recently Nyland et al. (2012) argued that the radio emission observed in the center of the dwarf galaxy NGC 404 originates in a low-luminosity active galactic nucleus (LLAGN) powered by a massive black hole ($M\sim<10^6$ M$_{\odot}$). High-resolution radio detections of MBHs are rare. Here we present sensitive, contemporaneous Chandra X-ray, and very long baseline interferometry (VLBI) radio observations with the European VLBI Network (EVN). The source is detected in the X-rays, and shows no long-term variability. If the hard X-ray source is powered by accretion, the apparent low accretion efficiency would be consistent with a black hole in the hard state. Hard state black holes are known to show radio emission compact on the milliarcsecond scales. However, the central region of NGC 404 is resolved out on 10 milliarcsecond (0.15-1.5 pc) scales. Our VLBI non-detection of a compact, partially self-absorbed radio core in NGC 404 implies that either the black hole mass is smaller than $3^{+5}_{-2}\times10^5$ M$_{\odot}$, or the source does not follow the fundamental plane of black hole activity relation. An alternative explanation is that the central black hole is not in the hard state. The radio emission observed on arcsecond (tens of pc) scales may originate in nuclear star formation or extended emission due to AGN activity, although the latter would not be typical considering the structural properties of low-ionization nuclear emission-line region galaxies (LINERs) with confirmed nuclear activity.Comment: Accepted for publication in the Astrophysical Journal. 7 pages, 2 figures, 1 tabl

Deep VLT spectroscopy of the z=2.49 Radio Galaxy MRC 2104-242: Evidence for a metallicity gradient in its extended emission line region

We present spectroscopic observations of the rest-frame UV line emission around radio galaxy MRC 2104-242 at z=2.49, obtained with FORS1 on VLT Antu. The morphology of the halo is dominated by two spatially resolved regions. Lya is extended by >12 arcsec along the radio axis, CIV and HeII are extended by ~8 arcsec. The overall spectrum is typical for that of high redshift radio galaxies. The most striking spatial variation is that NV is present in the spectrum of the region associated with the center of the galaxy hosting the radio source, the northern region, while absent in the southern region. Assuming that the gas is photoionized by a hidden quasar, the difference in NV emission can be explained by a metallicity gradient within the halo. This is consistent with a scenario in which the gas is associated with a massive cooling flow or originates from the debris of the merging of two or more galaxies.Comment: Accepted for publication in A&A Letter

A search for AGN in the most extreme UV-selected starbursts using the European VLBI Network

We have used the European VLBI Network (EVN) to observe a sample of Lyman Break Analogs (LBAs), nearby (z < 0.3) galaxies with properties similar to the more distant Lyman Break Galaxies (LBGs). The study of LBGs may help define the feed-back relationship between black holes (BHs) and their host galaxies. Previous VLA observations have shown that the kpc-scale radio emission from LBAs is dominated by starbursts. The main targets of this VLBI experiment were selected because they possessed emission-line properties between starbursts and Type 2 (obscured) AGN. Eight targets (three star-forming LBAs, four composite LBAs, and one Type 1 AGN) were observed at 5 GHz, four of which were also observed at 1.7 GHz. One star-forming LBA and one composite LBA were detected above 5 \sigma at 1.7 GHz (only), while the AGN was detected at 5 GHz. In both LBAs, the radio luminosity (LR) exceeds that expected from supernovae (remnants) based on a comparison with Arp220, Arp229A and Mrk273, by factors of 2 - 8. The composite LBA exhibits a compact core emitting around 10% of the VLA flux density. The high Tb of 3.5E7 K and excess core L_R with respect to the L_R/L_X relation of radio-quiet AGN indicate that this LBA possesses an obscured AGN (MBH ~ E5-E7 M_sun). While weak AGN may co-exist with the starbursts as shown in at least one of the LBAs, their contribution to the total radio flux is fairly minimal. Our results show that the detection of such weak AGN presents a challenge at radio, X-ray and optical emission-line wavelengths at z ~ 0.2, indicating the great difficulties that need to be overcome in order to study similar processes at high redshift when these types of galaxies were common.Comment: 10 pages, 4 figures, accepted for publication in MNRA

Discovery of a high-z protocluster with tunable filters: the case of 6C0140+326 at z=4.4

