ZBLLAC: A Spectroscopic Database of BL Lacertae Objects

BL Lacertae (BLL) are a peculiar class of low power ($L\sim {10}^{43}$ erg sec⁻¹) active galactic nuclei (AGN) whose relativistic jet, generated by the accretion of matter onto a supermassive black hole, is closely aligned with the observer's line of sight (e.g., Stannard & McIlwrath 1982). In this condition, the radiation produced by the jet is boosted due to the relativistic Doppler aberration and, in most cases, dominates the spectral energy distribution (SED) at almost any wavelength of the electromagnetic spectrum (see e.g., Maraschi et al. 1992; Ghisellini et al. 1993; Urry & Padovani 1995). A peculiar spectroscopic characteristic of BLL is that, at variance with other active galactic nuclei, the emission lines are absent or extremely weak (see, e.g., Falomo et al. 2014 for a recent review) and the boosted nonthermal continuum, in most of the cases, outshines the contribution from the starlight of the host galaxy, which is typically a giant elliptical with M_v  ∼ −22.50 (Sbarufatti et al. 2005a). More generally, BLL are a subclass of a larger parent population called blazars, which encompass even more powerful sources (namely flat spectrum radio quasars (FSRQ)) with bolometric luminosity of the order of 10⁴⁷ and 10⁴⁸ erg sec⁻¹ and optical spectra characterized by broad emission lines typical of quasars, suggesting the presence of a radiatively efficient accretion disk (Shakura & Sunyaev 1973).

The quasi-featureless continuum exhibited by BLL is a characteristic that made them rather elusive since the determination of the redshift, which is a fundamental parameter to determine the distance and derive physical quantities, is in many cases hindered. Historically, various groups made huge efforts to secure spectroscopic data with small-medium sized telescopes aiming to increase the number of BLL for which a firm determination of z was assessed; although for many objects this was limited to bright or moderately beamed sources due to the modest signal-to-noise ratio reachable with the available instrumentation (e.g., Miller et al. 1978; Stickel et al. 1988, 1989; Falomo & Treves 1990; Falomo et al. 2002).

Many surveys from the radio band (e.g., 1 Jy radio catalog; Stickel et al. 1991) to X-ray missions (for instance, the Einstein Observatory, ROSAT) allowed to increase the sample of known candidate BLLs stimulating spectroscopical followups in the optical band were carried out with various optical telescope facilities (see e.g., Stocke et al. 1985; Lawrence et al. 1996, and reference therein). The availability of deep optical surveys like the Sloan Digital Sky Survey (SDSS) also contributed to enlarge the number of BLLs. In particular, Plotkin et al. (2010) discovered about 700 BLLs candidates by combining data from SDSS and radio catalogs and constrained the z for 30% of them. More recently, at high energies, the advent of Fermi mission (Atwood et al. 2009) showed that BLLs represent the dominant extragalactic population in gamma-rays and, intriguingly, that for most of them, only poor quality spectroscopical data were available. Various methodologies, based on multiwavelength data (e.g., Massaro et al. 2011, 2012b, 2013; D'Abrusco et al. 2014; Nori et al. 2014; Massaro et al. 2014, 2015b, 2016; Massaro & D'Abrusco 2016; Paiano et al. 2017a and reference therein), were developed to associate low-energy counterparts of objects detected by Fermi further encouraged spectroscopic campaigns (e.g., Paggi et al. 2014; Landoni et al. 2015a; Massaro et al. 2015c; Ricci et al. 2015; Álvarez Crespo et al. 2016a, 2016b, 2016c; Paiano et al. 2017c; Marchesi et al. 2018; Peña-Herazo et al. 2019). In this context, hundreds of BLLs spectra have been successfully secured (see, e.g., the review from Massaro et al. 2016) but only for the brightest targets the redshift was measured. In fact, for the faintest sources or extremely beamed BLLs, only observations with large aperture optical telescope (like the Very Large Telescope, Keck Telescope, and Gran Telescopio Canarias) equipped with state-of-the-art instrumentation could secure high signal-to-noise ratio spectra of these targets and reveal weak spectral features (intrinsic or intervening) to firmly determine their redshift (see e.g., Sbarufatti et al. 2005b, 2006, 2009; Landoni et al. 2013; Sandrinelli et al. 2013; Landoni et al. 2015a, 2015b, 2018; Shaw et al. 2013; Pita et al. 2014; Paiano et al. 2017c, 2017b, 2018, 2019).

