PARSEC project

The central PARSEC of galaxies across the electromagnetic spectrum

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From kpcs to the central parsec: feeding star formation and a black hole at the same time (Prieto et al. 2004, 2019)

https://www.eso.org/public/images/eso0534a/

A panchromatic view of the star-forming ring and feeding process in the central kpc of the galaxy NGC 1097 is presented. The assembled IR to UV images at ~ 10 pc resolution allow us to characterize the population of circa 250 clusters in the ring and disentangle the network of filaments of dust and gas that enshroud and feed them.

The ring is a place of intermittent star bursts over the last 100 Myr. Four major episodes covering a proto-cluster phase of 11 mid-IR sources at the molecular clouds core, and two (three) previous bursts with a time separation of 20–30 Myr are identified.

The dust map of the inner few kpc resolves NGC 1097’s two major dust lanes in bundles of narrow, <25 pc width, filaments running along the galaxy’s bar. As they approach the ring, some circularize along it, others curve to the centre to produce a nuclear spiral. We believe these are kpc-scale dust-gas streamers feeding the ring and the black hole.

The total mass in clusters formed in the ring in the last 100 Myr is <10^7 Mo, i.e. less than 1%  of the 10^9 Moof molecular gas in the ring; yet, at its current star formation rate, ~ 1.8 Mo/yr, an order of magnitude more in stellar mass should have been produced over that period. This means that the availability of gas in the ring is not the main star formation driver, and other factors, as the rate at which dense gas accumulates in teh ring are more fundamental.

Figure above, lleft: RGB (VLT-NACO K-band-red, HST f814W-green and F336W-blue) image - 4 kpc accross - of NGC 1097 circumnuclear ring illustrating the ring of young star clusters, and the network of dust filaments entering the ring. Most dust filaments arrive to the ring via two major dust lanes, one at the North-East and one at the South-West of the ring (identified as the two dark, wide, long features in the image).

Figure above, right: a highly contrasted view (VLT-NACO J-band image, 30 arcsec accross) of the nuclear spiral of dust filaments which are traced up 10 pc distance from the BH ( at the center of the image). These filaments form part of the large scale dust lanes, they are interpreted as gas/dust stremers feeding the star forming ring and the hole.

 

In the figure above, 247 stellar clusters in the 1 kpc radius circumnuclear ring are identified with circles in the HST/F336W UV image. Some clusters are only seen from 0.8 um onwards becasue of extinction. North is up, East to the left, scale is in arcsecs. The parsecs-scale SED of some representative clusters in the ring are shown in three separate plots around the HST image. In each plot, the best stellar population fit to the SED is plot over the data points -with errors -in blue. All SEDs are sampled from 0.3 to 2.5 um. In colour, the up to three different bursts of star formation occurring along the ring are identified.

 

 

 

The nuclear dust lane of Circinus: collimation without a torus (Mezcua et al. 2016)

In some AGN, nuclear dust lanes connected to kpc-scale dust structures provide all the extinction required to obscure the nucleus, challenging the role of the dusty torus proposed by the Unified Model. In this letter we show the pc-scale dust and ionized gas maps of Circinus constructed using sub-arcsec-accuracy registration of infrared VLT AO images with optical Hubble Space Telescope images.

We find that the collimation of the ionized gas does not require a torus but is caused by the distribution of dust lanes of the host galaxy on 10 pc scales. This finding questions the presumed torus morphology and its role at parsec scales, as one of its main attributes is to collimate the nuclear radiation, and is in line with interferometric observations which show that most of the pc-scale dust is in the polar direction.

We estimate that the nuclear dust lane in Circinus provides 1=3 of the extinction required to obscure the nucleus. This constitutes a conservative lower limit to the obscuration at the central parsecs, where the dust filaments might get optically thicker if they are the channels that transport material from 100 pc scales to the centre.

 

In the figure above, left panel: VLT-NACO-Ks / HST-F814W continuum light ratio -efectively a dust map - with contours of [O III], centre: same dust map as in previous with HST-Ha line emission in green and warm VLT-NACO-H2 molecular gas in blue; right: same dust map with VLT-NACO-[Si VII]2.48 um coronal emission in contours. The V-shape defined by the two dust filaments at the centre is evidenced, the ionised gas morphology - its apparent collimation - is determined by the location of the dust filaments.

 

 

The central parsec of AGN: Challenges to the torus (Prieto et al. 2014).

 

Infrared VLT- Adapted-Optics-assisted images accurately registered with optical Hubble Space Telescope images allowed us to unambiguously identify with mill-arcsec accuracy the location of the nucleus of a reference sample of nearby, prototype 2 AGN. The nucleus is found in most cases shifted by several tens of parsec (pc) from the optical peak location. The optical peak, on the other hand, is found associated with either a young stellar cluster, a cloud in the extended narrow-line region, or the stellar photometric peak. Only in the galaxies with the lowest nuclear dust extinction, Av ~ 1 mag, the optical peak is the IR nucleus itself.