We present the first results obtained using a tunable narrowband filter in the search for high-z protoclusters. Using the recently commissioned red tunable filter on the Gran Telescopio Canarias we have searched for Lya emitters in a 75 arcmin^2 field centered on the z=4.413 radio galaxy 6C0140+326. With three different wavelength tunings we find a total of 27 unique candidate Lya emitters. The availability of three different wavelength tunings allows us to make estimates of the redshifts for each of the objects. It also allows us to separate a possible protocluster from structure in the immediate foreground. This division shows that the foreground region contains significantly fewer Lya emitters. Also, the spatial distribution of the objects in the protocluster field deviates from a random distribution at the 2.5 sigma level. The observed redshift distribution of the emitters is different from the expected distribution of a blank field at the ~3 sigma level, with the Lya emitters concentrated near the radio galaxy at z>4.38. The 6C0140+326 field is denser by a factor of 9+/-5 than a blank field, and the number density of Lya emitters close to the radio galaxy is similar to that of the z~4.1 protocluster around TNJ1338-1942. We thus conclude that there is an overdensity of Lya emitters around the radio galaxy 6C0140+326. This is one of few known overdensities at such a high redshift.Comment: 10 pages, 6 figures, accepted for publication in MNRA

The MUSE 3D view of feedback in a high-metallicity radio galaxy at z = 2.9

We present a detailed study of the kinematic, chemical and excitation properties of the giant Ly$\alpha$ emitting nebula and the giant \ion{H}{I} absorber associated with the $z = 2.92$ radio galaxy MRC 0943--242, using spectroscopic observations from VLT/MUSE, VLT/X-SHOOTER and other instruments. Together, these data provide a wide range of rest-frame wavelength (765 \AA$\,$ -- 6378 \AA$\,$ at $z = 2.92$) and 2D spatial information. We find clear evidence for jet gas interactions affecting the kinematic properties of the nebula, with evidence for both outflows and inflows being induced by radio-mode feedback. We suggest that the regions of relatively lower ionization level, spatially correlated with the radio hotspots, may be due to localised compression of photoionized gas by the expanding radio source, thereby lowering the ionization parameter, or due to a contribution from shock-heating. We find that photoionization of super-solar metallicity gas ($Z/Z_{\odot}$ = 2.1) by an AGN-like continuum ($\alpha$=--1.0) at a moderate ionization parameter ($U$ = 0.018) gives the best overall fit to the complete X-SHOOTER emission line spectrum. We identify a strong degeneracy between column density and Doppler parameter such that it is possible to obtain a reasonable fit to the \ion{H}{I} absorption feature across the range log N(\ion{H}{I}/cm$^{-2}$) = 15.20 and 19.63, with the two best-fitting occurring near the extreme ends of this range. The extended \ion{H}{I} absorber is blueshifted relative to the emission line gas, but shows a systematic decrease in blueshift towards larger radii, consistent with a large scale expanding shell.Comment: 25 pages, 18 figures, 10 tables. Accepted for publication in MNRAS. Published: 23 November 201

The kinematics of ionized gas in lyman-break analogs at z ~ 0.2

We present results for 19 “Lyman-break analogs” observed with Keck/OSIRIS with an adaptive-optics-assisted spatial resolution of less than 200 pc. We detect satellites/companions, diffuse emission, and velocity shear, all with high signal-to-noise ratios. These galaxies present remarkably high velocity dispersion along the line of sight (~70 km s^(−1)), much higher than standard star-forming spirals in the low-redshift universe. We artificially redshift our data to z ~ 2.2 to allow for a direct comparison with observations of high-z Lyman-break galaxies and find striking similarities between both samples. This suggests that either similar physical processes are responsible for their observed properties, or, alternatively, that it is very difficult to distinguish between different mechanisms operating in the low- versus high-redshift starburst galaxies based on the available data. The comparison between morphologies in the UV/optical continuum and our kinemetry analysis often shows that neither is by itself sufficient to confirm or completely rule out the contribution from recent merger events. We find a correlation between the kinematic properties and stellar mass, in that more massive galaxies show stronger evidence for a disk-like structure. This suggests a co-evolutionary process between the stellar mass buildup and the formation of morphological and dynamical substructure within the galaxy

Constraining the Thermal Dust Content of Lyman-Break Galaxies in an Overdense Field at z~5

We have carried out 870 micron observations in the J1040.7-1155 field, known to host an overdensity of Lyman break galaxies at z=5.16 +/- 0.05. We do not detect any individual source at the S(870)=3.0 mJy/beam (2 sigma) level. A stack of nine spectroscopically confirmed z>5 galaxies also yields a non-detection, constraining the submillimeter flux from a typical galaxy at this redshift to S(870)<0.85 mJy, which corresponds to a mass limit M(dust)<1.2x10^8 M_sun (2 sigma). This constrains the mass of thermal dust in distant Lyman break galaxies to less than one tenth of their typical stellar mass. We see no evidence for strong submillimeter galaxies associated with the ultraviolet-selected galaxy overdensity, but cannot rule out the presence of fainter, less massive sources.Comment: 5 pages, 2 figures. MNRAS in pres