The results of all these spectroscopic campaigns are frequently summarized across multiwavelength catalogs in the literature (e.g., Roma Blazar Catalog (BZCaz) and Teraelectronvolt Catalog (TevCAT); see Wakely & Horan 2008; Massaro et al. 2015a) but none of them allows scientists to access the fully calibrated spectra of the sources with a homogeneous set of figures that permit to identify the spectral features detected in the spectra and then reuse the data for many different scientific aims. Motivated by these facts, we developed a new web-based database of BLL objects, namely Z BL Lacertae objects (ZBLLAC; https://web.oapd.inaf.it/zbllac/), which is able to act as an online hub where optical spectra secured in the context of different publications with heterogeneous instrumentation are stored and made available to the community.

We present in this paper the properties of the database and of the web application. ZBLLAC includes a smart representation of the data to provide a facilitated access to the basic information of each source and, specifically, to the machine readable 1D calibrated spectra along with a .pdf figure that shows the spectroscopic identification of firmly detected emission or absorption lines. The paper is organized as follows: in Section 2 we give an overview of our database and the website, while in Section 3 we detail on the format of our data. In Section 4 we discuss properties of the data set and we report our conclusions and future perspectives in Section 5.

At the time of writing of this work, the ZBLLAC database contains 337 objects considered as BLLs or BLL candidates in the literature. The sources were selected among heterogeneous criteria such as their detection in gamma-rays at GeV or TeV band (see, e.g., Atwood et al. 2009; Paiano et al. 2017c), Wide Field Infrared Survey Explorer (WISE) infrared color (Massaro et al. 2012a; D'Abrusco et al. 2019; de Menezes et al. 2019, 2020), or the properties of their broadband SED (Padovani & Giommi 1995a, 1995b; Costamante & Ghisellini 2002; Costamante 2020). According to the properties of their optical spectra in ZBLLAC, we labeled 295 objects as BLLs by adopting spectral criteria based on the absence of emission features or, if detected, on the value of the equivalent width (EW), luminosities, and broadness of the lines. The remaining 42 targets exhibit spectra dominated by broad and intense emission lines, suggesting to us an alternative classification, and for this reason they have not been considered as BLLs.

For each object we give α, δ, the catalog name, the redshift, and the magnitude and provide a flux calibrated spectrum, ⁷ dereddened for Galactic extinction. The spectra are available both in text format and with a .pdf figure that reports both the flux calibrated and normalized spectra. The main detected features, if present, are marked and identified (see examples of Figure 1). We also note that for 37 BLLs, more than one spectrum, secured in different epochs and with different instrumentation, is reported in the database.

Zoom In Zoom Out Reset image size

The web interface of ZBLLAC is reported in Figure 2. The user can retrieve and interactively explore the data through the Spectroscopic Database page, which shows, by default, the full list of sources present in ZBLLAC. For each of them, the application displays basic information and a set of buttons to download the spectrum and the annotated .pdf figure. When more then one observation is associated to the same object, a further button is displayed allowing the user to select the spectrum, or figure, to download among all the available ones for the source. Finally, we implemented a Search Panel, shown on top of the page as reported in Figure 2, which permits us to actively filter the database according to various criteria such as being coordinated within a radius, target name, or redshift range.

Zoom In Zoom Out Reset image size

To store the data, we used the eXtensible Markup Language (XML ⁸ ) standard that allows us to define a proper data structure and model by combining the possibility to easily query the database while being both machine and human readable. More specifically, the XML is a markup language that defines a set of rules for encoding plain-text documents in which the data are marked by tags or attributes (see Bosak & Bray 1999 for a full review). Data within XML documents are organized using a tree-like data structure, where each node may posses one or more leafs.