The high angular scale and astrometry precision of the combined images allow us to trace the dust morphology at scales of few pc. Remarkably, in almost all cases, the IR nucleus is found behind a dust filament or lane, prompting to these being a major, if not the only, cause of the type 2 nucleus obscuration. Estimated extinctions –lower limits– are Av ~ 3 to 6 mag, an order of magnitude less than those inferred by X-rays, however sufficient to at least obscure the low-luminosity AGN class. The length scale of the nuclear dust lane or filaments is in the few 100 pc range, in some cases, seen connecting with kpc-scale dust structures. Their morphology and strategic location suggest these are the nuclear fueling channels. A net example of a ~ 100 pc dust filament hidden the nucleus of one of the galaxies in the study, NGC 1386, is shown bellow.

With equivalent astrometry accurazy, the precise location of the ionised gas Halpha and/or [SI VII] 2.48um coronal emission lines relative to those of the IR nucleus and dust was achieved. It is found that the often conical or collimated morphology of the ionised gas seen in AGN is not caused by a nuclear –torus– as it is widely thought but by the overall location of the dust filaments. The [SI VII] 2.48um emission, less subjected to dust extinction, reflects the truly, rather isotropic, distribution of the ionised gas. An example of this effect is shown bellow for the case of NGC 3169.

All together, the precise relative location of dust, ionised gas and the nucleus, is found compelling enough to cast doubts on the universality of the pc-scale torus in AGN and supports its vanishing in the low-luminosity class

The figure above shows the results for NGC1386. The top panels show HST/F814W image with Ks-band continuum contours in white. Blue circles mark the position of the point-like sources used for image alignment, which are also shown in the two inner panels at the top-right corner. The inner 4"x4" region (blue square) is shown in detail in the middle and bottom panels. Bright regions indicate the location of the dust. The middle right panel shows one of the filaments crossing straight into the nucleus (marked with a cross). The bottom left panel shows Halpha in green contours on top of the dust map: it can be seen that the apparent collimated morphology of the ionized gas is defined by the dust filaments. In the bottom right, [SI VII]2.48 um coronal emission shown in green contours on top of the dust map, shows the true isotropic morphology of the gas. North is up and East is to the left. From (Prieto et al. 2014).


In the figure above, top left: HST/F814W image with Ks-band continuum contours in white. Blue circles mark the position of the point-like sources used for image alignment. The blue square shows the inner 4"x4" in the middle and bottom panels. Top right is the Ks/F814W ratio or dust map. The bright arch at the centre is a dust lane crossing the nucleus. A zoom of this dust lane is shown in all other panels. The position of the nucleus and its error is marked with a cross in the middle panels. Bottom left shows the dust map with with H alpha in contours in green. The morphology of the gas - conical shape- is sharply defined by the large scale dust-lane. North is up and East is to the left. From (Prieto et al. 2014).

 

High ionization coronal lines: unique tracers of outflows in AGN (Muller-Sanchez et al. 2011).

High ionization coronal lines, i.e. lines arising from ionization species with ionization potential, IP > 100 eV, are unique tracers of AGN activity. Very high spatial resolution near-infrared integral field spectroscopy of the coronal line region in nearby Seyfert galaxies reveals for the first time the kinematics of this gas on scales of less than 300 pc from the centre. The near-IR coronal lines [Si VI], [Al IX] and [Ca VIII], all spatially resolved in the integral-field observations, reveal a kinematic pattern of rotation plus radial flow, the latter being the dominant motion. Several observational facts indicate that this radial motion corresponds to outflows: radial acceleration of the gas at different projected distances from the center, gas blueshifts in the direction of the jet pointing to us, redshifts in the direction pointing away from us.

The spatially resolved kinematics of the strongest line, [SiVI] 1.96 um, has been modeled as a combination of an outflow bicone and a rotating disk. The rotating disk appears to be coplanar with those seen on the same scales in lower ionized gas and in the central molecular H2 gas. But the outflow velocities could reach values of hundreds, range in the 120 to 400 km/s, at a few hundred parsec from the center. The measured velocity dispersion in the outflowing gas is also large, ~ 150 km/s, and all together implies relatively large kinetic power in the highly ionised gas outflows, of up to a few percent of the bolometric luminosity of the AGN (Muller-Sanchez et al. 2011).

 

The figure above shows the spatial resolved structure, up to scales of 150 pc from the center, of high ionization coronal gas in three Seyfert type 2 galaxies from the sample in (Muller-Sanchez et al. 2011). These spatial maps are produced from K-band SINFONI data-cube obtained with adaptive optics at the VLT: the first column is the continuum emission at 2.2 um; second, third and fourth illustrate the morphology of [Si VI] 1.96 um, [Al IX] 2.04 um and [Ca VIII] 2.32 um emission. The central cross marks the 2 um emission peak.

 

n6

Figure above shows the velocity tomography in [Si VI] 1.96 um in the Seyfert 1.5 galaxy NGC 3783 from Muller-Sanchez's et al. 2011 sample. The channel maps were obtained by integrating the flux within velocity bins of 80 km/s along the emission-line profile. The number in the upper part of each panel corresponds to the central velocity of the bin relative to systemic. The bottom right panel shows the integrated intensity image by summing the velocity slices. The acceleration in this galaxy is mostly observed in the north region.

 

 

 

Many stellar nurseries in the centre of an old galaxy: the elliptical NGC 1052 given birth again (Fernandez-Ontiveros et al. 2010)

High spatial resolution near-infrared images of the central 2 kpc of the near elliptical galaxy NGC 1052 reveal a total of 25 compact sources randomly distributed in the region. Fifteen of them exhibit Hα luminosities an order of magnitude above the estimate for an evolved population of extreme horizontal branch stars. Their Hα equivalent widths and optical-to-NIR spectral energy distributions are consistent with them being young stellar clusters aged <7 Myr. We consider this to be the first direct observation of spatially resolved star-forming regions in the central kiloparsec of an elliptical galaxy. The sizes of these regions are <~11 pc and their median reddening is E(B-V)~ 1 mag. According to previous works, NGC 1052 may have experienced a merger event about 1 Gyr ago. On the assumption that these clusters are spread with a similar density over the whole galaxy, the fraction of galaxy mass (5 x 10e-5) and rate of star formation (0.01 Mo yr-1) involved suggest the merger event as the possible cause for the star formation we see today (Fernandez-Ontiveros et al. 2010). NGC 1052 is at 18 Mpc distance.

In the figure above, left: SDSS colour composite (g- ,r- ,i- filters) image of NGC 1052; right: composite VLT/NACO Ks-band (red), HST/WFPC2 Halpha-continuum free (blue) of the central 2 kpc region. Star forming regions with Halpha-EW>~ 50A -15 in total- are marked with a circle. 10 more compact regions with Halpha-EW <50A are also in the same field. These ones are better seen in unsharp-masked VLT/NaCo Ks-band image (shown in Fernandez-Ontiveros et al. 2010)

 

Parsecs-scale spectral energy distributions of AGN (Prieto et al. 2010)

We are compiling spectral energy distributions (SEDs) with very high spatial resolution for some of the nearest, most well studied, active galactic nuclei (Prieto et al. 2010). These genuine AGN-core SEDs, mostly from Seyfert galaxies are characterized by two main features: an IR bump, with the maximum in the 2 -10 um range, and an increasing X-ray spectrum with frequency, in the 1 to ~200 keV region. These dominant features are common to Seyfert type 1 and 2 objects alike. In detail, type 1 AGNs are clearly distinguished from type 2s by their high spatial resolution SEDs: type 2 AGN exhibit a sharp drop shortward of 2 um, with the optical to UV region being fully absorbed; type 1s show instead a gentle 2 um drop ensued by a secondary, partially-absorbed optical to UV emission bump.

The figure above shows the average SED template of type 1 Seyfert nuclei in blue - it includes NGC 3783, NGC 1566 and NGC 7469 - and in red of type 2 Seyfert nuclei - Circinus, NGC 1068, NGC 5506 and NGC 7582. Prior averaging, each SED was set to its rest frame system and then normalized to the mean value of its nuFnu distribution. Tabular form of these templates can be retrieved in this link.

 

 

The location of warm dust in the nearest AGN (Reunanen et al. 2010)

In order to set an upper limit to the size of the putative torus, we have been imaging the nearest AGN with the VLT at 11 and 18 um, achieving at these wavelengths spatial resolutions of a few tens of parsec at the nucleus of these objects. For some of the objects, the obtained subarcsec images at 18 um are unique. Two major results were found:

1. The bulk of the mid-IR emission is concentrated on an unresolved central source within a size of less than 5 to 130 pc, depending on the object distance. Further resolved emission is detected in 70% of the sample in the form of circumnuclear star-forming rings or diffuse nuclear extended emission.

2. In all cases, the nuclear fluxes at 11 and 18 um represent minor contribution of that measured by IR satellites at the nearest energy bands of 12 and 25 um. This contribution ranges from 30% to less than 1%. Either a low surface brightness component extending over galactic scales or strong extra-nuclear IR sources – e.g. HII regions in spiral arms – have to be the main source of the IR satellites emission. In either case, the contribution of these components dwarfs that of the AGN at mid-IR wavelengths.