We decided to adopt a representation of the data by using the XML scheme reported in Figure 3. The root node of the structure is zbllac, which contains the set of all object of the ZBLLAC database as leafs. ⁹ Each object (see Figure 3) contains as attributes all the relevant information to identify the source (name, coordinates, etc.) and two mores nodes: the first one, named nedlink, contains the link to the object's NED page while the second (spectra) harbors a set of spectrum nodes where the information about each observation and the relative HTTP links to download the data are saved. To further illustrate the organization of the data, we show in Figure 4 a sample node of our .xml file for the object PKS 1553+113 for which three different spectra have been secured.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Regarding, the website back-end, we made use of standard PHP pages coupled with XQuery and XPath ¹⁰ protocols to retrieve the data throughout the .xml file.

4.1. General Properties

The data set of BLLs, which currently encompass 295 targets, can be retrieved using the Spectroscopic database page (see Figure 2) by selecting the flag BLL.

For 103 (∼35%) objects, intrinsic spectral features are revealed, allowing us to firmly measure the redshift. In detail, for 31 objects, only emission lines are detected in their spectra, while in 55 BLLs, only features from the host galaxy (mainly ascribed to the Ca ii and G bands) are present. In 17 cases, both emission and absorption features are revealed on the same spectrum. The median value of z is 0.40, ranging between 0.071 and 1.636. The distribution of the redshifts is given in Figure 5 where we also report an histogram of z for objects in which the host galaxy has been detected. As shown in Figure 5, the determination of the redshift for object at $z\gtrsim 0.80$ is assessed only through emission lines since the features from the host galaxy (e.g., Ca ii λλ 3934-3968) start to move outside the covered spectral range (which is, on average, between 4000 Å and 8000 Å for objects in ZBLLAC).

Zoom In Zoom Out Reset image size

Finally, for 35 BLLs that do not show intrinsic features, we detected intervening absorption systems along the line of sight, allowing us to establish a firm lower limit to their redshifts (see Section 4.3).

4.2. Emission Lines Properties

We detected emission features in 48 BLLs. The line identification and luminosities are given in Table 1.

Table 1. Properties of the Emission Lines Detected in the Spectra of BLLs that Belong to the ZBLLAC Database