The figure above shows narrow band continuum images at 18 um of two of the galaxies analyzed in our nearest-AGN study. Left is the type 2 Seyfert galaxy NGC 7582, right is the type 1 NGC 7469, both with circumnuclear star forming regions. The very central warm dust in these nuclei is distributed in a region less than 30 pc- in NGC 7582, less than 100 pc- in NGC 7469, FWHM (Reunanen, Prieto & Siebenmorgen 2010).

 

 

The centre of the prototype starburst galaxy NGC 253 at parsecs scale (Fernandez-Ontiveros et al. 2009)

VLT adaptive optics images of one of the nearest starburst galaxy NGC 253 - 4 Mpc - allow us to resolve the central 150 pc of this galaxy in more than 37 IR-bright star forming regions, a factor of three larger that is seen in the optical with the HST. The achieved angular resolution in the IR, down to 200 mas and thus comparable to that of available 2 cm maps, permitted us a very accurate IR--radio registration, with eight of these sources being found to have a radio counterpart.

These nuclear regions are characterized by Halpha equivalent widths of about 80 A, relatively large extinction of Av ~7 mag, luminosities, Lbol of about 10e7 Lo, and average sizes of ~3 pc. The compiled high spatial resolution spectral energy distributions (SEDs) of these regions look very similar, all are characterised by a maximum at ~20 um and a moderate emission bump in the 1 - 2 um range. We find that these features can be reproduced by considering a dominant contribution of very young stellar objects (YSO) to the integrated light from these regions: emission from the hot dust surrounding these protostars naturally accounts for the observed IR features and in turn for the overall SED shape. The current best estimate for the age and mas of these these nuclear regions are 0.1 - 3 Myr and 10e5 Mo respectively.

Due to their similar SEDs, and the fact that most of these regions are spatially resolved, an average SED of the 37 circumnuclear clusters has been produced. This template, spanning the radio - IR - optical range, is so far the most genuine representation of an extragalactic circumnuclear star-forming region (available here in tabular form)

Following the accurate IR-radio registration of the central 150 pc region of NGC 253, no X-ray, optical or IR counterpart to the radio core position are found. For the last twenty years this radio core has been assumed to harbor an AGN but the lack of a counterpart seen at IR wavelengths call into question its supposed AGN nature. The analysis of its SED suggests that this source may instead represents a scaled up version of Sgr A* at the Galactic Centre (Fernandez-Ontiveros et al. 2009).

We determined the position of the stellar kinematic center, from VLT / SINFONI 2D spectroscopy using the K-band CO band heads as tracers, and found it compatible within the errors, with either the historically presumed radio core position, 0.7" off-, or with an hard X-ray source, 0.4"off- the stellar kinematic center. As both could be a manifestation of nuclear activity, we consider the two as possible galactic nucleus candidates (Muller-Sanchez et al. 2010).

n253-composite-and-2cm

VLA / A-array 2 cm- and VLT adaptive optics J-, Ks- and L- bands composite- image of the central 400 pc region of NGC 253. A total of 37 regions above 3 sigma of the local background are identified within the central 150 pc -square- in the IR, eight of them have a radio counterpart at 2 cm. Most of the regions are spatially resolved in the VLT image, with median size FWHM ~ 3.3 pc (0.17 arcsecs). Best spatial resolution was achieved in L-band, FWHM ~ 0.13 arcsecs. North up, East left.

 

 

Unveiling the active nucleus in obscured AGN and the size of the torus (Prieto et al. 2010)

VLT adaptive optics images in the infrared of the nearest AGN are revealing the real spatial location of the active nucleus. Dust obscuration has being leading to wrong identifications in the optical in many cases. The subarcsec resolutions achieved at near-IR wavelengths is critical for identifying with high accuracy the true nucleus location. We find that in many, well known Seyfert 2 galaxies, the nucleus remains hidden up to about 1 um. Yet, from 2 um onward, it becomes the most outstanding source in the galaxy.

j-k

The figure above shows examples of obscured nuclei shortward of 1 um: e.g. NGC 1068 and NGC 5506. These are color images, J – K, of the central kpc region of these galaxies, the nucleus location is the bright source at the centre. In both cases, this bright source is unresolved down to the achieved resolution at the VLT with adaptive optics: FWHM < 0.06 arcsec in K-band. This value sets already an upper limit to the size of any putative torus at 2 um, and in turn of the location of the warmer dust, to be less than 4 pc in NGC 1068, 18 pc in NGC 5506, in diameter (Prieto & Meisenheimer 2004; Prieto et al. 2010).