Source	z	Line	EW Å	FWHM (km s−1)	L (erg s−1)	Type
ZBLL J0035+5950	0.467	[O ii] (λ3737)	0.3	350	$9.7\cdot {10}^{40}$	N
ZBLL J0050–0929	0.635	[O ii] (λ3727)	0.5	600	$1.0\cdot {10}^{41}$	N
		[O iii] (λ5007)	0.6	450	$9.4\cdot {10}^{40}$	N
		H $\alpha (\lambda$6563)	1.6	1300	$1.6\cdot {10}^{41}$	N
ZBLL J0048+4223	0.302	[O ii] (λ3727)	1.4	920	$4.6\cdot {10}^{40}$	N
		[O iii] (λ5007)	1.2	350	$4.2\cdot {10}^{40}$	N
ZBLL J0158+0101	0.4537	[O iii] (λ5007)	5.0	350	$7.0\cdot {10}^{40}$	N
ZBLL J0303–2407	0.2657	[O iii] (λ5007)	0.2	300	$8.5\cdot {10}^{40}$	N
		H $\alpha (\lambda$6563)	0.2	300	$7.7\cdot {10}^{40}$	N
		[N ii] (λ6585)	0.2	500	$1.0\cdot {10}^{41}$	N
		[O ii] (λ3727)	0.1	450	$1.2\cdot {10}^{41}$	N
ZBLL J0301–1652	0.278	[O iii]$(\lambda$5007)	1.5	400	$3.4\cdot {10}^{40}$	N
ZBLL J0305–1608	0.312	[O ii] (λ3727)	2.0	800	$1.7\cdot {10}^{41}$	N
ZBLL J0316–2607	0.443	[O ii] (λ3727)	0.52	1600	$1.3\cdot {10}^{41}$	N
ZBLL J0340–2119	0.223	[O ii] (λ3727)	1.5	1500	$2.9\cdot {10}^{40}$	N
		[O iii] (λ4960)	0.6	1200	$2.6\cdot {10}^{40}$	N
ZBLL J0428–3756	1.105	C iii] (λ1908)	2.3	3500	$6.2\cdot {10}^{42}$	B
		Mg ii (λ2800)	3.0	4500	$6.3\cdot {10}^{42}$	B
		[O ii] (λ3727)	0.5	650	$9.2\cdot {10}^{41}$	N
ZBLL J0509+0542	0.3365	[O ii] (λ3737)	0.07	500	$1.0\cdot {10}^{41}$	N
		[O iii] (λ5007)	0.05	600	$9.2\cdot {10}^{40}$	N
		[N ii] (λ6585)	0.05	300	$6.7\cdot {10}^{40}$	N
ZBLL J0550–3216	0.068	[N ii] (λ6585)	0.8	550	$2.0\cdot {10}^{42}$	N
ZBLL J0757+0956	0.266	[O ii] (λ3727)	0.6	850	$1.4\cdot {10}^{41}$	N
		[O iii] (λ5007)	0.9	1100	$1.9\cdot {10}^{41}$	N
ZBLL J0811+0146	1.148	C iii] (λ1908)	1.0	3000	$2.0\cdot {10}^{42}$	B
		Mg ii (λ2800)	1.5	4000	$3.0\cdot {10}^{42}$	B
ZBLL J0820–1259	0.539	[O ii] (λ3727)	1.2	1000	$5.8\cdot {10}^{41}$	N
		H $\beta (\lambda$4862)	0.5	850	$1.9\cdot {10}^{41}$	N
		[O iii] (λ5007)	2.5	650	$8.4\cdot {10}^{41}$	N
ZBLL J0930+5132	0.1893	[O iii] (λ4960)	1.0	450	$1.0\cdot {10}^{41}$	N
ZBLL J0942–0047	1.363	C iii] (λ1908)	4.8	200	$4.5\cdot {10}^{41}$	N
		Mg ii (λ2800)	5.0	1600	$1.0\cdot {10}^{43}$	N
ZBLL J1008–3139	0.534	[O ii] (λ3727)	1.0	600	$2.3\cdot {10}^{41}$	N
ZBLL J1012+0631	0.727	Mg ii $(\lambda$2800)	0.5	1200	$4.0\cdot {10}^{41}$	N
		[O ii] (λ3727)	0.3	900	$5.0\cdot {10}^{41}$	N
ZBLL J1046+5449	0.252	[O iii] (λ5007)	4.0	700	$1.2\cdot {10}^{41}$	N
ZBLL J1049+1548	0.326	[O ii] (λ3727)	0.3	1200	$1.2\cdot {10}^{41}$	N
ZBLL J1058–8003	0.581	Mg ii (λ2800)	1.2	2500	$4.6\cdot {10}^{42}$	B
		[O iii] (λ4960)	0.5	600	$8.3\cdot {10}^{41}$	N
		[O iii] (λ5007)	1.4	600	$2.5\cdot {10}^{42}$	N
ZBLL J1117+2014	0.140	[O ii] (λ3727)	0.8	1200	$5.2\cdot {10}^{40}$	N
ZBLL J1203–3926	0.227	[O ii] (λ3727)	2.4	600	$6.1\cdot {10}^{40}$	N
		[O iii] (λ4960)	1.4	800	$3.7\cdot {10}^{40}$	N
		[O iii] (λ5007)	2.82	550	$7.3\cdot {10}^{40}$	N
ZBLL J1215+0732	0.137	H $\alpha (\lambda$6563)	1.3	600	$3.3\cdot {10}^{40}$	N
ZBLL J1217+3007	0.129	[O ii] (λ3727)	0.2	600	$6.5\cdot {10}^{40}$	N
		[O iii] (λ5007)	0.2	650	$5.1\cdot {10}^{40}$	N
ZBLL J1221+2813	0.102	[O iii] (λ5007)	0.8	600	$4.8\cdot {10}^{40}$	N
ZBLL J1231+3711	0.219	[O ii] (λ3727)	5.0	1000	$1.4\cdot {10}^{41}$	N
ZBLL J1240+3445	1.636	Mg ii (λ2800)	4.5	3500	$4.2\cdot {10}^{42}$	B
ZBLL J1247+4423	0.569	[O iii] (λ5007)	1.4	400	$1.8\cdot {10}^{41}$	N
ZBLL J1259–2310	0.481	[O ii] (λ3727)	0.9	1100	$4.5\cdot {10}^{41}$	N
		[O iii] (λ5007)	0.4	600	$1.7\cdot {10}^{41}$	N
ZBLL J1309+4305	0.693	[O ii] (λ3727)	1.2	800	$2.0\cdot {10}^{42}$	N
		[O iii] (λ5007)	0.5	600	$5.5\cdot {10}^{41}$	N
ZBLL J1427+2348	0.604	[O ii] (λ3727)	0.1	300	$5.0\cdot {10}^{41}$	N
		[O iii] (λ5007)	0.2	400	$1.1\cdot {10}^{42}$	N
ZBLL J1522–2730	1.297	Mg ii (λ2800)	0.4	2000	$3.0\cdot {10}^{42}$	B
ZBLL J1541+1414	0.223	[O iii] (λ5007)	1.0	500	$3.4\cdot {10}^{40}$	N
ZBLL J1626–7638	0.1050	[O I] (λ6302)	1.5	1000	$3.7\cdot {10}^{40}$	N
		[S II] ($\lambda 6718-6732$)	2.0	700	$4.6\cdot {10}^{40}$	N
ZBLL J1637+1314	0.655	[O ii] (λ3727)	0.4	800	$1.5\cdot {10}^{41}$	N
ZBLL J1704+1234	0.452	[O ii] (λ3727)	2.3	900	$4.0\cdot {10}^{41}$	N
		[O iii] (λ4960)	0.7	600	$1.1\cdot {10}^{41}$	N
		[O iii] (λ5007)	3.0	600	$5.0\cdot {10}^{41}$	N
ZBLL J1917–1921	0.137	[O ii] (λ3727)	0.2	600	$3.5\cdot {10}^{40}$	N
		[O iii] (λ4960)	0.2	1000	$2.4\cdot {10}^{40}$	N
		[O iii] (λ5007)	0.5	900	$5.0\cdot {10}^{40}$	N
ZBLL J2009–4849	0.071	H $\alpha (\lambda$ 6563)	0.3	400	$2.0\cdot {10}^{40}$	N
ZBLL J2134–0153	1.284	C iii] (λ1908)	1.2	1800	$2.2\cdot {10}^{42}$	B
		C II] (λ2326)	0.5	1800	$9.0\cdot {10}^{41}$	B
		Mg ii (λ2800)	2.3	3500	$4.0\cdot {10}^{42}$	B
ZBLL J2152+1734	0.870	Mg ii (λ2800)	2.7	3000	$1.2\cdot {10}^{42}$	B
		[O ii] (λ3727)	2.2	1100	$9.5\cdot {10}^{41}$	N
ZBLL J2209–0451	0.3967	[O ii] (λ3727)	0.5	300	$7.7\cdot {10}^{40}$	N
ZBLL J2225–1113	0.997	[O ii] (λ3727)	1.8	800	$2.0\cdot {10}^{41}$	N
ZBLL J2246+1544	0.5965	[O ii] (λ3727)	0.9	850	$2.6\cdot {10}^{41}$	N
ZBLL J2250+1749	0.3437	[Ne V] (λ3426)	1.5	350	$4.3\cdot {10}^{40}$	N
		[O ii] (λ3727)	3.0	500	$9.5\cdot {10}^{40}$	N
		[O iii] (λ4960)	1.0	350	$5.4\cdot {10}^{40}$	N
		[O iii] (λ5007)	4.5	500	$2.3\cdot {10}^{41}$	N
ZBLL J2349+0534	0.419	Mg ii (λ2800)	4.6	3000	$6.6\cdot {10}^{41}$	B
		[O ii] (λ3727)	3.0	1000	$3.5\cdot {10}^{41}$	N
		[O iii] (λ4960)	1.6	750	$1.6\cdot {10}^{41}$	N
		[O iii] (λ5007)	3.5	700	$3.5\cdot {10}^{41}$	N
ZBLL J2357–0152	0.812	Mg ii (λ2800)	1.4	1500	$2.8\cdot {10}^{41}$	N

Download table as:  ASCIITypeset images: 1 2

In 28 cases, the only observed lines are narrow forbidden transition ascribed to [O ii] (λ 3934 Å) and [O iii] (λ 5007 Å) while broad spectral features, mainly associated to Mg ii (λ 2800) and C iii] (λ 1908), are revealed in just eight targets. We also note that, for only four cases, broad and narrow emission lines are present on the very same spectrum. These facts may suggest that, in the majority of the cases, there is no trace of the broad line region, possibly meaning that either the physical conditions for its appearance are absent or that the lines are so broad and swamped by the continuum that they are not detected. We report in Figure 6 the distribution of the luminosity of emission lines ascribed to [O ii] (λ 3934 Å) and [O iii] (λ 5007 Å). The median luminosity for [O ii] is $1.7\times {10}^{41}$ erg s⁻¹, while in the case of [O iii], we found $1.3\times {10}^{41}$ erg s⁻¹.

Zoom In Zoom Out Reset image size

Following the same approach described in Paiano et al. (2020), we compared our luminosities of [O ii] and [O iii] with those measured on spectra of low-redshift quasi-stellar objects (Shen et al. 2011, with similar luminosity on the continuum, assuming a beaming factor $\delta \,\sim$ 10) and find that their mean values are roughly similar. This result is in agreement with Paiano et al. (2020), but in this case, our conclusions are based on a data set that is significantly larger.

4.3. Intervening Absorption Systems

In the spectra of BLLs, absorption lines could arise intrinsically from the host galaxy, yielding directly to the determination of z, or from the intervening system if a cool gas cloud structure is intercepted along the line of sight. In this case, the detection of an intervening absorption gives a robust lower limit to the redshift of the source. We detected those systems in 35 objects and we report our measurements on Table 2. In the wavelength range covered by our collection of spectra, the main absorptions are those related to the Mg ii doublet transition (λλ 2796-2803 Å), when the redshift of the absorber is between $0.40\leqslant z\leqslant 1.9$. In fact, in 30 cases, we reveal spectral lines ascribed to Mg ii (λ2800 Å) that allow us to set a lower limit to z. Furthermore, in three sources, both at redshift $z\gtrsim 2.00$, we detected the onset of a Lyα forest (see Landoni et al. 2018; Paiano et al. 2017c for details) and a further intervening system, at a lower redshift, associated to Mg ii, C iv, and Fe ii (see Table 2). In a couple of targets, features arising from Ca ii (λ3934 Å) are detected in intervening systems along the line of sight.

Table 2. Properties of the Intervening Absorption Lines Detected in the Spectra of BLL that Allow Us to Derive a Redshift Lower Limit

Source	Line	EW (Å)	z abs
ZBLL J0003+0841	Mg ii (λ 2800)	1.50	1.5035
ZBLL J0008+4712	Mg ii (λ 2800)	2.00	1.659
ZBLL J0033−1921	Mg ii (λ 2800)	0.20	0.505
ZBLL J0038+0012	Mg ii (λ 2800)	0.70	0.80
ZBLL J0234−0628	Mg ii (λ 2800)	7.00	0.63
ZBLL J0251−1831	Mg ii (λ 2800)	3.50	0.615
ZBLL J0338+1302	Mg ii (λ 2800)	3.00	0.382
ZBLL J0441−2952	Mg ii (λ 2800)	2.15	0.68
ZBLL J0644+6038	Mg ii (λ 2800)	5.00	0.581
ZBLL J0649−3139	Mg ii (λ 2800)	3.00	0.563
ZBLL J0816−1311	Mg ii (λ 2800)	0.15	0.2882
	Mg ii (λ 2800)	0.60	0.2336
	Mg ii (λ 2800)	1.00	0.1902
ZBLL J0848+7017	Mg ii (λ 2800)	11.30	1.2435
ZBLL J1107+0222	Mg ii (λ 2800)	2.00	1.0735
ZBLL J1129+3756	Mg ii (λ 2800)	9.10	1.211
ZBLL J1231+0138	Ly α (1216)	15.00	3.140
	Mg ii (λ 2800)	6.00	2.004
	Fe ii (λ 2600)	5.00	2.004
	Mg ii (λ 2800)	4.00	2.004
ZBLL J1223+0820	Mg ii (λ 2800)	0.90	0.7187
ZBLL J1243+3627	Mg ii (λ 2800)	0.90	0.48
ZBLL J1312−2350	Mg ii (λ 2800)	2.50	0.462
ZBLL J1351+1114	Mg ii (λ 2800)	1.00	0.619
ZBLL J1450+5201	Ly α (λ 1216)	5.80	2.470
	C iv (λ 1908)	3.20	2.470
	C iv (λ 1908)	1.10	2.312
ZBLL J1511−0513	Mg ii (λ 2800)	2.10	0.451
ZBLL J1540+8155	Mg ii (λ 2800)	0.60	0.672
ZBLL J1730−0352	Mg ii (λ 2800)	7.50	0.776
ZBLL J1955−1603	Mg ii (λ 2800)	3.00	0.638
ZBLL J1959−4725	Mg ii (λ 2800)	2.30	0.519
ZBLL J1200+4009	Ly α (λ 1216)	13.00	3.367
	Mg ii (λ 2800)	2.50	1.484
	Fe ii (λ 2600)	2.00	1.484
	Mg ii (λ 2800)	4.50	1.142
ZBLL J2107−4828	Mg ii (λ 2800)	4.30	0.519
ZBLL J2115+1218	Mg ii (λ 2800)	4.00	0.497
	Mg ii (λ 2800)	0.90	0.525
	Mg ii (λ 2800)	0.90	0.633
ZBLL J2139–4235	Ca ii (λλ 3934-3968)	0.25	0.0087
ZBLL J2212+2759	Mg ii (λ 2800)	3.90	1.529
ZBLL J2236−1433	Mg ii (λ 2800)	0.70	0.490
	Mg ii (λ 2800)	0.90	0.493
ZBLL J2247+0000	Mg ii (λ 2800)	3.00	0.898
ZBLL J2255+2410	Mg ii (λ 2800)	0.70	0.8633
ZBLL J2319+1612	Mg ii (λ 2800)	1.50	0.970
ZBLL J2323+4210	Ca ii (λλ 3934-3968)	0.50	0.267
	Na I (λ 5892)	0.35	0.267

Download table as:  ASCIITypeset images: 1 2

The median value of our lower limits is $z\sim 0.64$ and we report their distribution in Figure 7. The peak around $z\sim 0.60$ is related to our spectral range, where the probability of the detection of Mg ii is maximized. We note that lower limits for sources at $z\leqslant 0.40$ are in one case still ascribed to Mg ii (ZBLL J0816−1311) because data has been obtained with the European Southern Observatory X-Shooter (Vernet et al. 2011) that provides increased spectral coverage in the UV (Pita et al. 2014), while the other two cases are associated with the intervention of Ca ii (λ3934 Å).

Zoom In Zoom Out Reset image size

Finally, note that on the remaining 157 BLLs that still appear to be featureless. In our data, the total number of absorbers of Mg ii, considering both multiple systems and those found in BLLs with z, is 46. However, according to Zhu & Ménard (2013) and Landoni et al. (2013), the expected number of detected Mg ii systems in our data set in the redshift range $0.40\leqslant z\leqslant 1.90$ (with EW ≥ 1.00 Å) should be of the order of ∼100. This consideration suggests that, in order to cope with this statistic, the 157 featureless BLLs should lie statistically at redshifts $z\lesssim 0.70$ (see also Paiano et al. 2020 for similar conclusions).

We described the ZBLLAC database that currently contains optical spectra for 295 BLLs. We discussed the spectroscopic properties of the objects in this data set, finding that, for 35% of them, intrinsic spectral features are revealed, allowing us to solidly measure the redshift. We reported on 35 targets in which, by detecting intervening absorption systems, we set tight lower limits on their z. Based on the absence of absorption lines ascribed to Mg ii in 157 featureless objects, we statistically suggest that they should lie at low redshifts $z\lesssim 0.70$.

The ZBLLAC spectroscopic database of BLL objects is an ongoing and still growing project. We encourage other groups to contribute by sharing their own published data. Instructions for joining our project and contributing to the data set can be found at https://web.oapd.inaf.it/zbllac/Instructions.pdf.

We thank B. Sbarufatti (Penn State University, USA), F. Massaro (University of Torino), N. Crespo (ESA), E. Marchesini (Universidad Nacional de La Plata, Buenos Aires), A. Paggi (University of Torino), H. Pena-Herazo (Instituto Nacional de Astrofísica Óptica y Electrónica, Mexico), F. Ricci (Pontificia Universidad Catolica de Chile), and S. Pita (Universite Paris Diderot) for their help in enriching the ZBLLAC database by sharing data.